Tidal stress is a globally acting perturbation driven primarily by the gravitational forcing of the Moon and the Sun. Understanding how tidal stresses can trigger seismic events is essential for constraining tectonic environments that are sensitive to small stress perturbations. Here, employing a spring-block with rate-and-state friction, we investigate tidal triggering on velocity-weakening stable sliding faults with stiffness slightly exceeding the critical stiffness. We first apply idealized step-like and boxcar normal stress perturbations to demonstrate a resonance-like amplification of slip rate when the perturbation period approaches the intrinsic frictional timescale of state evolution. Next, we perform nondimensional analyses and numerical simulations with harmonic tidal-like perturbations to identify the key parameters controlling tidal triggering and their admissible ranges. Triggered slip events are further characterized using physically interpretable quantities, including radiation efficiency and tidal phase. Our results show that even small stress perturbations can trigger periodic as well as complex slip events on stable sliding faults. The triggering behavior is primarily controlled by the normalized perturbation period and the normalized perturbation amplitude. An increase in the normalized period shifts event timing from the peak of tidal stress toward the peak of stress rate, whereas increasing the normalized amplitude promotes a transition from slow to fast events. The parameter space permitting triggered events suggests that the parameter which characterizes the instantaneous frictional strength of an interface, should not exceed tens to hundreds of kilopascals, and that the characteristic slip distance for frictional weakening is likely on the order of micrometers.

The recent observation of neighboring earthquakes with anti-correlated waveforms, referred to as anti-repeating earthquakes, or anti-repeaters, suggests that almost opposite fault slip may occur consecutively along the same or nearby structures in Earth. These earthquakes have so far been identified in a few areas only, although involving different seismogenic contexts, like tectonic, volcanic, and induced seismicity, so they could be a more common phenomenon than previously thought. Here, we analyze large and moderate-sized earthquakes worldwide in a systematic manner, scanning the Global Centroid Moment Tensor catalog over 48 yr in search of pairs of earthquakes with neighboring centroid locations and nearly opposite moment tensors. The results are somewhat surprising: thousands of nearby earthquake pairs with opposite source orientations can be found, clustering at hundreds of locations on Earth, occurring at different depths, in different tectonic settings, as well as in volcanic environments. We validate the detections by their waveform anti-correlation and report a global catalog of opposite faulting, as a first atlas for this new type of seismological observation.

Shallow tectonic tremors along the northeastern Japan subduction zone show regional differences in their spatiotemporal evolution, raising the question of whether their response to tidal stressing also varies along strike. We analyze the tremor catalogue obtained by Sagae et al. (JGR, 2025, e2025JB031348) for the period from August 2016 to August 2024. Based on their spatial distribution, tremor activity can be divided into three major regions: the southern end of the Kuril Trench (40.8–42°N; northern region), the northern Japan Trench (38.8–40.5°N; central region), and the southern Japan Trench (35–36.8°N; southern region). Here, we investigate the tidal sensitivity of tectonic tremors in these three regions. Our statistical analysis shows that tidal sensitivity is highest in the northern area, where tremors are clustered and occur in recurrent along-strike propagating bursts. Cluster-scale analyses in this northern region indicate that tidal sensitivity increases during the later stages of tremor clusters, consistent with the characteristics reported for deep tectonic tremors. Tidal sensitivity is intermediate in the southern area, where tremors appear more scattered. In the central region, where tremor activity has declined gradually since 2011 Mw 9.0 Tohoku-Oki earthquake, tidal sensitivity is lowest. In this region however, tremors and fast earthquakes occur in close spatial proximity. There, we further examine the relationship between tremor activity, fast earthquakes and tidal stress to explore potential interactions between slow and fast earthquakes.

