Between March 2018 and January 2021, fluids were repeatedly injected at the Geoven deep geothermal energy site near the city of Strasbourg, France. These injections caused seismic activity, which ultimately led to the interruption of operations after a ML 3.6 event. Despite the cessation of fluid injection, seismicity continued in the following months, culminating in a ML 3.9 earthquake six months after shut-in.
Seismicity occurred in different clusters, which exhibit a wide range of b-values (0.6–1.6) despite their close temporal and spatial proximity. In the context of induced seismicity, b-value variations are often attributed to the activation of faults of different sizes or faults under different stress conditions. However, the clusters observed in this study are of comparable size and, assuming they are subject to the same regional stress field, are likely under similar stress conditions. This raises the possibility that the observed b-value variations are instead linked to differences in the geomechanical properties of the faults. To explore this, we analyzed the spatio-temporal evolution of the clusters.
The clusters were intermittently active through multiple bursts of seismicity, with each burst typically activating a new area. This type of migration is consistent with the process of earthquake interactions and suggests the presence of geomechanical heterogeneities, such as variations in fault strength, along the fault zone.
Despite their similar overall migration behavior, the clusters have different activity characteristics. High b-value clusters are characterized by small burst areas and a stable maximum earthquake magnitude (Mmax) over time, indicating that rupture sizes remain limited throughout the cluster evolution. In contrast, low b-value clusters are characterized by larger burst areas and an increasing Mmax over time, suggesting less constrained rupture sizes. The difference in b-value between clusters may then be related to differences in rupture behavior, which in turn may be related to variations in the geomechanical properties of the faults, such as different degrees of strength heterogeneity.
How to cite: Minetto, R., Lengliné, O., and Schmittbuhl, J.: Fault geomechanical properties control the b-value at the Geoven deep geothermal site, France, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17510, https://doi.org/10.5194/egusphere-egu25-17510, 2025.
The Bedretto Underground Laboratory for Geosciences and Geoenergies (BedrettoLab) is a deep underground laboratory in the southern Swiss alps that allows to induce and study rock deformation processes in situ, from exceptionally short distances, at 1 km depth. In the context of the Fault Activation and Earthquake Rupture (FEAR) project, we have conducted a series of fluid injection experiments, with the goal of inducing an ML~0.0 earthquake. In this 'M-zero' experiment series, we have used a custom-built remote control system to implement fluid injection protocols that were informed by 2D continuum hydro-mechanical earthquake cycle models, with the target of increasing the likelihood of inducing a larger event. In the first of two major experiments, after 16 hours of high pressure (20 MPa) fluid injection, we have induced a Mw~ -0.4 "main shock", which triggered an aftershock sequence with >100 events in the first 3 seconds alone. The fluid injection was stopped 10 minutes after this main shock.
The experiment and earthquake sequence was recorded on an extensive multi-domain and multi-scale 3D monitoring array, which combines various types of seismic sensors with distributed acoustic, strain and temperature sensing, Fibre Bragg Grating strain sensors, as well as pressure and flow monitoring. Although the main shock magnitude is smaller than the target magnitude (which was unclear at the time of its occurrence), the wide range of observations made, including high-resolution micro-seismicity catalogues covering 5 units of magnitude, offer a detailed look at seismological and rock mechanic processes before, during and after the main shock. In this talk we summarize some of the key observations and insights including i) the foreshock sequence, which shows signatures of a triggering cascade but which did not accelerate before the main shock; ii) a distinct Gutenberg-Richter b-value anomaly where the main shock occurred, and which may reflect the structural inventory and stress state of the activated fracture network; iii) the first-order consistency between early aftershock distributions and kinematic rupture models from spectral fitting, and a pronounced aftershock gap across a 2-by-2 metre patch where the main shock occurred, iv) static deformation patterns resolved on the strain sensor network, and their consistency with the focal mechanisms inferred from seismic data. We also discuss the implications for the planned fault activation experiments in the coming three years at the BedrettoLab.
How to cite: Meier, M.-A., Gischig, V., Rinaldi, A., Jalali, M., Supino, M., Mosconi, F., Villiger, L., Selvadurai, P., Spagnuolo, E., Tinti, E., Dal Zilio, L., Zappone, A., Pozzi, G., Mesimeri, M., Scarabello, L., Hertrich, M., Amann, F., Cocco, M., Wiemer, S., and Giardini, D. and the FEAR Team: An Induced Mw -0.4 Earthquake Under a Microscope at the BedrettoLab, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17272, https://doi.org/10.5194/egusphere-egu25-17272, 2025.
