The Kolumbo submarine volcano in the southern Aegean (Greece) is associated with repeated seismic unrest since at least two decades and the causes of this unrest are poorly understood. We present a ten-month long microseismicity data set for the period 2006–2007. The majority of earthquakes cluster in a cone-shaped portion of the crust below Kolumbo. The tip of this cone coincides with a low Vp-anomaly at 2–4 km depth, which is interpreted as a crustal melt reservoir. Our data set includes several earthquake swarms, of which we analyse the four with the highest events numbers in detail. Together the swarms form a zone of fracturing elongated in the SW-NE direction, parallel to major regional faults. All four swarms show a general upward migration of hypocentres and the cracking front propagates unusually fast, compared to swarms in other volcanic areas. We conclude that the swarm seismicity is most likely triggered by a combination of pore-pressure perturbations and the re-distribution of elastic stresses. Fluid pressure perturbations are induced likely by obstructions in the melt conduits in a rheologically strong layer between 6 and 9 km depth. We conclude that the zone of fractures below Kolumbo is exploited by melts ascending from the mantle and filling the crustal melt reservoir. Together with the recurring seismic unrest, our study suggests that a future eruption is probable and monitoring of the Kolumbo volcanic system is highly advisable.

How to cite: Schmid, F., Petersen, G., Hooft, E., Paulatto, M., Chrapkiewicz, K., Hensch, M., and Dahm, T.: Swarms of Microseismicity Beneath the Submarine Kolumbo Volcano Indicate Opening of Near‐Vertical Fractures Exploited by Ascending Melts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1176, https://doi.org/10.5194/egusphere-egu23-1176, 2023.

Our understanding of dynamic earthquake triggering in submarine environments is limited due to the lack of offshore observations. Here, we analyze the triggering susceptibility of a magmatically robust, seismically active submarine volcano (Axial Seamount), located at the intersection of the Juan de Fuca ridge and the Cobb hotspot in the northeast Pacific Ocean. Axial Seamount hosts a cabled network of geodetic and seismic instruments since late 2014. Axial Seamount last erupted in April 2015 and has continued to inflate since. We utilize a high-resolution micro-seismicity catalog to evaluate the triggering response from July 2015 to July 2022 based on seismicity rate change estimates for potential triggering sources. We report statistically significant episodes of dynamic earthquake triggering for ~16% of cases, including instances of both instant (0 < 𝑡 < 2 ℎ𝑟) and delayed (2 < 𝑡 < 24 ℎ𝑟) increases in local earthquake rate following the arrival of teleseismic waves. Initial results do not show any obvious dependence of triggering strength on the amplitude of the peak ground velocity. To evaluate the possible influence of permeability change on dynamic earthquake triggering, we compute the phase lag between vent-fluid temperature and tidal loading for the 3-day periods before and after the arrival of teleseismic waves. We report permeability changes for both triggering and non-triggering cases. Our findings provide useful insights into the physical mechanisms controlling the dynamic earthquake triggering at submarine volcanic environments.

How to cite: Barkat, A., Tan, Y. J., Xu, G., Waldhauser, F., Tolstoy, M., and Wilcock, W. S. D.: Permeability and seismicity rate changes at an inflating submarine volcano caused by dynamic stresses, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5145, https://doi.org/10.5194/egusphere-egu23-5145, 2023.

The 2021 Fagradalsfjall volcanic eruption in the Reykjanes Peninsula, Iceland, was preceded by an intensive earthquake swarm lasting one month. At the end of July 2022, a new intensive earthquake swarm occurred, which was followed on 3 August 2022 by a new effusive flow at the extension of the 2021 effusive fissure. We analyze seismic data of both swarms recorded by the Reykjanet local seismic network to trace the processes leading to the eruption to understand the relation between seismic activity and magma accumulation.