Abstract Tectonic tremor is a long‐duration noise‐like seismic signal associated with slow fault motion at major plate boundaries. Due to their remarkable sensitivity to stress perturbations, tremors serve as invaluable probes of the mechanical state and stress conditions deep within fault zones. Near the Mendocino Triple Junction, a tremor cluster ceased abruptly following each of three Mw 6.0+ earthquakes (2020–2024), including the 2024 Mw 7.0 Mendocino fault earthquake. Moment tensor inversion reveals a high‐angle, right‐lateral strike‐slip mechanism for this tremor cluster. Coulomb stress modeling indicates the tremor cluster is located in a region where Coulomb stress was increased by the mainshock. During the recovery phase of tremor activity, we observed a clear migration trend. Integrating these observations with tomography, we suggest that fluid‐related processes may have contributed to tremor suppression and recovery. Our results provide a broader framework for understanding how large earthquakes modulate slow fault slip beyond stress shadow effects.

Japan's Shinkansen (bullet train) system has faced concerns regarding potential earthquakes occurring near its railways, motivating seismologists and Japan Railways Group (JR) leaders to keep developing its independent early warning system. Since the inception of Distributed Acoustic Sensing (DAS) technology, which transforms telecommunications fiber optic cables into seismometers, it has proven reliable in retrieving earthquake information and shows strong potential for early warning. This paper presents strategies demonstrating the effective use of DAS for real-time earthquake monitoring along the Shinkansen system. To accomplish this objective, we conducted an analysis of a specific segment of the JR Central Shinkansen line, spanning from Atami to Tokyo Station. We calibrated body-wave empirical scaling relations following two distinct approaches: 1) a method that determines earthquake location and magnitude to predict the S-wave peak strain-rate amplitude (PSRA), and 2) an empirical relationship between S-and P-wave amplitudes, allowing PSRA prediction without prior earthquake location information. All these procedures can be completed within the initial 3-4 s following the first P-wave arrival. Consequently, our methods represent a comprehensive early warning procedure.

We analyze similar to 7000 tectonic tremors recorded between 2012 and 2022 across five clusters along Taiwan's mountain belt at depths of 30-50 km, providing new insights into slow fault slip within an active continental collision zone. All clusters occur above the Moho, exhibit thrust-dominant focal mechanisms, and are distinct from crustal seismicity. Tidal sensitivity varies spatially, with Clusters 2-5, located in zones of active collision and subduction termination, showing strong modulation (alpha = 0.53-0.75), while Cluster 1, situated in a post-collisional extensional environment near the Okinawa Trough, exhibits weaker sensitivity (alpha approximate to 0.3). These variations correlate with differences in tidal stress amplitude and tectonic regime. Moment tensor inversions reveal consistent thrusting styles, but principal stress orientations vary with depth, with sigma(1) rotating from vertical in the upper crust to horizontal at tremor depths. This supports a two-layer deformation model shaped by orogenic collapse and lower crustal convergence-parallel shear. Our findings demonstrate that tremor generation in Taiwan reflects evolving stress regimes, fluid-assisted weakening, and structural heterogeneity associated with the interplay of collision, subduction, and back-arc extension. Tremors thus serve as sensitive indicators of deep-seated tectonic processes in dynamically evolving mountain belts.

California, as a transform plate boundary, provides a distinctive tectonic setting and an ideal natural laboratory for investigating tectonic tremors and the slow deformation associated with plate motion. By analyzing continuous seismic records across multiple stations with an envelope correlation method, we identified similar to 83,000 tremor events from 2000 to 2025. These events exhibit waveform characteristics consistent with tectonic tremors observed elsewhere. Beyond the previously documented central section of the San Andreas fault, we identify several new tremor clusters, primarily concentrated near the Mendocino Triple Junction and within the Big Bend segment. Our results suggest that tremor events near the Mendocino Triple Junction may mark the southern edge of the Cascadia subduction zone, while tremor events in the Big Bend region, located within the rupture zone of the 1857 M7.9 Fort Tejon earthquake, could have implications for regional seismic hazard.

