A high-resolution earthquake catalog and detailed quantification of earthquake source parameters are essential for constraining fault structure and earthquake interactions. As a candidate site for the next-generation gravitational wave detector (i.e., Einstein Telescope), the Lower-Rhine Embayment region requires a comprehensive assessment of the fault distribution inferred through seismicity and earthquake source properties. In this study, we build an enhanced earthquake catalog using both permanent seismic stations and new data from temporary deployments together with AI-based techniques, including signal enhancement with a decoder-autoencoder denoiser, seismic phase detection using PhaseNet, and event association with PyOcto. We further refine earthquake locations with the NLL-SSST-coherence algorithm and then apply an automatic quality-control filter using event association to remove false detections resulting from the misinterpretation of teleseismic signals as local ones due to event denoising. We detect 3900 events for the period 2019 to 2025, with 2101 of them being classified as earthquakes. The enhanced catalog shows increased hypocentral depths along the NW-trending Sandgewand fault, with a maximum depth of 20 km at the southern end of the fault system. We also present the first results of a source-parameter catalog for earthquakes that occurred between 2000 and 2025 based on the dataset of Hinzen et al. (2021). Focal mechanisms for selected earthquakes in the region are determined with SKHASH by combining S/P ratios and first-motion polarities obtained from PhaseNet+. We fit individual earthquake spectral parameters, including corner frequency and seismic moment, and calculate stress-drop values for ML≥1.5 events based on a circular crack model. Preliminary results indicate a median stress-drop value of 2.5 MPa across the region, with slightly higher stress-drop values observed on the Sandgewand fault relative to the Rurrand fault. In addition, we use the Distributed Acoustic Sensing (DAS) recordings to compute focal mechanism and corner frequency estimates and compare the results with broadband seismic stations for two earthquakes captured by DAS observations in the Netherlands in 2025. The enhanced spatial sampling density of DAS data provides additional constraints on earthquake source parameters and enables fault movement estimation that is difficult with seismic station data alone.

In this work, we present a new, high-resolution earthquake catalog for the Kefalonia region, Greece, together with the distributed acoustic sensing (DAS) waveform dataset used for its construction (10.60517/cv43p1601, Bocchini et al., 2025; 10.5281/zenodo.20558686, Bocchini, 2026). We build the catalog from DAS data recorded between 1 August 2024, 23:00 UTC and 15 August 2024, 23:00 UTC, combined with open-access seismic-station recordings from the Hellenic Unified Seismic Network (HUSN). The DAS dataset consists of continuous strain measurements acquired along a 15 km long telecommunications fiber-optic cable connecting northern Kefalonia and Ithaki. We use a semblance-based detector on the DAS waveforms to identify 5734 earthquakes within similar to 50 km of the cable origin. We jointly locate 356 high-SNR (SNR >12 dB) events with DAS and seismic stations and calculate their local magnitudes from seismic records. We then apply waveform cross-correlation to match unlocated detections with the most similar template events and estimate relative magnitudes from amplitude ratios to enhance the newly constructed catalog. Enhancement adds 2515 earthquakes, resulting in 2871 events with assigned locations and magnitudes and represents a similar to 32-fold increase in the number of earthquakes with respect to the official National Observatory of Athens (NOA) catalog. Most events (2790) cluster within a similar to 5 km radius offshore northwest of Kefalonia, where seismicity rates reach >100 events per hour. We achieve a similar to 38-fold increase in the number of earthquakes with respect to the official catalog from NOA in the region encompassing the earthquake cluster northwest of Kefalonia. Our dataset provides a detailed spatio-temporal view of seismicity in a region with limited station coverage and demonstrates the value of integrating DAS with conventional seismic networks to monitor intense earthquake sequences. The combination of high seismicity and open-access data from the HUSN makes this DAS dataset particularly valuable for the seismological community. We provide a 2-week-long catalog, the full detection list (local and distant events and false detections), and two weeks of continuous DAS recordings. Possible applications of the datasets include testing and benchmarking DAS processing algorithms for tectonic earthquakes, as well as studies of physical processes associated with complex seismic sequences.