Precise relocations of the 2021 swarm show two hypocenter clusters in the depth range of 1-6 km. The WSW-ENE trending cluster of the 2021 and previous swarms show a stepover of ∼1 km offset, forming a pull-apart basin structure at the intersection with the dyke. This is the place where the 2021 eruption occurred, suggesting that magma erupted at the place of crustal weakening. The pre-eruption seismic activity in 2021 started with a M_L 5.3 earthquake of 24 February 2021, which triggered the aftershocks on the oblique plate boundary and in the magmatic dyke area, in both cases in an area of elevated Coulomb stress. The co-existence of seismic and magmatic activity suggests that the past seismic activity weakened the crust in the eruption site area, where magma accumulated. The following M_L 5.3 earthquake of 24 February 2021 also triggered the seismic swarm and likely perturbed the magma pocket which led to the six-months lasting eruption that started on 19 March.

The relocations of the July 2022 earthquake swarm show that only the northern part of the dyke-related swarm was activated compared to the 2021 swarm and both eruptions are located at the southern tip of the 2022 swarm. We compare the space-time and statistical characteristics of the 24 February 2021 aftershock sequence and of the 2021 and 2022 swarms to relate them to the different expected origin of these seismic activities.

How to cite: Fischer, T., Vlček, J., Hrubcová, P., Doubravová, J., Ágústsdóttir, Þ., and Guðnason, E. Á.: Stress transfer between volcanic dyke and seismic activity accompanying the 2021 and 2022 Fagradalsfjall eruptions, Iceland, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16208, https://doi.org/10.5194/egusphere-egu23-16208, 2023.

Seismicity in Central Utah, USA, exhibits a remarkable diversity in swarm activity. Swarms in the study area may alter between high and low activity phases, show a comparably continuous moment release, or a rise and fall of magnitudes over time. While some swarms repeatedly occur in the same source area for several years, other large swarms occur in areas without any significant seismic activity. The diversity is attributed to the complex geo-tectonic transition zone between the Basin and Range province (BR) and the Colorado Plateau (CP) in Central Utah, which is manifested in tectonic forcing related to E-W extension, high heat flow, and hydrothermal processes. Based on the University of Utah Seismograph Stations' catalog, we analyzed forty years of seismic swarm activity within Central Utah, USA, regarding characteristic statistical features (e.g., duration, moment release over time, spatial variations). In-depth analyses of three seismic sequences, including event detections, relocations, MT inversions, waveform-based clustering, and repeater analysis, provide unique insights into the study area's complex and diverse faulting processes. This includes (1) swarm activity at a regional Basin and Range normal fault, (2) activation of a local fault deviating from the recent regional tectonic regime, and (3) the complex triggering of swarm activity by a mainshock. In a joint discussion of the single exemplary sequences and the characteristics of swarm activity, we aim to expand the discussion on swarm activity beyond the study area and shed light on its relation to geothermal and tectonic processes.

How to cite: Petersen, G., Whidden, K., and Pankow, K.: 40 years of seismic swarms in the the BR-CP transition zone in Central Utah, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1883, https://doi.org/10.5194/egusphere-egu23-1883, 2023.

The hypocenters of earthquake swarms and injection-induced seismicity usually show systematic migration, which is considered to be a manifestation of their triggering mechanism. In many cases, the overall growth of clusters is accompanied by short sequences of rapid migration events, the origin of which is still not sufficiently clarified. We review the possible triggering mechanisms of these migrating episodes and propose a graphical method for distinguishing internal and external triggering forces. We also analyze the theoretical relationship between the evolution of the cumulative seismic moment and the rupture area and propose two models, the crack model and the rupture front model, which can explain the spreading of hypocenters. We developed an automatic algorithm for detecting fast migration episodes in seismicity data and applied it to relocated catalogs of natural earthquake swarms in California, West Bohemia, and Iceland, and to injection-induced seismicity. Fast migration episodes have been shown to be relatively frequent during earthquake swarms (8-20% of cluster events) compared to fluid-induced seismicity (less than 5% of cluster events). Although the migration episodes were detected independently of time, they grew monotonically with time and square-root dependence of radius on time was found suitable for majority of sequences. The migration velocity of the episodes of the order of 1 m/s was found and it anticorrelated with their duration, which results in a similar final size of the clusters in the range of first kilometers. Comparison of seismic moment growth and activated fault area with the predictions of the proposed models shows that both the rupture front model and the crack model are able to explain the observed migration and that the front model is more consistent with the data. Relatively low estimated stress drops in the range of 100 Pa to 1 MPa suggest that aseismic processes are also responsible for cluster growth. Our results show that the fast migrating episodes can be driven by stress transfer between adjacent events with the support of aseismic slip or fluid flow due to dynamic pore creation.