The spatiotemporal characteristics and magnitude distributions of earthquake aftershocks provide critical insights into the dynamical processes within the Earth’s crust, carrying significant implications for seismic hazard assessment and mitigation. For example, the well-known Gutenberg-Richter law has been applied to characterize the frequency-magnitude distribution of aftershocks, with the b-value that reflects the relative frequency of small versus large events giving valuable information about heterogeneous crustal structures and regional stress conditions. Although great efforts have been made to study the mechanisms behind the b-value, no consensus has been reached, especially regarding its geometrical versus mechanical origin. Here, we develop physics-based analytical and numerical models to uncover the b-value’s origin within a fault network undergoing a mainshock-aftershock sequence. The analytical model relates the b-value to the power law exponents of the fault frequency-length distribution and the length-displacement scaling relation. High-fidelity 3D direct numerical simulations are then employed to model intricate mainshock-aftershock sequences in fault networks. In the model, the mainshock rupture initiated by a small stress drop at the hypocenter propagates spontaneously along the mainshock fault, triggering extensive aftershocks on spatially distributed secondary faults that obey a power law size scaling. The rupture and slip dynamics within the fault network assume a slip-weakening law captured by a static/kinetic friction model. We study aftershock statistics across various scenarios with different critical distances, which all show a two-branch frequency-magnitude scaling pattern. Through the analytical and numerical solutions, we show that the first branch of small-magnitude earthquakes, characterized by a lower b-value, is related to the faults that are partially-ruptured, while the second branch of large-magnitude earthquakes, with a higher b-value, is associated with faults that are (nearly) fully-ruptured. This provides a new perspective to interpret the commonly observed break in Gutenberg-Richter law that is conventionally attributed to catalogue incompleteness. We further interpret the mechanisms driving the emergence of this two-branch scaling statistics based on considerations of the energy budget. For events in the first branch, a significant portion of the available energy is dissipated as fracture energy, reducing the portion that sustains rupture propagation and leading to rupture arrest within a small area; for events belong to the second branch, only a small portion of the available energy is dissipated as fracture energy, leaving sufficient energy to drive spontaneous rupture and enabling faults to be fully ruptured. These findings and insights have important implications for understanding the origin of the b-value of earthquake aftershocks in the Earth’s crust.
How to cite: Pan, W., Lund, B., Zhang, Z., and Lei, Q.: Physics-based models indicate a combined geometrical and mechanical origin of the b-value of earthquake aftershocks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16912, https://doi.org/10.5194/egusphere-egu25-16912, 2025.
The Northern and Central Apennines of Italy are regularly struck by moderate to strong seismic activity, such as the 2009 L’Aquila (Mw 6.1 mainshock) and the 2016-2017 Amatrice-Visso-Norcia (AVN, Mw 6.0 to 6.5 mainshocks) sequences. Many studies have been conducted to understand the processes driving the seismicity of these two seismic sequences. However, much less is known about what happens during the interseismic period in this region. We focused on the seismic activity since the 2016-2017 AVN seismic sequence and retrieved the seismic catalogue from the INGV website for the period 01/01/2018 to 31/08/2024. A total of 55 278 events were detected. We relocated the seismicity using a double-difference relocation program and obtain 54 151 relocated events. Using a declustering method, we define 134 clusters (9 285 events) having more than 10 events. In order to gain insights into the physical processes driving the seismicity in each cluster, we use a set of parameters bearing information about their spatio-temporal evolution and magnitude distribution (e.g. Seismic-to-total-moment ratio, Skewness and Kurtosis, Seismic Active Volume, Estimated injected fluid volume). Based on this parametric analysis we can concluded that: 1) the majority of the clusters behaved like swarms, 2) the clustered seismicity did not occur on-fault but rather off-fault, activating a volume rather than discrete fault structures, 3) the clusters are likely driven by fluids. All these analyses helped us to characterize the seismic activity during the interseismic period.
How to cite: Baques, M., Poli, P., and Fondriest, M.: Characterization of seismic activity in Northern Apennines (Italy) during the interseismic period., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10909, https://doi.org/10.5194/egusphere-egu25-10909, 2025.
Recent advances in seismology through newer machine learning based tools, template matching and double difference relocation allow to build high quality earthquake catalogs from continuous waveforms. Here, we apply a workflow combining these approaches and reducing computational cost to the Corinth Gulf, Greece. This extensional rift is characterized by a prolific seismicity mainly occurring as swarms and driven by a complex interplay between tectonic loading, fluid diffusion and aseismic slip. This area also benefits from the long-standing and dense instrumentation of the Corinth Rift Laboratory (EPOS NFO). Focusing on the year 2017, the catalog contains more than 100k events, that illuminate three different active areas and sequences (mainshock-aftershock sequences and earthquake swarms). This high quality catalog allows us to identify and define seismogenic structures, but also to gain insights on the driving mechanisms of these seismic sequences (fluids, aseismic slip, etc.), how they evolve in time and relate to known geological and tectonic features. In particular, we focus on the migration of the swarm fronts and bursts within the swarms, by studying both fast and slow migration patterns. As both types of migration are related to fluid-induced aseismic slip, this allows us to refine our general understanding of the driving processes of swarms.
How to cite: Danré, P., Lengliné, O., and De Barros, L.: Complex seismicity features observed in a high-resolution seismicity catalog for the Gulf of Corinth, Greece, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8849, https://doi.org/10.5194/egusphere-egu25-8849, 2025.
