SM1.1

EDI
General Contributions on Earthquakes, Earth Structure, Seismology

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.

Convener: Philippe Jousset | Co-convener: P. Martin Mai
vPICO presentations
| Tue, 27 Apr, 13:30–17:00 (CEST)

Session assets

Session materials

vPICO presentations: Tue, 27 Apr

Chairpersons: Philippe Jousset, P. Martin Mai
13:30–13:35
Case study
13:35–13:37
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EGU21-1581
|
Highlight
Alexandra Moshou, Antonios Konstantaras, and Panagiotis Argyrakis

On 30th October 2020, at 11.51 (UTC), a very strong earthquake of magnitude Mw = 7.0 struck north of the Greek island of Samos in the Aegean coast of Turkey, south of Izmir. The epicentre was determined 17km north of Samos, in the Gulf of Ephesus and was felt in many parts of Greece and western Turkey. The geographical coordinates as calculated of the manual analysis of the National Observatory of Athens (http://bbnet.gein.noa.gr/Events/2020/10/noa2020vipzs_info.html) was determined as  φ= 37.9001⁰N, λ=26.8167⁰E at a focal depth at 11.8km. The earthquake triggered a tsunami that flooded the coastal district of Seferihisar (Turkey), Cesme, Izmir and the port of Samos (Greece). In the next 8 minutes after the detection of the earthquake, tsunami bulletins were issued to national focal points by the Tsunami Service Providers accredited by UNESCO’s IOC Intergovernmental Coordination Group for the Tsunami Early Warning and Mitigation System in the North-eastern Atlantic, the Mediterranean and connected seas (ICG/NEAMTWS). Greece and Turkey were put on Tsunami Watch (highest level of alert). In Seferishar the tsunami swept away many boats in the marina and the water level reached 1.5 meters causing damage to shops.

Three hours later, 15:14 (UTC) a second strong event (Mw = 5.3) occurred in the same region some kilometres south of the main earthquake (φ=37.8223⁰N,λ=26.8652⁰E, http://bbnet.gein.noa.gr/Events/2020/10/noa2020viwsi_info.html). By the end of the same day that the earthquake took place, there were 65 aftershocks while a total of 576 aftershocks up to 31/12 with magnitude greater than 1.0. For the aftershocks with 3.7<ML<7.0 we applied the moment tensor inversion to determine the focal mechanism, the Seismic Moment (M0) and the Moment Magnitude (Mw). For this purpose, 3–component broadband seismological data from the Hellenic Unified Seismological Network (HUSN) at epicentral distances less than 3˚ were selected and analysed. The preparation of the data, includes the deconvolution of instrument response, following the velocity was integrated to displacement and finally the horizontal components rotated to radial and transverse. Finally, an extensive kinematic analysis from data provided by two private sector companies networks was done.

References:

Athanassios Ganas, Penelope Kourkouli, Pierre Briole, Alexandra Moshou, Panagiotis Elias and Isaak Parcharidis. Coseismic Displacements from Moderate-Size Earthquakes Mapped by Sentinel-1 Differential Interferometry: The Case of February 2017 Gulpinar Earthquake Sequence (Biga Peninsula, Turkey), Remote Sensing, 2018, pp. 237 – 248

Athanassios Ganas, Zafeiria Roumelioti, Vassilios Karastathis, Konstantinos Chousianitis, Alexandra Moshou, Evangelos Mouzakiotis. The Lemnos 8 January 2013 (Mw=5.7) earthquake: fault slip, aftershock properties and static stress transfer modeling in the north Aegean Sea J Seismol (2014) 18:433–455 DOI 10.1007/s10950-014-9418-3

Konstantaras A. Deep Learning and Parallel Processing Spatio-Temporal Clustering Unveil New Ionian Distinct Seismic Zone. Informatics, 7(4), 39, 2020

KONSTANTARAS, A. Expert knowledge-based algorithm for the dynamic discrimination of interactive natural clusters. Earth Science Informatics 9, (2016), 95-100

How to cite: Moshou, A., Konstantaras, A., and Argyrakis, P.: The major (Mw=7.0) earthquake of 30th October 2020 north Samos Island, Greece: Analysis of seismological and geodetic data , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1581, https://doi.org/10.5194/egusphere-egu21-1581, 2021.

13:37–13:39
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EGU21-9320
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ECS
Karen Lythgoe, Muzi Muzli, Win Oo, Hongyu Zeng, Rahmat Triyono, Phyo Maung Maung, Dwikorita Karnawati, and Shengji Wei

Supershear earthquakes have significant implications for seismic hazard, in terms of  ground shaking and aftershock pattern. It has been suggested that supershear ruptures are associated with fewer aftershocks on the supershear rupture segment, however this needs to be tested using high resolution event locations. Current aftershock catalogues for the M7.5 Palu 2018 supershear rupture are not of sufficient resolution to identify any characteristic aftershock pattern. Additionally it is unclear whether the supershear rupture speed occurred from the time of earthquake initiation, or at a later time on a certain segment of the fault.

We deployed a nodal array to record aftershocks following the main event. The array comprised of twenty short-period nodes, which can be deployed rapidly, making them ideal for post-rupture investigations in areas of sparse coverage. We expand the earthquake catalogue by applying template matching to the nodal array data. We then relocate seismicity recorded by the array using a double difference method. We also relocate seismicity that occurred before the array was active, using a relative relocation method. To do this, we calibrate the more distant permanent stations using events well-located by the nodal array. We further derive moment tensors for the largest events by waveform modelling using short-period and broadband records.

Our results show that the aftershocks cluster at the northern and southern extents of rupture. There is a relative dearth of aftershocks in the middle part of the rupture, particularly in the Palu valley, where rupture terminated to the surface. The fault here is a long and straight distinctive geomorphic feature. Many secondary faults were triggered, particularly in the southern Sapu valley fault system. An earthquake swarm was triggered 1 month after the main event on a strike-slip fault 200km away.

How to cite: Lythgoe, K., Muzli, M., Oo, W., Zeng, H., Triyono, R., Maung Maung, P., Karnawati, D., and Wei, S.: Aftershock signature of the M7.5 Palu 2018 supershear rupture from a rapidly deployed nodal array , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9320, https://doi.org/10.5194/egusphere-egu21-9320, 2021.

13:39–13:41
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EGU21-10516
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ECS
Guido Maria Adinolfi, Elvira Battimelli, Ortensia Amoroso, and Paolo Capuano

The Adriatic region has always attracted the interests of researchers involved in the study of the tectonic processes that controlled the evolution of the Alpine-Mediterranean area. It has been considered as an undeformed area, an aseismic, rigid block located between two active orogenic belts, the Apennines and External Dinarides thrust belts. Nevertheless, new scientific evidences reveal a complex structural framework in which active faults are capable to produce seismic activity not only along the borders of Adriatic Sea, but also in the offshore areas. In fact, the outer thrusts of Apennines and Dinarides orogenic belts propagated from the coasts to the offshore areas originating active, NW-SE trending anticlines and thrust faults that affects the Plio-Quaternary sequences.

Defining the seismotectonics of Adriatic domain and studying the active tectonics of the area with its seismogenic potential represent a challenge because the sea prevents direct observation of main geological and structural lineaments and the deployment of standard seismic networks for a more accurate analysis of seismicity. Despite the existence of new evidences, derived from seismic profiles and borehole data, by hydrocarbon exploration, correct seismic hazard estimates of Adriatic Sea require original and accurate data on the seismic activity that can allow to depict the number, size and geometry of seismogenic sources.

In this work, we focused our attention on the seismic sequence, consisting of about 230 events,  which occurred along the Central Adriatic coast, in the Conero offshore, during the 2013-2104, with a ML 4.9 mainshock located at 20 km far away from city of Ancona, the main city of Marche region. After a careful and innovative selection of the data recorded from the Italian National Seismic Network, operated by the Istituto Nazionale di Geofisica e Vulcanologia, the earthquakes were relocated according to a probabilistic approach. By the inversion of the polarity of the P-wave first arrivals, the focal mechanisms were estimated and finally the local magnitudes were re-calculated. Moreover, in order verify if there has been a migration of seismicity with the activation of different faults during the seismic sequence, the analysis of spatio-temporal evolution of the seismic sequence was performed. Preliminary results show that the seismic sequence was originated mainly at small depths (< 10 km) along NW-SE trending thrust fault structures as evidenced by fault plane solutions, consistent with NE-SW horizontal, maximum compression of the outer front of Apennines thrust belt, still active in the Central Adriatic offshore.

How to cite: Adinolfi, G. M., Battimelli, E., Amoroso, O., and Capuano, P.: Insights into seismic activity of Central Adriatic offshore (Italy) evidenced by the 2013-2014, Conero seismic sequence, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10516, https://doi.org/10.5194/egusphere-egu21-10516, 2021.

