SM1.1 | General Contributions on Earthquakes, Earth Structure, Seismology
General Contributions on Earthquakes, Earth Structure, Seismology
Convener: Alice-Agnes Gabriel | Co-conveners: João Fontiela, Philippe Jousset, Joana Carvalho
Orals
| Wed, 26 Apr, 14:00–18:00 (CEST)
 
Room D1
Posters on site
| Attendance Wed, 26 Apr, 10:45–12:30 (CEST)
 
Hall X2
Posters virtual
| Attendance Wed, 26 Apr, 10:45–12:30 (CEST)
 
vHall GMPV/G/GD/SM
Orals |
Wed, 14:00
Wed, 10:45
Wed, 10:45
The session General Contributions on Earthquakes, Earth Structure, Seismology features a wide range of presentations on recent earthquakes and earthquake sequences of local, regional, and global significance, as well as recent advances in characterization of Earth structure using a variety of methods.
We will have a special section this year about challenges associated to monitoring seismology in volcanic islands.

Orals: Wed, 26 Apr | Room D1

Chairpersons: Alice-Agnes Gabriel, João Fontiela
14:00–14:05
14:05–14:15
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EGU23-1264
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ECS
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On-site presentation
Pepen Supendi, Tom Winder, Nicholas Rawlinson, Conor Bacon, Daryono Daryono, Andrean Simanjuntak, Kadek Hendrawan Palgunadi, Hasbi Ash Shiddiqi, Andri Kurniawan, Priyobudi Priyobudi, Sri Widiyantoro, Andri Dian Nugraha, Suko Prayitno Adi, and Dwikorita Karnawati

A destructive earthquake (Mw 5.6) struck Cianjur, West Java, Indonesia, on 21 November 2022, resulting in at least 321 deaths, damage to 47,000 buildings, and economic losses of up to 7.7 trillion Indonesian Rupiahs (∼US $546 million). The causative fault that generated this earthquake was not previously recognised, therefore making further analysis crucial for assessing future seismic hazard in the region. In this study, we undertake automated event detection and location using QuakeMigrate from 10 days before to 23 days after the mainshock, prior to relocation using a double-difference method. We also determine the source mechanism for selected aftershocks from waveform inversion. Our result show that the mainshock was preceded by three clear foreshocks, and intriguingly the aftershocks appear to reveal the presence of a conjugate fault pair trending northwest-southeast with a length of ~8 km and southwest-northeast with a length of ~5 km.

How to cite: Supendi, P., Winder, T., Rawlinson, N., Bacon, C., Daryono, D., Simanjuntak, A., Palgunadi, K. H., Shiddiqi, H. A., Kurniawan, A., Priyobudi, P., Widiyantoro, S., Nugraha, A. D., Adi, S. P., and Karnawati, D.: Identification of a hidden fault associated with the Mw 5.6 (November 21, 2022) Cianjur Earthquake, West Java, Indonesia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1264, https://doi.org/10.5194/egusphere-egu23-1264, 2023.

14:15–14:25
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EGU23-5367
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On-site presentation
Mario Ruiz, Jordi Diaz, Ariadna Canari, Maria Ortuño, and Jaume Vergés

Although the seismic activity at the eastern Pyrenees is nowadays moderate and sparse, with events usually not exceeding magnitudes 4.5, this area has been affected in the past by the most destructive event occurred in the Pyrenees, reaching an intensity of IX, whose seismogenic structure is not well  understood and still under debate. In order to progress in the knowledge of these structures, we present here the results derived from a 14 months-long broad-band seismic deployment focused on the Cerdanya Basin, but encompassing the eastern termination of the Pyrenees. The dense station coverage has allowed us to obtain accurate hypocentral locations, as well as up to 23 focal mechanisms from local low-magnitude earthquakes. In addition to a relatively sparce seismicity, several clusters of seismic events located in well-defined, small areas and depth ranges have been identified. The results show a few low-magnitude seismic events located in the southern limit of the Cerdanya Basin that could be related to oblique secondary faults within the footwall of the Têt Fault, the major tectonic structure in the area. Our data shows also that the Capcir Fault has associated seismicity, with some of the events located out of the fault plane, perhaps on secondary fault branches. To the east, a cluster of low-magnitude events is detected in the epicentral area of two relatively large earthquakes occurred recently, probably indicating the development of the preparatory phase. Further west, the Maladeta Massif has a sustained seismic activity, although its origin does not seem to be related to the most relevant structure in the area, the North Maladeta Fault. Regarding focal mechanisms, most of them show normal fault solutions with nodal planes NW-SE oriented, which are in agreement with the extensional regime perpendicular to the axis of the chain derived from the seismic and geodesic data.

 

How to cite: Ruiz, M., Diaz, J., Canari, A., Ortuño, M., and Vergés, J.: Advances in the knowledge of seismogenetic structures in the eastern edge of the Pyerenees, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5367, https://doi.org/10.5194/egusphere-egu23-5367, 2023.

14:25–14:35
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EGU23-17567
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On-site presentation
Natalia Poiata, Bogdan Grecu, Dragos Tataru, and Felix Borleanu

Vrancea seismic zone is the most important seismically active region in Romania, representing the main source of seismic hazard in the area and neighbouring countries. The largest significant earthquakes of the past century, M 7.7 and 7.4 in 1940 and 1977, caused major and widespread destruction. The intermediate-depth earthquakes from Vrancea have a particular space distribution, being constrained to a compact volume (60-180 km in depth and 20x50 km areal extent) and falling into the category of, so called, “seismic nests”, with a peculiar and not well understood seismogenic mechanisms.

We present first results of the repeating events identification for both the crustal and intermediate-depth activity from the Vrancea seismic region, obtained by multi-channel waveform-similarity (cross-correlation - cc) analysis for earthquakes extracted from the ROMPLUS catalogue. The analysed events cover the time-period of about 2.5 years (August 2016 – December 2018) and contains 1229 earthquakes with magnitude of 2.5 – 5.9. Of these events 630 correspond to crustal and 599 – to the intermediate-depth earthquakes. Our analysis identifies 37 families of similar events (cc > 0.7) with the largest one composed of 30 events. We observe that most of the repeating events families are located in the deeper portion of the Vrancea intermediate-depth seismic volume, at the depth greater than 100 km. The identified most active family corresponds to the average depth of 140 km. Most of the intermediate-depth families are characterized by the activity that is persistent in time over the analysed time-period. For the crustal seismicity, we identified over 35 similar event families, most of which corresponding to the short-lived activations of the local (fault) structures.

In the presentation, we will discuss how these results, in relation to the structural composition and tectonic regime of the region, can help to better understand the depth-dependent activity of the Vrancea seismic zone and whether there exists a potential correlation between the crustal and intermediate-depth seismicity.

How to cite: Poiata, N., Grecu, B., Tataru, D., and Borleanu, F.: Repeating crustal and intermediate-depth earthquakes from the Vrancea seismic region., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17567, https://doi.org/10.5194/egusphere-egu23-17567, 2023.

14:35–14:45
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EGU23-1449
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Highlight
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On-site presentation
Matteo Picozzi, Daniele Spallarossa, Dino Bindi, Antonio Giovanni Iaccarino, and Eleonora Rivalta

We consider approximately 23,000 microearthquakes occurred between 2005 and 2016 in central Italy to investigate the crustal strength before and after the three largest earthquakes of the 2016 seismic sequence (i.e., the Mw 6.2, 24 August 2016 Amatrice, the Mw 6.1, 26 October 2016 Visso, and the Mw 6.5, 30 October 2016 Norcia earthquakes). We monitor the spatio-temporal deviations of the observed radiated energy, ES, with respect to theoretical values, ESt, derived from a scaling model between ES and M0 calibrated for background seismicity in central Italy. These deviations, defined here as Energy Index (EI), allow us to identify the onset of the activation phase one week before the mainshock. We show that foreshocks are characterized by a progressive increase in slip per unit stress, in agreement with the diffusion of highly pressurized fluids before the LAquila earthquake proposed by previous studies. Our results suggest that the largest events occur where EI is highest, in agreement with the existing link between EI and the mean loading stress.

Furthermore, our results show a progressive evolution of the dynamic properties of microearthquakes in the years following the Mw 6.1, 2009 LAquila earthquake, and the existence of high EI patches close to the Amatrice earthquake hypocenter. We show the existence of a crustal volume with high EI even before the Mw 6.5 Norcia earthquake. Our results agree with the previously suggested hypothesis that the Norcia earthquake nucleated at the boundary of a large patch, highly stressed by the two previous mainshocks of the sequence. We highlight the mainshocks interaction both in terms of EI and of the mean loading shear stress associated to microearthquakes occurring within the crustal volumes comprising the mainshock hypocenters. Our study shows that the dynamic characteristics of microearthquakes can be seen as beacons of stress change in the crust, and, thus, be exploited to monitor the seismic hazard of a region and help to intercept the preparation phase of large earthquakes.

How to cite: Picozzi, M., Spallarossa, D., Bindi, D., Iaccarino, A. G., and Rivalta, E.: Temporal and spatial evolution of radiated energy to seismic moment scaling during the preparatory phase of the Mw 6.1, 2009 L’Aquila earthquake (Italy) and the 2016 Central Italy Seismic Sequence., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1449, https://doi.org/10.5194/egusphere-egu23-1449, 2023.

14:45–14:55
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EGU23-1534
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Virtual presentation
Lucia Lozano, Juan Vicente Cantavella, Elisa Buforn, Carolina López-Sánchez, Resurección Antón, Jaime Barco, María Victoria Manzanedo, Roberto Cabieces, and Maurizio Mattesini

The Alboran Sea is a complex tectonic region in the westernmost Mediterranean Sea, dominated by the present-day NW-SE convergence between Eurasia and Nubia plates. This compression regime accomodates long strike-slip active fault systems, together with several inverse structures, crossing the Alboran crust in a NE-SW trending shear deformation belt which mainly controlls the shallow seismicity in the area. In fact, the southern sector of the Alboran domain has experienced two large earthquakes in the last two decades, the Mw 6.3 2004 Alhoceima and the Mw 6.4 2016 Alboran events. Since mid-april 2021, and over the following 20 months, tens of moderate-magnitude shallow earthquakes (4≤Mw≤5.3, h<20 km) have been registered in this area, to the northwest of Melilla, between the 2016 main shock and the African coast. The two largest events, a Mw 5.1 on August 28, 2021 and a Mw 5.3 on May 20, 2022, were widely felt in Melilla city (maximum EMS-98 intensities of IV and IV-V, respectively) and along the southern Spanish and the Moroccan coasts. These moderate seismicity occurs together with thousands of low-magnitude events (M<3) in a swarm-type distribution, in contrast to previous seismic sequences in 2004 and 2016 which showed a more typical foreshock-mainshock-aftershock pattern. An accurate hypocentral location of this seismicity is a key point to better image the seismicity distribution and rupture area and, hence, to improve our knowledge of the active tectonics of this region, contributing to improve seismic and tsunami hazard assessments. In this study we perfom a high-precision relocation of a selected good-quality subset of moderate-magnitude earthquakes of the 2021-2022 seismic sequence and we compare them to a similar set of relocated earthquakes of the 2004 and 2016 series, using all the available seismic data. We apply a non-linear probabilistic location algorithm jointly with a 3-D velocity model for the Alboran-Betic-Rif system, to account for differences in wave propagation in the laterally heterogeneous crust. This approach is a powerful tool to improve the hypocentral parametres.