The 2021-2022 seismic swarm in the southern tip of the Alboran Shear Zone provides a unique opportunity to explore the interplay between tectonic, fluid-driven, and possibly mantle-related processes within the tectonically complex western Mediterranean. The swarm lasted more than 2 years and is characterized by low magnitudes up to M5 and the lack of a dominant mainshock. The relocation with HypoDD revealed a distinct spatial clustering and temporal migration patterns. The activity is concentrated along the southern termination of the Al Idrisi Fault Zone, a left-lateral strike-slip system, and the Trougout Fault, a normal-left lateral strike-slip fault, as major onshore-offshore structures. Focal mechanism analysis reveals predominant strike-slip and transtensive faulting, consistent with the left-lateral motion and the ongoing convergence between African and Eurasian plates governing deformation along the Alboran Ridge. Furthermore, the seismicity reaching depths of 30–35 km points to deeper lithospheric processes, suggesting fluid migration, mantle dynamics, or crustal densification, as potential triggers for fault reactivation. Our analysis of the distance-time (R-T) diagrams indicates that the swarm is likely fluid-driven supporting the hypothesis of fluid migration as a key factor in fault reactivation. The localization of this activity along a shear zone highlights the role of distributed deformation and stress redistribution in regions dominated by transpressional and transtensional regimes. The absence of large seismic events suggests that the fault system may accommodate stress through slow-slip events or fault creep, where deformation occurs gradually rather than through abrupt ruptures. This mechanism is often observed in weak or immature fault zones, where strain accumulates and releases in smaller, episodic bursts instead of a single catastrophic failure. Such behavior is typical of regions undergoing lithospheric thinning or mantle upwelling, where thermal anomalies and stress perturbations influence fault dynamics.
How to cite: Akka, H., Rigo, A., Tahayt, A., Timoulali, Y., and Jabour, N.: Unraveling mechanism of the 2021-2022 South Alboran Seismic Swarm: Insights into Fault Reactivation and Lithospheric Dynamics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13177, https://doi.org/10.5194/egusphere-egu25-13177, 2025.
Oceanic transform faults (OTFs) accommodate deformation through a combination of quasi-periodic Mw > 6 earthquakes, aseismic creep, and microseismicity (Mw 0–5), including swarms, foreshocks, aftershocks, and background events. These seismic patterns result from the interplay of tectonic processes and fluid circulation, as well as magmatism within intra-transform spreading centers. Hydrothermal and magmatic fluid circulation at OTFs significantly alters the lithological and rheological properties of fault systems, shaping their seismic and aseismic behavior.
In this study, we analyze seismic data from a 55-station ocean-bottom seismometer (OBS) network (X9: 2012–2013) deployed around the Blanco Fracture Zone (BFZ), located offshore Oregon. Using advanced machine-learning pickers trained on OBS data, the PyOcto 1D associator, and high-resolution location algorithms, we significantly enhance the detection of low-amplitude / low-SNR microseismic events (Ml 0–2), augmenting the seismic catalog by approximately 20,000 events.
Our high-resolution seismic catalog reveals significant along-strike variations in earthquake depths, density and moment release, with main part of the earthquake density concentrated near the transform fault offsets (East Blanco Depression, Surveyor Depression, Cascadia Depression, and Gorda Depression), some of which may be spreading centers. Comparing our seismic catalog with recent 3D thermal and shear-wave velocity models highlights the along-strike variations in fault properties, providing new insights into the geological, lithological, and rheological complexities of the BFZ system. While most of the seismicity lies above the ~600°C isotherm located in the upper mantle, some deeper events might suggest deep seawater infiltration further in the mantle. Additionally, spatial correlations between slow shear-wave velocity anomalies and microseismic activity may indicate partial melting beneath intra-transform spreading centers and/or active hydrothermal fluid circulation through fault networks.
Spatio-temporal analyses of the seismic catalog identify over 1,000 earthquake clusters, including migrating seismic swarms with rates of up to 1 km/hr. These complex swarms, observed at the East Blanco Depression, West Blanco Depression, Cascadia Depression, and Blanco Ridge Transform, point to the involvement of hydrothermal and possibly magmatic fluid circulation, as well as slow-slip transients.
The western segment of the BFZ system is characterized by prominent swarms, including those associated with Mw 5 earthquakes in 2008, 2015, and 2021. Building on insights from the 1-year OBS dataset, we plan to refine and validate methods that use only permanent land-based stations along the coast. By developing this workflow, we aim to produce a multi-year seismic catalog that enhances the spatio-temporal resolution of BFZ seismicity. This future work will allow us to revisit the recurrent West Blanco swarms and explore their triggering mechanisms, which might involve slow-slip transients, fluid dynamics, and the interplay between tectonic and magmatic processes.
How to cite: Journeau, C., Thomas, A., Abercrombie, R., Hirao, B., Toomey, D., Hooft, E., Liu, M., Barbot, S., and Kuna, V.: OBS Data Mining and Earthquake Swarms Analysis Reveal the Complex Structure and Dynamics of the Blanco Fracture Zone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14331, https://doi.org/10.5194/egusphere-egu25-14331, 2025.
Krafla is one of the five central volcanoes of the Northern Volcanic Zone in north-east Iceland and has been utilised for decades for geothermal energy production. Thus, the volcano and its geothermal system have been monitored and imaged extensively with various geophysical methods to better understand this complex geological system not only for scientific, but also industrial interests.
With a ten-year dataset of 30.000 manually picked events from a local permanent 12 station seismic network owned by Landsvirkjun and operated by Iceland GeoSurvey, and a very dense temporary array of 98 seismic nodes deployed for one month in 2022 in the center of Krafla caldera, we imaged its subsurface P- and S-wave velocity structures by using local earthquake tomography and analysed seismicity patterns.