13:41–13:43
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EGU21-1759
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ECS
Pınar Büyükakpınar, Mohammadreza Jamalreyhani, Mehdi Rezapour, Stefanie Donner, Nima Nooshiri, Mirali Hassanzadeh, Pouria Marzban, and Behnam Maleki Asayesh

In May 2020 an earthquake with Mw 5.0 struck at ~40 km east of Tehran metropolis and ~15 km south of the Damavand stratovolcano. It was responsible for 2 casualties and 23 injured. The mainshock was preceded by a foreshock with Ml 2.9 and followed by a significant aftershock sequence, including ten events with Ml 3+. The occurrence of this event raised the question of its relation with volcanic activities and/or concern about the occurrence of larger future earthquakes in the capital of Iran. Tehran megacity is surrounded by several inner-city and adjacent active faults that correspond to high-risk seismic sources in the area. The Mosha fault with ~150 km long is one of the major active faults in central Alborz and east of Tehran. It has hosted several historical earthquakes (i.e. 1665 Mw 6.5 and 1830 Mw 7.1 earthquakes) in the vicinity of the 2020 Mw 5.0 Tehran earthquake’s hypocenter. In this study, we evaluate the seismic sequence of the Tehran earthquake and obtain the full moment tensor inversion of this event and its larger aftershocks, which is a key tool to discriminate between tectonic and volcanic earthquakes. Furthermore, we obtain a robust characterization of the finite fault model of this event applying probabilistic earthquake source inversion framework using near-field strong-motion records and broadband seismograms, with an estimation of the uncertainties of source parameters. Due to the relatively weak magnitude and deeper centroid depth (~12 km), no static surface displacement was observed in the coseismic interferograms, and modeling performed by seismic records. Focal mechanism solution from waveform inversion, with a significant double-couple component, is compatible with the orientation of the sinistral north-dipping Mosha fault at the centroid location. The finite fault model suggests that the mainshock rupture propagated towards the northwest. This directivity enhanced the peak acceleration in the direction of rupture propagation, observed in strong-motion records. The 2020 moderate magnitude earthquake with 2 casualties, highlights the necessity of high-resolution seismic monitoring in the capital of Iran, which is exposed to a risk of destructive earthquakes with more than 10 million population. Our results are important for the hazard and risk assessment, and the forthcoming earthquake early warning system development in Tehran metropolis.

How to cite: Büyükakpınar, P., Jamalreyhani, M., Rezapour, M., Donner, S., Nooshiri, N., Hassanzadeh, M., Marzban, P., and Maleki Asayesh, B.: Rupture process of the 7 May 2020 Mw 5.0 Tehran earthquake and its relation with the Damavand stratovolcano, and Mosha Fault, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1759, https://doi.org/10.5194/egusphere-egu21-1759, 2021.

13:43–13:45
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EGU21-10625
Paola Cusano, Pierdomenico Del Gaudio, Danilo Galluzzo, Guido Gaudiosi, Anna Gervasi, Mario La Rocca, Claudio Martino, Girolamo Milano, Lucia Nardone, Simona Petrosino, Vincenzo Torello, Luciano Zuccarello, and Francesca Di Luccio

FURTHER – “The role of FlUids in the pReparaTory pHase of EaRthquakes in Southern Apennines” is an INGV Departement Strategic Project devoted to define the role of fluids in earthquake genesis. One of the target areas of the multidisciplinary study is Mefite d’Ansanto, which is the largest area of non-volcanic low temperature CO2 emission field on the Earth. In particular, Work Package 1.4 is dedicated to the application of analysis methodologies in time and frequency domains, aimed to intercept eventual variations in fluid behavior before or in correspondence of local and regional earthquakes, using recordings from the INGV National Seismic Network (IV) and local networks. For this purpose, temporary acquisition surveys have been locally deployed.

On November 20, 2020, a stand-alone seismic station equipped with a Guralp CMG40T 60s broadband sensor, was installed close to the Mefite emission field. In this study we analyze some characteristics of the local seismicity, e.g., frequency content, energy temporal pattern (RMS) and polarization (Montalbetti et al., 1970), and estimate site effects (Nakamura, 1989; http://www.geopsy.org/). Here we present the first results of the ongoing investigation of the seismic noise wavefield in the Mefite area. The temporal pattern of the retrieved seismological observables is compared with the meteorological parameters, such as temperature and rainfall, to find possible relationships with exogenous factors.

Preliminary analysis of the waveforms acquired by the stations of the (IV) have been also performed. We selected the stations inside a radius of 30 km from Mefite area to eventually retrieve  the fluid dynamics footprint in the recorded wavefield.

The identification of the wavefield and site characteristics will be useful to define the features of the next survey planned in the area.

 

References

Montalbetti, J. R., Kanasevich, E. R. (1970): Enhancement of teleseismic body phase with a polarization filter. Geophys. J. Int. 21 (2), 119–129.

Nakamura, Y. (1989). A method for dynamic characteristics estimation of subsurface using microtremor on the ground surface, Railway Technical Research Institute, Quarterly Reports, 30 (1), 25-33.

How to cite: Cusano, P., Del Gaudio, P., Galluzzo, D., Gaudiosi, G., Gervasi, A., La Rocca, M., Martino, C., Milano, G., Nardone, L., Petrosino, S., Torello, V., Zuccarello, L., and Di Luccio, F.: Analysis of background seismicity recorded at Mefite d’Ansato CO2 emission field in the framework of FURTHER project: first results., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10625, https://doi.org/10.5194/egusphere-egu21-10625, 2021.

Rupture Processes/ Source mechanism
13:45–13:47
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EGU21-15620
Andreea Craiu, Marius Craiu, Mariu Mihai, Elena Manea, and Alexandru Marmureanu

The Vrancea zone is an unique area with both crustal and intermediate-depth seismic activity and constitutes one of the most active seismic area in Europe.  An intense and persistent seismicity is generated between 60 and 180 km depth, within a relic slab sinking nearly vertical in the Earth’s mantle due to the increasing of the stress state within this volume. At intermediate-depths, large magnitude events are frequent, i.e. four earthquakes with moment magnitudes (Mw) >7 occurred in the last century. An unique slab geometry, likely preserved until the present, causes stress localization due to the slab bending and subsequent stress release resulting in large mantle earthquakes in the region.

In this study, we evaluate the current stress field along the Vrancea subcrustal region by computing the fault plane solutions of 422 seismic events since January 2005. The continuous development of the National Seismic Network allows us to constrain the fault plane solutions and subsequently to evaluate the current stress field.

The main style of faulting for Vrancea subcrustal events presents a predominant reverse one, with two main earthquakes categories: the first one with the nodal planes oriented NE-SW parallel with the Carpathian Arc and the second one with the nodal planes oriented NW-SE perpendicular on the Carpathian Arc. The main axis of the moment tensor may indicate a predominant compressional stress field (Tpl>450 Ppl<450). Another characteristic of  the Vrancea subcrustal zone is the tendency of the extension axis T to be almost vertical and the compression axis P being almost horizontal.

The results of stress inversion indicate a dominant reverse faulting style, with an average stress regime index of 2.9. Other tectonic regimes were observed in the present dataset as normal and strike-slip but they are retrieved for a restrained number of events.

The stress patterns obtained from formal stress inversion of focal mechanism solutions reveal many features of the current stress field that were not captured by large-scale numerical models.

How to cite: Craiu, A., Craiu, M., Mihai, M., Manea, E., and Marmureanu, A.: Tectonic stress patterns along the Vrancea subcrustal zone from the inversion of focal mechanisms data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15620, https://doi.org/10.5194/egusphere-egu21-15620, 2021.

13:47–13:49
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EGU21-8209
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ECS
Carolina López-Sánchez, Elisa Buforn, Maurizio Mattesini, Simone Cesca, Juan Vicente Cantavella, Lucia Lozano, and Agustín Udías

One of the characteristics of the seismicity in the Ibero-Maghrebian region is the occurrence of intermediate depth earthquakes (50<h<100 km), their largest concentration located at the western part of the Alboran Sea, with epicenters following an NNE-SSW alignment. In this study, we have relocated over 200 intermediate depth earthquakes (M≥3) occurred in this region in the period 2000-2020, using a non-linear probabilistic approach (NonLinLoc algorithm) together with a recent regional 3D tomography lithospheric velocity model for the Alboran-Betic Rif Zone. Maximum likelihood hypocenters confirm the NNE-SSW distribution in a depth range between 50 and 100 km. We have determined the focal mechanisms of 26 of these earthquakes with magnitudes (mb) greater than 3.9. We first derived focal mechanisms using the P-wave first motion polarity method and then performed a moment tensor inversion, using a probabilistic inversion approach based on the simultaneous fit of waveforms and amplitude spectra of P and S phases. We performed an accurate resolution study, by repeating the inversion using different 1-D velocity models and testing different moment tensor (MT) constraints: a full moment tensor, a deviatoric moment tensor and a pure double couple (DC). Misfit values are similar for different MT constraints. Most solutions have a non-DC component larger than 30%. This may be due to the tectonic complexity of the region and the use on the inversion of 1-D Earth model. The DC components obtained from the inversion show different orientations of the nodal planes. A first group of events to the northern part with epicenters inland on south Spain have horizontal tension axes in NE-SW direction. A second group of earthquakes with epicenters off-shore, but close to the Spanish coast, presents near-vertical pressure axes. The third group, formed by deeper earthquakes, with epicenters on the center of the Alboran sea have dip slip focal mechanisms of either normal or reverse motion with planes either vertical or dipping 45º plane oriented in NNE-SSW direction, approximately the same orientation as the alignment of their epicenters. The distribution of these intermediate depth earthquakes and their focal mechanisms evidence the seismotectonic complexity of the region related with a possible subduction.