To reproduce characteristics of empirically known seismic activity, such as magnitude-frequency (M-F) statistics that follow a power law, the temporal evolution of aftershocks following the Omori formula, and repeating earthquakes with a constant recurrence interval, as well as the rupture initiation process of small and large earthquakes over a long period under a simple model, we conducted quasi-dynamic numerical simulations of earthquake generation cycles. In the framework of the rate- and state-dependent friction law, we adopted a multiscale circular patch model to represent the spatial heterogeneity of friction in the northern Japan Trench. We divided historical earthquakes with a magnitude greater than 5.6, recorded since 1896, into four groups according to magnitude. We set the characteristic slip distance to be proportional to the patch radius. The M-F statistics of the simulated earthquakes followed a power law with a smaller magnitude range than that of the characteristic earthquake. Large earthquakes occurred as combined ruptures of nearby patches with different triggering patterns due to foreshocks and slow slip. The rupture initiation points of large, moderate, and small earthquakes were nearly in the same location, suggesting a cascade-up rupture process. The number of foreshocks and aftershocks of the largest earthquakes statistically followed the modified Omori formula. Small repeating earthquakes occur at isolated patches with recurrence intervals of a few years. The more hierarchical the model, the more complex and varied the seismic cycles. The multiscale circular patch model effectively reproduced these observed characteristics in quasi-dynamic earthquake cycle simulations.

Distributed Acoustic Sensing (DAS) is popular in seismological research for its high-resolution, spatially distributed seismic records. Unlike conventional seismic instruments, DAS measures strain rate, which requires appropriate scaling to interpret the data in terms of ground motion. This article presents two perspectives on Swave DAS-based scaling relations for earthquakes recorded in Mexico City. The first perspective explores two models. (1) a frequency-independent relation that uses earthquake magnitude, hypocentral distance, and average site response to explain the peak strain rate amplitude. (2) a frequency-dependent relation that incorporates the source spectrum, hypocentral distance, and a frequency-dependent site response to derive the strain-rate spectrum. The second perspective explores the well-known proportional relationship between strain rate and acceleration records using a nearby seismic station data. Our results show that, for the first-perspective models, the source and geometrical spreading coefficients match theoretical expectations, while M0 retrieval from (2) is well constrained within in a frequency band limited by the signal-to-noise ratio (suggesting reliability for Mw <= 5.0), and the site effect terms correlate with the geological and infrastructural features. For the second perspective, as anticipated, we confirm that the strain rate-acceleration proportionality is governed by shallow S-wave velocity in minimally stratified structures. Analogous to a puzzle, the first-perspective models provide practical tools for estimating earthquake magnitude and characterizing site effects. In conjunction with the rapid PGA estimations derived from the second perspective, DAS can be positioned as a powerful tool with for seismic studies in Mexico City.

What controls the occurrence of giant earthquakes remains a central question in seismology. We reveal a notably simple—but previously overlooked—relationship between earthquake size and fault dip in subduction zones. Global earthquake statistics show a robust dependence of earthquake size on fault dip, with the probability of extreme events peaking on ultralow-angle faults. Through three case studies, we demonstrate that earthquakes tend to grow larger when the fault geometry is aligned with the regional stress field. We further discuss how such favorable stress configurations are generated along subduction interfaces and emphasize the critical importance of monitoring the temporal evolution of stress in subduction zones to better forecast future megaearthquakes.