How to cite: Vlcek, J., Fischer, T., and Hainzl, S.: Fast migrating sequences within earthquake swarms, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12252, https://doi.org/10.5194/egusphere-egu23-12252, 2023.

Both laboratory experiments and friction theory predicts that earthquake ruptures do not begin abruptly but are preceded by an aseismic slip acceleration over a finite nucleation zone. Such a nucleation phase may also trigger precursory ruptures known as foreshocks. Therefore, the scalability of the nucleation phase and its detectability before actual earthquakes is an important question with direct implications for earthquake prediction and seismic hazard assessment. Both Slow Slip Events (SSEs) and seismicity rate increase have already been identified before a few large earthquakes and are often interpreted as evidence of their nucleation process. However, such observations still remain scarce and are associated with different characteristic lengths that raise doubt on the actual preparatory nature of these signals. Here, we further study the case of the 2017 Valparaiso Mw= 6.9 earthquake that was preceded both by an SSE and an intense seismicity suspected to reflect the nucleation phase. We further investigate seismic and aseismic interplay over the complete earthquake sequence, from foreshock up to post-mainshock times, to search for a possible connection with the mainshock occurrence. For that, we build a high-resolution catalog (Mc=2) of the region using cutting edge picking tools, reporting more than 100 000 events from 2015 to 2021 (compared with the ~8000 events reported by the Centro Sismológico Nacional over the same time-period). First, we search for anomalous seismicity rate increases in the vicinity of the mainshock compared to usual earthquake to earthquake triggering models. Using a modified Epidemic Type Aftershock Sequences model that accounts for short-term incompleteness (Hainzl 2021) and MISD declustering (Marsan and Lengliné 2008), we highlight a significant over-productive earthquake rate starting within the foreshock sequences and persisting continuously after the mainshock for several days. Then, thanks to repeating earthquakes, we show that the slow slip event is continuously decelerating from the foreshock sequences up to months after the mainshock. The estimated slip rate is lightly impacted by large magnitude occurrences and does not accelerate toward the mainshock or any large magnitude earthquake. The slip estimate from repeaters is also compared with original high-rate GPS observations during the complete 2017 sequence, further supporting the continuity of the slow slip from the foreshock up to post-mainshock times. The joint observation of an SSE and a transient background seismicity continuously from the foreshock up to post mainshock suggests a close connection between the SSE and the seismicity. Results suggest that the unusual seismic and aseismic activity observed do not reflect the nucleation phase accelerating to the mainshock dynamic rupture. The SSE would rather underlie the complete 2017 earthquake sequence, mediating a part of the seismicity, possibly by stress transfer. The resulting seismicity is then further enhanced with usual earthquake to earthquake triggering, building up the sequence. This suggests that high resolution analysis of seismic and aseismic processes over the complete earthquake sequence is needed to properly assess the significance of signals preceding mainshocks.

How to cite: Moutote, L., Itoh, Y., Lengliné, O., Duputel, Z., and Socquet, A.: Evidence of an aseismic slip event continuously driving the 2017 Valparaiso earthquake sequence, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6581, https://doi.org/10.5194/egusphere-egu23-6581, 2023.