How to cite: López-Sánchez, C., Buforn, E., Mattesini, M., Cesca, S., Cantavella, J. V., Lozano, L., and Udías, A.: Focal mechanism of intermediate depth earthquakes in the Alboran Sea (Western Mediterranean), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8209, https://doi.org/10.5194/egusphere-egu21-8209, 2021.

13:49–13:51
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EGU21-3020
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ECS
Javier Fernandez Fraile, Elisa Buforn, Maurizio Mattesini, and Juan Vicente Cantavella

The aim of this study is to make a review, actualization and homogenization of the seismic parameters of the Seismic Catalogue of the National Seismic Network of Spain, which belongs to the National Geographic Institute. Our analysis focusses on the region that spans from 36.0 to 39.5° N and from 3.25° W to 1° E, which is a seismically very active region. The studied time period refers to earthquakes occurred between 1900 and 1923, where most information comes from macroseismic data and macroseismic effects.

The study begins by searching and collecting information from seismic bulletins and seismic catalogues, seismograms, seismic surveys, photographs, specific studies, historical newspapers and different digital archives. Then, the achieved information from all the different sources were reviewed and, whenever possible, the seismic parameters such as localization, seismic intensity and magnitude were recalculated.

The objective of this work is, from one hand, to establish the study methodology that allow to develop an overall review of all the earthquakes occurred in Spain from 1900 to date, and on the other hand, to provide good quality seismic data (improving the completeness and homogeneity of this seismic catalogue). Seismic data is important because it is used to make seismic hazard maps, studies of seismic risk, to update the seismic building standards and it is also used to make seismic characterization of the territory.

How to cite: Fernandez Fraile, J., Buforn, E., Mattesini, M., and Cantavella, J. V.: Focal parameter analysis of earthquakes of the S-SE of the Iberian Peninsula (1900-1923), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3020, https://doi.org/10.5194/egusphere-egu21-3020, 2021.

13:51–13:53
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EGU21-3256
Elisa Buforn, Carmen Pro, Hernando Tavera, Agustin Udias, and Maurizio Mattesini

We analyze the differences in the rupture process for twelve very deep earthquakes (h>500 km) at the Peruvian-Brazilian subduction zone. These earthquakes are produced by the contact between the Nazca and the South America Plates. We have estimated the focal mechanism from teleseismic waveforms, using the slip inversion over the rupture plane, testing rupture velocities ranging from 2.5 km/s to 4.5 km/s, and analyzing the slip distribution for each  rupture velocity. The selected 12 earthquakes have occurred in the period 1994- 2016, with magnitudes between 5.9 and 8.2 and focal depth 500- 700 km. They can be separated in two groups attending to their epicentral location. The first group is formed by 9 events located, in the Peruvian-Brazil border, with epicenters following a NNW-SSE alignment, parallel to the trench. Their focal mechanisms present solutions of normal faulting with planes oriented in NS direction, dipping about 45 degrees and with vertical pressure axis. The second group is formed by three earthquakes located to the south of the first group in northern Bolivia. Their mechanisms show dip-slip motion with a near vertical plane, oriented in NW-SE direction and the pressure axis dipping 45º to the NE. The moment rate functions correspond to single ruptures with time durations from 6s to 12s, with the exception of the large 1994 Bolivian earthquake (Mw = 8.2) which presents a complex and longer STF. The different mechanisms for the two groups of earthquakes confirm the different dip of the subducting Nazca plate at the two areas, with the steeper part at the southern one.  

How to cite: Buforn, E., Pro, C., Tavera, H., Udias, A., and Mattesini, M.: Differences in the rupture process for very deep earthquakes at the Peru-Brazil and Peru-Bolivia borders , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3256, https://doi.org/10.5194/egusphere-egu21-3256, 2021.

13:53–13:55
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EGU21-3305
Natalia Shikhova and Andrey Patonin

In laboratory experiments, acoustic emission (AE) caused by the deformation of geomaterial reflects changes in the strength and stress state of the sample. By analogy with the solution of focal mechanisms of earthquake sources, there are several methods for determining the mechanisms and types of AE sources using the amplitudes and signs of the first arrival of an elastic wave on sensors that register acoustic signals. With 16 receiving acoustic sensors, the number of polarity determinations of the incoming wave usually does not exceed 5-10, while the sign determination on some sensors is often incorrect due to the omission of the first half-period of the weak signal by the automatic registration algorithm. This reduces the reliability of determining the mechanism of the focus in laboratory tests of rocks by wellknown methods based on the distribution of signs of the first arrival of the AE wave. We propose a method for determining the directions of the axes and the values of compression and tension in the AE source. The algorithm uses information about the coordinates of events and receivers, values of amplitudes and signs of the first half-period of P-waves coming to the receivers. In this case, the model of the AE source is assumed as a quadrupole with compression and tension axes. The source-receiver distance, the directional diagram of the receiver, and the emission diagram of the source are taken into account for each of the receivers to calculate the value of displacements in the source. To test the proposed algorithm and compare it with the known methods, there was developed a program for generating an acoustic signal source of a given type with random coordinates and directions of the compression and tension axes. An array of signs and amplitudes of the first arrivals coming to the receivers was calculated from simulated data. The high efficiency of the proposed algorithm was shown. The usage of this method together with the determination of AE event types [Zang et.al., 1998] in real laboratory experiments allows us to characterize the prevailing processes of destruction during separate phases of the experiment on triaxial loading of rocks in more detail. The developed algorithm makes it possible to determine the directions of the axes and the values of compression-tension with a minimum number of signs of the arrivals of P- waves, to estimate the components of the seismic moment tensor and obtain more complete information about the mechanism of the AE source.

The work was supported partly by the mega-grant program of the Russian Federation Ministry of Science and Education under the project no. 14.W03.31.0033 and partly by the state assignment of the Ministry to IPE RAS.

How to cite: Shikhova, N. and Patonin, A.: Methods for determining focal mechanisms in laboratory experiments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3305, https://doi.org/10.5194/egusphere-egu21-3305, 2021.

13:55–13:57
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EGU21-12225
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ECS
Berkan Özkan, Tuna Eken, Peter Gaebler, and Tuncay Taymaz

Reliable magnitude estimates of the earthquakes are of utmost important for seismic hazard studies, particularly, in tectonically active areas such as the Marmara region of NW Turkey. The region is highly populated and contains a major fault associated with destructive earthquakes. In this study we apply a coda wave modelling approach based on acoustic radiative transfer theory to calculate the source displacement spectrum, and thus to obtain moment magnitudes of small earthquakes within the Marmara region. We examine three-component waveform data extracted from local earthquakes with magnitudes 2.5 ≤ ML ≤ 5.7 recorded in a radius of 150 km. For each event in the region, an inversion is performed in several different frequency bands. Our results indicate significant similarity with the local magnitude values reported by the KOERI. Consequently, we focus on establishing a novel relation between Mw and ML in the Marmara region.

How to cite: Özkan, B., Eken, T., Gaebler, P., and Taymaz, T.: Moment Magnitude Estimates in the Marmara Sea Region (NW Turkey) Using Coda Wave Analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12225, https://doi.org/10.5194/egusphere-egu21-12225, 2021.

Catalogues and b-values
13:57–13:59
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EGU21-14608
Anthony Lomax, Pierre Henry, and Sophie Viseur

We present a high-precision, absolute earthquake location procedure (NLL-SSST-coherence) based on waveform similarity between events and using the probabilistic, global-search NonLinLoc (NLL) location algorithm. NLL defines a posterior probability density function (PDF) in 3D space for absolute hypocenter location and invokes the equal differential-time (EDT) likelihood function which is very robust in the presence of outlier data. For NLL-SSST-coherence location we take initial NLL locations and iteratively generate smooth, 3D, source-specific, station travel-time corrections (SSST) for each station and phase type and an updated set of locations. Next, we greatly reduce absolute location, aleatoric error by combining location information across events based on waveform coherency between the events. This absolute coherency relocation is based on the concept that if the waveforms at a station for two or more events are very similar (have high coherency) up to a given frequency, then the distance separating these “multiplet” events is small relative to the seismic wavelength at that frequency. The NLL coherency relocation for a target event is a stack over 3D space of the event’s SSST location PDF and the SSST PDF’s for other similar events, each weighted by the waveform coherency between the target event and the other event. Absolute coherency relocation requires waveforms from only one or a few stations, allowing precise relocation for sparse networks, and for foreshocks and early aftershocks of a mainshock sequence or swarm before temporary stations are installed.

We apply the NLL-SSST-coherence procedure to the Mw5.8 Lone Pine CA, Mw5.7 Magna UT and Mw6.4 Monte Cristo NV earthquake sequences in 2020 and compare with other absolute and relative seismicity catalogs for these events. The NLL-SSST-coherence relocations generally show increased organization, clustering and depth resolution over other absolute location catalogs. The NLL-SSST-coherence relocations reflect well smaller scale patterns and features in relative location catalogs, with evidence of improved depth precision and accuracy over relative location results when there are no stations over or near the seismicity.