Establishing a quantitative framework to evaluate the spatiotemporal patterns of slow earthquakes and to detect their anomalous activities is essential for understanding diverse slip behaviors on plate boundaries and seismic hazard assessment. In this study, we focus on deep tectonic tremors, which are one manifestation of slow earthquakes and are detectable through seismological observations along the Nankai Trough. We develop a probabilistic model based on a multivariate Hawkes process to describe both the temporal and spatial characteristics of their activity. Our analysis shows that more than half of the tremors are attributable to interactions with neighboring regions, which underlines the importance of incorporating spatial interactions into the forecasting model. Along-dip and along-strike variations in background tremor seismicity and the effective durations of inter-tremor interactions revealed in our model are consistent with previous geophysical and geological observations and conceptual frameworks, including tremor migration, depth-dependent slip modes, partial overlap with slow slip events (SSEs), and along-strike variations in plate convergence rate. We further compare the model-predicted tremor activity with observations and quantify their differences in cumulative event counts using the Kolmogorov-Smirnov test to identify transient anomalies. The detected anomalies include both short-duration (similar to 0.1 days) and long-duration (similar to 100 days) activations and quiescences. Although their spatial extent is much smaller (25 km) than that of SSEs, half of the short-term anomalies partially correlate with geodetically detected short-term SSEs. These results demonstrate the potential of our approach to provide a robust framework for forecasting slow earthquake activity and detecting its changes.

It is widely acknowledged that predicting the final size of an earthquake from the P-wave onset in seismograms is nearly impossible. We confirm this fact by measuring the slope (parameter B ) of the acceleration record shortly after onset assuming a constant rupture speed. Since 2007, B has been used in the Japanese earthquake early warning system as an indicator of epicentral distance, yet it may also provide deeper insights into earthquake dynamics and wave propagation. Utilizing high-sensitivity seismograms from approximately 800 borehole stations in Hi-net and combining manually picked P-wave arrival times and focal mechanisms for about 1800 earthquakes, we estimate B for each station and earthquake pair within a 0.1 s window. We confirm that B decreases from the square to the fourth power of the P-wave travel time, a phenomenon not explainable by simple geometric decay or intrinsic attenuation alone. Residuals between observed values and travel time dependencies are further decomposed into event and site terms, alongside radiation pattern dependency, which is close to a power of 0.5—possibly reflecting complex rupture processes that begin at a minute scale. The event term primarily represents the initial stress drop, showing minimal dependency on final size and a clear dependency on event depth, mirroring observations of average stress drop. This term also shows a statistically significant negative trend with the moment-normalized duration estimated independently using S-waves, suggesting limited predictability of the rupture process. The site terms, which correlate with tectonic structure, help reduce errors in estimating arrival times, especially for nearby earthquakes, thus offering practical benefits for early warning systems.

Tidal modulation of tectonic tremors provides a sensitive measure of fault response to small stress perturbations, yet how this response varies in a mixed fast and slow earthquake system remains unclear. Here we present the first systematic investigation of tremor tidal sensitivity in such a system, focusing on tectonic tremors along the northeastern Japan subduction zone. Using a tremor catalog from 2016 to 2024, we show that the southern end of the Kuril Trench, characterized by tremor migration and relatively weak seismicity, exhibits the strongest tidal sensitivity, whereas the northern Japan Trench shows the weakest response. Spatial analysis further reveals that areas with weaker tidal sensitivity tend to coincide with more earthquakes (M_j ≥ 4) and denser tremor activity. In addition, tidal sensitivity at the southern end of the Kuril Trench increases from the early to later stages in tremor migration, potentially reflecting changes associated with underlying slow slip processes. Together, these spatial and temporal patterns suggest that tremor tidal sensitivity may be influenced by the relative contribution of other ongoing perturbations. These results highlight tidal sensitivity as a useful probe of the underlying perturbation environment and provide insight into the possible influence of slow slip processes, earthquakes, and other stress changes on tremor-generating regions.

Repeating earthquakes have overlapping rupture patches, similar focal mechanism and magnitudes. They are often detected based on highly similar waveforms using template matching techniques, which help to reconstruct complex sequences and swarms. Here, we investigate earthquakes with highly anti-correlated waveforms. Such poorly known observation implies the occurrence of reversed seismogenic processes at close hypocentral locations. We introduce the terms true and quasi anti-repeating earthquakes to denote cases affecting the same rupture patch or neighboring patches, respectively. We report about a number of observations of anti-repeating earthquakes in different environments, such as volcano, induced and intermediate-depth seismicity, and then review conceptual models to explain them. Some of these observations occurred during seismicity unrests, in the form of seismic sequences and swarms. Both true and quasi anti-repeating earthquakes are indicators for stress perturbation transients or local stress heterogeneities, often controlled by fluid migration processes. Therefore, their analysis may help the identification and tracking of fluids in the subsurface.