The velocity structures retrieved in the high-resolution 3D models for P- and S-wave velocities offer a glimpse into the subsurface of the volcanic system with the two wave types being responsive to distinct rock/fluid properties and their phases. The relocated seismicity underscores active structures pinpointed through the tomography.
The seismogenic zone hosting the largest, rather diffuse cluster of earthquakes at Krafla is located at the interface of high to low Vp/Vs close to where magma was repeatedly encountered by wells. Even though these events are located at the same boundary, their focal mechanisms vary widely from double-couple mechanisms with normal and thrusting earthquakes striking in different directions, to non-double-couple explosions and implosions. To decipher if events can be attributed to different sources, we use an unsupervised machine learning approach to cluster the events based only on the polarity of the P-onset, to make sure that path effects in the clustering are minimized. With this approach, events originating from diffuse seismicity clouds can be attributed to different sources, using existing focal mechanisms, available GPS data and variations in the re-injection rates at wells of the geothermal powerplant to validate the clustering.
By applying this method to the ten-year data set, we hope to gain a better understanding of when and where structures are, and thus offer insights if volcanic forcing such as inflation/deflation or external forcing such as regional seismicity and anthropogenic influence trigger certain seismicity patterns.
How to cite: Glück, E., Carthy, J., Garambois, S., Vandemeulebrouck, J., Benitez, C., Gudnason, E. A., Agustsdottir, T., and Mortensen, A. K.: Seismicity patterns and their source regions at Krafla (N-E Iceland) , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7961, https://doi.org/10.5194/egusphere-egu25-7961, 2025.
The Korea Polar Seismic Network (KPSN) was installed with 4 broadband seismometers in 2011 around Mt. Melbourne, located in Northern Victoria Land, Antarctica, ~30 km North of the Jang Bogo station (JBG). The KPSN was extended to the David Glacier with 7 broadband seismometers to the South. Currently, the KPSN consists of 21 broadband seismic stations, including the seismic station at the JBG, and covers the David Glacier from South to Mt. Rittmann and Coulman Island to North.
The Extreme Geoscience Group (EGG) at the Korea Polar Research Institute (KOPRI) has been developing a local earthquake catalog utilizing continuous seismic data from the KPSN, particularly focusing on data recorded since 2016, as the network achieved stable recordings post-2017. To compile this catalog, the team employed the 'scautopick' module of SeisComP, utilizing a Short-Term Average/Long-Term Average (STA/LTA) detector within the 0.7 to 2 Hz frequency range to identify P-wave arrivals. Following initial detections, a secondary picker was applied for S-waves using the 'spicker' module. This methodology led to the identification of over 3,900 local seismic events from the 2016 and 2017 data. Event locations were determined using the NonLinLoc software with the "ISAP91" velocity model, and subsequent relocations were performed using the hypoDD algorithm.
The detected events predominantly cluster into two groups: one in the upstream region of the David Glacier (1,370 events) and another around the Mt. Melbourne area (858 events). Current research is focused on understanding the factors influencing seismicity changes in the David Glacier region, considering variables such as air temperature fluctuations, tidal movements, the glacier's geometric structure, etc. The results are preliminary; efforts are underway to extend the earthquake catalog up to 2024 and to conduct detailed studies on the focal parameters and mechanisms of the recorded events.
How to cite: Park, Y., Jung, J., Lee, W. S., and Lee, C.-K.: Preliminary results of seismicity around the Jang Bogo Station in Antarctica: induced by ice stream and volcanic activities, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14347, https://doi.org/10.5194/egusphere-egu25-14347, 2025.
Posters on site: Tue, 29 Apr, 16:15–18:00 | Hall X1
A growing body of in-situ experimental data suggests that stimulating faults through the injection of pressurized fluids can trigger both aseismic and seismic slip. The simultaneous occurrence of aseismic and seismic slip can be explained by a fault pressurization mechanism that facilitates the nucleation of aseismic ruptures. Stress concentration at the leading edges of these advancing aseismic cracks triggers seismic slip on brittle asperities. This mechanism has also been proposed to explain the behavior of shallow (<10 km) faults during well-documented slow slip events (SSEs). Observations of these SSEs, particularly in extensional and transform settings, reveal a power-law scaling relationship between seismic and aseismic moments across several orders of magnitude.
We have extended the scaling of crustal SSEs to encompass induced seismicity, in-situ experiments, and laboratory earthquakes. Our findings show that power-law scaling remarkably applies to lower magnitude slow slip transients, including those observed up at the laboratory scale. However, there is an observational gap for SSEs with magnitudes between Mw 0-4. To address this gap, we employed numerical simulations of SSEs and their associated seismicity, integrating a poroelastic model with a stochastic earthquake simulator. The simulated distributions of aseismic and seismic moments align well with the natural and induced cases, maintaining the established scaling. Our results support the hypothesis that stress changes resulting from aseismic slip are the primary driving mechanism, while fluid-fault pressurization is crucial for the nucleation of aseismic slip. Furthermore, varying levels of confining stress help explain the depth-dependent variability in the ratio between aseismic and seismic slip. Overall, our findings suggest that the process governing the transition from aseismic to seismic release due to fluid pressurization are consistent across both nature to laboratory scales.