For all three western US sequences in 2020 the NLL-SSST-coherence relocations show mainly sparse clusters of seismicity. We interpret these clusters as damage zones around patches of principal mainshock slip containing few events, larger scale damage zone and splay structures around main slip patches, and background seismicity reactivated by stress changes from mainshock rupture. The Monte Cristo Range seismicity (Lomax 2020) shows two, en-echelon primary slip surfaces and surrounding, characteristic shear-crack features such as edge, wall, tip, and linking damage zones, showing that this sequence ruptured a complete shear crack system. See presentation EGU21-13447 for more details.

Lomax (2020) The 2020 Mw6.5 Monte Cristo NV earthquake: relocated seismicity shows rupture of a complete shear-crack system. Preprint: https://eartharxiv.org/repository/view/1904

 

How to cite: Lomax, A., Henry, P., and Viseur, S.: High-precision, absolute earthquake location based on waveform similarity between events and application to imaging foreshocks, fault complexity and damage zones for recent western US earthquakes., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14608, https://doi.org/10.5194/egusphere-egu21-14608, 2021.

13:59–14:01
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EGU21-8035
Boris Rösler and Seth Stein

Catalogs of moment tensors form the foundation for a wide variety of studies in seismology. Despite their importance, assessing the uncertainties in the moment tensors and the quantities derived from them is difficult. To gain insight,  we compare 5000 moment tensors in catalogs of the USGS and the Global CMT Project for the period from September 2015 to December 2020. The GCMT Project generally reports larger scalar moments than the USGS, with the difference between the reported moments decreasing with magnitude. The effect of the different definitions of the scalar moment between catalogs, reflecting treatment of the non-double-couple component, is consistent with that expected. However, this effect is small and has a sign opposite to the differences in reported scalar moment. Hence the differences are intrinsic to the moment tensors in the two catalogs. The differences in the deviation from a double-couple source and in source geometry derived from the moment tensors also decrease with magnitude. The deviations from a double-couple source inferred from the two catalogs are moderately correlated, with the correlation stronger for larger deviations. However, we do not observe the expected correlation between the deviation from a double-couple source and the resulting differences in scalar moment due to the different definitions. There is essentially no correlation between the differences in source geometry, scalar moment, or fraction of the non-double-couple component, suggesting that the differences reflect aspects of the inversion rather than the source process. Despite the differences in moment tensors, the reported location and depth of the centroids are consistent between catalogs.

How to cite: Rösler, B. and Stein, S.: Analysis of Differences in Seismic Moment Tensors between Global Catalogs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8035, https://doi.org/10.5194/egusphere-egu21-8035, 2021.

14:01–14:03
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EGU21-7913
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ECS
Andres Felipe Peña Castro, Sophie Lambotte, Marc Grunberg, Pierre Arroucau, Jessi Mayor, Guillaume Daniel, and Jean Letort

Locating earthquakes has been a longterm problem in seismology that depends on multiple parameters like station density and spacing, azimuthal gap, velocity models, and phase pick precision. Here, we analyze the current state of the earthquake French catalog for the time period between 2010 until 2018, which we divide into different regions: the Alps, Massif Central, the West, the Pyrenees, the Grand-East and the North. We perform multiple location synthetic tests using as benchmark the earthquake catalog and the evolution of the French seismic network to quantify the improvements in 1) earthquake location through time and 2) the error locations and their uncertainties. For such endeavors, we use NonLinLoc to perform the synthetic tests varying, as input, the stations, the number of stations and phase picks, 1D velocity models and 3D velocity models, and to understand the changes in 1) earthquake hypocenters, 2) ellipsoidal errors and 3) posterior density functions. Then, we relocate the entire catalog using NonLinLoc including 3D velocity models (where available) and compare the hypocentral location differences when we relocate the catalog with 1D velocity models. Additionally, we estimate a quality factor for each of the located earthquakes and report the changes on the quality factor with the temporal evolution of the national seismic network. The resulting catalog and its associated error location will help future seismic hazard estimations in the Metropolitan French area.

How to cite: Peña Castro, A. F., Lambotte, S., Grunberg, M., Arroucau, P., Mayor, J., Daniel, G., and Letort, J.: A comprehensive quantification of error location uncertainties for the French earthquake catalog, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7913, https://doi.org/10.5194/egusphere-egu21-7913, 2021.

14:03–14:05
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EGU21-5436
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ECS
Felipe Vera, Frederik Tilmann, and Joachim Saul

We present a teleseismic earthquake back-projection method parameterized with multiple arrays and combined P and pP waveforms, improving the spatiotemporal resolvability of rupture complexity. The contribution of each array to the rupture image is weighted depending on the multi-array configuration. Depth phases also contribute effectively to earthquakes at 40 km depth or deeper.

We examine 31 large earthquakes with moment magnitude greater than 7.5 from 2010-2020, which were back-projected in the 0.5-2.0 Hz band, giving access to the high-frequency rupture propagation. An algorithm estimates rupture length, directivity, and speed based on the back-projection results.

Thrust and normal earthquakes showed similar magnitude-dependent lengths and consistent subshear ruptures, while strike-slip earthquakes presented longer ruptures (relative to their magnitude) and frequently reached supershear speeds. The back-projected lengths provided scaling relations to derive high-frequency rupture lengths from moment magnitudes. The results revealed complex rupture behavior, for example, bilateral ruptures (e.g., the 2017 Mw 7.8 Komandorsky Islands earthquake), evidence of dynamic triggering by a P wave (e.g., the 2016 Mw 7.9 Solomon Islands earthquake), and encircling asperity ruptures (e.g., the 2010 Mw 7.8 Mentawai and 2015 Mw 8.4 Illapel earthquakes). The latter is particularly prevalent in subduction megathrust earthquakes, with down-dip, up-dip, double encircling, and segmented patterns. The automated choice of array weighting and the extraction of basic rupture parameters makes the approach well suited for near-real-time earthquake monitoring.

How to cite: Vera, F., Tilmann, F., and Saul, J.: Multi-Array Multi-Phase Back-Projection: Improving the imaging of earthquake rupture complexities, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5436, https://doi.org/10.5194/egusphere-egu21-5436, 2021.

14:05–14:07
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EGU21-9664
Rodrigo Estay and Claudia Pavez

The Gutenberg – Richter’s b-value is commonly used to analyze the frequency-magnitude distribution of earthquakes, describing the proportion of small and large seismic events as the first estimation of seismic hazard. Additionally, the b-value has been used as a stress meter, giving some insights into the stress regime in different regions around the world. In this research, a grid-based spatial distribution for the b – value was estimated in three different areas of Norway: northern (74°-81° N/ 12°-26° E), southern (57°-64°N/3°-12° E), and the ridge zones of Mohns and Knipovich. For this, we used a complete catalog from the years 2000 to 2019, which was obtained from the Norwegian National Seismic Network online database. The magnitude of completeness was estimated separately for each zone both in time and space, covering a total area of ~425,000 km2. Our results show a regional variation of the mean b-value for northern (bnorth = 0.79) and southern (bsouth = 1.03) Norway, and the Ridge (bridge = 0.73), which can be interpreted in terms of the predominant stress regime in the different zones. So far, a few calculations regarding the b-value were previously done in Norway to analyze local intraplate sequences. Then, according to our knowledge, this research corresponds to the first estimation of a regional spatial variation of the b – value in the country.

How to cite: Estay, R. and Pavez, C.: A grid-based b-value approximation through Southern and Northern Norway: preliminary results , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9664, https://doi.org/10.5194/egusphere-egu21-9664, 2021.

14:07–14:09
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EGU21-9249
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ECS
Po-Yuan Chen, Sean Kuanhsiang Chen, and Yih-Min Wu

Recent studies show that earthquake b values gradually decrease before large earthquakes at the epicenters and then immediately increase after the earthquakes. Temporal b-value variations may result from crustal stress changes associated with a large earthquake. However, the physical process is rarely observed and remains unclear. Taiwan island is a young orogeny leading to frequent earthquakes with magnitudes greater than ML 6.0, which provides an excellent laboratory to examine the physical process. We calculated b-value variation before and after ML ≥ 6.0 Taiwan earthquakes at the epicenters from 2012 to 2019. The time period is based on an enhancement of earthquake detection capability from the Central Weather Bureau Seismic Network in Taiwan, which allows the magnitude of completeness (Mc) down to 1.5 in the inland region. We used a relocated earthquake catalog to precisely estimate b value and Mc by the maximum likelihood method and maximum curvature method, respectively. We designed three steps in our research. First, we calculated the b value and Mc at the epicenters of the ML ≥ 6.0 earthquakes in overall 8 years to know the background seismic activity. Based on this, second, we calculated b values and Mc per half year to test the sensitivity between the radius from epicenters (r) and the number of earthquakes with magnitudes greater than Mc (n). Finally, we will apply moving window approach with specific criteria to continuously calculate temporal b-value variations. Our results showed that spatial b values in Taiwan in overall 8 years have an average of 1.0. The b values are systematically lower in the epicenters of ML ≥ 6.0 earthquakes from 2012 to 2019. We have determined suitable r and n values for each earthquake at the epicenters and some epicenters share similar r and n values. We preliminarily observed temporal b-value decreases before the 2018 Mw 6.4 Hualien earthquake. Considering temporal b-value variation by moving windows, we aim to realize whether temporal b-value variation by a large earthquake can be frequently observed in Taiwan.

