Volcanic eruptions often result from the failure of rocks surrounding magma pathways. A decrease in b-value typically precedes such failures, indicating increased stress and eruption likelihood. Here, we analyze b-value time series of seismic data from 8 well-monitored volcanoes across various tectonic regions: Eyjafjallajökull, Fagradalsfjall, Grímsvötn, and Holuhraun (Bárðarbunga) in Iceland; Mauna Loa in Hawaii (USA); and Kirishima, Ontake, and Usu in Japan. We introduce a 'Volcano Traffic Light Alert System (VTLAS)' to assess priming timescales and potential improvements to near‑real‑time forecasting. We define red light as instances when the b-value drops exceed 10% below normal background levels of activity and examine whether eruptions occur within 10 days. Of the 25 identified cases, 39% are followed by eruptions, whereas 61% are not. Among the latter, 71% involve red lights without subsequent eruptions but are mostly linked to large earthquakes, and 29% show no drop before an eruption. Despite the relatively low percentages, our forecasting success rate is comparable to that reported in studies employing InSAR, GNSS deformation, or thermal anomaly analyses. This suggests that the proposed VTLAS may help anticipate impending eruptions. However, to improve reliability, we recommend combining b-value analysis with other geophysical observables to effectively distinguish volcanic unrest from imminent eruptions.

Fluid migration before and after a large megathrust earthquake

Earthquake magnitude is controlled by where and when rupture propagation stops. However, rupture arrest has rarely been directly observed in near-field seismic records of natural earthquakes. Here, we present systematic near-field observations of ground-motion stopping phases from large strike-slip earthquakes. Analysis of 12 global events shows that transient overshoot in fault-parallel ground surface displacement is a robust diagnostic signature of abrupt termination of rupture propagation. Dynamic rupture simulations reveal that near-field ground motions are strongly amplified by low-wavespeed rocks at shallow depth, which enhances the amplitude of displacement overshoot recorded at the surface. The occurrence of stopping phases at near-fault locations far from mapped rupture termini implies that large strike-slip earthquakes rupture in a segmented manner, with dynamic rupture propagation punctuated by abrupt arrest and reinitiation at internal fault-segment boundaries.

ABSTRACT Bangkok, located on a deep alluvial basin of very soft clay, is highly susceptible to long-period ground motions generated by distant large earthquakes. On 28 March 2025, an Mw 7.7 teleseismic earthquake produced unusually long-period vibrations across the city, providing a unique opportunity to examine the dynamic behavior of tall buildings under distant strong shaking. Ambient and strong-motion measurements from multiple reinforced concrete high-rise buildings were analyzed to evaluate variations in natural periods and damping ratios before, during, and after the event. Although the peak ground acceleration was only about 0.02 g, spectral amplitudes in the long-period range were amplified by more than ten times, causing significant vibration in tall buildings. Several structures exhibited permanent period elongation of up to 40%, reflecting the strong influence of deep soft-soil amplification. These results highlight the seismic vulnerability of tall buildings to teleseismic long-period motions and the importance of continuous monitoring in deep-basin environments.