Combining traditional seismic networks with Distributed Acoustic Sensing (DAS) to record ground-motion on telecommunications cables provides new opportunities to study small earthquakes with unprecedented spatial and temporal resolution. Here, we investigate an earthquake sequence offshore northwest of Kefalonia, Greece that initiated in March 2024 and returned to background levels by November–December of the same year. The sequence was recorded by a permanent seismic network throughout its duration and by DAS deployed along a ~15 km fiber-optic cable connecting northern Kefalonia and Ithaki between July and September 2024. We focus on a two-week period of elevated seismicity (1–15 August 2024) identified during routine earthquake catalog monitoring by the National Observatory of Athens. The integration of seismic and DAS observations increases the number of detected earthquakes by approximately a factor of 40 and reveals detailed source and statistical properties of the sequence. The enhanced catalog resolves clear mainshock–aftershock sequences and captures source spectra up to ~50 Hz for events with M < 3, frequencies not well-resolvable using seismic stations alone. DAS waveforms exhibit signal-to-noise ratios exceeding 3 at frequencies up to ~70 Hz for representative events, enabling spectral stress drop estimates consistent with typical earthquake values of 1–10 MPa.We apply semblance-based detection to DAS data and manually review 5,734 events occurring within ~50 km of the cable to construct an initial catalog. By combining DAS and seismic-station data, we locate 356 events with signal-to-noise ratios greater than 12 dB and determine their local magnitudes from seismic stations. Waveform cross-correlation is then used to associate additional detections with template events, enabling relative magnitude estimation and further expansion of the catalog. This approach yields 2,871 earthquakes with assigned locations and magnitudes and a completeness magnitude between -0.4 and -0.3. Approximately 97% of events (2,780 of 2,871) cluster within a ~5 km radius located ~10 km offshore of northwestern Kefalonia, with peak seismicity rates exceeding 100 events per hour.Our results demonstrate how integrating DAS with conventional seismic networks can substantially increase event detection rates and improve constraints on earthquake locations and source properties in regions with sparse station coverage. The enhanced resolution resolves clear mainshock–aftershock clustering that would likely be misclassified as swarm-like activity in standard catalogs, highlighting how limited observations can bias interpretations of earthquake sequence behavior.

Abstract. The western Peloponnese exhibits a continuum of earthquake faulting styles that reside in a complex stress field near the western termination of the Hellenic Subduction System. Here we present a detailed study of recent seismicity in western Peloponnese, including the spatiotemporal distribution of earthquake sequences and their clustering and migration properties to infer possible driving mechanisms. We build a detailed earthquake catalog from December 2023 – September 2024 with a magnitude-of-completeness Mc ~1 and a location precision on the order of 100 s of meters by combining new data from temporary seismic station deployments with publicly available data from permanent stations. Catalog statistical and clustering analysis shows increased background seismicity rates and seismic moment release in the northern part of the study area that is consistent with larger strain rates and a higher stressing rate reported from geodetic data. The seismicity distribution and focal mechanism solutions suggest that the predominantly north-south extensional regime in the north changes to east-west extension in the central-western Peloponnese near the town of Zaharo. Nearest-neighbor cluster analysis reveals mainshock-aftershock-type sequences in the northwest near the town of Vartholomio that are consistent with tectonic loading. Conversely, clustering properties in the central-western Peloponnese near Zaharo are consistent with swarm-like sequences driven by external forcing, such as pore-fluid pressure changes, and potential aseismic slip. Independent studies of slab dewatering suggest fluid-driven pore-pressure gradients that might be responsible for migrating seismicity and swarm-like behavior in the upper plate.