How to cite: Passarelli, L., Rinaldi, A. P., De Barros, L., Danré, P., Cappa, F., Selvadurai, P., and Wiemer, S.: Scaling of seismic and aseismic moments of natural and induced earthquakes , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13595, https://doi.org/10.5194/egusphere-egu25-13595, 2025.
Stress drop is one of the fundamental physical parameters of earthquake sources. It is usually estimated by measuring the corner frequency of the earthquake spectrum, which is not an easy task due to the seismic attenuation and the station's site effect, particularly for small earthquakes. Here, we propose an alternative method to estimate the stress drop without measuring the corner frequency. We employed the Frequency Index (FI), defined as the logarithm of the ratio of the average spectral amplitude between the low- and high-frequency ranges. FI has been frequently used to distinguish low-frequency earthquakes from ordinary ones. We introduced theoretical FI, in which the average spectral amplitude is expressed as the numerical integration of the product of the source spectrum and attenuation term. By employing a commonly used source model and the relations between the corner frequency and stress drop, the theoretical FI is a function of S-wave velocity, attenuation factor, and stress drop. We expect a slight spatial variation of S-wave velocity and attenuation factor in a small area. Assuming these parameters, we estimate an optimal stress drop to minimize the difference between the observed and theoretical FI.
We applied and verified the proposed method to the triggered earthquake swarm by the great 2011 Tohoku earthquake on the border of Yamagata and Fukushima prefectures in northeastern Japan. We confirmed that our results are consistent with those of Yoshida et al. (2017) in both spatial distribution and temporal variation. Our method's advantage is its robustness, even for smaller earthquakes. The traditional method of stress drop estimation uses spectral inversion for event pairs to measure corner frequencies. The number of suitable pairs is limited to a small number. Our method avoids selecting event pairs, which is why we can get a large number of stress drops. We obtained the stress drop for more than 6600 earthquakes with magnitudes ranging from 1.5 to 4.0. The increased number of stress drops enables a detailed investigation of the spatiotemporal evolution of earthquake swarms and frequency-magnitude analysis. In the case of the analyzed earthquake sequence, the b-vales for events with lower stress drop are evidently larger than those with higher stress drop. Thus, the proposed method is promising to deepen our understanding of the underlying physical processes of seismic activity.
How to cite: Kosuga, M. and Maeda, T.: Estimate of stress drop through Frequency Index and its application to a triggered earthquake swarm in northeastern Japan, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3990, https://doi.org/10.5194/egusphere-egu25-3990, 2025.
Seismic activity on immature fault systems is known to consist of earthquakes with various fault plane orientations. Preceding seismic activity to a large earthquake is sometimes observed in such area. The size distribution of minor earthquakes ( b-value) has been reported to be indicative about the hazard information of major earthquakes. In addition, a study suggested that proximity of large earthquake can be evaluated by fault orientation distribution. In this study, we consider the proximity evaluated from ratio of stacked seismic moment tensor to total moment release in a volume. The ratio indicates efficiency of inelastic strain due to seismic moment release by earthquakes, meaning close to the strength of the medium based on the Mohr diagram and Coulomb failure condition. We examined the change in b-value and the efficiency in the focal regions of large earthquakes in immature fault systems: 2016 Kumamoto and 2019 Ridgecrest earthquakes. The large mainshocks over magnitude of 7 were accompanied by large fore shocks and preceding small seismic activity. Both parameters show specific temporal variation for pre-foreshock, between foreshock and mainshock, and aftershock sequences. Especially, the parameters between the two shocks are different from it just after the main shocks, showing a possibility of identification whether large fore shock is followed by an even larger earthquake.
How to cite: Matsumoto, S.: Large earthquake at immature fault system characterized by seismic moment efficiency and frequency-number distribution of small earthquakes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4933, https://doi.org/10.5194/egusphere-egu25-4933, 2025.
The Tjörnes Fracture Zone (TFZ) in North Iceland links the offshore Kolbeinsey Ridge to the onshore Northern Volcanic Zone. About 98% of the seismicity in the TFZ concentrates along the Húsavík-Flatey Fault (HFF), a ~120-km-long right-lateral strike-slip fault, and the Grímsey Oblique Rift (GOR), an oblique rift with associated volcanism. Many earthquake swarms have occurred in the TFZ during the past decades. A notable swarm began on June 19, 2020, on the HFF and lasted until October 2020. It included three earthquakes of magnitude 5.4, 5.7 and 6.0, making it the most energetic earthquake sequence in North Iceland since the 1976 M6.2 Kópasker earthquake. The seismicity began on the western HFF and propagated to the northwest and southeast along the HFF. The first large event (M5.4) occurred on June 20 at 15:05h and was followed by the M5.7 earthquake four hours later at 19:26h. While the earthquake activity on the western HFF increased significantly, these events and their aftershocks also show a distinct N-S lineation, roughly perpendicular to the HFF, indicating that a conjugate fault may have been activated. The third large event and the largest of the sequence (M6.0) struck on the following day at 19:07h. It occurred at the eastern shoulder of the N-S oriented Eyjafjarðaráll Rift (ER), which is the southern continuation of the Kolbeinsey Ridge. This M6.0 event is the largest normal faulting earthquake recorded in Iceland, and thus constraining the parameters of this sequence is crucial for seismic hazard assessment in the region. However, the offshore location of these events makes this task challenging. Here, we attempt to overcome this challenge by combining waveform data from broadband stations up to 4,000 km away from the earthquakes and co-seismic GNSS offsets from 20 continuous and 10 campaign stations in North Iceland to estimate the earthquake source parameters, utilizing Bayesian inference. An initial double couple source was estimated from waveform data only as a prior for the joint geodetic and waveform data inversion. The M5.4 and M5.7 events correspond to strike-slip faulting on a WNW-ESE right-lateral fault or a N-S left-lateral fault, while the M6.0 event indicates normal faulting on a N-S fault. Our preliminary results show that incorporating GNSS data yields shallower depths than with waveform data alone (7.9 km for the first event, 5.1 km for the second event, and 10.2 km for the third event). However, previous results from moment tensor inversions of local broadband data indicate shallower depths for the event in the ER than for the strike-slip earthquakes, emphasizing the difficulty in constraining the depths of these offshore events. For their magnitudes, the first event was estimated as M5.3, with and without geodetic data, and the latter two events were found slightly larger in the joint inversion (M5.8 and M6.1) than in the seismic data solution, indicating limited aseismic moment release. These findings highlight the importance of combining diverse datasets to improve our understanding of complex tectonic settings and to constrain the proportion of seismic/aseismic moment release in earthquake sequences.