Large earthquakes may be associated with traumatic stress and eating-related outcomes, but evidence from Turkey remains limited. This study examined the associations between direct earthquake exposure, post-earthquake traumatic stress, and eating-related outcomes approximately two years after the 6 February 2023 Kahramanmaraş-centred earthquakes. This cross-sectional comparative online survey was conducted in Turkey between March and June 2025. Adults were classified as directly exposed (n = 602) or comparison (n = 615) based on residence in the 11 affected provinces and self-reported earthquake experience. Participants completed the Post-Earthquake Trauma Level Determination Scale (PETLDS), Salzburg Emotional Eating Scale (SEES), Mindful Eating Questionnaire (MEQ), and SCOFF eating disorder screening tool, together with sociodemographic and self-reported anthropometric measures. Adjusted group differences were estimated using multivariable regression models controlling for age, BMI, gender, marital status, education, employment, income status, and earthquake-related loss. Additional adjusted models examined whether PETLDS total score was independently associated with eating-related outcomes. Direct exposure was strongly associated with higher traumatic stress (PETLDS total: B = 12.30, 95%CI 10.66 to 13.94; p < 0.001) and with higher scores on most PETLDS subscales, whereas the PETLDS emotional subscale was higher in the comparison group (B=-1.09, 95%CI -1.37 to -0.81; p < 0.001). Direct exposure was not independently associated with SEES total score (p = 0.153), MEQ total score (p = 0.584), or a positive SCOFF screen (OR = 1.286, 95%CI 0.969 to 1.706; p = 0.081), although small exploratory subscale differences were observed. In additional adjusted models, higher PETLDS total score was associated with lower MEQ total score and greater odds of a positive SCOFF screen (both p < 0.001), but not with SEES total score. Direct exposure was strongly associated with traumatic stress, whereas eating-related associations were limited. Traumatic stress severity was more clearly associated with lower mindful eating and higher eating disorder risk screening positivity, supporting trauma-informed post-disaster care and nutritional screening in highly stressed individuals.

Abstract Compared to continental strike‐slip faults, oceanic transform faults (OTFs) are thought to mainly slip aseismically and host significantly more foreshocks triggered by precursory aseismic slip which enhance the mainshocks' short‐term predictability. However, long‐term high‐resolution observational constraints remain limited. In December 2024, one of the largest ever OTF earthquakes occurred offshore California on the Mendocino OTF. Here we show that foreshock activity is very limited within the rupture zone of this moment magnitude ( M w ) 7.0 earthquake and does not indicate accelerating aseismic slip in the preceding month. The 2016 M w 6.6 and 1994 M w 7.0 Mendocino OTF earthquakes share similar characteristics. The 15 historical M w ≥ 5.5 mainshocks also have significantly fewer foreshocks on average compared to continental strike‐slip earthquakes. Therefore, there is no clear evidence of aseismic‐nucleation‐related seismicity preceding large Mendocino OTF earthquakes. Our results further demonstrate that enhanced foreshock activity is not a general characteristic of OTFs despite prevalent aseismic slip.

Predicting large earthquakes remains a significant challenge due to the complexity of fault systems and the variability of preparatory processes. We introduce an unsupervised machine learning framework to categorize seismicity patterns and identify, when present, seismicity transients preceding large earthquakes. We focus on five large earthquakes and extract seismo-mechanical features per families of events, defined as clustered events in space, time and magnitude. Here we show that for those cases displaying a preparatory phase, specific long-lasting families belonging to a critical category signalling an upcoming earthquake occur during the preparatory phase. Compared to other periods, critical categories reflect a higher spatial-temporal localization, earthquake interaction and strain release. The method will not detect such a transient for earthquakes with no detectable seismic preparatory phase. Finally, we demonstrate that the method is capable of identifying preparatory phases (when present), showing potential for operational earthquake forecasting.

Re-examination of TEC anomalies in the ionosphere before large earthquakes

Interseismic deformation and strain rates derived from Global Navigation Satellite Systems (GNSS) or Interferometric Synthetic Aperture Radar (InSAR) data are often used to assess a fault’s earthquake potential. Deformation rates, commonly derived from short-term geodetic observations, are assumed to be indicative of long-term strain accumulation, and variations in strain rate throughout the interseismic period are poorly studied. Many regions have limited spatial and temporal coverage of GNSS data, and using InSAR presents challenges on low strain rate faults, where the signal-to-noise ratio is low. As a case study, we examine the interseismic period prior to a large intraplate left-lateral strike-slip earthquake: the 2021 M W 7.4 Maduo Earthquake. Using InSAR, we derive eastward velocities and maximum shear strain rates, whilst minimising the influence of near-field GNSS data, for two-year, three-year and six-year time periods. We investigate the sensitivity of geodetic deformation rate to the temporal observation window, and find that transient variations in deformation vary in a spatial pattern that cannot be unequivocally attributed to tectonic motion, given the uncertainty of InSAR data in the region. We do, however, observe a consistently low strain rate on the seismogenic fault. This study highlights the challenges faced when trying to obtain meaningful geodetic measurements on low strain rate faults, bringing to question the utility of strain rate as a proxy for seismic hazard in continental block interiors. • InSAR and GNSS data constrain strain rate in several observation windows prior to the 2021 Maduo earthquake. • Low strain rates are consistently observed on the seismogenic fault, irrespective of observation window. • Changing the observation window leads to fluctuations in deformation rates that can not be unequivocally attributed to tectonic motion given the signal-to-noise ratio of InSAR in the region.