Aftershocks of megathrust earthquakes at subduction zones may be driven by stresses arising from the topography of the forearc. However, the effect of topographic stresses on aftershock triggering is quantitatively not well understood and has been neglected in Coulomb failure stress models that assess whether the stress change caused by an earthquake promotes or inhibits failure on nearby faults. Here we use analytical and numerical models to examine the importance of topographic stresses on Coulomb failure stress changes caused by megathrust earthquakes. We show that topographic stresses are a prerequisite for widespread aftershock seismicity in the forearc and that their superposition with tectonic stresses leads to a dependence of the forearc stability on the stress state before and after the earthquake. The dependence can be taken into account by determining the Coulomb failure stress change between optimal failure planes before and after the earthquake, which requires constraining the total stresses in the forearc. Applying our modelling approach to the 2011 Mw 9.0 Tohoku-Oki and 2010 Mw 8.8 Maule megathrust earthquakes yields coseismic Coulomb failure stress changes of up to similar to 40 MPa, which promoted the majority of aftershocks in the Japanese and Chilean forearcs. The model results further reveal that the spatial distribution of aftershocks was not only influenced by the megathrust stress drop and the proximity of faults to failure but also by local differences in forearc topography and pre-earthquake stress state. Our analysis highlights the significance of total stresses in Coulomb failure stress calculations, enabling a better estimation of seismic hazard at subduction zones.

In January-February 2023 a seismic sequence took place in the Central Mediterranean Sea, ~90 km south of the island of Malta, and ~200 km from the coast of Sicily. The seismicity started in Mid January, in a region that experienced only sparse seismicity in the past. In the following days several M4+ events occurred. The largest earthquake, with moment magnitude Mw 5.3, took place a couple of weeks after the unrest onset, on January 30. The seismicity continued for several weeks, before fading down. Later seismicity was observed and still going on. This recent, unusual seismicity offers a unique opportunity to investigate seismogenic processes in this region with unprecedented detail. However, analyzing seismic sequences in offshore environments presents significant challenges due to the absence of optimal seismic monitoring conditions. These limitations compromise the effectiveness of conventional data analysis techniques, hindering the characterization of offshore seismic sequences. We tackled these limitations through the adoption of advanced, waveform-based seismic data analysis techniques that allow to investigate offshore seismic sequences, with the aim to provide insights into their origin. We combine full-waveform based detection and template matching methods to enhance the detection of events, advanced location techniques based on Distance Geometry Solvers (DGS), and probabilistic waveform-based methods for seismic source characterization. We combine the seismic source analysis for the 8 largest earthquakes in the sequence, with magnitude exceeding ML 4.5, with waveform-based and statistical analysis of the seismicity. About 500 events are identified. Their locations map a narrow lineament extending ~NW-SE. Full moment tensors for the largest events identify normal faulting mechanisms with a similar orientation, and shallow centroids of ~5 km depth. This result, combined with a waveform similarity analysis, suggests a predominant mechanism for the entire sequence. Using different seismicity indicators we classify the 2023 sequence as a seismic swarm. Indeed, the largest events in the sequence occur weeks after the unrest onset. Compared to previous seismicity, the sequence was outstanding in terms of maximum magnitude, seismicity rate and moment rate. While normal faulting earthquakes are not unusual in the Central Mediterranean, they differ from the few focal mechanisms previously proposed for the swarm focal region, which has important implications, considering that normal faulting earthquakes at shallow depth pose a tsunami hazard in the region.

Earthquake stress drop (Delta sigma) may increase with depth and stress in the brittle lithosphere. However, the range of uncertainty in Delta sigma and the lack of constraints on absolute stress make it difficult to establish whether they are correlated. Here, we investigate Delta sigma dependence on depth and maximum shear stress (tau(max)) based on similar to 11 years of seismicity in the northeastern Japanese forearc following the 2011 Tohoku-Oki megathrust earthquake. We interpret Delta sigma estimates computed using both individual spectra and spectral-ratio methods and find that Delta sigma exhibits a clear depth dependence within the seismically active upper similar to 60 km of the forearc lithosphere (similar to 0.8 MPa per 10 km). We further compare Delta sigma values with quantitative tau(max) estimates from finite-element models of force balance. We find that median Delta sigma values increase with tau(max) in the brittle forearc lithosphere and that earthquake stress release is proportional to tau(max). The dependence of Delta sigma on tau(max) explains the apparent depth dependence of Delta sigma and suggests that average Delta sigma values provide a relative measure of the stress at failure. In the northeastern Japanese forearc, Delta sigma values remained roughly constant in the decade following the Tohoku-Oki earthquake, suggesting negligible changes in failure stress in the forearc since the mainshock.