How to cite: Barreto, A., Viltres, R., Jónsdóttir, K., Guðmundsson, G. B., and Jónsson, S.: The most significant earthquake sequence in North Iceland for almost 50 years, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12421, https://doi.org/10.5194/egusphere-egu25-12421, 2025.
We investigate the relationship between the cumulative number of earthquakes and ground uplift at the Campi Flegrei caldera (South Italy) during the ongoing unrest (2005–present). While previous studies have explored this correlation, we propose a nonlinear epidemic model that captures new features of the caldera system. More precisely, our model describes earthquake occurrence as a cascading process (similarly to what observed for tectonic earthquakes) influenced by ground deformation. The nonlinearity reflects the reduced efficiency of the triggering mechanism, which contributes to the short duration of seismic swarms. This mechanism, we hypothesize, may represent a general framework for understanding volcanic earthquake occurrence globally.
How to cite: Godano, C., Convertito, V., Tramelli, A., and Petrillol, G.: How the ground deformation drives the earth-quake occurrence during the 2005-presenttime unrest at Campi Flegrei - Italy, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2339, https://doi.org/10.5194/egusphere-egu25-2339, 2025.
The Beibu Gulf and its surroundings are a significant intraplate seismic zone in the northwest of the South China Sea. At least 15 M>5 earthquakes and 4 M>6 events have occurred historically. In 1605, a destructive earthquake with M7.5 occurred in the southern Beibu Gulf, causing villages to submerge into the sea with an area exceeding 100 km2 of land subsidence into the sea. This seismic zone has serious earthquake risk, but few studies focused on the moderate to strong earthquake sequence in this area, leading to unclear seismic structures, mechanisms, and triggers of earthquakes in the region. In this work, waveform data of the 2023 Beibu Gulf earthquake sequence (ML5.4) are well analyzed based on 4-month data recorded at 51 broadband stations. One foreshock and 73 aftershocks were detected by template matching techniques. The entire earthquake sequence is relocated using the HypoDD relocation method and the focal mechanism solutions of 12 M>1.5 earthquakes are inverted. The results show that the 2023 Beibu Gulf earthquake sequence forms a belt in the NWW-SEE direction, with a lateral extent of ~5.0 km and a width of ~1.5 km. Vertical slices show that the earthquake sequence is distributed continuously and densely with a dip angle of about 50° toward the NNE direction. Most foreshocks and aftershocks have similar source mechanisms to the main shock including the NWW-trending fault plane having a strike of 288°, a dip angle of 51°, and a slip angle of 177°. The location and attitude of the seismic fault match well with the NWW-trending Wei-2 Fault (F1) identified by petroleum exploration. The stereographic projection of the P-axes indicates that the principal compressive stress direction is NW-SE in the source area, which intersects obliquely with the F1, resulting in the right-lateral strike-slip faulting of the 2023 Beibu Gulf earthquake sequence. The seismic images in the research area show that the location of the 2023 Beibu Gulf earthquake sequence corresponds well to the deep columnar low-velocity zone, indicating that the triggering of the event may be related to the magmatic-hydrothermal fluids produced by the Hainan mantle plume. By comparing the seismic activity characteristics of the surrounding areas, and combining geological with geophysical survey information, we believe that deep hydrothermal fluids and fault structures or weak zones are the main driving factors of seismicity in this area.
How to cite: Lin, J., Xie, X., and Liu, S.: Investigating the 2023 Beibu Gulf ML5.4 earthquake sequence: seismicity, faulting, and the influence of the Hainan mantle plume fluids?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5460, https://doi.org/10.5194/egusphere-egu25-5460, 2025.
Seismicity and fault networks often display a high degree of complexity and segmentation at multiple scales. At shallow crustal levels, fault geometries and rupture processes are typically characterized using seismic exploration methods and precise earthquake relocation, which delineates fault surfaces. However, standard absolute location methods can produce scattered event distributions, necessitating more advanced techniques. Here, we reconstruct the fine-scale complexity of the Corinth Gulf fault system with unprecedented detail by applying a new probabilistic relocation method, NLL-SSST-WC. This technique combines source-specific, station travel-time corrections (SSST) with waveform-coherence (WC) stacking of probabilistic locations for closely spaced events.