Strain partitioning in the Central American convergent margin between the subducting Cocos Plate and Caribbean Plate is accommodated along the Middle America Trench and faults in the forearc-arc regions. In Nicaragua northwest-directed (margin parallel) forearc motion occurs on northeast (margin normal) and northwest (margin parallel) trending faults within the arc. The proximity of active faults and magmatic systems has historically led to magma-tectonic interactions. We investigate the active tectonics of Nicaragua, including a triggered sequence of earthquakes and a volcanic eruption. We use GPS-derived co-seismic displacements and relocated earthquake aftershocks to study the April 10, 2014 (M_w 6.1), September 15, 2016 (M_w 5.7), and September 28, 2016 (M_w 5.5) as a triggered sequence of earthquakes on faults that accommodate forearc motion. Our analyses and modeling indicate that the April 10, 2014 earthquake ruptured a previously unmapped margin parallel right-lateral strike-slip fault in Lago de Managua (Xolotlan) and that the September 2016 earthquakes ruptured mapped arc-normal, left-lateral and oblique-slip faults. The April 10, 2014 earthquake promoted failure of the September 2016 earthquake faults by imparting static Coulomb failure stress changes of 0.02 MPa to 0.07 MPa. Additionally, the September 15, 2016, earthquake promoted failure (static Coulomb failure stress change of 0.08 MPa to 0.1 MPa) on a sub-parallel fault that ruptured five hours after the mainshock. The April 10, 2014 earthquake displaced the flank of Momotombo volcano ~6 cm coseismically and dilated (10s of µStrain) the shallow magma system of Momotombo Volcano, which led to magma injection, ascent, and eruption on December 1, 2015, after ~100 years of quiescence. In total, this sequence represents the potential for cascading hazards in a forearc-arc system, with earthquake and magmatic triggering over short spatial (10’s km) and temporal (yrs) scales.

How to cite: Higgins, M., La Femina, P. C., Saballos, A. J., Ouertani, S., Fischer, K. M., Geirsson, H., Strauch, W., Mattioli, G., and Malservisi, R.: Cascading Hazards in a Migrating Forearc-Arc System: Earthquake and Eruption Triggering in Nicaragua, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10172, https://doi.org/10.5194/egusphere-egu23-10172, 2023.

23 November, 2022 Mw 6.0 Düzce Earthquake occurred in the west of the North Anatolian Fault Zone (NAFZ) representing a nascent transform fault between the Eurasian Plate and Anatolian Plate. Well-documented seismic sequence along this fault zone started in the east with the 1939 M7.9 Erzincan Earthquake and migrated westward with M>7 earthquakes. Following 1999 Mw7.4 Izmit and Mw7.2 Düzce failures, the next major earthquake was expected to be on the branch of the NAFZ in the Sea of ​​Marmara. However, the 2022 Düzce Earthquake activated already broken Karadere segment during the 1999 Izmit earthquake but release the accumulated strain energy at the north-eastern end of this segment. In this study, we aimed to measure co-seismic surface deformation caused by the 2022 Düzce rupture and determine the source parameters by combining geodetic and geophysical measurements. The co-seismic deformation is analyzed by using Interferometric Synthetic Aperture Radar (InSAR) technique performed on the Sentinel-1 data. The pre- and post-earthquake ascending and descending SAR images were processed using the TopsApp module of the InSAR Scientific Computing Environment (ISCE) software to obtain interferograms of the co-seismic deformation. Our preliminary results show ~30 mm surface deformation. Our preliminary inversion, based on Okada elastic dislocation modeling, resulted in a fault geometry with ~267, 68 and -172 for strike, dip, and rake angles, respectively. This identifies a dominant right-lateral strike slip motion and is fairly compatible with both surface morphological properties of this segment as well as with initial seismology data-based mechanism solutions of various national/international monitoring centers (e.g. KOERI, GEOFON).

How to cite: Kaya-Eken, T., Zoroğlu, Ç. S., Havazlı, E., and Özener, H.: Co-seismic Deformation and Source Parameters of the Mw6.0 Düzce Earthquake by InSAR, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9466, https://doi.org/10.5194/egusphere-egu23-9466, 2023.