How to cite: Lozano, L., Cantavella, J. V., Buforn, E., López-Sánchez, C., Antón, R., Barco, J., Manzanedo, M. V., Cabieces, R., and Mattesini, M.: The 2021-2022 seismic sequence in southern Alboran Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1534, https://doi.org/10.5194/egusphere-egu23-1534, 2023.

14:55–15:05
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EGU23-5528
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ECS
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On-site presentation
Annie Jerkins, Andreas Köhler, and Volker Oye

CCS (Carbon Capture and Storage) is becoming an increasingly important technology to reduce global greenhouse gas emissions. The Norwegian part of the North Sea is an ideal place to conduct such projects as much of the infrastructure and experience is already in place due to current oil and gas operations. For safe CO2 storage an improved understanding of the natural background seismicity in the North Sea is still required. Currently, earthquakes in the region are monitored using onshore stations deployed on mainland Norway, resulting in low azimuthal sensor coverage and poorly constrained earthquake locations. However, permanent reservoir monitoring systems (PRMs), which are deployed offshore to surveil oil and gas fields, have the potential to reduce these station gaps and thus improve earthquake locations.

In this study we test the potential of incorporating offshore sensors at the Grane oil field for earthquake locations and detection utilizing array processing techniques. The advantage of array processing is that it can enhance seismic signals, decrease the detection threshold, and put additional constraints on direction and apparent velocities. Out of the 3400 sensors deployed at the Grane field, we have access to two subsets of data: i) Continuous data from 10 sensors spread along the boundaries of the field and, ii) segments of data from 30 sensors optimized for the purpose of array processing. Since the distances between the 10 sensors are large (6 km), traditional array processing methods are not applicable, and we therefore test and develop a new method for incoherent array processing using the kurtosis characteristic function. The kurtosis function is applied to the seismic signal prior to FK-analysis to make the signal more coherent.  The method showed great potential and worked for the majority of earthquakes analyzed in this study. The 30 sensor array was superior to the 10 sensor array and could potentially decrease the detection threshold of seismic events if continuous data are available. We conclude that the Grane sensors could be implemented as a part of a system for passive seismic monitoring in the North Sea. We recommend using the 30-sensors for this purpose. However, as we only have access to continuous data from 10 sensors, we found that these sensors are an appropriate replacement when the 30 sensors are not accessible. 

How to cite: Jerkins, A., Köhler, A., and Oye, V.: On the potential of offshore sensors and array processing for improving seismic event detection and locations in the North Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5528, https://doi.org/10.5194/egusphere-egu23-5528, 2023.

15:05–15:15
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EGU23-4905
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On-site presentation
P. Martin Mai, Laura Parisi, and Sigurjón Jónsson

The NEOM multi-billion-dollar project on the eastern coast of the Gulf of Aqaba will bring underground infrastructures, new cities, and tourist destinations. This project will dramatically increase the seismic risk associated with active faults in the Gulf of Aqaba and the northern Red Sea. The Gulf of Aqaba, located between northern Saudi Arabia and the Sinai peninsula and formed by the transtension at the southern termination of the Dead Sea Transform, is a 180-km-long fault system that can generate earthquakes of magnitude at least 7.3 (as occurred in 1995). South of the gulf, the fault system connects to the Red Sea rift, where an earthquake of magnitude larger than 5 occurred in 2020. To investigate the regional tectonics and to better understand the associated seismic hazard, we have run a temporary network of 12 broadband seismic stations in the area since 2019.

In this contribution, we present a new local magnitude scale calibrated by using more than 10,000 half-peak-to-peak amplitudes, automatically measured and Wood-Anderson-corrected, from earthquakes recorded by our network from May 2019 to February 2021. We used the amplitudes from the two horizontal components of each station to constrain the constants of the distance-dependent correction term of the local magnitude formula (n, related to the geometrical spreading, and k, related to the attenuation), magnitudes, and station corrections.

We used a least-square regression scheme in two steps to ensure the convergence of the solution and independence of the results from the initial values. In the first step, we only inverted for n, k, and magnitudes. In the second step, we also inverted for station corrections and we used the magnitudes obtained in the first step as initial values for the second step. Conversely to most previous studies, we did not introduce any constraints on the station corrections. We run several regressions in a grid search approach to tackle the trade-off between n and k and find the best solution.

We found that the estimated station corrections, because of the lack of constraints on them, are strongly correlated with the rock properties and topographic attributes. We also compared the frequency-magnitude distributions obtained with our best solution, including the station corrections (case A), the Hutton and Boore (1967) formula (case B), and Hutton and Boore formula with our station corrections (case C). We found that magnitudes for A are lower than for B and C. However, differences in statistical parameters, such as b-values, between A and C are neglectable.

Our work provides NEOM with a reliable and locally calibrated earthquake magnitude scale. This new magnitude scale can also be applied in surrounding regions with similar geological features (e.g., Egypt, Jordan, and Israel). Moreover, this work highlights that estimations of station corrections are critical, and at least, as important as a locally calibrated magnitude scale.

 

 

How to cite: Mai, P. M., Parisi, L., and Jónsson, S.: The importance of station corrections for local earthquake magnitudes: the example from the seismicity in NEOM (Gulf of Aqaba and northern Red Sea), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4905, https://doi.org/10.5194/egusphere-egu23-4905, 2023.

15:15–15:25
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EGU23-10540
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On-site presentation
Andres Tassara, Martin Riedel, Javiera Rioseco, and Iñigo Echeverria

Temperature controls the maximum depth of seismicity because it governs the transition between seismogenic brittle deformation at shallow levels to thermally-activated ductile creep at higher depths. We investigate this relationship for the Andean margin of western South America, an archetype of subduction-related active margins that host the largest megathrust earthquakes ever recorded and dense upper plate seismicity associated to crustal faults. We develop our own analytical formulation of the thermal state of the upper plate based on a 1D conductive geotherm with crustal heat production and known temperature at the base of the plate. To the east of the intersection of the subducted slab with the continental lithosphere-asthenosphere boundary (LAB), temperature at the LAB depth is prescribed by an asthenospheric adiabat. To the west of this intersection, temperature along the megathrust is calculated with the analytical expression of England (2018) that depends on the age and subduction velocity of the slab, megathrust friction, thermal conductivity and upper plate heat production. Because the shape of the megathrust and continental LAB for the study region are well constrained with geophysical data (Tassara and Echaurren, 2012), we can extrapolate the 1D approach to 3D and cover the entire margin between the trench and the eastern foreland. We select a preferred set of involved parameters (the preferred thermal model) by fitting a new compilation of surface heat flow measurements. Our results are then compared with the depth of earthquakes recorded along the study region by the catalogue of the Chilean Seismological National Center. For crustal earthquakes (located above the Moho depth), we computed the seismicity cutoff depth (SCD) at each location on a grid as the depth where 90% of the events have hypocentral depths shallower than SCD (for details see Riedel et al. in this meeting). As expected, the spatial distribution of SCD is positively correlated with temperature, although the actual average temperature for a given SCD can be different for different geological regions. This is explained by the dependency of the brittle-ductile transition (BDT) on rock type throughout creep properties of different lithologies, a concept that we use to infer the main rock type implied by the SCD-Temperature relation at each location. Thus, we convert the SCD map and thermal structure in a geological map of the middle-lower crust, which seems to be correlated with the geological structure and geophysical images of the Andean crust.

How to cite: Tassara, A., Riedel, M., Rioseco, J., and Echeverria, I.: Thermomechanic control on crustal seismicity along the Andean margin, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10540, https://doi.org/10.5194/egusphere-egu23-10540, 2023.

15:25–15:35
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EGU23-10456
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ECS
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On-site presentation
Martin Riedel, Andrés Tassara, Catalina Cabello, Denisse Leal, and Mauro Castillo

The thickness of the seismogenic crust (Ts) controls the location and magnitude of crustal earthquakes. Its upper limit is generally found near the surface and correlates to crustal seismicity onset depth (SOD) while its base correlates to the brittle-ductile transition in the crust and the seismicity cutoff depth (SCD) (Chiarabba & De Gori, 2016; Wu et al., 2017 and Zuza & Cao, 2020). Thus, it is a proxy of the brittle crust thickness and limits how deep earthquake ruptures may propagate, influencing their magnitude. Furthermore, crust with a thin Ts is inherently weaker and may concentrate more earthquakes (Burov, 2010 and Zuza & Cao, 2020). Given these factors, knowledge of Ts can help constrain future earthquake’s locations and magnitudes, aiding in seismic hazard assessment and mitigation.

Previous authors have used seismic data to calculate Ts in Italy, California and Taiwan considering the depth distribution of earthquakes (eg., Chiarabba & De Gori; 2016, Wu et al., 2017 and Zuza & Cao, 2020). However, the Chilean case presents a special and complex scenario. Here, the Nazca plate subducts below the South American plate producing an abundance of subduction earthquakes. Comparatively, crustal seismicity is sparse which presents a challenge. Adding to the complexity of the problem, the geometry of subduction as well as crustal thickness change considerably in latitude and longitude.

In this work, we present the first attempt at a Ts map of Chile. Following the methodologies of Chiarabba & De Gori, 2016; Wu et al., 2017 and Zuza & Cao, 2020 we divided the study area (ie. the Chilean margin between 15º and 45ºS) into a grid of square cells superposed by 2/3 of their width and calculated the depth distribution of earthquakes in each cell. As no consensus on which depth percentile to use for SOD and SCD exists, we calculated the percentiles 1, 5 and 10 for SOD and 90, 95 and 99 for SCD. Ts was then calculated as the difference between SCD and SOD. We compared the different outcomes.

Furthermore, we test a new methodology, relaying on cells of variable radius. Here, cell size changes according to earthquake density. We believe this approach is optimal for heterogenous catalogues, such as is the case in Chile.