How to cite: Chen, P.-Y., Chen, S. K., and Wu, Y.-M.: Temporal b-value variation before and after ML ≥ 6.0 Taiwan earthquakes from 2012 to 2019, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9249, https://doi.org/10.5194/egusphere-egu21-9249, 2021.

14:09–14:11
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EGU21-13895
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ECS
Pei-Ying Wu, Sean Kuanhsiang Chen, and Yih-Min Wu

Earthquake b value is primarily controlled by differential stress in the crust. Pore pressure has also been reported influencing b value locally. In nature, the influence can only be observed in the subsurface crust by injection wells. It remains unclear whether the influence of pore pressure on b value can be observed in the scale of the entire crust. To this end, we assume that pore pressure increases proportionally with VP/VS ratio, which is derived from seismic tomography studies, to examine correlation between VP/VS ratio and b value. We investigated this correlation in Japan because it is one of the most earthquake-prone countries with dense seismic networks and high-quality earthquake catalogs. We used an earthquake catalog from the Japan Meteorological Agency from 1998 to 2011 Feb to calculate the b values in the inland region of Japan above the 30 km depth. The selected period is based on a stable completeness of magnitude (Mc) since 1998 and the strong clustering effects by the both 2011 Tohoku and 2016 Kumamoto earthquakes. We then calculated Mc and b value by maximum curvature method and maximum likelihood method, respectively, in the grids of 0.1  0.1  10 km with a radius of 30 km from the center of the grids. The b value determination requires the number of earthquakes with magnitudes greater than the Mc over 150 within the radius. For the VP/VS ratios, we used the latest data derived from the National Research Institute for Earth Science and Disaster Resilience, Japan, to resample them to the same grids as b values. We simply resampled the VP/VS ratios by either averaging them into the grids of b values, or weighting them through a triangular function to the grids center of b values in depths. We analyzed b value as a function of VP/VS ratio and binned the b values within every 0.01 VP/VS interval to calculate the means and medians for liner regressions. Our preliminarily results show that there is little correlation between entire b values and VP/VS ratios among different depth ranges (0-10 km, 10-20 km, 20-30 km). We observed a linear negative relation in the binned data at the 10-20 km depth, however, this relation is not likely observed in the other depths. It may imply that the influence of pore pressure on b value could vary with depths. We’ll calculate the b values using entire magnitude range method and compare the results to the other localized geophysical observations.

How to cite: Wu, P.-Y., Chen, S. K., and Wu, Y.-M.: Correlation between earthquake b value and VP/VS ratio in Japan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13895, https://doi.org/10.5194/egusphere-egu21-13895, 2021.

14:11–15:00
Break
Tomography and attenuation
15:30–15:32
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EGU21-3511
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ECS
Susini Desilva, Ebru Bozdag, Guust Nolet, Rengin Gok, Ahmed Ali, and Yahya Tarabulsi

High-resolution seismic images of the crust and mantle beneath regions of complex surface geological structures are necessary to gain insights on the underlying geodynamical processes. One such region embodying various plate boundary motions and intraplate deformations is the Middle East, and consequently the region is prone to significant seismic activity. Hence a tomographic investigation using a more recent and reliable data set is vital in understanding the ongoing complicated deformation process driven by the African, Arabian and Eurasian plates. The purpose of our study is to retrieve a detailed  model of the crust and mantle beneath the Middle Eastern region using teleseismic P arrival times from the ISC-EHB bulletin (Engdahl et al., 1998).

Starting with AK135 as the reference model we invert for tomographic models of compressional wavespeed perturbations down to lower mantle depths in an area bounded by longitudes 22E–66E and latitudes 8N–48N.  The data set used in this study consists of regionally observed P-phase arrival times from over 1000 global events from 1996–2016 culminating in a larger dataset than other similar studies. Selection of a reliable data, ray tracing, preconditioning and inversion steps are carried out using the BD-soft software suite (https://www.geoazur.fr/GLOBALSEIS/Soft.html).

Preliminary inversion results are consistent with the previous regional tomographic studies. In checkerboard tests, cell sizes as low as ∼ 2.8° × 2.8° ( ∼ 240 × 240 km at surface) are generally well recovered down to a 1000 km depth beneath the Anatolian plateau where we currently have the densest coverage. Additionally the Caucasus region and northern parts of the Iranian plateau shows good recovery of ±4% Vp perturbation amplitudes at depths ∼ 70 – 135 km. There is fair recovery for a minimum cell size of ∼ 2.8° × 2.8° beneath the Iranian Plateau, Zagros mountain region, Persian gulf, and northeast Iraq, along with quite good recovery of cell amplitudes towards the Anatolian-Caucasus region at depth ranges 380 – 430 km, 650 – 700 km, and around 950 km. Tomographic inversions unveil a low P velocity zone stretching from the Afar region to Sinai Peninsula consistent with S wave velocity observations of a similar feature by Chang and van der Lee 2011.

We are able to further improve coverage especially down to lithospheric depths within the Arabian peninsula using first arrival times measured from waveform data collected from regional networks. Addition of first arrival time delays from waveforms highlights a prominent low velocity in the tomographic inversions beneath the volcanic fields of western Saudi Arabia. Our ultimate goal is to perform full-waveform inversion of the region constrained by the constructed P-wave model.

How to cite: Desilva, S., Bozdag, E., Nolet, G., Gok, R., Ali, A., and Tarabulsi, Y.: Direct P-Wave Travel Time Tomography of the Middle East, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3511, https://doi.org/10.5194/egusphere-egu21-3511, 2021.

15:32–15:34
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EGU21-507
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ECS
John Keith Magali, Thomas Bodin, Navid Hedjazian, Yanick Ricard, and Yann Capdeville

Large-scale seismic anisotropy inferred from seismic observations has been loosely interpreted either in terms of intrinsic anisotropy due to Crystallographic Preferred Orientation (CPO) development of mantle minerals or extrinsic anisotropy due to rock-scale Shape Preferred Orientation (SPO). The coexistence of both contributions misconstrues the origins of seismic anisotropy observed in seismic tomography models. It is thus essential to discriminate CPO from SPO in the effective anisotropy of an upscaled/homogenized medium, that is, the best possible elastic model recovered using finite-frequency seismic data assuming perfect data coverage. In this work, we investigate the effects of upscaling an intrinsically-anisotropic and highly-heterogeneous Earth's mantle. The problem is applied to a 2-D marble cake model of the mantle with a binary composition in the presence of CPO obtained from a micro-mechanical model. We compute the long-wavelength effective equivalent of this mantle model using the 3D non-periodic elastic homogenization technique. Our numerical findings predict that overall, upscaling purely intrinsically anisotropic medium amounts to the convection-scale averaging of CPO. As a result, it always underestimates the anisotropy, and may only be overestimated due to the additive extrinsic anisotropy from SPO. Finally, we show analytically (in 1D) and numerically (in 2D) that the full effective radial anisotropy ξ* is approximately just the product of the effective intrinsic radial anisotropy ξ*CPO and the extrinsic radial anisotropy ξSPO:

ξ= ξ*CPO × ξSPO

Based on the above relation, it is imperative to homogenize a texture evolution model first before drawing interpretations from existing anisotropic tomography models. Such a scaling law can therefore be used as a constraint to better estimate the separate contributions of CPO and SPO from the effective anisotropy observed in tomographic models.

How to cite: Magali, J. K., Bodin, T., Hedjazian, N., Ricard, Y., and Capdeville, Y.: Quantifying Intrinsic and Extrinsic Contributions to Elastic Anisotropy Observed in Seismic Tomography Models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-507, https://doi.org/10.5194/egusphere-egu21-507, 2021.

15:34–15:36
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EGU21-9852
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ECS
Rûna van Tent, Arwen Deuss, Andreas Fichtner, Lars Gebraad, Simon Schneider, and Jeannot Trampert

Constraints on the 3-D density structure of Earth’s mantle provide important insights into the nature of seismically observed features, such as the Large Low Shear Velocity Provinces (LLSVPs) in the lower mantle under Africa and the Pacific. The only seismic data directly sensitive to density variations throughout the entire mantle are normal modes: whole Earth oscillations that are induced by large earthquakes (Mw > 7.5). However, their sensitivity to density is weak compared to the sensitivity to velocity and different studies have presented conflicting density models of the lower mantle. For example, Ishii & Tromp (1999) and Trampert et al. (2004) have found that the LLSVPs have a larger density than the surrounding mantle, while Koelemeijer et al. (2017) used additional Stoneley-mode observations, which are particularly sensitive to the core-mantle boundary region, to show that the LLSVPs have a lower density. Recently, Lau et al. (2017) have used tidal tomography to show that Earth's body tides prefer dense LLSVPs.