Using S-wave records at epicentral distances less than 60 km we determine the apparent stress for 62 Mw≥4.5 earthquakes in southern California since 2000. All earthquakes have reliable network moment tensor solutions. We compute seismic radiated energy with two methods: a time domain method by Kanamori et al. (2020) and a frequency domain method by Boatwright et al. (2002). The Kanamori approach (GR) is a modified Gutenberg-Richter in which attenuation and near surface effects are not considered. The Boatwright method uses path attenuation, near surface kappa0, and a station specific radiation pattern. With Boatwright we compute seismic energy 1) with an average radiation pattern (F0) and 2) with station specific radiation pattern (F1). The geometric means of apparent stress are 0.48, 0.40 and 0.57 MPa for GR, F0 and F1, respectively. Apparent stress is independent of seismic moment for these earthquakes. Converting apparent stress to Brune’s stress drop (Andrews, 1986), we find stress drops of 2.1, 1.7 and 2.5 MPa for GR, F0 and F1, respectively. From the perspective of seismic radiated energy, a Brune stress drop is nearly the same as that when using Madariaga (1976) and Kaneko and Shearer (2014) models (Ji, Archuleta and Wang, 2022). The standard deviation of stress drop (log10) is 0.35—almost the same for GR, F0 and F1. Cotton et al. (2013) show the standard deviation from stochastic vibration theory used in ground motion prediction equations is 0.15 for Mw>5.5 earthquakes. Seismic moment/corner frequency methods produce a standard deviation of 0.61, though the magnitude range is larger in some studies. Apparent stress (and consequently stress drop) shows a statistically significant depth dependence (~0.05 MPa/km).

How to cite: Archuleta, R., Ji, C., and Peyton, A.: Apparent stress of moderate sized earthquakes in southern California, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10099, https://doi.org/10.5194/egusphere-egu23-10099, 2023.

Understanding mechanical processes occurring on faults and catching the preparation phase of large magnitude events require a detailed characterization of the microseismicity, which can be enhanced today using advanced techniques for earthquake detection. These techniques decrease the detection threshold of seismic networks and provide augmented catalogs, which enable improved statistical analysis associated with event occurrence and size. However, seismic events recorded at the level of the noise typically emerge only at a few stations, making earthquake characterization challenging. This issue is further complicated in areas where seismicity occurs deep in the crust, as happens in the normal fault system of the Irpinia region, Southern Italy, where earthquakes occur at depths between 8 and 15 km.

In this work we focus on the detection and characterization of seismic sequences occurring in the Irpinia region featuring low magnitude mainshocks (Ml∼3), using data from the Irpinia Near Fault Observatory.

Event detection for the sequences is performed through the integration of a machine learning based detector (EQTransformer, Mousavi et al., 2020) and a template matching technique (Chamberlain et al., 2018), with the former providing a wider set of templates for the similarity search. This strategy outperforms auto-similarity techniques based on fingerprints (FAST, Yoon et al., 2015) and template matching grounded in manual catalogs. On average, the final catalog of the analyzed sequences increases the manually revised network bulletin by a factor 7. We compared P- and S- arrival time estimates, grounded in the machine learning phase picking and cross-correlation for template matching, using manual identifications to assess the reliability of automatic picks; the mean residual between manual and automatic values is ~0 for both P- and S-waves, with a larger residuals standard deviation for the latter.
We apply a double-difference location technique using both catalog and cross-correlation differential travel times for locating the events, with the goal of resolving and highlighting fault structures where seismicity takes place. We finally track the spatio-temporal evolution of the seismicity, and apply a mechanical model based on static stress, to discriminate whether sequences in the area are mainly triggered by static stress change, dynamic stressing, or aseismic mechanisms such as fluid diffusion.

How to cite: Scotto di Uccio, F., Festa, G., Michele, M., Beroza, G. C., Chiaraluce, L., Picozzi, M., Scala, A., and Supino, M.: Detection and characterization of seismic sequences in the normal fault system of the Irpinia region, Southern Italy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12165, https://doi.org/10.5194/egusphere-egu23-12165, 2023.