The Hellenic Subduction System (HSS) in the eastern Mediterranean is the oldest active subduction margin on earth. It is a segmented boundary that hosts the continuum of faulting styles over a ~200km range in depth and can generate large earthquakes with high tsunamigenic potential. The complexity of deformation styles and rates leave key aspects of the system poorly understood. For example, historical records of Mw

The Armutlu Peninsula in north western Türkiye, a horst zone in an active transtensional pull-apart basin, is bounded by two major sub-branches of the North Anatolian Fault zone and host high rates of seismicity in the northern part. The ~25-station SMARTnet surface seismic network was installed in 2019-2020 with the purpose of augmenting permanent seismic stations in the northern part of the Armutlu Peninsula and to help increase the detection of small-magnitude earthquakes. Here, we employ a waveform-based clustering method that integrates detailed information from the seismicity and focal mechanism distribution enabled by the added station coverage to investigate the geometry and kinematics of the seismically active structures. We start by using an enhanced earthquake catalog of >4,000 double-difference-relocated events and >150 focal mechanisms obtained using P-wave polarities and amplitudes in the time period between January 2019 and February 2020. We perform a formal inversion of the stress field orientation from focal mechanisms to investigate the regional deviatoric stress field and its relation with activated fault structures. The stress-field inversion uses input data that combines the enhanced focal-mechanism catalog from background seismic events together with published focal mechanisms of M≥2.5 events that occurred between 1999 and 2019. Stress inversion results show an extensional stress regime for the broader northern Armutlu Peninsula and a transtensional stress regime for a narrow region of ~80 km2, referred to as the Esenköy Seismic Zone (ESZ). Within the ESZ, the minimum principal stress (σ3) is approximately horizontal and NE-trending, while the maximum (σ1) and intermediate (σ2) principal stresses are close in magnitude and vary between near vertical and near horizontal. We observe clusters of normal and strike-slip faulting events identified in the ESZ through waveform-based clustering analysis that are optimally oriented with respect to the stress field we derive for the area. The minimum principal stress in the ESZ is rotated clockwise by ~10-15° with respect to the minimum principal stress inferred for the broader Armutlu Peninsula and eastern Sea of Marmara. Based on the Mohr-Coulomb failure criterion, we quantify the relative and absolute magnitudes of the principal stresses, determine the local crustal stress and strength conditions, and will present a discussion of the implications for regional tectonic forces.

Abstract. Aftershocks of megathrust earthquakes at subduction zones may be driven by stresses arising from the topography of the forearc. However, the effect of topographic stresses on aftershock triggering is quantitatively not well understood and has been neglected in Coulomb failure stress models that assess whether the stress change caused by an earthquake promotes or inhibits failure on nearby faults. Here we use analytical and numerical models to examine the importance of topographic stresses on stress changes caused by megathrust earthquakes in the forearc. We show that the superposition of topographic and tectonic stresses leads to a dependence of the stress change on the stress state of the forearc. The dependence on the forearc stress state largely determines the coseismic stress change induced by a megathrust earthquake and must be considered when calculating Coulomb failure stress changes. We further show that increases in Coulomb failure stress promoting widespread failure in the forearc are only possible if topographic stresses dominate the regional stress field after the megathrust earthquake. Applying our modelling approach to the 2011 Mw 9.0 Tohoku-Oki and 2010 Mw 8.8 Maule megathrust earthquakes shows that the effect of topographic stresses caused Coulomb failure stress changes of up to ~40 MPa, which promoted the majority of aftershocks in the Japanese and Chilean forearcs. The model results further reveal that the spatial distribution of aftershocks was influenced by local differences in pre-earthquake stress states, fault strength and megathrust stress drop. Our analysis highlights the significance of topographic stresses in Coulomb failure stress calculations, enabling a better estimation of seismic hazard at subduction zones.