Large earthquakes commonly generate surface rupture accompanied by both localized on-fault slip and spatially distributed off-fault deformation. Capturing both components is essential for understanding rupture processes and improving earthquake hazard assessment, yet field mapping alone often fails to fully document diffuse deformation. Here we evaluate the applicability of high-resolution Korea Multi-Purpose Satellite (KOMPSAT)-3 and -3A optical imagery for mapping near-field co-seismic deformation using sub-pixel optical image correlation (OIC), through two case-study areas affected by the 6 February 2023 Kahramanmaraş, Türkiye, earthquake sequence. We processed pre- and post-event stereo-mode KOMPSAT imagery using a MicMac-based workflow to generate orthorectified products and displacement fields, and compared the results with published Sentinel-2 OIC products and independent airborne Light Detection and Ranging (LiDAR) measurements. In the Hatay Airport area, KOMPSAT-3/3A OIC recovered a displacement pattern consistent with Sentinel-2, indicating ~5 m of relative motion across the fault, while the ~1 m effective spatial resolution enabled identification of localized infrastructure offsets (runway displacement) that were not detectable in 10 m Sentinel-2 imagery. In the Elbistan near-epicenter area, KOMPSAT-3/3A OIC resolved block motions of ~6 m and ~2 m in opposing directions. Swath profile analysis indicates an average on-fault slip of 6.8 m, whereas the total slip including distributed deformation reaches 9.3 m, implying that approximately 27% of the deformation is accommodated off-fault. Airborne LiDAR mapping provides an independent benchmark, with on-fault net slip of ~6.13 m and horizontal slip of 5.57 ± 1.40 m, consistent with the KOMPSAT-derived on-fault estimates and supporting the quantitative validity of the OIC results. However, the rupture geometry inferred from OIC is simpler than LiDAR-derived mapping, and absolute geolocation uncertainty remains a limiting factor with a post-correction Root Mean Square Error (RMSE) of 10.25 m and Circular Error with 90% Confidence (CE90) of 11.34 m, requiring cautious interpretation of absolute displacement magnitudes. Overall, our results demonstrate that KOMPSAT-3/3A imagery can serve as an effective resource for rapid rupture mapping and quantifying both on-fault and distributed deformation, while highlighting key requirements for improving geolocation control and integrating complementary datasets for robust three-dimensional deformation assessment.

Understanding how stress and strength conditions evolve before and after large earthquakes remains a fundamental challenge in seismology. Here, we introduce an observationally grounded metric-the ratio of the summed moment tensor to scalar seismic moment (Mstk/M₀)-to track temporal changes in deformation behavior surrounding earthquake faults. Using high-resolution focal mechanism datasets, we analyze two well-instrumented sequences with M6-class foreshocks followed by M7-class mainshocks: the 2016 Kumamoto and 2019 Ridgecrest events. We find that Mstk/M₀ remains elevated after M6-class foreshocks but decreases sharply after M7-class mainshocks, indicating a transition toward a more heterogeneous deformation state. To evaluate broader applicability, we examined nine inland M6-M7 earthquake sequences in Japan (2000-2020). Among ten events that exhibited high Mstk/M₀ during foreshock activity, five maintained high values after the M6-class event, and three of these were subsequently followed by even larger earthquakes. In contrast, all other sequences not followed by larger events showed clear decreases in Mstk/M₀ after the initial large earthquake. These observations suggest that sustained high Mstk/M₀ after an M6-class earthquake may indicate conditions favorable for continued rupture growth, whereas decreases may reflect reduced likelihood of further large rupture. Relationships between Mstk/M₀ and inelastic strain rate further support a physical interpretation linking deformation consistency with the ambient stress field to the potential for large earthquake occurrence. Monitoring Mstk/M₀ in the immediate aftermath of large earthquakes may therefore provide useful information for assessing whether an even larger event is likely to follow.