The analysis of highly coherent seismic signals produced during tremor episodes has been recently gained interest as a mean to study the structure of volcanic systems, and the underlying physical mechanism producing its activity. Volcanic tremor signals usually appear with a non-impulsive gradual onset and can last for long periods of time, ranging from hours to months, during which individual waves cannot be recognized. The lack of identifiable arrivals during tremor episodes and the long duration of the registered signals, renders ineffective classical methods based on the analysis of the travel times, making difficult the location of its sources. 

We present observations showing that during tremor episodes, the relative phase of inter-station cross-correlations is approximately constant, which is directly linked to the stability of the source position and mechanism. We propose a new method to identify the relative phase stability (and therefore, wavefield coherence) in recordings obtained from a seismic network, that can also be applied to recordings from a single pair of stations. Then, we use a new approach based on the relative phase measurements to find the position of the source of a tremor episode in 2015 at the Klyuchevskoy Volcanic Group.  In general, we show several of the advantages of extracting information from the relative phase as opposed from methods relying on the identification of arrival phases.

How to cite: Barajas, A. and Shapiro, N. S.: Relative phase analysis for volcanic tremor detection and source location, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2557, https://doi.org/10.5194/egusphere-egu23-2557, 2023.

Earthquake swarms are bursts of relatively small to moderate earthquakes lasting from hours to months without a clear triggering mainshock. To shed new light on the physical processes driving the seismic swarm that occurred in 2013 along the Alto Tiberina low angle normal fault, we investigate the strain sensitivity to seismic velocity variations (dv/v). For that, we use continuous recordings of ambient noise recorded at 18 stations located in the vicinity of the Alto Tiberina fault for a period of four years. We then retrieve daily dv/v with a time lapse approach by applying the stretching technique. After decomposing our dv/v into tectonic and non-tectonic (thermoelastic and rain induced changes) components, we find a velocity drop (0.035%) coinciding with the seismic swarm. Our observations and the deduced strain sensitivity of roughly 1000 suggest that the triggering of the swarm is mainly caused by an aseismic slip enhanced by the presence of fluids at seismogenic depth (3 - 5 km).  

How to cite: Mikhael, N., Poli, P., and Garambois, S.: Non-linear seismic velocity variations observed during a seismic swarm in the Alto Tiberina low angle normal fault from ambient noise correlation measurements, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-981, https://doi.org/10.5194/egusphere-egu23-981, 2023.

The site response input in the Groningen seismic hazard assessment is based on modelled shear-wave velocity (V_S) profiles. Two sets of data were used to compare in situ (field) and model data of V_S. The first set consists of data from several blocks of ~ 400 nodes. Inversion of passive seismic data from a coarse grid of ~ 6 km x 10 km resulted in V_S profiles to a depth of 800 m and from a denser grid of ~ 1 km x 1 km more detail to a depth of 100 m. The field V_S profiles were a combination these two depth ranges. The site response analysis based on either the field or model V_S profiles showed on average similar amplification factors over periods relevant for seismic risk. The model V_S profiles are therefore a good representation. The second set consists of V_S data from MASW surveys on dwelling mounds. The local detailed field V_S profiles reach a depth of 18 m. Site response analyses using the full model V_S profiles or profiles with the top 18 m replaced by field V_S showed that the amplification on dwelling mounds is underestimated significantly, on average by 7 to 28 %. Because of this, a frequency-dependent Penalty Factor has been derived. In the risk calculations, this Factor is to be applied to buildings on dwelling mounds to transform the estimated motions at the ground surface (based on model V_S) into motions at the top of the dwelling mound.

How to cite: Kruiver, P., Pefkos, M., Campman, X., Meijles, E., and van Elk, J.: Using measured and modelled shear-wave velocity profiles for the assessement of site response in Groningen, the Netherlands, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15577, https://doi.org/10.5194/egusphere-egu23-15577, 2023.

Rupture processes of large earthquakes have been studied by seismic waveform analysis and directivity effects have also been observed in moderate and small earthquakes.