Distributed Acoustic Sensing (DAS) is becoming increasingly popular in microseismic monitoring operations. This data acquisition technology converts fiber-optic cables into dense arrays of seismic sensors that can sample the seismic wavefield produced by active or passive sources with a high spatial density, over distances ranging from a few hundred meters to tens of kilometers. However, standard microseismic data analysis procedures have several limitations when dealing with the high spatial (inter-sensor spacing up to sub-meter scale) sampling rates of DAS systems. Here we propose a semblance-based seismic event detection method that fully exploits the high spatial sampling of the DAS data. The detector identifies seismic events by computing waveform coherence of the seismic wavefield along geometrical hyperbolic trajectories for different curvatures and positions of the vertex, which are completely independent from external information (i.e. velocity models). The method detects a seismic event when the coherence values overcome a given threshold and satisfies our clustering criteria. We first validate our method on synthetic data and then apply it to real data from the FORGE geothermal experiment in Utah, USA. Our method detects about two times the number of events obtained with a standard method when applied to 24h of data.

Abstract Numerous normal‐faulting aftershocks in subduction forearcs commonly follow large megathrust earthquakes. Postseismic normal faulting has been explained by stress changes induced by the stress drop along the megathrust. However, details of forearc stress changes and aftershock triggering mechanisms remain poorly understood. Here, we use numerical force‐balance models combined with Coulomb failure analysis to show that the megathrust stress drop supports normal faulting, but that forearc‐wide aftershock triggering is feasible within a narrow range of megathrust stress drop values and preseismic stress states only. We determine this range for the 2011 Tohoku earthquake (Japan) and show that the associated stress changes explain the aftershock seismicity in unprecedented detail and are consistent with the stress released by forearc seismicity before and after the earthquake.

Large megathrust earthquakes like the 2011 Mw 9.0 Tohoku earthquake (Japan) are followed by numerous aftershocks in the subduction zone forearc overlying the seismogenic fault. The aftershocks in the forearc can include normal-faulting events despite the thrust mechanism of the main shock. Postseismic normal faulting has been explained by stress changes induced by the coseismic stress drop along the megathrust. However, details of stress changes in the forearc and aftershock triggering mechanisms remain poorly constrained. Here we use numerical force-balance models combined with Coulomb failure analysis to show that the megathrust stress drop indeed supports normal faulting, but that forearc-wide triggering of aftershocks is feasible within a narrow range of megathrust stress-drop values and forearc stress states only. We determine this range for the Tohoku earthquake and show that the associated stress changes explain the aftershock seismicity in unprecedented detail. In particular, our analysis reveals that ~78% of the aftershocks and ~92% of the seismic moment release occurred in areas where the Tohoku earthquake caused a stress increase, and that the detailed aftershock distribution was also governed by spatial variability in fault strength and forearc topography. Our findings provide new insights into aftershock triggering and help to understand where aftershocks occur after great earthquakes at subduction zones.

Earthquake stress drop values estimated from ground-motion spectra commonly vary by several orders of magnitude, particularly for small earthquakes (~M < 3). Stress-drop values have been found to vary with faulting style, faulting type (intraplate, interplate), depth, and to exhibit differences between natural and induced earthquakes. Nevertheless, distinguishing uncertainties from real trends across data sets is challenging, in part due to the variation in methodological approaches and observational constraints. However, the proliferation of high-quality, dense seismic data in recent years has shown that at least some of the variability in stress drop values almost certainly reflects diversity in fault strength and geological conditions. Coupling well-constrained observations to a variety of modeling approaches will help uncover what controls earthquake rupture processes, but deconvolving observational constraints from real variation in rupture behavior is key.We present our stress drop estimates from data sets representing a wide range of fault loading conditions and geological environments, from interplate, intraslab and forearc subduction faults, to volcanic, intraplate, and human induced events. Stress-drop values range primarily between 1 – 100 MPa for events that meet the criteria for spectral-ratio analysis. We present correlations of low relative stress drop values in areas of high seismic attenuation indicative of lower rock strength, and a slight correlation with depth that corresponds to modeled deviatoric stress values. We also show one notable subset of induced events near active injection wells that exhibit stress drop values of ~0.1 MPa and have distinctive low-frequency content. Their spatial distribution, waveform, and source spectral characteristics suggest either slower rupture, lower stress drop values, or a combination of both, and may represent part of the transition between aseismic and seismic slip. We show using a Large-n array that while stress drop values are roughly constant (within 2 orders of magnitude), estimates can vary by roughly 25% when station coverage is limited to 15 stations or less with a maximum azimuthal gap of 90°. Our findings highlight the importance of using modeling approaches to explore relative influence of fault strength and methodological approaches in stress drop variation. In particular, models that incorporate both frictional and thermoelastic approaches may provide clues to the variability of conditions that can activate faults, both within stable sliding and seismic rupture conditions.