Our results indicate that Ts in Chile varies latitudinally and longitudinally. Longitudinally it is generally thin at or close to the subduction trench, becomes thicker towards the east, reaching a maximum thickness below the central valley and then becomes thinner once again towards the volcanic arc. Latitudinally, it varies with crustal thickness as well as with subduction geometry (ie. it is thicker above the flat slab region).

How to cite: Riedel, M., Tassara, A., Cabello, C., Leal, D., and Castillo, M.: Seismogenic Thickness of the Andean Crust, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10456, https://doi.org/10.5194/egusphere-egu23-10456, 2023.

15:35–15:45
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EGU23-12988
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ECS
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Virtual presentation
Ángela María Gómez García, Álvaro González, Mauro Cacace, Magdalena Scheck-Wenderoth, and Gaspar Monsalve

Most crustal earthquakes occur between two depths: an upper boundary marked by the onset of seismicity and a lower one defined by the seismicity cut-off. The depth difference between them is the crustal seismogenic thickness (CST). As these boundaries are diffuse, they are usually determined from thresholds (percentiles) of the statistical distribution of earthquake hypocentral depths. This spatial earthquake distribution can be used as a proxy for the rheological conditions of the hosting rock, because earthquake generation is controlled by the mechanical rock properties, and in-situ temperature, pressure and strain rates.

Laboratory friction experiments with representative rocks and major rock-forming minerals indicate that earthquakes are expected to nucleate at <350±50°C in crustal rocks, and <700±100°C in ultramafic rocks typical of the mantle. However, the small spatial and temporal scales of these experiments hinder up-scaling their results to the geological conditions found in Nature.

In this contribution, we propose a solution for such upscaling: we use a 3D lithospheric-scale model of northwestern South America to compute the corresponding temperature field, and calculate the hypocentral temperatures of crustal earthquakes recorded in the region. The model is constrained by the integration of available geophysical data and 3D gravity modelling. For each layer, lithology-constrained thermal parameters (conductivity and radiogenic heat production) are assigned, either using direct samples when available, or from representative values. We use an S-wave tomography to set the temperature at 75 km depth as the lower boundary condition, and the measured average temperature at the Earth’s surface as the upper one. Furthermore, we use available measurements of heat flow and downhole temperatures to calibrate the model.

According to our results, most crustal earthquakes nucleated at <350°C, in agreement with laboratory experiments. The relatively few outliers are likely due to uncertainties in the Moho depths and/or in the earthquake hypocentral location. Also, they may be due to the presence of ultramafic rocks (which allow larger nucleation temperatures for seismicity) within the allochthonous crustal terranes accreted to this complex margin.

We map the depths of the upper and lower boundaries of the seismogenic crust using a spatial sampling procedure, defining them as the 10th and 90th percentiles of the hypocentral depths (D10 and D90, respectively). We find that D10, D90 and the resulting CST have significant spatial variations. Some of these correlate with crustal-scale faults which apparently separate crustal domains with different seismogenic behaviors.

Moreover, we point out that the two largest earthquakes recorded in the region (Ms = 7.3 and Ms = 6.8, of the Murindó sequence in 1992) nucleated at the lower boundary of the seismogenic crust, highlighting the importance of considering this lower boundary into account when characterizing seismogenic sources for hazard assessments.

Our approach could effectively bridge the scale gap between the laboratory rock friction experiments and Nature, as it enables to integrate the full geological complexity, including a realistic present-day lithospheric structure, the three-dimensional heat flow in the lithosphere, and the mantle temperatures imprint into the crustal thermal configuration.

How to cite: Gómez García, Á. M., González, Á., Cacace, M., Scheck-Wenderoth, M., and Monsalve, G.: At which temperatures do crustal earthquakes nucleate? Northwestern South America as a case study, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12988, https://doi.org/10.5194/egusphere-egu23-12988, 2023.

Coffee break
Chairpersons: Philippe Jousset, Joana Carvalho
16:15–16:25
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EGU23-11829
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On-site presentation
Ben D.E. Dando, Bettina P. Goertz-Allmann, Quentin Brissaud, Andreas Köhler, Johannes Schweitzer, and Tormod Kværna

Since the invasion of Ukraine in February 2022, daily media reports have shown the shocking effects of fighting and the inevitable devastation associated with war. However, getting a comprehensive and unbiased overview of the ongoing military attacks remains a challenge. The availability of geophysical data that can identify individual attacks provides much needed objectivity to this problem. The pressure waves generated by an explosion travel through the atmosphere and subsurface as sound and seismic waves, and their signature can be recorded by arrays of seismometers for ground motion or microbarometers for sound propagation. In this work, we demonstrate the first known case of using seismological data to detect conflict-related explosions in near-real-time. Using the Ukrainian primary station of the International Monitoring System (IMS), the Malin array (AKASG), we automatically locate explosions around the Kyiv and Zhytomyr provinces. We show how our resulting catalogue of explosions correlates with key events in the Ukraine conflict and how these data can be used to both verify and improve accurate reporting of military attacks. We analyze events with a variety of seismo-acoustic signatures and significant differences in explosive yield. These can be associated with various types of military attacks, including artillery shelling, cruise missile attacks and airstrikes. This work opens-up the possibility for future conflict monitoring using geophysical data.

How to cite: Dando, B. D. E., Goertz-Allmann, B. P., Brissaud, Q., Köhler, A., Schweitzer, J., and Kværna, T.: Real-time monitoring of the Russia-Ukraine conflict using seismic and infrasound array data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11829, https://doi.org/10.5194/egusphere-egu23-11829, 2023.

16:25–16:35
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EGU23-13651
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ECS
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On-site presentation
Bernd Trabi and Florian Bleibinhaus

Predicting the peak ground velocity (PGV) of blast vibrations is important for blast mining in order to set the right amount of charge weights so that they do not exceed certain thresholds. One problem is the large dispersion in the observed PGV due to unknown complexity of seismic waves spread. Classical prediction methods most often use one of several empirical formulas. One very common method is the Scaled Distance (SD) approach, which has the fewest parameters to calibrate, is widely used and works for a single sensor. In this study, we use a dataset of 55 mining production blasts recorded by 81 seismic sensors to compare the performance of the different methods. The large array allows us to apply multi-sensor inversion, which gives more information about the physical meaning of various parameters. Our results show that classical SD methods are less suitable, at least on the site we reviewed, as the data contradicts the previous link between the radial amplitude decay constant b and the load weight exponent c. For the last we find a value of 0.5, which we express as an expression of the physical relationship between the charge, energy and amplitude, suggesting that it may be a global value independent of the specific site.

How to cite: Trabi, B. and Bleibinhaus, F.: Blast Vibration Prediction, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13651, https://doi.org/10.5194/egusphere-egu23-13651, 2023.

16:35–16:45
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EGU23-16905
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On-site presentation
Jon Karapetyan, Eduard Geodakyan, Roza Karapetyan, Lilya Hovhannisyan, and Gurgen Matevosyan

The real-time monitoring of the seismicity of the territory of the Taurus-Caucasus region shows that at present, since 2021, geodynamic processes of significant intensity have been taking place in the earth's crust of the Taurus-Caucasus region, which can form anomalous areas of increased seismotectonic deformations on separate segments of seismically active faults. The article studies the activation of weak seismicity in the study area during the period of instrumental observations from 2021 to the present.  The structure of the set of sources of weak earthquakes in five-dimensional space (hypocenter, time, energy) is considered.  Based on the methods of statistical analysis of instrumental data, clusters of earthquake sources interconnected in space and time with magnitudes M≥3.0 (Figure 1).

Figure 1.

A detailed analysis shows that almost all types of anomalous manifestations of seismicity (swarms, earthquake doublets, pseudo-gaps, etc.) are observed in the same period in the Taurus-Caucasus region.  It was revealed that during the same period, the earthquake epicenters in real time line up mainly along trajectories that have a quasi-perpendicular direction with respect to the main seismically active faults of the Caucasus strike.  In the same period, in the Tavrokavkaz region, there is an alternation of seismotectonic stress variations in tense series of earthquakes in the magnitude range M=3.0-5.4 by sequential occurrence in the region of Eastern Turkey, Northern Armenia, Iran, Azerbaijan and the Greater Caucasus. The results obtained indicate that in the Taurus-Caucasus region there are regional and local significant changes in the stress-strain state of the earth's crust that are complex in their manifestation, which must be taken into account in a detailed study of seismic geodynamic processes in the Taurus-Caucasus region in order to identify possible zones of occurrence of strong earthquakes and medium-term earthquake forecast. This work was funded by the Science Committee of the Republic of Armenia, as part of the research project (No. ACH-01/22, 21SCG-1E021).

How to cite: Karapetyan, J., Geodakyan, E., Karapetyan, R., Hovhannisyan, L., and Matevosyan, G.: Study of modern regional and local anomal variations of seismicity in the Tavro-Caucasian region, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16905, https://doi.org/10.5194/egusphere-egu23-16905, 2023.

16:45–16:55
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EGU23-5874
|
On-site presentation
Quentin Bletery and Jean-Mathieu Nocquet

The existence of a potentially observable precursory phase of slip on the fault before large earthquakes has been debated for decades. Though observations preceding a handful of large events have been proposed as possible indicators of precursory slip, these observations do not directly precede the earthquakes, are not seen before most events and are commonly observed without being followed by earthquakes. Here we present a global analysis of geodetic measurements prior to large earthquakes, based on high-rate time series recorded before 78 large (Magnitude ≥ 7) seismic events. Our analysis highlights a 2 hour-long exponential signal consistent with acceleration of precursory slip on the fault. Our observation indicates that precursory phases of slip exist and that future improvements in measurement precision could allow to monitor them, making earthquake prediction potentially achievable.

How to cite: Bletery, Q. and Nocquet, J.-M.: Geodetic evidence that earthquakes start with precursory slip, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5874, https://doi.org/10.5194/egusphere-egu23-5874, 2023.

16:55–17:05
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EGU23-9148
|
ECS
|
On-site presentation
Rebekka Steffen, Holger Steffen, Ambrus Kenyeres, Tobias Nilsson, and Martin Lidberg

The European continent is divided into several tectonic plates and velocity variations appear along plate boundaries. However, velocity changes inside a tectonic plate can also occur due to local effects or other geodynamic processes, which is of interest for researchers trying to understand intraplate deformations in the horizontal and vertical directions. These changes can be observed by a dense network of GNSS (Global Navigation Satellite System) stations or more recently by the usage of InSAR (Interferometric Synthetic Aperture Radar). However, a dense GNSS network cannot be maintained over large areas due to, e.g., high costs and topographical obstacles, thus a regional velocity model to study intraplate deformation has to be obtained via an interpolation of scattered GNSS station velocities. The obtained velocity models can be used to estimate strain rates, which can be compared to existing seismic hazard models. In addition, in areas with a limited amount of seismic information, strain rates obtained from GNSS velocity models can provide a useful input for seismic hazard models.

