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Every gigantic earthquake begins as a tiny rock failure at almost a point, followed by successive slip of the complex fault system, before radiating strong shaking from a vast rupture area extending over hundreds of kilometres. Whether the growth process of the rupture of a large earthquake is predictable and whether it produces observable signatures different from that of smaller events 1 – 5 are fundamental questions related to the potential for earthquake early warning and probabilistic forecasting. Inspired by a recent discovery that large earthquakes might have seismic waves, and probably rupture processes, that are almost identical to those of smaller events 6 – 8 , we show that such similarity characterized by large cross-correlation is a common feature of earthquakes in the Tohoku–Hokkaido subduction zone, Japan. A systematic comparison of 15 years of high-sensitivity seismograph records for approximately 100,000 events reveals 80 extremely similar and 390 very similar pairs of large (moment magnitude M  > 4.5) and small ( M  < 4.0) earthquakes, co-located within about 100 metres. An extremely high similarity is observed for pairs of subduction-type earthquakes (170 of 899 large events) separated by a long period of up to 15 years, whereas for pairs of other types of large earthquakes only the foreshocks and aftershocks are similar. This frequently occurring similarity between different-sized subduction-type earthquakes suggests repeated cascading rupture processes in a widespread hierarchical structure 9 – 12 along the plate interface and indicates a specific but probabilistically limited predictability of the final size of the earthquake (that is, the location and a set of possible sizes of an earthquake are well predicted, but its final size is not at all well constrained). Analysis of a dataset of high-sensitivity Tohoku–Hokkaido seismograph records shows that pairs of subduction-type earthquakes of different sizes have very similar initial characteristics, implying that the final size of an earthquake cannot be reliably predicted from these.

Deep tectonic tremor occurs at various sites worldwide, and the source characteristics are heterogeneous, even at small scales. In this study, tremor sources were determined using data sets of seismic waveforms from various locations, some of which are not well recognized as being a site of tremor activity. The regions of interest are subduction zones at Nankai and Kyushu, Japan, Cascadia in western North America, Mexico, southern Chile, and New Zealand. Tremor locations are consistent with the geometry of subducting plates, and tremor depth tends to be shallower where younger plate is subducting. Tremor duration shows a negative correlation with tidal sensitivity, as reported previously in western Shikoku in the Nankai subduction zone. The duration is also related to the width of the tremor zone in the dip direction of the subducting plate. Linear structures, resembling striations, are observed in hypocenter distributions in western Shikoku and in the probability of short‐term migration direction. These spatial heterogeneities in tremor activity may be controlled by the heterogeneous frictional properties of the downgoing slab due to long‐term plate subduction, thereby providing information on the maturity of the plate interface. Key Points Quantitative comparison of tremor activity in various subduction zones Common characteristics: duration, tidal sensitivity, and striations Spatial heterogeneity is controlled by long‐term plate subduction