This effect leads to azimuthal and spectral variations in ground motion that can be used to estimate the fault plane orientation or a predominant rupture propagation direction in a particular region or during a seismic sequence. For moderate-to-strong events, directivity at low frequencies can result in potentially destructive pulses with large ground motions, while at high frequencies and for small-to-moderate events, the most pronounced effect is the shift in corner frequencies that results in high-frequency energy arriving in short time intervals.

It is therefore of the utmost importance to estimate the directivity effects in engineering applications and seismological studies of earthquake sources. While some methods appear to work well for high magnitude earthquakes, determining directivity and source parameters for small to moderate magnitude earthquakes remains a challenge.

One of the most common methods to estimate the directivity from moderate to small earthquakes relies on measuring the duration of the source pulse (the apparent source time function) at each location and then modeling it using a line source. Some approaches rely on the deconvolution of waveforms by an empirical Green’s function (eGf), to overcome the problems associated with the presence of path and site effects.

A promising approach for estimating the rupture directivity effect and associated source properties is based on the calculation of the second seismic moments. In this study we apply the method based on the calculation of the second seismic moments to estimate the rupture process and source parameters to study a Mw 4.7 earthquake that occurred in central Italy during the 2016 - 2017 seismic sequence recorded by the RAN (Rete Accelerometrica Nazionale) and RSN (Rete Sismica Nazionale) italian networks.

We first used synthetic apparent source time functions calculated from a geometric source model obtained from a real event to test the robustness of the method. Then, we applied the second-seismic moment method and the approach based on high-frequency S wave amplitude variations versus source azimuths analysis with an empirical Green's function deconvolution approach and compare the results.

How to cite: Cuius, A., Meng, H., Saraò, A., and Costa, G.: Estimating the rupture directivity and source parameters of moderate to small earthquakes using the second seismic moment , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14089, https://doi.org/10.5194/egusphere-egu23-14089, 2023.

Ample observations attest to the importance of pore pressure dynamics and fault-zone fluid flow in producing earthquake swarms, foreshocks, and aftershocks. However, poroelasticity effects incorporating the two-way coupling of solid and fluid phases where pore-pressure evolves due to e.g. slip, dilatancy, and compaction, are rarely considered in simulations of fault slip. Here we study how coupled dynamics of frictional slip, pore pressure, permeability and porosity evolution control the occurrence of precursory slow-slip and foreshocks leading to the mainshock. We model fully dynamic seismic cycles with a newly-developed Hydro-Mechanical Earthquake Cycles (H-MECs) code where uniform velocity-weakening rate-and-state friction and a constant and far-field loading rate are applied on a 2-D anti-plane fault model embedded in a poro-visco-elasto-plastic medium. We also include the effect of permeability barriers, represented by regions of low permeability and high pore-fluid pressure with wavelengths similar to the nucleation length. Despite the relatively simple model setup, a complex fault behavior arises from the numerical experiments, including small seismic events, complete ruptures, as well as aseismic slip transients. Our results indicate that permeability barriers – where pore-fluid pressure is high – lead to fault creep, whereas foreshocks and mainshocks unzip from locked asperities with relatively lower pore-fluid pressure. We find that the ratio between the size of locked asperity and the wavelength of permeability barriers controls both the nucleation and propagation of aseismic creep, slow-slip transients, cascade of foreshocks, and full seismic rupture. Further numerical experiments accounting for both permeability enhancement due to fault slip and permeability reduction due to healing and sealing show that, once the permeability seals break, fluid is injected and redistributed through the fault zone, which diffuses primarily on-fault, thus leading to the nucleation of a complete rupture. As a result, permeability evolution and pore-fluid pressure variations modulate the ratio of seismic to aseismic moment release of seismic swarms. Our results, compared to observations, demonstrate that pore-fluid pressure evolution and poroelastic effects on- and off-fault play a critical role in the dynamism of earthquake swarms, as they control the stability of faults and whether slip is seismic or aseismic.

How to cite: Ye, J. and Dal Zilio, L.: The role of poroelasticity and permeability barriers in governing the interplay between precursory slow slip and foreshocks, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2003, https://doi.org/10.5194/egusphere-egu23-2003, 2023.