The increased availability of GNSS station velocities in Europe via the EUREF Permanent Network Densification (EPND) project (https://epnd.sgo-penc.hu) allows to obtain a complete picture of the horizontal and vertical deformation in Europe via an interpolation. Here, we apply a new interpolation technique to a velocity field solution from EPND. The homogenized and quality-checked velocity field is interpolated via a least-square collocation including the knowledge of existing plate boundaries to avoid a smoothing of nearby velocities on different tectonic plates. We also apply a moving variance approach to avoid effects of non-stationarity, which arise due to the variable station densities. The new 3D GNSS velocity model EuVeM2022 is used to estimate the strain rates and a comparison to seismic risk maps shows a clear correlation. However, in some areas increased shear strain rates as well as anomalies in the velocity model are visible where peak ground acceleration is not increased for example in Serbia.

How to cite: Steffen, R., Steffen, H., Kenyeres, A., Nilsson, T., and Lidberg, M.: EuVeM2022: a new European GNSS velocity model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9148, https://doi.org/10.5194/egusphere-egu23-9148, 2023.

Special section on volcanic islands
17:05–17:15
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EGU23-4587
|
On-site presentation
Marco Calò, Francesca Di Luccio, Patricia Persaud, Guido Ventura, and Mimmo Palano

We applied ambient noise tomography to continuous data recorded by a dense seismic array deployed on the volcanic island of Lipari in the southern Tyrrhenian Sea. Since most of Lipari’s seismicity occurs offshore and is not evenly distributed, this technique allowed us to obtain the first high-resolution images beneath the island down to ~2.5 km depth. Results show a complex seismic structure related to the various ages and compositions of the volcanic products characteristic of the different regions of the island. High shear wave velocities are found in western Lipari where active hydrothermal vents and N-S faults are mapped. Low wave speeds are revealed beneath southern and north-eastern Lipari, where more recent volcanic activity developed along N-S dike-like structures that are aligned with rhyolitic vents. We suggest these dikes likely represent the probable pathways of future volcanic eruptions.

 

Work funded by the Istituto Nazionale di Geofisica e Vulcanologia, sezione di Roma 1 and the Department of Geology and Geophysics of Louisiana State University and partially supported by the “Pianeta Dinamico” all, 2023-2025 CAVEAT Project. M.C. was supported by UNAM PASPA – DGAPA. 

How to cite: Calò, M., Di Luccio, F., Persaud, P., Ventura, G., and Palano, M.: Ambient Noise Tomography of the Lipari Volcanic Island (Southern Italy) from a Dense Nodal Array, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4587, https://doi.org/10.5194/egusphere-egu23-4587, 2023.

17:15–17:25
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EGU23-5787
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ECS
|
On-site presentation
Apsara Sharma Dhakal, Lapo Boschi, and Simone Cesca

The study of long-period events in a volcanic setting is of fundamental importance to better understand the physics of volcanic plumbing systems. We locate such events using a source-imaging method developed by our team, and successfully applied, e.g., to the great Sumatra earthquake (Dhakal et al. 2022). Our approach combines seismic time reversal with a surface-wave ray tracing algorithm based on generalized spherical-harmonic parameterization of surface-wave phase velocity, and accounting for azimuthal anisotropy. We present a new application, to recordings of a suite of Mayotte events that Cesca et al. (2020) have already studied and interpreted in terms of the drainage of a magma reservoir.

We first conduct synthetic tests to quantify the resolving power of our method, given the available data coverage for the events of interest. We then use low-frequency Rayleigh wave signals recorded by different stations, reverse them in time and back propagate them through a surface-wave phase-velocity model. The time-reversed wave field has a prominent maximum at the spatial location(s) and time(s) where and when the recorded signal had been generated. From the time- and space-distribution of such maximum, we can make inferences on the nature of the source. Results so obtained are compared with those determined by Cesca et al. (2020) via moment tensor inversion and found to be in good agreement. We infer that our methodology is applicable to volcanic settings, possibly providing new insights into the nature of long-period seismic sources related to volcanic activity. The precise location of such events can provide better constraints on the depth interpretations and the extent of the seismic source.

How to cite: Sharma Dhakal, A., Boschi, L., and Cesca, S.: Imaging of seismic sources by surface-wave time-reversal: long-period earthquakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5787, https://doi.org/10.5194/egusphere-egu23-5787, 2023.

17:25–17:35
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EGU23-2598
|
On-site presentation
|
Lingling Ye, Thorne Lay, Hiroo Kanamori, Emily Brodsky, Takao Ohminato, Mie Ichihara, Kazuya Yamakawa, Hiroshi Tsuruoka, and Kenji Satake

The 2018 Kilauea eruption and East Rift Zone dike intrusion have been accompanied by more than 60 earthquakes with magnitude 4.7 – 5.3 near the summit. Corresponding long-period moment tensor solutions (constrained to have no isotropic component) have nearly vertical P-axes with moderate to large non-double-couple components (~20%-80%). The predominantly normal faulting is consistent with summit deflation as magma drains to the rift zone. Ground velocities recorded by local broadband seismic stations (<5 km) along the edge of the crater show distinct behavior between mid-May events and subsequent events. From May 16 to 26, twelve M5 events with large eruptive plumes below the SE edge of Halema'uma'u crater generated very long-period (VLP) seismic pulses with durations of about 20-40 s at all azimuths, suggesting distinct static outward displacements. This group can be represented by either isotropic or CLVD source. Almost all VLP pulses ended with sharp arrivals that are likely from small collapses. Similar VLP signals had been observed during the 2000 Miyake-jima eruption (Kikuchi et al., 2001; Kumagai et al., 2001). After May 28, M5 events were located below both north and south edges of the crater and generated broadband ground motions at the same stations; with large amplitude fracture-generated high-frequency signals (often clipped). Seismicity at the summit was low during the VLP events but increased to have 20-40 events per hour before later M5 events followed by several hours of reduced activity. After May 28, seismicity has been quasi-periodic, with intervals from ~2 to ~1 day. Comparison with solid earth tides and volume strain variations suggests that they are not direct driving factors, because the time interval between collapse events varied significantly. We consider a model with two stages, stage 1 (from May 16 to 26) with inflation transients that causes VLP events with large ash/gas explosion, and stage 2 (after May 28) dominated by collapse with extensive shallow fracturing and weaker gas explosions. The rapid expansion of the VLP events may be due to water flashing to steam, or partial collapse into the magma causing abrupt inflation. We investigate these VLP events comprehensively with local broadband seismic data, infrasound signals, GPS and tilt signals in this study, along with comparisons with published studies for the 2018 Kilauea sequences. 

How to cite: Ye, L., Lay, T., Kanamori, H., Brodsky, E., Ohminato, T., Ichihara, M., Yamakawa, K., Tsuruoka, H., and Satake, K.: Very Long-Period Seismic Signals and Collapse Events at the Kilauea Summit Crater in 2018, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2598, https://doi.org/10.5194/egusphere-egu23-2598, 2023.

17:35–17:45
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EGU23-14218
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ECS
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Virtual presentation
Eduardo Andrés Díaz-Suárez, Itahiza Dominguez Cerdeña, Carmen Del Fresno, Antonio Villaseñor, and Sergio Sainz-Maza Aparicio

We have analyzed the seismic series that preceded La Palma 2021 eruption using a purely automatic methodology based on Deep Learning. A new catalog has been retrieved, firstly 5797 absolute locations were obtained using NLLoc algorithm and considering a 3D velocity model of the island. At a further stage, these results were improved computing relative locations with HypoDD code. Our final catalog has doubled the number of events of the original manual catalog of the Instituto Geográfico Nacional. We observe new detailed features of the seismicity migration throughout the crust. We can clearly differentiate two phases within the sequence: the first one from September 11th to 16th and the second one from September 16th until the eruption onset. This differentiation can be described in terms of independent seismic parameters: inter-event time dispersion coefficient, b-value spatio-temporal distribution, hypocentral migration, magnitude distribution). Our results are in agreement with previous geodetic studies. Finally, we have developed a conceptual model describing the magmatic unrest and how the magma made its way to the surface prior to this eruption. 

How to cite: Díaz-Suárez, E. A., Dominguez Cerdeña, I., Del Fresno, C., Villaseñor, A., and Sainz-Maza Aparicio, S.: New Insights on the seismic unrest of La Palma 2021 eruption through a purely automatic analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14218, https://doi.org/10.5194/egusphere-egu23-14218, 2023.

17:45–18:00

Posters on site: Wed, 26 Apr, 10:45–12:30 | Hall X2

Chairpersons: João Fontiela, Philippe Jousset
X2.61
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EGU23-14699
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ECS
Henning Lilienkamp, Rémy Bossu, Fabrice Cotton, Francesco Finazzi, Matthieu Landès, and Graeme Weatherill

Rapid assessment of an earthquake’s impact on the affected society is a crucial step in the early phase of disaster management, determining the further organization of civil protection measures. In this study, we demonstrate that felt-reports containing macroseismic observations, collected via the LastQuake service of the European Mediterranean Seismological Center, can be utilized to rapidly estimate the probability of a felt earthquake to be “damaging” rather than “harmless” on a global scale. In our fully data-driven, transparent, and reproducible approach, we first map the reported observations to macroseismic intensities according to the EMS-98 macroseismic scale. Subsequently, we compare the distribution of felt-reports to documented earthquake impact in terms of economic losses, number of fatalities, and number of damaged or destroyed buildings. Using the distribution of felt-reports as predictive parameters and an impact measure as the target parameter, we infer a probabilistic model utilizing Bayes’ theorem and Kernel Density Estimation, that provides the probability of an earthquake to be “damaging”. For 22% of felt events in 2021, a sufficient number of felt-reports to run the model is collected within 10 minutes after the earthquake. While a clean separation of “damaging” and “harmless” events remains a challenging task, correct and unambiguous assessment of a large portion of “harmless” events in our dataset is identified as a key strength of our approach. We consider our method an inexpensive addition to the pool of earthquake impact assessment tools, that can be utilized instantly in all populated areas on the planet. Being fully independent of seismic data, the suggested framework poses an affordable option to potentially improve disaster management in regions that lack expensive seismic instrumentation today and in the near future.

How to cite: Lilienkamp, H., Bossu, R., Cotton, F., Finazzi, F., Landès, M., and Weatherill, G.: Utilization of crowdsourced macroseismic observations to distinguish damaging from harmless earthquakes globally within minutes of an event, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14699, https://doi.org/10.5194/egusphere-egu23-14699, 2023.