Megathrusts host Earth's largest earthquakes. Understanding the physical conditions controlling their rupture dynamics is critical for assessing seismic and tsunami hazards. These earthquakes often display complex rupture dynamics, exemplified by the 2011 Tohoku-Oki earthquake, which exhibited multiple rupture episodes, depth-dependent seismic radiation, and substantial tsunamigenic slip near the trench. However, how such complexity arises from preexisting physical conditions remains uncertain. Here, we demonstrate that the observed rupture complexity of the Tohoku-Oki earthquake can spontaneously and self-consistently emerge, driven by rapid coseismic frictional restrengthening and data-informed fault heterogeneity. We use an ensemble of 3D dynamic rupture simulations to identify that mixed downdip pulse-like and updip crack-like rupture are driven by dynamic stress redistribution with episodic rupture reactivation. By featuring low fault strength compared to its dynamic stress drop, a preferred model can consistently reproduce the observed complex depth-dependent propagation speeds, multiple rupture fronts as imaged by back-projection, and large tsunamigenic slip at the trench. Our findings demonstrate that preexisting fault heterogeneity conjointly with dynamic frictional weakening and restrengthening drives seemingly unexpected megathrust rupture complexity, highlighting the need to include dynamic effects into physics-based seismic and tsunami hazard assessments of future earthquakes.

Abstract Large earthquakes can activate complex aftershock fault networks. In such systems, what controls the spatiotemporal evolution of early aftershocks remains a critical yet unresolved problem. Here, using the 2019 M 7.1 Ridgecrest earthquake as an example, we partition the first 10 days of aftershocks onto 15 branching faults activated by the mainshock. For each branching fault, we assess the spatial and temporal influence of mainshock‐induced Coulomb failure stress change (ΔCFS) on aftershock patterns and b values. We find that positive ΔCFS may promote aftershock occurrence across multiple faults during the initial 3 days following the mainshock, but this effect diminishes afterward. Moreover, on most branching faults, higher aftershock b values tend to be associated with areas experiencing negative ΔCFS and reduced differential stress. These findings indicate that mainshock‐induced stress plays an important role in controlling initial aftershock generation and size‐frequency characteristics, providing constraints on aftershock forecasting in complex fault systems.

Summary The source characteristics of moderately large earthquakes in the moment magnitude range Mw 6.0–6.5 are essential for understanding earthquake source physics and regional structures, yet often remain less constrained than those of larger events. Here, we develop an improved method for characterising regional moderately large earthquakes by simultaneously resolving the centroid moment tensor (CMT) and source time function (STF). Four earthquakes in regions where the geological structure can be approximated by a 1-D model or a composite of 1-D models for waveform modelling periods down to 10 s are analysed: the 2020 Mw6.5 Idaho (USA), 2020 Mw6.5 Nevada (USA), 2015 Mw6.5 Alor (Indonesia), and 2020 Mw6.4 Petrinja (Croatia) events. Furthermore, by incorporating uncertainty from data noise and Earth model error within a Bayesian inversion framework, we obtain improved CMT and STF solutions compared to those from existing catalogues. We show that while non-double-couple components exist in the Idaho and Nevada earthquakes, their contributions are smaller than previously estimated. The Alor and Petrinja events align with pure double-couple mechanisms. Notably, the Petrinja earthquake exhibits shallower strike-slip faulting (at 5 km depth) and a simple, short rupture, as reported in the SCARDEC catalogue. Its inferred fault geometry diverges from most existing seismogenic fault models. For all four cases, our CMT and STF solutions provide enhanced waveform fits up to 0.1 Hz, offering refined constraints on source characteristics and regional fault geometries. These findings underscore the importance of accounting for data and model uncertainties in source inversions, particularly at higher frequencies, where STFs are most sensitive. The reliable source parameters could have broad implications for seismic tomography, hazard assessment, and emergency response.