Seismicity migration is one of the most remarkable features of earthquake swarms because of its ubiquity and the wide range of migration durations, velocities and shapes observed. The dynamic properties of swarms, like seismic moment or number of events, are often attributed to fluid circulation, directly or indirectly. However, classical models of fluid pressure diffusion aiming at explaining seismicity triggering and migration show some limitations.  An increasing body of evidence points at an important contribution of fluid-induced aseismic slip during swarms. Moreover, parameters like injection history and fault criticality are expected to intervene. In this work, we use a fracture mechanics framework to show that  earthquake migration can be explained as driven by the propagation of a fluid-induced aseismic slip transient on a rate-and-state fault. This theoretical model predicts a simple linear relation between the seismic migration distance and the square root of the injected fluid volume. This relation is validated by observations in two well-studied seismic sequences induced by injections for geothermal purposes (Basel and Soultz-sous-Forêts), in which the seismicity is mainly clustered around a single surface. In addition, the model helps constrain frictional, hydraulic and structural properties of the fault hosting aseismic slip, and can be reasonably generalized to all fluid-induced earthquake swarms, natural and anthropogenic.

How to cite: Danré, P., Garagash, D., De Barros, L., Cappa, F., and Ampuero, J.-P.: Propagation of a fluid-induced aseismic crack leads to earthquake swarm migration controlled by fluid volume, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2618, https://doi.org/10.5194/egusphere-egu23-2618, 2023.

One of the major challenges in seismology is the lack of a unified view of earthquake dynamics embracing all the spatial and temporal scales at which it takes place: while several models describe successfully how single seismic events develop and statistical laws have been set up to characterize the distribution of seismicity in magnitude, space and time, at least as a consequence of mainshocks, such dichotomic information is rarely put into communication to better understand how seismogenesis occurs. Although a comprehensive, cross-scale theory is intrinsically impossible because of the different levels of physical complexity involved in the seismological processes, it is possible to derive several well-known links, as well as interesting new ones, between coseismic properties and long-term statistical patterns. We introduce a simple model based on optimization criteria to explain such mathematical relationships. Given an initial energy perturbation localised on a fault segment, the interface breaks down if the perturbation increases its energy beyond the breakdown level. The slip occurs and the fracture spreads rapidly. Not only that (which is not enough to explain how the fault system will behave at large scales), since the fault zone is in an unstable and frustrated state, i.e., a configuration forced by tectonic stress: meanwhile the fracture propagates, the adjoining interfaces and volumes move towards a more stable energy level, amplifying energy release by a certain amount. Then, the latter can be interpreted as a measure of the triggering power of fracture and applied to connect local and collective dynamics. We focus on the role of tectonic setting and the differences between in-fault and off-fault seismicity. Our model can reproduce several features of seismicity, such as the dependence of the b-value of the Gutenberg-Richter law on the tectonic setting, the correlation between b- and p-value of the Omori-Utsu law, the fractal dimension of hypocentral time series, duration of seismic sequences and the efficiency of the seismogenic process. We utilise the same framework to analyse the composition of moment tensor solutions of global and regional shallow seismicity in terms of double-couple vs non-double-couple contributions. We find that thrust events are characterized by higher double-couples with respect to normal and strike-slip fault earthquakes. Our results are also coherent with the broadly studied stress dependence of the scaling exponent b-value of the Gutenberg-Richter law, which turns out to be anticorrelated to the double-couple contribution. Our work suggests that the structural and tectonic complexity of the seismogenic source marked by the roughness of faults and the width of the dislocation zones has a significant impact on coseismic dynamics, which should be considered in the routinely applied observational procedures to avoid systematic biases in the estimation of the parameters of the seismic source, e.g., the seismic moment.

How to cite: Zaccagnino, D., Telesca, L., and Doglioni, C.: One to many seismogenic sources: from single earthquakes to seismic sequences, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-211, https://doi.org/10.5194/egusphere-egu23-211, 2023.