X2.62
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EGU23-7745
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ECS
Clara Duverger, Merlin Keller, and Gloria Senfaute

We propose a methodology to assist in the selection or definition of weights for components of seismic hazard models, which are combinations of seismic source models and ground motion models, in the context of probabilistic seismic hazard assessment (PSHA). The methodology uses Bayes's theory by optimally exploiting available observations that are the seismic catalogues and accelerometric databases. When compared to the current method of calculation, the proposed approach, simple to implement, allows a more exhaustive use of the data and discriminates between inputs without expert judgements to weigh branches of the PSHA logic tree.

We implement the proposed methodology in a Python package called Phebus to process the seismic source models (the first of the two main ingredients of the seismic hazard model) as a first step. The main purpose of this package is to estimate earthquake recurrence parameters and confidence intervals using a full Bayesian approach, and perform a Bayesian model averaging (BMA) amongst multiple seismic source models. More particularly, we focused our study on area-source models, which consist of subdivisions of a particular region of interest into zones that are assumed homogeneous in terms of seismic activity rate, and that are systematically used in PSHA calculations for low-to-moderate seismic regions. We conducted sensitivity analyses on the selection performances and the adjustments performances of recurrence parameters for simplistic toy models against a synthetic model-generated seismicity catalogue. We also illustrate the application of Phebus to the metropolitan France, a low-strain region, where at least four national, competitive and published area-source models are used by engineers and researchers for seismic hazard evaluation.

How to cite: Duverger, C., Keller, M., and Senfaute, G.: How to discriminate between area-source models for probabilistic seismic hazard assessment in a more objective manner?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7745, https://doi.org/10.5194/egusphere-egu23-7745, 2023.

X2.63
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EGU23-13526
Sylvert Paul, Tony Monfret, Françoise Courboulex, Bertrand Delouis, Anne Deschamps, Roby Douilly, David Ambrois, Steeve Julien Symithe, Sadrac St Fleur, Eric Calais, and Jérôme Chèze

On August 2021, 14th, a Mw 7.2 earthquake struck Haiti’s southern peninsula, eleven years after the devastating Mw 7 January 12, 2010 earthquake that occurred near Port au Prince. This large event, called the Nippes earthquake, has been recorded locally by the citizen network composed of low-cost raspberry shake stations. A precise analysis of the mainshock rupture from geodetic and seismic data revealed both left lateral strike slip and trust motion.

Few days after the mainshock, a temporary network consisting of 12 broadband stations was deployed in the vicinity of the epicentral zone in order to better record the aftershock sequence. Data from August 20 to December 31, 2021, were used to determine a suitable 1D velocity model of the zone and relocate about 2500 aftershocks that highlight the activation of several structures.

In this study, we focus our analysis on the region of Miragoâne situated between the ruptures of the 2021 and the 2010 earthquakes.  Before the Nippes earthquake, only a few events were detected there. Then, the Nippes earthquake triggered a burst of seismicity that lasted two months and stopped on November 2, 2021 and resumed in January 14 until March 10, 2022 with the occurrence on January 24th of two earthquakes in less than an hour of magnitude 5.3 and 5.1 respectively. We use the new 1D velocity model and the NonLinLoc using Source-Specific Station Term Corrections method (NLL-SSST) to relocate this seismic sequence. We find that the two larger earthquakes of magnitude slightly greater than 5 are closely located and confirm their reverse faulting mechanism using the waveform inversion method FMNEAR. The relocated seismicity is distributed from the surface to 20 km deep between the coast and the Enriquillo left-lateral strike-slip fault and along a plane which dips southward at ~50°, in agreement with the reverse faulting mechanism of the two larger magnitude > 5 earthquakes.

How to cite: Paul, S., Monfret, T., Courboulex, F., Delouis, B., Deschamps, A., Douilly, R., Ambrois, D., Symithe, S. J., St Fleur, S., Calais, E., and Chèze, J.: The Miragoâne seismic clusters in southern Haiti triggered by the Mw 7.2 Nippes earthquake of August 14, 2021, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13526, https://doi.org/10.5194/egusphere-egu23-13526, 2023.

X2.64
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EGU23-17388
Boyan Liu

On May 22, 2021, a MS7.4 earthquake occurred in Madoi County, Guoluo Prefecture, Qinghai Province. The epicenter was located at 98.34°E and 34.59°N (Officially determined by China Earthquake Networks Center). The earthquake occurred inside the Bayan Har block with a focal depth of 17 kilometers. The regional stress adjustment after a major earthquake directly causes the surrounding faults to undergo Coulomb stress changes, which affects the rate of seismic activity, off-fault aftershocks, and probability changes of occurrence of impending earthquakes. This paper uses the fault slip model to calculate Coulomb stress changes of main faults in Madoi area of Qinghai. Using the Dieterich earthquake occurrence rate model, a formula for the occurrence probability of an earthquake exceeding a certain magnitude under the Coulomb stress disturbance is obtained. We calculate the probability changes of MS≥7.0 and MS≥6.0 earthquake occurrence of the surrounding 8 faults (segments) which caused by the Coulomb stress increase. Affected by the Qinghai Madoi MS7.4 earthquake, the probability of the 8 faults earthquake occurrence has increased to varying degrees. For the Gander South Margin fault, Madoi-Gander fault and Tibet Dagou- Changmahe fault, the probability of earthquakes occurrence has increased rapidly in a short period of time (within about 10 years) after the main shock and it has stabilized later. There is a potential for destructive earthquakes, especially for earthquakes with MS≥6.0. For the Dari fault, since the increase in Coulomb stress has little effect, it is unlikely to occur MS≥7.0 or MS≥6.0 in the short term. However, as time goes by, the possibility of potentially destructive earthquakes occurring decades later cannot be ruled out. Particular attention to the possibility of earthquakes with MS≥6.0 should be paid to. The East Kunlun fault, especially the Maqin-Maqu section, is still a possible section for strong earthquakes in the future. This section still needs to focus on and prevent earthquakes with MS≥6.0 and even MS≥7.0. An earthquake with MS≥6.0 occurred on the Yushu-Ganzi fault, especially the risk of an earthquake with MS≥7.0 is not high, while the Ulan-Ula Lake-Yushu South fault has a potential risk of destructive earthquakes. It is necessary to strengthen prevention for earthquakes with MS≥6.0.

How to cite: Liu, B.: Effect of Qinghai Madoi MS7.4 earthquake on Coulomb stress and earthquake probability increment of adjacent faults, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17388, https://doi.org/10.5194/egusphere-egu23-17388, 2023.

X2.65
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EGU23-5751
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ECS
Giovanni Messuti, Silvia Scarpetta, Ortensia Amoroso, Ferdinando Napolitano, and Paolo Capuano

The knowledge of the crustal stress field is essential in the evaluation of the seismic hazard of an area.To this aim, it is necessary to derive reliable focal mechanisms mainly when small earthquakes have to be included in the computation. The first motion focal mechanism solution techniques are still widely used in modern softwares. The determination of P-wave polarities with manual procedures can lead to human errors and it is time-consuming. Automatic procedures can avoid these drawbacks. Polarity identification is not a classification task easily expressed in terms of mathematical procedures, in fact classical automatic procedures can lead to worse results than those obtained by human operators. For this reason, the use of machine learning approaches results necessary to accomplish this task.With low computational costs, real-time analysis capabilities, no need for complicated pre-processing procedures, and truly competitive results, properly designed convolutional networks can be the answer to various problems, including those related to seismology. 

In our work, we present the Convolutional First Motion (CFM) network, a Deep Convolutional Neural Network (DCNN) used to classify seismic traces based on first motion polarities of P-waves. We used waveforms contained in two datasets. We prepared the first dataset selecting approximatively 150˙000 waveforms contained in the Italian seismic catalogue INSTANCE, specifically designed for the application of machine learning techniques. To this end we devised an analysis procedure using Principal Component Analysis and Self-Organising Maps, through which a clustering process individuated groups of suitable traces. A second dataset, not specifically designed for machine learning techniques, is prepared manually picking approximatively 4˙000 waveforms of earthquakes occurred between 2010 and 2014 at Mt. Pollino area in Italy, avoiding possible overlapping of waveforms between the two datasets. The network, trained on ~130˙000 time windows centred on P-wave arrival times of waveforms in the INSTANCE catalogue, achieved accuracies of 95.7% and 98.9% on two test sets: the Mt. Pollino dataset and part of the INSTANCE catalogue. Further testing showed that if we give the network waveforms with uncertain arrival times, it acquires robustness to this type of noise, still showing high-level of performance.

We infer that the CFM network would be suitable in succession to automatic techniques that derive P-wave arrival times, for example techniques in which deep learning is used, in order to cover the entire data processing phase with machine learning. Given the incredible ability of DCNNs to model and process large volumes of data and their remarkable performance, it is reasonable to assume that deep learning will soon become the norm even in the context of first-motion polarity determination. 

This work was partially supported by the PRIN-2017 MATISSE project (no. 20177EPPN2), funded by the Ministry of Education and Research.

How to cite: Messuti, G., Scarpetta, S., Amoroso, O., Napolitano, F., and Capuano, P.: CFM: a Convolutional network for First Motion polarity classification of earthquake waveforms., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5751, https://doi.org/10.5194/egusphere-egu23-5751, 2023.

X2.66
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EGU23-15186
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ECS
Automatic Picking of Teleseismic SH- and SV-Phases using an Autoregressive and Singal Analytic Approach
(withdrawn)
Johannes Stampa and Thomas Meier
X2.67
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EGU23-14763
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On Ki Angel Ling, Simon Stähler, David Sollberger, and Domenico Giardini

Single-station polarization analysis allows us to extract wave parameters, such as inclination, azimuth, and ellipticity angle, directly from a recorded seismic signal theoretically. In reality, however, seismic data are not purely polarized in the finite analysis window due to varying noise levels, complex wavefield interactions, and calibration errors. Hence, this would potentially influence the observation window of phases of interest. In order to minimize these systematic errors, the involvement of arrays and array processing techniques can further increase the signal-to-noise ratio of coherent signals in a wavefield, which allows us to identify different seismic phases, especially the weaker phases that are usually difficult to observe in a single waveform, even after filtering for a desired wave type. In this study, we present a new approach that combines polarization analysis and filtering in the time-frequency domain using the S-transform with conventional array analysis such as beamforming to enhance seismic signals and distinguish different phases based on their expected slownesses and backazimuth. We apply this approach on AlpArray data and demonstrate wavefield separation in vespagrams using various polarization filters. We also discuss the benefits of our approach especially on small amplitude inner core phases (e.g., PKIKPPKIKP) and their applications for advancing seismological study of Earth’s inner core.

How to cite: Ling, O. K. A., Stähler, S., Sollberger, D., and Giardini, D.: Enhancement of Seismic Phase Identification using Polarization Filtering and Array Analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14763, https://doi.org/10.5194/egusphere-egu23-14763, 2023.