Small stress changes such as those from sea level fluctuations can be large enough to trigger earthquakes. If small and large earthquakes initiate similarly, high-resolution catalogs with low detection thresholds are best suited to illuminate such processes. Below the Sea of Marmara section of the North Anatolian Fault, a segment of approximate to $\approx $150 km is late in its seismic cycle. We generated high-resolution seismicity catalogs for a hydrothermal region in the eastern Sea of Marmara employing AI-based and template matching techniques to investigate the link between sea level fluctuations and seismicity over 6 months. All high resolution catalogs show that local seismicity rates are larger during time periods shortly after local minima of sea level, when it is already rising. Local strainmeters indicate that seismicity is promoted when the ratio of differential to areal strain is the largest. The strain changes from sea level variations, on the order of 30-300 nstrain, are sufficient to promote seismicity.

Stress in active subduction forearcs is controlled primarily by friction along the megathrust and the gravitational force. The competing deviatoric compressive and tensional stresses generated by megathrust friction and gravity, respectively, are of the same order of magnitude and result in very low deviatoric stress in the forearc. The near neutral stress state in subduction forearcs is supported by observations of stress reversal, that is a change from deviatoric compression to deviatoric tension, caused by very small megathrust shear stress drops (<10MPa) after recent large megathrust earthquakes. However, studies that quantify and compare the stress state in subduction forearcs at various stages of the seismic cycle are still limited. Here, we use two-dimensional finite-element force-balance models to quantify and constrain forearc stresses at different locations along the Chilean and Japanese subduction margins that are at different stages of the seismic cycle. The models consider forearc topography, slab geometry, crustal thickness, and water load to quantify the elastic stress in the forearc due to gravity and friction along the megathrust. The models indicate low deviatoric stress values (10s of MPa) in the subduction forearcs, which imply a weak forearc crust in areas of active seismic deformation. We validate the model results by estimating seismic stress drop values of forearc earthquakes from high-quality seismic waveform recordings. We estimate spectral corner frequency using both single-spectrum and spectral-ratio estimates and depth-dependent shear-wave velocity models. Spectral-ratio estimates provide more robust corner-frequency estimates that we employ to validate and interpret the increasing stress drop trend down to depths of ~50-60 km. The slight depth dependency of seismic stress drop values is consistent with depth dependency of deviatoric stress obtained from the finite-element models. Moreover, we find that average seismic stress drop values are systematically lower or similar to maximum deviatoric stress obtained from our models, which is consistent with a partial stress release during earthquakes in the forearc. Our results suggest a relation between seismic stress drop values and ambient stress in subduction forearcs.

Abstract Earthquakes and slow‐slip events interact, however, detailed studies investigating their interplay are still limited. We generate the highest resolution microseismicity catalog to date for the northern Armutlu Peninsula in a ∼1‐year period to perform a detailed seismicity distribution analysis and correlate the results with a local, geodetically observed slow‐slip transient within the same period. Seismicity shows a transition of cluster‐type behavior from swarm‐like to burst‐like, accompanied by an increasing relative proportion of clustered (non‐Poissonian) relative to background (Poissonian) seismicity and gradually decreasing b‐value as the geodetically observed slow‐slip transient ends. The observed slow‐slip transient decay correlates with gradually increasing effective‐stress‐drop values. The observed correlation between the b‐value and geodetic transient highlights the influence of aseismic deformation on seismic deformation and the impact of slow‐slip transients on local seismic hazard.