A large number of new normal-mode splitting function measurements has become available since the last density models of the entire mantle were published. Here, we show the models from our inversion of these recent data and compare our results to previous studies. We find areas of high as well as low density at the base of the LLSVPs and we find that inside the LLSVPs density varies on a smaller scale than velocity, indicating the presence of compositionally distinct material. In fact, we find low correlations between the density and velocity structure throughout the entire mantle, revealing that compositional variations are required at all depths inside the mantle.

How to cite: van Tent, R., Deuss, A., Fichtner, A., Gebraad, L., Schneider, S., and Trampert, J.: A new 3-D mantle density model from recent normal-mode measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9852, https://doi.org/10.5194/egusphere-egu21-9852, 2021.

15:36–15:38
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EGU21-2829
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ECS
Sujania Talavera-Soza and Arwen Deuss

Seismic tomographic models based solely on wave velocities are unable to distinguish between a temperature or compositional origin for Earth’s 3D structure variations, such as the Large Low Shear Velocity Provinces (LLSVPs) beneath the lower mantle of Africa and the Pacific. Seismic attenuation or damping is able able to provide additional information that may help to unravel the origin of the LLSVPs, which is fundamental to understand mantle convection evolution. For example, a thermal origin for the LLSVPs will point to them being short-lived anomalies, whereas a compositional origin will point to them being long-lived, forming mantle 'anchors' and influencing the pattern of mantle convection for a large part of Earth’s history. Seismic attenuation is able to make that distinction, because it is directly sensitive to temperature variations. So far, global 3D attenuation models have only been available for the upper mantle, with only two regional body waves studies exploring the lower mantle (Lawrence and Wysession, 2006; Hwang and Ritsema, 2011).
Here, we use normal mode data to measure elastic splitting functions (dependent on velocity and density) and anelastic splitting functions (dependent on attenuation). The advantage of normal modes is that they allow us to include focussing and scattering due to the velocity structure without the need for approximations, because we measure the elastic splitting function jointly with the anelastic splitting function. In our measurements for upper mantle sensi- tive modes, we find anti-correlation between the elastic and anelastic splitting functions, suggesting a thermal origin for low velocity spreading ridges, and agreeing with previous studies. On the other hand, for lower mantle sensitive modes, we find correlation, suggesting the averagely attenuating LLSVPs are surrounded by strongly attenuating regions potentially due to the presence of post-perovskite.

How to cite: Talavera-Soza, S. and Deuss, A.: Global observations of 3D mantle attenuation using normal modes , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2829, https://doi.org/10.5194/egusphere-egu21-2829, 2021.

15:38–15:40
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EGU21-4843
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ECS
Kaushik Kumar Pradhan and Supriyo Mitra

Lg waves are formed by the superposition of shear waves trapped within the crustal waveguide and are the most destructive at regional distances. Excitation of Lg waves, its propagation and lateral variability determine the intensity of ground shaking from regional earthquakes. Spatial decay of spectral amplitude of Lg waves have been used to quantify the attenuation characteristics of the crust. In this study we use regional waveform data from the Jammu and Kashmir Seismological NETwork (JAKSNET) to study Lg wave propagation across the Indian Peninsula, Himalaya, Tibetan Plateau and Hindu Kush regions. We compute Lg/Sn wave ratio to distinguish regions with efficient Lg propagation from those with Lg blockage. These results are categorised using earthquake magnitude and depth to study Lg wave excitation and propagation across these varying geological terrains. We further use the two-station method to study Lg wave quality factor and its frequency dependence for the NW Himalaya. Seismograms recorded at two stations of the network, which are aligned within 15 degrees of the event, are used for analysis. The spectral ratio of Lg wave amplitude recorded at the two stations will be used to estimate the Q (quality factor) as a function of frequency. This will provide Q0 along all inter-station paths, which will then be combined to form Q0 tomography maps for the region. Checkerboard tests will be performed to estimate the resolution of the tomographic maps and accordingly the results will be interpreted.

How to cite: Pradhan, K. K. and Mitra, S.: Lg wave propagation and attenuation characteristics of the NW Himalaya, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4843, https://doi.org/10.5194/egusphere-egu21-4843, 2021.

15:40–15:42
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EGU21-5940
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ECS
Moloud Rahimzadeh Bajgiran and Lorenzo Colli

In recent years, several types of Machine Learning (ML) methods have been employed by Earth scientists to extract patterns and structures from multi-dimensional feature spaces. In this regard, images of the mantle obtained by different seismic tomography (ST) models are diverse datasets with varying structures due to their different theoretical approximations and input data. In this work, we apply an unsupervised ML method, K-means clustering, on ST models to explore their similarities and differences to improve our physical understanding of the Earth’s interior. The K-means clustering method requires ST models to be standardized in a three-dimensional domain. For this purpose, we implement a weighted average technique to resample ST models to radial structural zones with uniform horizontal grid resolutions. However, the homogenized ST models still have 103-104 parameters, which need to be distilled into a small number of summary features. Feature selection is thus a key part of this study: features should be independent from unphysical effects of inversion choices (e.g., the damping factor) and should instead capture the essence of the geological structure. Preliminary results obtained using the center of mass as the attribute to represent the longest wavelength part of the mantle structure show that P-wave and S-wave models do not cluster separately. Therefore, compositional anomalies do not play an essential role at these spatial scales. We plan to expand our analysis by including more summary attributes from both the spatial as well as the frequency domain. 

How to cite: Rahimzadeh Bajgiran, M. and Colli, L.: Applying cluster analysis to seismic tomography models: Uncovering similarities and differences in the spatial and spectral domains, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5940, https://doi.org/10.5194/egusphere-egu21-5940, 2021.

15:42–15:44
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EGU21-11829
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ECS
Marcel van Laaten, Tom Eulenfeld, and Ulrich Wegler

Seismic attenuation provides valuable information about the structure of the crust. For the analysis of seismic attenuation in the central part of the Leipzig-Regensburg fault zone in Germany, where numerous areas of intracontinental earthquake swarms are located, we use 18 of the region's strongest earthquakes from the period 2008 to 2019 with a magnitude between 1.4 and 3.0 in the frequency range between 3 and 34 Hz. Two different methods were used to determine the frequency-dependent scattering and the intrinsic attenuation on one hand and to compare the two methods with respect to their results on the other hand. Both methods, the Multiple Lapse Time Windows Analysis (MLTWA) and the Qopen method use the acoustic radiative transfer theory for forward modelling to generate synthetic data and fit them to the observed data. As a by-product of Qopen, we also obtain the energy site amplifications of the stations used in the inversion, as well as the estimated moment magnitudes of the inverted earthquakes. In addition, factors that influence the inversion were investigated. Different combinations of inversion parameters were tested for the MLTWA, as well as the influence of the window length on the result of Qopen. The results from both methods provide similar results within their error bars, with intrinsic attenuation being stronger than scattering and overall, rather low attenuation values compared to other regions.

How to cite: van Laaten, M., Eulenfeld, T., and Wegler, U.: Seismic attenuation analysis in the central part of the Leipzig-Regensburg fault zone using the Multiple Lapse Time Window Analysis and Qopen, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11829, https://doi.org/10.5194/egusphere-egu21-11829, 2021.

Seismic hazard and risk
15:44–15:46
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EGU21-8183
Petr Spacek, Pavel Zacherle, Götz Bokelman, Sven Schippkus, Rita Meurers, Jana Pazdírková, and the AlpArray Working Group

Earthquakes in the Eastern Alps are characterized by strongly elongated isoseismals, documenting significantly more efficient propagation of seismic waves towards the foreland (F) than into the orogen (O). In an effort to understand this phenomenon we analysed the local to regional wavefield of a single earthquake with ML4.2 / mb3.6 and epicenter WSW of Vienna (Alland) using instrumental data with unprecedented dense coverage (including AlpArray) and rich macroseismic observations. This earthquake with characteristic asymmetry of isoseismals and with the source located in the basement of the European plate just beneath the frontal thin-skinned thrust of the Penninic units is considered a representative example of the stronger historical and potential future earthquakes from this regionally important seismogenic source area. The analysis of macroseismic intensities and PGA, PGV and spectral content within time windows tied to Sg+Lg wavetrains and other interpreted phases indicates a very good match of smoothed high precision instrumental and high resolution macroseismic wavefields, which allows their joint interpretation. In the F-direction, a very small decrease of intensity and PGA values at an epicentral distance range between 30-50 km and 130-180 km is well approximated by intensity prediction equations derived for central and eastern North America. On the other hand, a sudden drop of respective values is observed at a distance of 20-30 km in the O-direction, correlating with the seismically active fault zone of Mur-Mürz line. The geographic distribution of regional distance-corrected PGA perturbations (dPGA) reveals several well-defined domains with internally limited variance whose boundaries partly correlate with known major geologic structures. Special attention has been paid to description of contrasts between the Foreland domain (Bohemian Massif + autochthonous sediments), the North Alpine domain (between the frontal thrust and Mur-Mürz line + its WSW continuation, i.e. close to southern limits of stable European plate) and the South Alpine domain (south of the former to the southern limits of the region of interest at latitude 46.2°N). The ratio of mean dPGA values observed in these three neighbouring domains is 1.00 : 0.27 : 0.05, respectively. Furthermore, significant contrast between the three domains is observed in terms of spectral content. High frequency signal above 10Hz is characteristic for the Foreland domain and strongly reduced in the South Alpine domain, suggesting that the structures related to the margin of stable European plate act here as an efficient high-cut frequency filter. While map isolines of high frequency spectral amplitude are strongly elongated in F-direction, in agreement with PGA and macroseismic intensity, for frequencies below ~5Hz the isolines of spectral amplitude are quasi-isometric around the epicenter at least within distance of ~120 km. Combination of several mechanisms is considered to explain the wave energy propagation, including intrinsic attenuation at fault zones, blockage at waveguide inhomogeneities and Q(f) contrasts between the crustal domains. Numerous other interesting observations from the whole region including the Carpathian and Pannonian domains, demonstrate the strong potential of densely sampled earthquake wavefields for studies of crustal structure and seismic hazard in the generally low-rate seismicity areas.