X2.68
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EGU23-2507
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Satoshi Matsumoto, Yoshihisa Iio, Shinichi Sakai, and Aitaro Kato

“b-value”, which degree of power law decay in the earthquake-size distribution, is known showing spatial and temporal variation based on observational studies. Especially, the temporal variation sometime have detected before a large earthquake occurrence, showing that it can be an indicator for the occurrence and may help earthquake hazard mitigation. The b-value changes due to tectonic stress regime. In addition, laboratory experiments have revealed acoustic emission size distribution depends on differential stress magnitude and criticality of failure condition. However, it is unclear in natural earthquake activity that which factor controls b-value condition. In this study, we show b-value change of small earthquake sequences in normalized shear and normal stress.

We carried out dense seismic observation composed by over 1000 stations deployed in hypocentral area (with diameter about 35 km) of the 2000 Western Tottori Earthquake (M7.3). About one year observation enabled us to obtain hypocenters and focal mechanisms about 5000 small earthquakes. Relative stress tensor have been inverted by the focal mechanism data in spatial bins and relative shear and normal stress for individual earthquake also estimated.  We investigated b-value change in relative shear and normal stress condition of small earthquake dataset. The b-value dependency on relative shear stress were detected, showing that the b-value decrease with increasing shear stress. In addition, the b-value takes minimum value at normal stress at critical point where line following Coulomb Failure condition with friction coefficient of 0.6 touches the unit Mohr circle. This suggests that the b-value become small in case of fault plane in optimal direction.

How to cite: Matsumoto, S., Iio, Y., Sakai, S., and Kato, A.: Fault strength dependency of natural earthquake-size distribution based on the precise focal mechanism data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2507, https://doi.org/10.5194/egusphere-egu23-2507, 2023.

X2.69
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EGU23-9201
Gabi Laske, William Frazer, and Adrian Doran

Surface wave arrival angles are an important secondary set of observables to constrain Earth's 3-dimensional structure. These data have also been used to refine information on the alignments of horizontal seismometer components with the geographic coordinate system. In the past, particle motion has been inspected and analyzed on single 3-component seismograms, one at a time. But the advent of large, dense seismic networks has made this approach tedious and impractical. Automated toolboxes are now routinely used for datasets where station operators cannot determine the orientation of a seismometer upon deployment, such as conventional free-fall ocean bottom seismometers. 

In a previous paper, we demonstrated that our automated Python-based toolbox DLOPy compares favorably with traditional approaches to determine instrument orientations. But an open question has been whether the technique also provides  individual high-quality measurements for an internally consistent dataset to be used for structural imaging. For this feasibility study, we compared long-period Rayleigh-wave arrival angles at frequencies between 10 and 25 mHz for 10 earthquakes during the first half of 2009  that were recorded at the USArray Transportable Array (TA), a component of the EarthScope program. After vigorous data vetting, we obtained a high-quality dataset that compares favorably with an arrival angle database compiled using our traditional interactive screen approach, particularly at frequencies 20 mHz and above. 

On the other hand, the presence of strong Love waves may hamper the automated measurement process as currently implemented. 
While the proper choice of the start time of the analysis window may depend on a particular geographic location of a seismic network, our observations for USArray data suggest that a slightly later start time than is currently used may yield more high-quality Rayleigh wave measurements.

How to cite: Laske, G., Frazer, W., and Doran, A.: Benchmarking Automated Rayleigh-Wave Arrival Angle Measurements for USArray Seismograms, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9201, https://doi.org/10.5194/egusphere-egu23-9201, 2023.

X2.70
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EGU23-4855
|
ECS
Kyungmin Min, Mikyung Choi, Seolhan You, Yeongjae Choi, Gyeongdon Chai, and Suncheon Park

The Korea Meteorological Administration(KMA) uses seismic and infrasound networks data identifying  explosion, collapse, and earthquakes.
We identify events using various methods . First, Vp/Vs spectral amplitude ratio in the frequency domain is calculated by applying Fast Fourier Transform (FFT) and Power Spectrum Density (PSD) analysis methods. The Vp/Vs ratio more than 50% is distinguished as an explosion.
Second, Explosion, collapse, and earthquakes are distinguished by (using) the body wave method (Walter et al., 2018). The Pn/Lg, Lg/Lg ratios are used for studying the DPRK 6th test, collapse and induced earthquakes at Chuncheon, Sokcho, and Seohwa KMA seismic networks.
Finally, the KMA operates five infrasound networks for  monitoring DPRK nuclear test and Mt. Baekdu volcano in addition to the seismic networks.
Infrasound analysis calculates the apparent speed and azimuth of the infrasound source by applying Progressive Multi-Channel Correlation(PMCC)  algorithm.
The calculated azimuth and apparent velocity are  important factors in determining that the seismic and infrasound signals occurred at the same point during an explosion.

How to cite: Min, K., Choi, M., You, S., Choi, Y., Chai, G., and Park, S.: A Study on the Identification of Explosions, Collapses and Earthquakes using KMA Seismic and Infrasound Networks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4855, https://doi.org/10.5194/egusphere-egu23-4855, 2023.

X2.71
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EGU23-9964
|
ECS
|
William Frazer and Jeffrey Park

SS precursor imaging has long been used to detect sharp interfaces within Earth’s mid-Mantle. The topography of the 410- and 660-km discontinuities, the major interfaces in the mantle transition zone (MTZ), can provide valuable insight into the temperature of and material flow within the mantle. Additionally, negative velocity gradients and possible partial melt surrounding the MTZ in some regions provide evidence for a hydrogen-enriched mid-mantle, a feature that may have implications for global water circulation and long-term (~100 Ma) ocean-mass regulation. Here, we apply a novel SS-precursor deconvolution technique based on multiple-taper correlation (MTC). Typical SS-precursor techniques require tightly bandpassed signals (e.g., 0.02-0.1 Hz), limiting both vertical and horizontal resolution. Higher-frequency content allows for the detection of finer structure in and around the MTZ. MTC-based SS-precursor estimates can increase the frequency cutoff to above 0.5 Hz, thereby increasing vertical resolution to under 10 km. We conduct this analysis on a global data set of over 300,000 SS waveforms recorded on the permanent GSN, GEOSCOPE, and GEOFON networks as well as the temporary EarthScope TA and AlpArray. Such a large dataset provides unprecedented bounce-point density, particularly in the North Pacific Ocean. Preliminary results suggest a global average depth of ~409 km and ~665 km for the 410- and 660-km discontinuities respectively. In this work we used time-delays calculated for the 1-D ak135 velocity model. In general, we find moderate agreement with previous low-frequency SS precursor analysis. Additionally, we identify a sharp feature above the MTZ, north of the Hawaiian Islands, that was interpreted previously from an asymmetry in sidelobe amplitudes, suggesting a low-velocity zone with a sharp interface (<10-km thickness), rather than a thick wavespeed gradient. Further results will include corrections for 3-D structure with various mantle tomography models and focus on potential impacts to the solid-Earth water cycle.

How to cite: Frazer, W. and Park, J.: Global High-Resolution mid-Mantle Imaging with Multiple-Taper SS-Precursor Estimates, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9964, https://doi.org/10.5194/egusphere-egu23-9964, 2023.

X2.72
|
EGU23-4706
Resolving the rupture directivity and seismogenic structure of the 2018 Taiwan Shoal MW5.7 earthquake
(withdrawn)
Lanlan Tang
X2.73
|
EGU23-9542
Pierre Arroucau, Stéphane Drouet, Guillaume Daniel, Paola Traversa, and Kévin Manchuel

In this work, we present a new earthquake catalogue for metropolitan France (i.e. the part of France located in Europe), which can be used to derive parameters of interest for seismic hazard modeling in that region. The catalogue is built from the amalgamation of existing catalogues for France but also neighboring countries, for both historical (here, ante-1965) and instrumental times. It covers a period ranging from 250 to 2020. Magnitudes are homogenized as moment magnitudes (Mw) using adequate conversion laws when needed. Uncertainties on location and magnitude are also provided so they can be used to realistically quantify epistemic uncertainties in hazard models. This catalogue somehow represents an updated version of the catalogue used in Drouet et al. (2020), augmented from recently published information. It extends from -8.1°E to 11.°E in longitude and from 38°N to 51°N in order to encompass the three area source models that were used in that study. The core of this catalogue is FCAT-17 (Manchuel et al., 2018), completed using the most recent ESHM2020 catalogue, FCAT-17 being given priority on the French territory plus a 20 km buffer beyond its borders and exclusive economic zone. Then, national catalogues for neighboring countries (Portugal, Spain, United Kingdom, Ireland, Belgium, Netherlands, Luxembourg, Germany, Austria, Switzerland, Italy) are also incorporated and are given full priority over their territory. The final model contains more than 45,000 events with magnitudes as low as Mw=2.0. Such low magnitudes were considered in order to provide as much constraint as possible to recurrence models, despite the fact low magnitude events are -per se- of little interest for seismic hazard models.

How to cite: Arroucau, P., Drouet, S., Daniel, G., Traversa, P., and Manchuel, K.: A new earthquake catalogue for seismic hazard assessment in metropolitan France and neighboring countries, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9542, https://doi.org/10.5194/egusphere-egu23-9542, 2023.

X2.74
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EGU23-11857
Lucia Fojtíková, Jiří Málek, Ivan Prachař, Renata Lukešová, Róbert Kysel, Jiří Vackář, Jan Valenta, and Barbora Lachová

The Czech Republic is situated in an intraplate region with low seismicity. The seismic hazard is relatively low but not negligible. A new map of the seismic hazard of the Czech Republic computed using a probabilistic approach is being compiled and will be released in the second half of 2023. As a part of this project new catalogues of earthquakes, both historical (based on macroseismic observations) and instrumental (based on seismograms) were compiled. These include earthquakes on the territory of the Czech Republic and neighboring areas of Slovakia, Austria, Germany and Poland.
       When analyzing these new enhanced catalogues, we recognized a period of increased seismicity at the beginning of the 20th century. The annual seismicity rate in this time interval for earthquakes with a magnitude 4 and greater is several times higher than at present. One possible explanation is the inconsistent magnitude determination between the beginning of the 20th century and the present time. Therefore, we re-examine historical seismograms at the beginning of the 20th century and verify their magnitudes. We also compared macroseismic observations of historical earthquakes with modern ones having almost the same magnitude.
       We found out that the increased seismicity at the beginning of 20th century is real. During this period, relatively large earthquakes were observed in various source zones in the investigated region. Such earthquakes have not occurred since 1920-1930 to the present. 