Our SSST-WC approach iteratively computes smoothed source-specific station travel-time corrections, which significantly improve relative relocation accuracy and event clustering by accounting for three-dimensional lateral heterogeneities in the Earth. Waveform coherence groups similar events, enhancing relative locations through stacking the corresponding probability density functions.
We analyzed more than 3,500 earthquakes (Mw0.5–5.4) that occurred during 2020 – 2021 and recorded by the 49 stations of HUSN (Hellenic Unified Seismic Network) located at distances less than 135 km from the study area. For each event, we used manually picked P- and S-phase arrivals (weighted by signal-to-noise ratio) and employed a regional 1D velocity model. During NLL-SSST relocations, we initially used a large smoothing distance (D = 999 km) to treat station residuals as static corrections (given the small source area compared to station distances). Subsequently, we iteratively reduced the smoothing distance to 2.5 km to capture finer-scale heterogeneities.
The resulting high-precision earthquake locations (with errors under 100 m) reveal detailed structures of the main fault segments, spanning roughly 10 km in an east–west orientation, dipping northward, and exhibiting extensional kinematics as indicated by focal mechanisms. The fault surface curves eastward, consistent with previous focal mechanism solutions for large earthquakes in the region. The spatiotemporal distribution of the events highlights multiple clusters and distinct migration patterns over time, revealing interactions among smaller faults adjacent to those associated with the main events, within a complex fault network identified in our analysis.
This refined earthquake imaging provides improved constraints on fault geometries, rupture initiation points, and kinematic properties, which are crucial for seismic hazard assessment. It also supports more accurate modeling of rupture processes by providing detailed input data, including precise hypocenter locations, fault orientations, and spatiotemporal clustering patterns.
How to cite: Sollai, A., De Landro, G., Zollo, A., Lomax, A., Emolo, A., Bonatis, P., Papadimitriou, E., and Karakostas, V.: High-Precision Earthquake Locations Reveal Detailed Fault Geometries in the western Corinth Rift, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21500, https://doi.org/10.5194/egusphere-egu25-21500, 2025.
In 2021, a remarkable natural earthquake sequence initiated in the Haute-Ajoie region (Canton Jura, Switzerland), which provides new insights on how present-day deformation is accommodated in the Jura fold-and-thrust belt (JFTB) in the northern foreland of the Alps. The ML 4.1 mainshock of December 24. 2021, located south of the village of Réclère, triggered an unusual earthquake sequence that has been lasting for at least 3 years. Initially, few aftershocks occurred in the days after the ML 4.1 mainshock, and aftershock activity rapidly ceased within the first week. On March 22. 2023, the sequence was reactivated by an ML 4.3 earthquake, which was followed by an intense sequence of aftershocks. Additional seismic stations were installed in the epicentral area to improve detection thresholds and hypocenter location qualities of aftershocks. Earthquake activity remained high between March 2023 and October 2024, with 14 earthquakes of ML ≥ 2.5. By December 2024, the Swiss Seismological Service (SED) detected and located more than 430 earthquakes with standard methods and derived high-quality focal mechanisms for 17 earthquakes of this sequence.
Relative relocations in combination with the focal mechanisms image a complex fault-zone structure within the pre-Mesozoic basement at depths of about 5-6 km. It consists of a system of reverse-to-transpressive faults hosting the ML 4.1 and ML 4.3 earthquakes, which are limited to the west by an adjacent, roughly N-S oriented strike-slip fault zone. The spatio-temporal analysis of the sequences suggests that the reverse-to-transpressive ruptures activated the adjacent strike-slip fault zone and seismicity migrates predominantly southward since March 2023. We complement our study by an enhanced earthquake catalog derived from a cross-correlation based template-matching procedure and the analysis of the stress field at various scales.
Our preliminary results indicate significant differences in the b-value parameter over distances of few hundred meters and between the reverse-to-transpressive and strike-slip fault zones. Similarly, a Mohr-circle analysis points to differences in the distance to criticality between the two fault zones. Our results therefore provide new insights into the variability of stress and fracture conditions across upper-crustal fault zones at scales of few hundred meters and raise the question on the possible role of fluids to explain these variations. Besides adding new constraints to present-day seismotectonic processes within the JFTB, our observations are also of general relevance for geothermal exploration targeting complex fault zones.
How to cite: Diehl, T., Kraft, T., Mosar, J., Toledo, T., Lanza, F., and Simon, V.: The ML4 Réclère Sequence (Switzerland): Evidence for Interaction of Reverse and Strike-Slip Faults in the Basement of the Jura Fold-and-Thrust Belt, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16899, https://doi.org/10.5194/egusphere-egu25-16899, 2025.