How to cite: Spacek, P., Zacherle, P., Bokelman, G., Schippkus, S., Meurers, R., Pazdírková, J., and AlpArray Working Group, T.: Crustal shear wave blockage in and around the Eastern Alps from the 2016 Alland earthquake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8183, https://doi.org/10.5194/egusphere-egu21-8183, 2021.

15:46–15:48
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EGU21-13043
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ECS
Georgia Giannaraki, Zafeiria Roumelioti, and Nikolaos S. Melis

The stochastic method is applied for the finite-fault modelling of strong ground motion from the October 30, 2020, shallow earthquake of Mw=6.9 which occurred offshore the northern part of Samos island in the Aegean Sea, Greece. The earthquake resulted to several human casualties and many injuries, to considerable infrastructure damage in Samos island and Western Turkey, especially in the city of Izmir, and a tsunami affecting both the Greek and Turkish coast. We focus this research on reproducing the ground motion field and damage pattern observed in Vathy, the capital of Samos Island. Different source representations, based on preliminary finite-fault slip distribution models, are tested against their capability to reproduce the two acceleration records available in Vathy. Site effects are incorporated in our modelling in the form of empirical amplification factors assigned according to a Vs30 distribution for the Samos island, which we constructed based on local geology and terrain-based proxies and on the Vs profiles at the sites of the two permanent accelerometric stations. The analysis further focuses on the empirical assessment of structural vulnerability for an estimated exposure model per building block in Vathy, which suffered structural damage due to the mainshock, mainly to a number of old and monumental buildings. The estimated exposure model in Vathy, when combined with the synthetic ground motion derived from the validated stochastic model, provides results in good agreement with available macroseismic intensities and damage reports. Our results contribute to better understanding the observed spatial distribution of damage in Vathy with respect to variations in the quality of buildings, the foundation soil and the frequency content of the excitation motion as radiated from the seismic source. The usefulness of our validated stochastic model is further demonstrated through blind predictions at sites of considerable earthquake effects, at which no record of the Mw=6.9 earthquake is available, such as in the town of Karlovasi in Samos and in the port of Chios Island.

How to cite: Giannaraki, G., Roumelioti, Z., and Melis, N. S.: The Samos Mw6.9 event: Damage investigation in the town of Vathy incorporating a stochastic finite-fault source with site and structural information, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13043, https://doi.org/10.5194/egusphere-egu21-13043, 2021.

15:48–15:50
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EGU21-8564
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ECS
Javier Ojeda, Sebastian Arriola, Christian Flores, Cristian Otarola, and Sergio Ruiz

The 2016 Kumamoto earthquake Mw 7.0 occurred in Japan reveal a multisegment shallow fault rupture that was well recorded by the KiK-net stations in accelerographs placed inside boreholes and on the surface. The numerous damaged buildings due to this earthquake reflect the critical implications for seismic hazard estimation and improvement of earthquake-resistant design for a shallower event. Herewe generate synthetic accelerograms at high frequencies implementing a stochastic method that allow us to simulate horizontal and vertical strong ground-motion accelerograms in azimuthal well-distributed stations. We included multisegment finite fault geometries estimated by independent authors as input for source model. From each sub-fault we calculated the incident and azimuthal angles arriving at each seismic station, we determined free surface effect, energy partition, radiation pattern and dynamic frequency corner for sources effect. Besideswe adopted region-specific attenuation parameters such as geometrical spreading and anelastic attenuation for path effect, and site effect parameters such as generic amplifications, soil amplification transfer functions for body waves, and high-frequency attenuation kappa filter. Our simulated acceleration time series show similarities in time and frequency with the observed records in the frequency band between 1 – 10 Hz. We obtained a good agreement between peak ground accelerations for both horizontal and vertical components, and we reproduce the amplitude and attenuation trend for the horizontal component of the GMPE models in the region. Finallywe are capable to simulate the high-frequency band of engineering interest using physics-based parameters to improve our knowledge about the sourcepath, and site effect and their impact on a seismic hazard assessment in earthquake-prone regions.

How to cite: Ojeda, J., Arriola, S., Flores, C., Otarola, C., and Ruiz, S.: High-frequency strong ground-motion simulation for the 2016 Mw 7.0 Kumamoto earthquake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8564, https://doi.org/10.5194/egusphere-egu21-8564, 2021.

15:50–15:52
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EGU21-3092
Márta Kiszely, Bálint Süle, and István Bondár

Contamination of earthquake catalogues with anthropogenic events largely complicates seismotectonic interpretation. It is especially true for relatively low seismicity areas, such as Hungary. In the present study, we analyze the characteristics of earthquakes and blasts of quarries occurred between 2015 and 2020 in the Mecsek Mountains in southern Hungary within 120 km to MORH and KOVH stations.

The objective of this study was to determine the linear discrimination line between the two classes earthquakes and explosions. We investigated the effectiveness of P/S amplitude ratios using filtered waveforms at different ranges of frequencies. We applied waveform cross-correlation to build correlation matrices at both stations and performed hierarchical cluster analysis to identify event clusters. Because most of the quarry blasts were carried out by ripple-fire technology, we computed spectrograms and examined the spectral ratio between low and high frequencies and the steepness of spectra.

Classes of earthquakes and quarry blasts have separated well from each other by combining the amplitude ratio, waveform similarity and the different spectral methods. We compare the discrimination parameters and capability of both stations to identify the explosions in analyzed quarries that were misclassified as earthquakes in the Hungarian National Bulletins.

How to cite: Kiszely, M., Süle, B., and Bondár, I.: Discrimination of earthquakes and quarry blasts in Mecsek Mountain region (Hungary) and its vicinity by using a linear discrimination function , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3092, https://doi.org/10.5194/egusphere-egu21-3092, 2021.

15:52–15:54
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EGU21-14253
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ECS
Atınç Özgün Akgün, Zeynep Gülerce, and Atilla Arda Özacar

Site-specific decay in the Fourier Amplitude Spectrum (FAS) at high frequencies, a.k.a. the zero-distance kappa (κ0), is frequently used in seismic analysis of critical infrastructure; especially for the host-to-target adjustment of the design spectrum and the site response analysis. The zero-distance kappa value for hard rock sites is more crucial but harder to constrain because the amount of strong-motion stations on hard-rock sites is limited in the global datasets. The objective of this study is to calculate the zero-distance kappa value for the hard rock strong-motion stations operated by the Disaster and Emergency Presidency of Turkey (AFAD). For this purpose, 6463 recordings from 22 strong-motion stations with measured average shear wave velocities at the first 30 meters (VS30) higher than 740m/s and having at least 100 records have been analyzed. The slope of the decay in the S-wave portion of the FAS (kappa) at high frequencies is determined for a carefully selected and record-specific frequency range. Variation of the kappa with epicentral distance is evaluated to determine the median zero-distance kappa and its uncertainty for each recording station. Estimated median zero-distance kappa values vary between 0.01s to 0.06s and are consistent with the limited amount of previously published data. Only a weak reduction in median zero-distance kappa is observed with increasing VS30 and a rather large scatter in kappa for the same VS30 values is observed. More robust results might be attained by isolating the site amplification effects of weak surficial layers and subcategorization based on available geological and geographical information.

How to cite: Akgün, A. Ö., Gülerce, Z., and Özacar, A. A.: Estimation Of High-Frequency Attenuation Parameter (Kappa) For Hard Rock Stations Of Turkish Strong Motion Network, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14253, https://doi.org/10.5194/egusphere-egu21-14253, 2021.