This study investigated the spatiotemporal evolution of a deep short-term slow slip event (SSE) in southwest Japan using three geodetic data types: tilt, strain, and GNSS. Because tilt and strain data are more sensitive to crustal deformation than GNSS data, their joint use enables us to estimate the detailed spatiotemporal evolution of SSE slip. In addition, a higher sampling rate of tilt and strain data improves their temporal resolution down to sub-daily time scales. We developed a geodetic inversion method jointly using the three types of geodetic data, and obtained the spatiotemporal evolution of an SSE slip, which occurred beneath the western Shikoku region in November 2022. Comparing results from all possible combinations of data types, we found that the small earlystage SSE slip could be resolved only when strain data was included in the dataset, which is considered to have the highest sensitivity to crustal deformation among the sensors. With the improved spatiotemporal resolution, we compared the spatiotemporal evolution of the SSE slip with that of tremor activities. The initiation timing of the SSE was consistent with the timing when tremor activities were initiated. SSE slips were spatiotemporally collocated with tremor activities, with a moment evolution quantitatively consistent with tremor energy radiation. The scaling factor between the aseismic moment of SSE and the seismic energy of tremor was estimated to be 10-11. However, it is unclear whether we can regard this value as the scaled energy of a slow earthquake because some tremor energy might be missed through the tremor detection process. Our results show that joint use of tilt and strain is especially useful in obtaining not only spatial, but also spatiotemporal evolution of small SSE slip in southwest Japan and that SSE and tremors are representations of a single broadband slip phenomenon in different frequency bands.

Here, we present the energy budget analysis that links seismic focal mechanisms to the development of geological structures in numerical granular rock box experiments, utilizing a discrete element method simulation approach. The model simulates the horizontal shortening of a thin 3D granular rock layer on a geological-scale (100 km x 0.25 km x 2 km), with a maximum element radius of 12.5 m. This simulation reproduces the millimeter-scale fault displacements caused by the rapid and intermittent motions of elements generating elastic waves, that is, virtual earthquakes. The simulation also captures early postseismic processes occurring within several tens of seconds, during which popup structure between the active faults is uplifted. We analyzed over 190 earthquake events during the simulation, with a total shortening length of 14.72 m. The change in energy balance starts with a local fault potential drop that generates the main shock, followed by a regional potential release induced by propagated waves. Statistical analysis reveals positive correlations, ranging from linear to quadratic, between the magnitudes of energy changes and fault slip displacement. Notably, approximately 0.01 % to 60 % of the local contact potential drop in the seismogenic fault is converted into kinetic wave energy, and the efficiency of this conversion increases with the earthquake size. Our results reveal that the uplift energy of the popup structure, which considerably exceeds the wave energy by several orders of magnitude, cannot be explained solely by the local seismogenic fault potential release. Instead, off-fault regional potential release should also be taken into account. We also demonstrate the scaling law for earthquake energy and seismic moment. Our findings suggest that the inherent diversity of virtual earthquakes partially captures earthquake behaviors.

The subduction of active spreading centers is an unusual phenomenon along subduction zones. In southern Chile, the Nazca‐Antarctic spreading system (Chile Rise) subducts beneath the South America plate at the Chile Triple Junction (CTJ), forming the Patagonian slab window. The beginning (spatially youngest portion) of the slab window has been estimated based on plate kinematic reconstructions, but direct observations remain insufficient. To investigate this tectonic feature in detail, an Ocean Bottom Seismometer array was deployed south of the CTJ between 2019 and 2021. Using these continuous data and the envelope correlation method, we searched for tectonic tremors to complement the seismic observations and detected more than 500 events in this period. A notable separation between fast seismicity and tremors is observed at the current location of the subducted Chile Rise segment. We interpret this seismic gap as evidence of the Patagonian slab window formation within the last 0.3 Myr.