How to cite: Fojtíková, L., Málek, J., Prachař, I., Lukešová, R., Kysel, R., Vackář, J., Valenta, J., and Lachová, B.: Increased seismicity at the beginning of 20th century in the Czech Republic and adjacent areas, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11857, https://doi.org/10.5194/egusphere-egu23-11857, 2023.

X2.75
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EGU23-12078
|
ECS
Cristina Caricato, Pauline Galea, Paola Baccheschi, Elisa Tinti, Fabio Villani, Matthew Agius, and Sebastiano D'Amico

The Maltese archipelago lies in the centre of the Sicily Channel (Central Mediterranean), a crustal domain subject to extension since the Miocene and characterised by the presence of several active graben systems in the Plio-Quaternary. This region is affected by diffuse seismicity, in the form of isolated swarms interspersed with long pauses of quiescence and events of modest magnitude, in general not exceeding 4.0. The key features of seismicity are poorly constrained by unfavourable geographical conditions and the scarcity of seismic stations in the area between southern Sicily and Tunisia. Until a few years ago, the only Maltese station operating (WDD of the MedNet network) was the only source of information to characterise local Maltese seismicity. On the other hand, the localisations of major events (M > 4) made by the other monitoring agencies (e.g. Istituto nazionale di Geofisica e Vulcanologia - INGV) are based on traveltime readings from seismograms recorded at great distances from Malta (> 100 km) and are therefore affected by considerable formal errors. Moreover, the only published catalogue of offshore seismicity around the Maltese islands (Seismic Monitoring and Research Group, University of Malta) is based on single station location at WDD up to 2014. This is characterised by considerable uncertainties in hypocentral parameters, in particular a lack of depth information. In consequence, therefore, it is very difficult to obtain information on the geometry and kinematics of active tectonic structures, both offshore and possibly on.

In recent years, there has been a considerable increase in the number of seismic stations on the Maltese archipelago: there are currently eight active stations, that make up the Malta Seismic Network (MSN). The study we present is an example of the application of 3-D localisation techniques of Maltese seismicity, which benefits from the recent implementation of the local seismic network. In particular, we focus on the swarm occurring offshore in the period September-October 2020, and characterised by an unusual number of events (> 100), including a main event (M > 4.0) that was strongly felt over the archipelago. We have handpicked P- and S-wave traveltimes for all the events using recordings from the MSN and a selected number of Italian stations according to quality criteria, and inverted them using the Hypoellipse code. The precise earthquake localisations allow us to obtain details of the structures activated during the swarm, together with a more detailed insight into the time evolution of the sequence. Accompanying these analyses, we calculated the moment tensor solutions for some of the largest events (M > 3) and performed a spectral analysis to distinguish the waveform characteristics of events occurring at shallow depths (< 10 km) from those nucleating in the mid-crust (> 15 km depth), as a function of different wavepaths through the crust. The prospect is that in the future it will be possible to better constrain the seismotectonics of the Maltese archipelago and have a more accurate picture of the seismogenic potential of active faults in this sector of the Mediterranean.

 

How to cite: Caricato, C., Galea, P., Baccheschi, P., Tinti, E., Villani, F., Agius, M., and D'Amico, S.: Recent improvements in seismic monitoring of the Maltese archipelago: A case study from the 2020 seismic swarm., EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12078, https://doi.org/10.5194/egusphere-egu23-12078, 2023.

X2.76
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EGU23-13752
Naiara Fernandez, Mauro Cacace, Magdalena Scheck-Wenderoth, and Oliver Heidbach

The North Anatolian Fault (NAF) is a right-lateral continental transform fault that extends from eastern Anatolia to the northern Aegean in the eastern Mediterranean. It is characterized by strong and frequent seismic activity, posing a high seismic hazard level to the region. The Main Marmara Fault (MMF), the northern branch of the NAF along the Marmara Sea (NW Turkey), has produced several major earthquakes (M7+) in the past with a recurrence rate of about 250 years. At present, there is a 150 km seismic gap along the MMF that has not ruptured since 1766. The MMF seismic gap shows distinct variability in its along-strike interseismic strain-accumulation with locked and creeping segments, but it is unclear which are the controlling parameter of this observation. Thus, the interseismic evolution of the MMF, especially its frictional state and its inter-to-pre-seismic behavior, is still a matter of debate. It has been proposed that the observed along-strike variation in strain localization around the MMF might be linked to a heterogeneous off-fault crustal and sedimentary rheological configuration.

Here, we use a forward numerical approach with visco-elastic rheology to investigate the space and time scales of the long-term seismic behavior of the Main Marmara Fault and its main controlling factors. The MMF is modelled following a Coulomb frictional constitutive law. The spatially variable rock properties are derived from a lithospheric-scale 3D structural model of the region around the MMF. This model has been generated with a data-driven approach in a previous stage of the work. The forward model is used to test the effect of varying boundary conditions (i.e. kinematic) and fault strength properties (i.e. coefficient of friction). Our modelling approach highlights the first order role of crustal rheology and fault-strength in the long-term behavior of the MMF (spatial distribution and recurrence of seismic events), as well as their potential to explain the along fault locking degree variability.

How to cite: Fernandez, N., Cacace, M., Scheck-Wenderoth, M., and Heidbach, O.: Factors influencing the long-term interseismic behavior of the Main Marmara Fault, NW Turkey, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13752, https://doi.org/10.5194/egusphere-egu23-13752, 2023.

X2.77
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EGU23-12243
|
ECS
|
Doyoung Kim, Kwang-Hee Kim, Yu Jin Sohn, and Young-Cheol Lee

The ML 5.2 earthquake occurred in Mt.Sokri (September 16, 1978), the center of South Korea. It was the fourth largest earthquake in South Korea since the modern seismic observation began. The Korea Meteorological Administration (KMA), the United States Geological Survey (USGS), and the International Seismological Centre (ISC) announced the location of the hypocenter respectively, but they were different. In this study, we analyzed the subsurface fault structure using current micro-earthquakes. We have used data collected by temporary seismic stations installed in the Mt.Sokri area by Pusan National University since May 2019 and the permanent seismic stations installed by KMA since 1978. KMA reported 188 earthquakes from 2007 to 2021 in the study area. We detected additional 280 micro-earthquakes using STA/LTA and template matching methods. The initial result of earthquake locations using HYPOELLIPSE was scattered across the study area. To obtain reliable locations, we relocated earthquakes using HypoDD. As a result, 468 earthquakes were relocated, about twice as many as those reported by KMA. We recognized earthquakes have occurred along WNW-ESE subsurface faults at the depth of 14 to 18km. We determined the focal mechanisms of 15 earthquakes with magnitudes greater than 2. The location of the Mt.Sokri earthquake was reviewed by comparing these results with the locations announced by the three institutions. Joint analysis of the focal mechanisms, distribution of earthquakes, and geological setting, the WNW-ESE plane was interpreted as the major fault plane. Apparently, the micro-seismicity locations in this study better correlated with the epicenter announced by USGS. However, it is difficult to confidently specify the location of the 1978 earthquake only to the current earthquakes.

How to cite: Kim, D., Kim, K.-H., Sohn, Y. J., and Lee, Y.-C.: Revisiting the 1978 ML 5.2 Mt.Sokri earthquake, Korea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12243, https://doi.org/10.5194/egusphere-egu23-12243, 2023.

X2.78
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EGU23-11450
|
ECS
Yen Joe Tan, Zilin Song, and Yiyuan Zhong

At volcanic islands, seismometers are the main tool for monitoring various active processes including seismicity that could forewarn impending eruptions. However, the sparse seismic networks, high background noise levels, large diversity of seismic signals, and high event rates during unrest episodes make automatic detection and classification of volcano seismicity a difficult challenge. In this paper, we use the Alaska Volcano Observatory’s catalogue of ~120,000 long-period (LP) and volcano-tectonic (VT) earthquakes at 34 volcanoes from 1989-2018 to evaluate the efficacy of automatic detection and classification methods. For each event, we calculate the frequency index (FI) based on the ratio of mean spectral amplitudes in the higher and lower frequency bands of the recorded waveforms. Using the local minima in the FI distribution at each volcano as the classification boundary, we find that our labels generally agree with the catalogue’s manual labels. The classification boundaries separating LP and VT earthquakes are also relatively consistent (FI = -1) between volcanoes. Therefore, the FI method is an effective method for automatic classification of volcano seismicity. We then evaluate the performance of two machine-learning-based models (PhaseNet and EQTransformer) and the cross-correlation-based template-matching method for automatic detection. While the template-matching method is computationally more expensive, we find that most of the catalogue events can be detected by using another event as a template, with relatively low false positive rates. In comparison, both machine-learning-based models’ performances are worse than previously reported results and deteriorate systematically with decreasing FI index values. The bias might have resulted from the models having been trained using earthquake catalogues from non-volcanic regions that lack LP events. Therefore, these models should be retrained with a dataset of volcano seismicity before being applied for automatic earthquake detection at volcanic regions.  

How to cite: Tan, Y. J., Song, Z., and Zhong, Y.: Efficacy of automatic detection and classification methods for volcano seismicity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11450, https://doi.org/10.5194/egusphere-egu23-11450, 2023.

X2.79
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EGU23-9184
Laura Scognamiglio, Mimmo Palano, Francesca Di Luccio, Giuseppe Pezzo, Sofia De Gregorio, and Filippo Greco and the CAVEAT Team

In retreating subduction zones the proposed lithosphere tearing processes at slab edges are typically related to segmentation of subducting plate. A direct response to lithosphere tearing is the channeling of new asthenospheric mantle that can initiate magmatism. Tearing mechanisms have also been proposed for the Calabrian Arc, where slab migration led to the formation of the southeastern Tyrrhenian basin and was progressively accommodated by inherited and newly formed vertical tear faults whose oldest (~2 Ma) and youngest (~0.8 Ma) tectonic expressions in the upper plate are the Sisifo-Alicudi and the Aeolian-Tindari-Letojanni fault systems, respectively. The Western and Central-Southern Aeolian Islands nested along these structures as highlighted by a large number of geological and geophysical studies in the last decades. CAVEAT aims to apply a multidisciplinary approach to study in detail the local lithospheric structure, the pattern of crustal deformation and the geochemical signature of the Central-Southern Aeolian Islands. The acquisition of new data will provide a broad overview of the ongoing geodynamics of the southern Tyrrhenian region and will allow us to properly study the interplay of present-day tectonics and volcanic deformation and the related role and nature of the fluids and the hydrothermal activity.

How to cite: Scognamiglio, L., Palano, M., Di Luccio, F., Pezzo, G., De Gregorio, S., and Greco, F. and the CAVEAT Team: Volcanism and tearing in the Tyrrhenian subduction system: the CAVEAT project, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9184, https://doi.org/10.5194/egusphere-egu23-9184, 2023.