Summary A temporary array of seven ocean bottom seismometers (OBS) was deployed offshore the Guerrero subduction zone in Mexico to monitor previously unreported shallow seismicity. These OBS instruments are especially valuable for studying earthquake activity in the Guerrero seismic gap, where a future large event could severely impact densely populated regions of Mexico. This study investigates the shallow seafloor structure, including site effects, shear wave attenuation, and velocity models, using both earthquake data and ambient seismic noise. We employed spectral inversion to estimate the quality factors of shear wave attenuation and site effects. Additionally, we calculated the microtremor horizontal-to-vertical spectral ratio (HVSR) as a proxy for site response and invert it using constraints from hydroacoustic seafloor profiles, parametric sub-bottom profile system (TOPAS), to derive the shallow velocity structure beneath the stations. The inclusion of TOPAS data in the inversion significantly improved convergence, reduced misfit, and resulted in more reliable subsurface models. The HVSR inversions indicate the presence of water-saturated sediments within the upper 250 m, characterized by shear-wave velocities ranging from 55.2 to 1950 m/s and Vp/Vs ratios between 1.80 and 27.84. Strong attenuation effects, typical of marine environments, were observed, with Q(f) values as low as Q = 86f0.62 in the forearc accretionary wedge. Our attenuation estimates are consistent with those found in other offshore subduction zones, contributing to a broader understanding of shallow structures in similar tectonic settings worldwide. We found strong agreement between the estimated site effects and HVSR results, underscoring their close relationship and supporting the reliability of our site response estimates. This is the first study in Mexico to use OBS data to characterize offshore attenuation, site effects, and velocity structure, information that will support future seismological analyses, including earth structure imaging and investigations of both large earthquakes and shallow slow earthquakes in the Guerrero seismic gap.

Summary Seismic sources are typically characterized as stochastic slip distributions on complex fault geometries, which pose significant challenges for computational modelling. Source scaling laws, however, offer a streamlined alternative by correlating simplified fault geometry and slip characteristics with earthquake magnitude. So far, distinct scaling laws have been developed for different tectonic settings and fault mechanisms. However, regional variations in source parameters have not been explicitly quantified. For example, it remains unclear whether earthquakes occurring in similar tectonic environments (e.g. subduction zones) and fault mechanisms (e.g. reverse faulting), but in different regions such as Japan, South America, or Indonesia, exhibit comparable source characteristics, or how such variability should be incorporated into scaling relations. To address this gap, the present study performs a comprehensive exploratory analysis of earthquake source attributes derived exclusively from finite-fault models, including stress drop, alongside standard fault geometry and slip parameters. The analysis spans multiple groupings defined by tectonic setting, fault mechanism, seismic region, focal depth, crustal type, as well as fault-plane inversion modality and spatial resolution, which are examined to account for modelling-related variability across datasets. Stress-drop proxy and slip-parameter estimates, particularly for large magnitude earthquakes, display systematic deviations from self-similar scaling assumptions. Fault-plane modality, defined by the type of seismic and/or geodetic data used in the inversion, and fault-plane resolution, quantified by subfault discretization, are found to be associated with systematic differences in inferred slip and asperity parameters, and help explain part of the intra-event variability observed when multiple models exist for the same earthquake. These factors are therefore incorporated explicitly to isolate physical variability from modelling effects. Based on these findings, existing source scaling laws are revised using a mixed-effects regression framework. Tectonic setting, inversion modality, and fault-plane resolution are treated as fixed effects, while fault mechanism and seismic region are modelled as random and nested-random effects, respectively. The refined scaling relations provide more robust estimates of fault geometry (length, width, area, and asperity dimensions) and slip statistics (mean slip, maximum slip, and slip standard deviation), and are particularly valuable for region-specific computational source modelling and physics-based seismic hazard analysis.