15:54–15:56
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EGU21-8781
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ECS
Helena Latečki, Josip Stipčević, and Irene Molinari

In order to assess the seismic shaking levels, following the strong Zagreb March 22nd 2020 earthquake, we compute broadband seismograms using a hybrid technique. In a hybrid technique, low frequency (LF, f < 1 Hz) and high frequency (HF, f = 1–10 Hz) seismograms are obtained separately and then merged into a single time series. The LF part of seismogram is computed using a deterministic approach while for the HF part, we adopt the semi-stochastic method following the work of Graves and Pitarka (2010). For the purposes of the simulation, we also assemble the 3D velocity and density model of the crust for the city of Zagreb and its surrounding region. The model consists of a detailed description of the main geologic structures that are observed in the upper crust and is embedded within a greater regional EPCrust crustal model (Molinari and Morelli, 2011). To test and evaluate its performance, we apply the hybrid technique to the Zagreb March 22nd 2020 Mw = 5.3 event and four smaller (3.0 < Mw < 5.0) events. We compare the measured seismograms with the synthetic data and validate our results by assessing the goodness of fit for the peak ground velocity values and the shaking duration. Furthermore, since the 1880 Mw = 6.2 historic earthquake significantly contributes to the hazard assessment for the wider Zagreb area, we compute synthetic seismograms for this event at two different hypocenter locations. We calculate broadband waveforms on a dense grid of points and from these we plot the shakemaps to determine if the main expected ground-motion features are well-represented by our approach. Lastly, due to the events that occured in the Petrinja epicentral area at the end of 2020, we decided to extend our 3D model to cover the area of interest. We will present the preliminary results of the simulation for the December 29th 2020 Mw = 6.4 strong earthquake, as well as our plans for further research.

How to cite: Latečki, H., Stipčević, J., and Molinari, I.: Seismic shaking scenarios for city of Zagreb, Croatia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8781, https://doi.org/10.5194/egusphere-egu21-8781, 2021.

15:56–15:58
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EGU21-14796
Nikolaos S. Melis, Eustratios Liadopoulos, Georgia Giannaraki, Ioannis Kalogeras, and Konstantinos Boukouras

An extended strong motion array comprised mainly of low cost sensors has been deployed in the Achaia region: the Patras city and the Aigion, Kalavrita towns, Greece. It combines: 4 standard accelerometric stations operated by the National Observatory of Athens, Institute of Geodynamics (NOA), 15 P-Alert MEMS acceleration devices, already deployed and operated in public sector buildings, schools and private dwellings (the Patras P-Alert Array) and 40 Raspberry Shake 4D sensors, which are deployed through the newly established Test Bed 4 region (TB4) for the H2020 financed TURNkey project. Principal aim, in an operational approach, to estimate rapidly the intensity of a felt event in a highly populated urban environment and inform local Civil Protection Agencies and through them the final responders and the general public. Moreover, the deployment of these low cost sensors, especially in schools of the Achaia region, aims to involve the pupils/students, in primary and secondary education, towards exploring School Seismology exercises, in a region where strong felt earthquakes are very frequent. Simple exercises in class, using the recorded data after a felt event have been completed such as: locating the event, estimating the magnitude, show the distribution of max PGA values in the region etc. Taking advantage of the school – local community link, the resilience increase has been already demonstrated in the local communities through happenings, popularized seminars and local press postings. A connection with the Municipalities and the Communal public sector allows the expansion of the citizen involvement (Citizen Seismology) through the use of dedicated smartphone app (i.e. LastQuake@EMSC). Citizens are informed and also pass their felt experience. This allows improved estimation and distribution of the shaking in a second phase, useful for Civil Protection Agencies. The increase of the resilience and public awareness are under monitoring with the collaboration of local media. All data will be also used as input to a TURNkey under development central platform, serving as an EEW system, mainly focusing to schools in an application for the TB4 project region in Greece.

How to cite: Melis, N. S., Liadopoulos, E., Giannaraki, G., Kalogeras, I., and Boukouras, K.: The TURNkey TB4 Achaia Array: Bridging School and Citizen Seismology through Earthquake Alerting, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14796, https://doi.org/10.5194/egusphere-egu21-14796, 2021.

15:58–16:00
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EGU21-6771
Hoseon Choi and Seung Gyu Hyun

According to strict criteria step by step for site selection, design, construction and operation, the seismic safety of nuclear power plant (NPP) sites in South Korea are secured by considering design basis earthquake (DBE) level capable of withstanding the maximum ground motions that can occur on the site. Therefore, it is intended to summarize DBE level and its evaluation details for NPP sites in several countries.

Similar but different terms are used for DBE from country to country, i.e. safe shutdown earthquake (SSE), design earthquake (DE), SL2, Ss, and maximum calculated earthquake (MCE). They may differ when applied to actual seismic design process, and only refer to approximate comparisons. This script used DBE as a representative term, and DBE level was based on horizontal values.

The DBE level of NPP sites depends on seismic activity of the area. Japan and Western United States, where earthquakes occur more frequently than South Korea, have high DBE values. The DBE level of NPP sites in South Korea has been confirmed to be similar or higher compared to that of Central and Eastern Unites Sates and Europe, which have similar seismic activity.

How to cite: Choi, H. and Hyun, S. G.: Current status of design basis earthquake level for nuclear power plant sites in several countries, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6771, https://doi.org/10.5194/egusphere-egu21-6771, 2021.

16:00–16:02
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EGU21-11008
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ECS
Florian Fuchs, Götz Bokelmann, and AlpArray Working Group

Consistent and monochromatic signals appear as sharp peaks in frequency spectra or as continuous lines in spectrograms on many permanent and temporary seismic stations in Central Europe, especially in South-Eastern Germany, Austria and the Czech Republic. Similar observations have already puzzled the seismic community more than 20 years ago. Here we report on new observations of such monochromatic seismic signals within a 1 – 10 Hz range across central Europe using the dense AlpArray network.

We identify several monochromatic signals on both permanent and temporary stations. The respective frequencies of e.g. 1.72 Hz, 2.08 Hz, 2.77 Hz or 4.16 Hz are generally stable even over long time spans (months to years). Strikingly, all such signals at any given station show identical and simultaneous short-term (minutes to days) frequency variations of up to 0.4% of the central frequency. These variations precisely correspond to fluctuations of the frequency of the European electric power network, which is regulated to 50 Hz +/- 0.4%. In fact, all persistent seismic signals that follow this behavior have frequencies of 50 Hz / n with n being an integer number (50 Hz / 29 = 1.72 Hz, 50 Hz / 24 = 2.08 Hz, 50 Hz / 18 = 2.77 Hz, 50 Hz / 12 = 4.16 Hz). We show that if the frequency of an observed spectral line is an integer fraction of the power network frequency (and only in that case) it will perfectly follow the fluctuations of the power network. This obviously raises questions about the nature of the signal itself, in particular if it is of seismic or maybe electro-magnetic origin.

We confirm that the signals are of seismic origin and we have identified water turbines inside river power plants as the source. The observed frequencies correspond well to reported rotation frequencies of water turbines at several different river power plants in Southern Germany and Austria. The seismic signals may propagate to almost 100 km from the corresponding plant. We analyze the spatial distribution of signal amplitudes for a selected river power plant in Austria, and show that it is similar to expected isolines of seismic shaking for an earthquake in the region.

Knowing the source of those exotic signals potentially enables us to use them for seismo-tectonic purposes. The long-term (several years) stability and the permanent availability (24h operation of water turbines) render them very interesting sources e.g. for studying temporal seismic velocity variations in the shallow crust.

How to cite: Fuchs, F., Bokelmann, G., and Working Group, A.: Persistent monochromatic seismic signals across central Europe: AlpArray data indicate a man-made seismic source for regional wave propagation studies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11008, https://doi.org/10.5194/egusphere-egu21-11008, 2021.

16:02–16:04
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EGU21-14708
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ECS
Ramees Mir, Imtiyaz Parvez, and Vinod Gaur

We used regional as well as global Rayleigh wave signals (source-receiver distance: 5°-175°; M≥ 6, Depth ≤ 150 km) recorded at 12 broadband seismic stations in northwestern Himalaya to compute arrival angles of surface waves at each station, assuming orthogonality of the horizontal components, and error-free levelling of the instrument. The average of all measurements at a station with cross-correlation values > 0.8, between Hilbert transformed vertical and radial components, was interpreted as the degree of misalignment of the horizontal components in a geographic frame of reference.

Out of the 12 station data used in this analysis, 3 were found to have instrument misorientation errors between 5° and 10° w.r.t geographic north, 2 between 10° and 15° and the remaining 7 < 5°. The number of measurements at each of these stations ranged from 75 to 331, with 11 stations having more than 90 measurements, assuring high reliability. We also analysed data from two nearby broadband instruments located in Ladakh Himalaya. One of these (LEH) with 46 measurements showed a misorientation error of 14.87°±4.87° and the other (HNL) with 48 showed an error of 0.75°±3.48°. Since misorientation errors based on less than 90 data elements are considered to be unstable, these were not used for further analysis.

We evaluated the effect of seismograph misorientations on the inverted solutions for P-wave receiver functions (RFs) and core-refracted shear waves (SKS). The errors in Moho depths and those of other intra-crustal features were within ±2 km for instrument misorientations of up to ~15°, that is close to the resolution errors. But, the SKS results, notably the azimuths of the fast component, were, found to be quite sensitive to instrument misalignment. For example, a ~14° error in orientation was found to cause a shift of up to 20° in the calculated azimuth of the fast component. Corrections of misorientation errors in both cases showed reduction of variance in the inverted solutions.

How to cite: Mir, R., Parvez, I., and Gaur, V.: Orientation of broadband seismographs in the Kashmir Himalaya: Effect on vector-based studies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14708, https://doi.org/10.5194/egusphere-egu21-14708, 2021.

16:04–17:00