X2.80
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EGU23-6588
|
ECS
Investigation of 1-D and 3-D Crustal seismic velocity structure in the broader area of Crete (Greece)
(withdrawn)
Andreas Karakonstantis and Filippos Vallianatos
X2.81
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EGU23-11359
|
ECS
Célia Barat, Geneviève Savard, Francisco Muñoz, Douglas Stumpp, Salvatore Alparone, Tullio Ricci, Andrea Ursino, Mimmo Palano, Maria-Paz Reyes Hardy, Lucia Pruiti, Thomas Planes, Federica Sparacino, Costanza Bonadonna, Joël Ruch, Luca Caricchi, and Matteo Lupi

The Vulcano-Lipari System is a volcanic complex located in the central sector of the Aeolian Archipelago in southern Italy. The edifice is affected by complex tectonics and develops upon the trans-extension of the Aeolian-Tindari-Letojanni Fault System. This fault is proposed to control magmatism and acts as a preferential pathway for upwelling of magmatic and supercritical fluids. Over the last three decades, Vulcano Island underwent several volcanic crises and since September 2021 it has been showing signs of increasing activity and volcanic unrest. Temperature, degassing, seismic activity, and deformation rapidly increased causing temporal evacuation of the inhabitants of the most affected regions. During the unrest in October 2021, we deployed 196 3C geophones all around Vulcano and south of Lipari to record the seismic signals for a full month, as part of the VulcaNODES project. The resulting seismic catalog confidently contains more than 7000 volcano-seismic and volcano-tectonic events with an average local magnitude of -0.32. This catalog is used to produce an unprecedented travel-time time-lapse tomography of the unrest. Seismic tomography is a powerful tool for observing structures at depth beneath volcanic systems, using seismic waves generated by earthquakes. Such a dense network combined with the exceptional seismic signal recorded will provide tomographic time series of the plumbing system every 1-3 days for a month. The still ongoing VulcaNODES project aims at observing fluids evolution along the volcano’s tectonic structures on a daily basis, providing new insights on processes usually difficult to record on short timeframes, and shedding light on the plumbing system on a high resolution.

How to cite: Barat, C., Savard, G., Muñoz, F., Stumpp, D., Alparone, S., Ricci, T., Ursino, A., Palano, M., Reyes Hardy, M.-P., Pruiti, L., Planes, T., Sparacino, F., Bonadonna, C., Ruch, J., Caricchi, L., and Lupi, M.: Deployment of dense nodal network during unrest: investigation of the Vulcano-Lipari System (Italy) with Local Earthquake Tomography, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11359, https://doi.org/10.5194/egusphere-egu23-11359, 2023.

X2.82
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EGU23-12460
Cristina Totaro, Marco Aloisi, Carmelo Ferlito, Barbara Orecchio, Debora Presti, and Silvia Scolaro

Seismic tomography represents a very powerful and effective tool to look at depths beneath volcanic systems thus helping to better understand their behavior. In particular, a key parameter useful to discriminate the presence of gas, fluids and melts is represented by the P-wave and S-wave velocity ratio. In the present study, we collected ~ 4400 crustal earthquakes that occurred in the last thirty years and we used the LOcal TOmography Software LOTOS to estimate the first 3D overall model of Vp, Vs and Vp/Vs for the Lipari–Vulcano complex belonging to the Aeolian islands system (southern Tyrrhenian sea). The investigated area has been characterized both in old and recent times by fumaroles, hydrothermal activity and active degassing. In particular, in the past decades several episodes of anomalous increases of fumarole temperature and strong degassing have interested the Vulcano Island, the latter of which started in September 2021.

The results of the tomographic investigation indicate the presence of two main anomalies of low Vp and low Vp/Vs, clearly depicted up to ~ 8 km depths, and related to gas-rich materials beneath the central-northern sector of Vulcano and the western off-shore of Lipari, respectively.

The anomaly beneath Vulcano is located in close correspondence with La Fossa caldera area and with the sector where fumaroles, hydrothermal activity and active degassing are widely documented. Moreover, beneath the western Lipari off-shore a new, previously undetected, volume of strong gas-concentration has been identified. Even if these two anomalies show almost the same intensity, no evidence of degassing activity is available for the latter one because of its location at sea depths where the relevant water column pressure may inhibit the observation of possible degassing processes.

The obtained results furnished a picture of the spatial distribution of gas-filled volumes feeding the main degassing activity of the Lipari-Vulcano complex and allowed to highlight the main role played by volcanic gas in the whole system, thus furnishing invaluable constraints for improved modelling of the volcanic system and of its possible evolution.

How to cite: Totaro, C., Aloisi, M., Ferlito, C., Orecchio, B., Presti, D., and Scolaro, S.: Seismic tomography investigation at the Lipari-Vulcano complex (South Italy) provides new insights on the active degassing system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12460, https://doi.org/10.5194/egusphere-egu23-12460, 2023.

X2.83
|
EGU23-14189
João Fontiela, Nuno Afonso Dias, Graça Silveira, Mário Moreira, and Luís Matias

We present the analysis of the local and regional seismicity recorded in 2019-2021 by a temporary seismic network installed on Terceira Island. This new seismic dataset allow us to study the induced seismicity caused by fluid extraction in a geothermal powerplant and to image the subsurface seismic structure with local earthquake tomography. 

From the distribution of the seismicity, it is possible to highlight two regions with a high number of events amongst other areas. The first one, located in the central part of the island, is associated with the volcanoes of Pico Alto and Guilherme Moniz, with  low magnitude earthquakes, and hypocentre's depths less than 10 km. The geothermal power plant is located in the transition between these two volcanic systems. We identify a cluster of earthquakes in the neighbourhood of the geothermal power plant at depths ranging from 1 to 3 km, consistent with the induction by the powerplant operation. The second seismicity region is located on the island's western sector, at the Santa Bárbara volcanic system. There, the seismicity pattern is more complex, mainly by the occurrence of both tectonic and seismo-volcanic earthquakes.  

Local earthquake tomography allows imaging of the crust from the surface to the upper-middle crust, up to 8 km depth. The Santa Bárbara and Pico Alto volcanoes are characterized by low Vp and high Vp/Vs anomalies, stronger in the first and typically related to active volcanoes. In the transition between the two volcanoes, we observe shallow strong Vp and very low Vp/Vs anomalies typical of geothermal fields. On the other hand, the Guilherme Moniz volcano exhibits high Vp anomaly and normal Vp/Vs values.

The eastern sector of the Terceira is characterized by low seismicity and, consequently, low tomographic resolution. 

This work is a contribution to projects GEMMA (PTDC/CTA-GEO/2083/2021) and RESTLESS (PTDC/CTA-GEF/6674/2020), and it was also supported by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020-IDL, UIDB/04683/2020 - ICT and UIDP/04683/2020 - ICT

How to cite: Fontiela, J., Afonso Dias, N., Silveira, G., Moreira, M., and Matias, L.: Seismicity and crustal structure of the Terceira Island, Azores, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14189, https://doi.org/10.5194/egusphere-egu23-14189, 2023.

X2.84
|
EGU23-9245
Joana Carvalho, Graça Silveira, Virgílio Bento, Martin Schimmel, and Resurrección Antón

Volcanic eruptions are, generally, characterized by several recordable signals that can occur before, during and/or after the eruption. Such signals may reflect temporal changes in the seismic structure that can be associated with alterations in the seismic activity, fluid migration or outgassing, useful to track the volcano evolution through its eruptive phases.

Cumbre Vieja ridge, in La Palma, is the most active volcanic field in the Canaries. The last eruption occurred at the Tajogaite cone, lasting from 19 September to 13 December 2021. It is considered one of the best-monitored volcanic crises in the Canary Islands, thus allowing us to study the eruption from different scientific perspectives.

Ambient seismic noise interferometry has been widely applied to monitor temporal velocity changes, especially at volcanoes before the eruption. However, the causes of these velocity changes are not always fully understood. In this study we use a dataset of three years (2020 to 2022) recorded in two stations operating in La Palma to identify and characterize different physical processes before and after the eruption. For this purpose, we computed phase auto- and cross-correlations and stacked them through the time-frequency phase-weighted stack to create hourly cross-correlation functions.

Seismic velocity changes cause phase shifts in the auto- and cross-correlation functions waveforms. The phase shifts can be measured through the waveform similarity, which consists in comparing each correlation function with a reference correlation function trace, in a defined window. The reference trace corresponds to a calm period, in this case, the period before the eruption. We also compared the waveform similarity results with the ground deformation inferred from GPS.

Tajogaite eruption provides an opportunity to study the behaviour within the volcanic structure prior to the eruption and its recovery after the eruption and to improve our knowledge of the magmatic fluid migration. This recent and well-documented eruption may serve as a proxy for future eruptions.

This work is a contribution to project RESTLESS (PTDC/CTAGEF/6674/2020) and it was also supported by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020-IDL.

How to cite: Carvalho, J., Silveira, G., Bento, V., Schimmel, M., and Antón, R.: Insights into the fluid migration dynamics of Tajogaite eruption, La Palma, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9245, https://doi.org/10.5194/egusphere-egu23-9245, 2023.

Posters virtual: Wed, 26 Apr, 10:45–12:30 | vHall GMPV/G/GD/SM

vGGGS.4
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EGU23-9358
Vahan Davtyan, Armen Kazarian, and Haik Kazarian

The quality of the earthquakes’ catalogs plays the most important role in the study of seismicity, of active faults, in seismic hazard and risk assessment, etc. The quality of the catalog is evaluated by the absence of duplicate records or explosions in itself. For example, in catalogs of historical earthquakes, often the same earthquake, described by different historical sources, appears as two different earthquakes. The situation is much more complicated in instrumental catalogs, where often tremors of the surface of the earth's crust related to human vitality appear as natural earthquakes.

The study of the instrumental earthquakes’ catalogs using daily histograms and remote sensing methods made it possible to identify a large number of events that, in our opinion, are not natural earthquakes. In particular, the catalog of The International Seismological Center (ISC), on the territory of Northern Kazakhstan and South-East Russia, contains more than 10,000 events that are most likely explosions, and not earthquakes. Likewise, the catalog of the National Survey for Seismic Protection of Armenia (NSSP) in the region of the Armenia-Georgia border contains more than 1,000 events that are also explosions. Similar events were recorded in Spain, Egypt, Turkey, Syria.

The used methodology for the detection of false earthquakes is recommended for cleaning the instrumental catalogs of seismic events.

How to cite: Davtyan, V., Kazarian, A., and Kazarian, H.: Certain problems related to earthquakes’ catalogs, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9358, https://doi.org/10.5194/egusphere-egu23-9358, 2023.