Most seismicity in Latin America is controlled by the subduction process. Different zones have hosted earthquakes of magnitudes larger than Mw 8.5 that repeat every several centuries. Events around Mw 8.0 are more frequent; since the beginning of the twentieth century, some collocated earthquakes have occurred with differences of decades, which allows for comparison of old and modern seismological records. The rupture zones that have hosted mega-earthquakes continue to produce smaller earthquakes after three centuries. Therefore, the process of unlocking in the Latin America subduction zone occurs by giant (≥Mw 9.0), mega- (9.0 > Mw ≥ 8.5), and large (8.5 > Mw ≥ 7.5) earthquakes, and interaction between these events is not yet fully understood. We have less understanding of the earthquakes that occurred in the oceanic plates, which have not been correctly recorded due to poor seismological instrumentation and lack of knowledge about subduction during the first half of the twentieth century in Latin America. Slow earthquakes have been observed in some zones of Latin America, several of them with recurrence periods of a few years, as well as tectonic (nonvolcanic) tremors and low-frequency and very low-frequency earthquakes. How do these slow slip manifestations relate to ordinary earthquakes? This question is still difficult to answer for Latin America given the lack of dense geodetic and seismic networks that allow identification of all the slow earthquakes that likely occur more frequently than currently reported. ▪ Latin America subduction zones share similar seismic characteristics. They can host large-magnitude earthquakes and exhibit a variety of slow earthquakes. ▪ Giant earthquakes, with a magnitude greater than 9, have occurred so far in Chile, and mega-earthquakes have occurred in several Latin American countries. ▪ Additional slow earthquakes will be detected in Latin America as seismic and geodetic networks become denser.

Earthquakes may seem random, but are often concentrated in some localized areas. Thus, they are likely controlled by fault materials and stress heterogeneity, which are little understood. Here, we provide high-resolution observations of fault material and stress heterogeneity in the Japan subduction zone through an integration of material and source imaging with numerical simulations. Our results present evidence for localized, anisotropic structures with a near-zero Poisson's ratio in the medium surrounding 1 to 2 kilometer-sized earthquake clusters, suggesting that the fault medium is damaged, foliated, and enriched with fluid. Such localized structures may cause stress perturbations on faults that in turn favor the frequent occurrence of deep interplate earthquakes at depths of 60 to 70 kilometers. Therefore, identifying the distribution and properties of fault material heterogeneity is important for more informed assessment of earthquake hazards.

The subduction of active spreading centers is an unusual phenomenon along subduction zones. In southern Chile, the Nazca-Antarctic spreading system (Chile Rise) subducts beneath the South American plate at the Chile Triple Junction (CTJ), forming the Patagonian slab window. The onset of the slab window has been estimated based on plate kinematic reconstructions, but direct observations remain insufficient. To study this tectonic feature in detail, an Ocean Bottom Seismometer (OBS) array was deployed south of the CTJ between 2019 and 2021, and many earthquakes were detected and located around the CTJ. Using these continuous data and the envelope correlation method, we searched for tectonic tremors to complement the seismic observations and detected more than 500 events in this period. The tremors detected are mainly located beneath the Taitao Ridge, where no fast earthquakes were observed. The tremors exhibit burst and episodic activity, reaching depths less than 20 km. A notable separation between fast seismicity and tremors is observed at the current location of the subducted Chile Rise segment. We interpret this seismic gap as evidence of the Patagonian slab window formation within the last 0.3 Myr. The shallow tremor activity is likely triggered by the migration of fluids, introduced by the subduction of the spreading ridge, into the accretionary prism preserved along the Taitao Ridge.