Abstract The decomposition of earthquake moment tensors into isotropic and deviatoric components is standard in seismology. The deviatoric part is further separated into double‐couple (DC) and non‐double‐couple (NDC) components, with NDC providing insights into source complexity. However, existing methods often yield inconsistent results with different physical interpretation. We propose a novel decomposition method that models an earthquake as two DC subevents, in which the first is determined by P‐wave first‐motion data and the second is derived analytically via moment conservation. This method is simple, efficient, and physically interpretable. We evaluate its stability and apply it to eight global M w ≥ 7.5 earthquakes (2000–2023) with &gt;40% NDC components. The results align well with previous studies, demonstrating robustness. This method can be extended to multiple subevents when additional constraints, such as aftershock mechanisms, are available. It offers a powerful tool for exploring complex rupture processes and understanding the physical mechanisms of large earthquakes.

Abstract The Yingxiu–Beichuan Fault Zone (YBFZ), located in the Longmen Shan Thrust Belt (LSTB) on the eastern Tibetan Plateau, is highly seismically active. However, direct evidence of large earthquakes at depth is scarce. Here, we present rock magnetic, microstructural, and geochemical analyses of four fault zones from the Wenchuan Earthquake Fault Scientific Drilling borehole 2. Results show that fault gouges have high magnetic susceptibility, contain neoformed magnetite and monoclinic pyrrhotite, providing direct evidence for repeated large earthquakes with frictional heating temperatures of ∼500–900°C at depth. These earthquakes occurred in a reducing, sulfide‐bearing fluid environment throughout the seismic cycle. The increasing abundance of neoformed ferromagnetic minerals with depth could be related to more recent large earthquakes propagating at greater depths, supporting a structural model of vertical stacking and eastward propagation of the LSTB. This study provides key constraints on the deep seismic history and tectonic evolution of the LSTB.

Abstract Earthquake magnitude‐frequency distributions exhibit significant space‐time variations, which can provide critical insights into the physical processes driving seismicity. Understanding these variations is crucial for assessing seismic hazards and uncovering the physical processes driving earthquakes. One key parameter, the b ‐value, describes the relative proportion of small to large earthquakes and is thought to reflect factors such as stress conditions and fault properties. However, empirical evidence linking b ‐value variations to physical processes in real tectonic settings is still limited. Here, we show that b ‐value is systematically higher in regions with elevated heat flow, consistently across different tectonic settings and faulting style. This suggests that thermal conditions play a fundamental role in controlling earthquake size distributions, controlling the likelihood of large earthquakes in the different areas.

Abstract Distributed acoustic Sensing (DAS) data collected along a 30 km length of telecommunications fiber crossing the Alpine Fault near Haast enable analysis of interactions between fluvioglacial and seismotectonic processes. Here we use DAS recordings of 25 earthquakes to probe near‐surface structure beneath the Haast river valley. For each earthquake, delayed P‐ and S‐wave arrivals incompatible with a regional velocity model are observed at common locations. We show using a planar slab model that these delayed arrivals are proportional to the thickness of subjacent low‐velocity structures. 3D ray‐tracing reveals basin‐like low‐velocity features of approximately ∼1–4 km width and 200–650 m depth, consistent with independent seismic and gravity measurements collected along partly co‐located lines. We infer these low‐velocity structures to be sediment‐filled erosional basins that may exacerbate ground shaking in large earthquakes. The method developed here is general and highlights the potential of dense DAS measurements to provide high‐resolution subsurface characterization.