Intraplate seismicity has been poorly understood relative to interplate seismicity, which may be partly due to low seismicity and long earthquake recurrence intervals and ambiguity in the responsible structures. Locally clustered earthquakes provide essential information on the nature of intraplate seismicity such as earthquake-spawning mechanisms, earthquake source properties, seismogenic depths and structures, stress accumulation and release patterns, rupture processes, and potential seismic hazards. The Korean Peninsula, located in a stable intraplate region around the eastern margin of the Eurasian Plate, offers a unique opportunity for studying intraplate seismicity due to the dense seismic network and diverse geological and paleo-tectonic structures. We investigate clustered earthquakes from 2018 to 2024 at 39 sites in the Korean Peninsula using a matched filter analysis based on 1057 stations. The earthquake locations are refined using a double-difference method (hypoDD) based on interevent phase traveltime differences. The earthquake magnitudes are determined using an amplitude-scaling method based on the waveform amplitude ratios between earthquakes. We determine the focal mechanism solutions using a phase polarity analysis (FOCMEC). The analysis detects 4 to 1644 earthquakes by site, identifying a total of 6957 clustered earthquakes with magnitudes of ML–0.9 to 3.7. The earthquake distributions and focal mechanism solutions reveal NNE-SSW to NE-SW and WNW-ESE directional subvertical strike-slip faults at most sites, which agree with the ambient stress field. Coast-parallel reverse faults are identified around the eastern margin of the Korean Peninsula, which may be associated with the reactivation of paleo-rifting structures. Normal faults and relatively low-angle faults are observed around the western margin of the peninsula, which may be related to the paleo-collision structures. The earthquake clusters are generally populated at upper crustal depths of ~5–15 km, consistent with typical seismogenic depths in the region. We also observe earthquakes at mid-crustal depths of ~18–25 km in low-heat-flow regions, which may indicate relatively deep brittle-ductile transition zones. Either episodic or one-off earthquake swarms are observed depending on the site, suggesting site-dependent stress accumulation and release patterns. The one-off swarms may be associated with fluid diffusion along faults. The earthquakes at most sites present prominent temporal migration that is potentially due to the stress transfer and fluid diffusion, which may expand the fault ruptures with time. Some sites exhibit up-dip migration of seismicity, implying potential seismic hazards. These observations suggest that intraplate seismicity is controlled by combined effects of various geological and geophysical factors, including the ambient stress field, geological and tectonic structures, earthquake interactions, and medium properties.
How to cite: Park, S., Hong, T.-K., Lee, J., Kim, B., Lee, J., and Kim, D. G.: Properties of clustered earthquakes in stable intraplate region: a case study for the Korean Peninsula, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5287, https://doi.org/10.5194/egusphere-egu25-5287, 2025.
Posters virtual: Mon, 28 Apr, 14:00–15:45 | vPoster spot 1
EGU25-11849 | ECS | Posters virtual | VPS21
A complex deposit sequence from a small, southern Cascadia lake suggests a previously unrecognized subduction earthquake immediately followed a crustal earthquake in 1873 CEMon, 28 Apr, 14:00–15:45 (CEST) | vP1.12
Here, we disentangle a complex disturbance deposit sequence attributed to the ~M 7 1873 CE Brookings earthquake from lower Acorn Woman Lake, Oregon, USA, using sedimentological techniques, computed tomography, and micro-X-ray fluorescence. The lower portion of the sequence is derived from schist bedrock and has characteristics similar to a local landslide deposit, but is present in all cores, suggesting that it is the result of high frequency (>5 Hz) ground motions from a crustal earthquake triggered the landslide. In contrast, the upper portion of the sequence is similar to a deposit attributed to the 1700 CE Cascadia subduction earthquake (two-sigma range of 1680-1780 CE): the base has a higher concentration of light-colored, watershed-sourced silt derived from the delta front followed by a long (2-5 cm) organic tail. The soft lake sediments are more likely to amplify the sustained lower frequency accelerations (<5 Hz) of subduction earthquakes, resulting in subaquatic slope failures of the delta front. The upper portion of the 1873 CE deposit, however, has an even higher concentration of watershed-sourced silt as compared to the 1700 CE deposit, which is suspected to be the result of shaking-induced liquefaction of the lake’s large subaerial delta. The tail of both the 1873 CE and 1700 CE deposits is explained as the result of flocculation that occurred during sustained shaking. A preliminary literature search suggests that flocculation may occur during low frequency (<4-5 Hz) water motion that is sustained for an extended period of time (~minutes). The subduction interpretation of the upper portion of the 1873 CE deposit is supported by the observation of a small local tsunami offshore and the presence of a possible seismogenic turbidite attributed to the 1873 CE Brookings earthquake in southern Oregon sediment cores.
These results are important to regional seismic hazards for several reasons. Southern Cascadia crustal earthquakes, not previously recognized as a threat in southern Oregon, have the potential to cause damage to infrastructure, including the Applegate dam and buildings and other structures at Oregon Caves National Monument. They also identify a previously unrecognized recent southern Cascadia subduction earthquake. Finally, the close temporal relationship between these two types of earthquakes, not observed elsewhere in the downcore record, may be early evidence of the transition of the Walker Lane belt into a transform fault as predicted.
How to cite: Morey, A. E., Shapley, M. D., Gavin, D. G., Goldfinger, C., and Nelson, A. R.: A complex deposit sequence from a small, southern Cascadia lake suggests a previously unrecognized subduction earthquake immediately followed a crustal earthquake in 1873 CE, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11849, https://doi.org/10.5194/egusphere-egu25-11849, 2025.