Deep tremor at subduction zones Deep seismic tremor provides valuable information on deep plate motion and shallow stress accumulation on the fault plane of megathrust earthquakes. It has been suggested that in subduction zones, deep seismic tremor repeats at regular intervals, is migrated at various velocities and is modulated by tidal stress. Satoshi Ide now presents evidence that a time-invariant interface property seems to control tremor behaviour in the Nankai subduction zone of Japan. In areas where tremor duration is short, tremor is more strongly affected by tidal stress and migration is inhibited; where tremor lasts longer, diffusive migration occurs with a constant diffusivity. He also finds that the spatial variation of the controlling property is characterized by striations in tremor source distribution, which follow either the present or previous plate subduction directions. Such interface properties might therefore be controlled by the subduction of inhomogeneous structures, such as seamounts. Deep seismic tremor in subduction zones has been suggested to repeat at a regular interval, migrate at various velocities and be modulated by tidal stress. Here, evidence is presented that a time-invariant interface property — possibly the ratio of brittle to ductile areas — controls tremor behaviour in the Nankai subduction zone, Japan. Where tremor duration is short, tremor is more strongly affected by tidal stress and migration is inhibited. Where tremor lasts longer, diffusive migration occurs with a constant diffusivity. Deep tremor in subduction zones is thought to be caused by small repeating shear slip events on the plate interface with significant slow components 1 , 2 , 3 , 4 . It occurs at a depth of about 30 kilometres and provides valuable information on deep plate motion and shallow stress accumulation on the fault plane of megathrust earthquakes. Tremor has been suggested to repeat at a regular interval 1 , 2 , migrate at various velocities 4 , 5 , 6 , 7 and be modulated by tidal stress 6 , 8 , 9 . Here I show that some time-invariant interface property controls tremor behaviour, using precise location of tremor sources with event duration in western Shikoku in the Nankai subduction zone, Japan. In areas where tremor duration is short, tremor is more strongly affected by tidal stress and migration is inhibited. Where tremor lasts longer, diffusive migration occurs with a constant diffusivity of 10 4  m 2  s −1 . The control property may be the ratio of brittle to ductile areas, perhaps determined by the influence of mantle wedge serpentinization on the plate interface. The spatial variation of the controlling property seems to be characterized by striations in tremor source distribution, which follows either the current or previous plate subduction directions. This suggests that the striations and corresponding interface properties are formed through the subduction of inhomogeneous structure, such as seamounts, for periods as long as ten million years.

The magnitude and rate of seismicity differ between subduction zones. Calculations of background seismicity rates, based on a global model of subduction zone seismicity, reveal a positive correlation between relative plate velocity and background seismicity, yet only the seismically quieter zones seem capable of generating magnitude 9 earthquakes. Maximum earthquake magnitude and the rate of seismic activity apparently differ among subduction zones. This variation is attributed to factors such as subduction zone temperature and stress, and the type of material being subducted 1 , 2 , 3 , 4 , 5 . The relative velocity between the downgoing and overriding plates controls their tectonic deformation. It is also thought to correlate with seismicity 1 , 2 , 6 , 7 , 8 . Here I use the epidemic type aftershock sequence model 9 , 10 to calculate the background seismicity rate—the frequency of seismic events above magnitude 4.5—for 117 sections of subduction zones worldwide, during the past century. I demonstrate a proportionality relationship whereby relative plate velocity correlates positively with seismicity rate. This relationship is prominent in the southwestern Pacific Ocean. However, although seismically active, this region has not experienced a magnitude 9 earthquake since 1900. In contrast, the Cascadia, Nankai, southern Chilean and Alaskan subduction zones exhibit low background seismicity rates, yet have experienced magnitude 9 earthquakes in the past century. Slow slip occurs in many of these regions, implying that slow deformation may aid nucleation of very large earthquakes. The proportionality relationship could be used to assess the seismic risk between two endmembers: active subduction zones that generate moderate earthquakes and quiet subduction zones that generate extremely large earthquakes.

Distributed acoustic sensing (DAS) is a new method that measures the strain change along a fiber-optic cable and has emerged as a promising geophysical application across a wide range of research and monitoring. Here we present the results of DAS observations from a submarine cable offshore Cape Muroto, Nankai subduction zone, western Japan. The observed signal amplitude varies widely among the DAS channels, even over short distances of only ~ 100 m, which is likely attributed to the differences in cable-seafloor coupling due to complex bathymetry along the cable route. Nevertheless, the noise levels at the well-coupled channels of DAS are almost comparable to those observed at nearby permanent ocean-bottom seismometers, suggesting that the cable has the ability to detect nearby micro earthquakes and even tectonic tremors. Many earthquakes were observed during the 5-day observation period, with the minimum and maximum detectable events being a local M1.1 event 30–50 km from the cable and a teleseismic Mw7.7 event that occurred in Cuba, respectively. Temperature appears to exert a greater control on the DAS signal than real strain in the quasi-static, sub-seismic range, where we can regard our DAS record as distributed temperature sensing (DTS) record, and detected many rapid temperature change events migrating along the cable: a small number of large migration events (up to 10 km in 6 h) associated with rapid temperature decreases, and many small-scale events (both rising and falling temperatures). These events may reflect oceanic internal surface waves and deep-ocean water mixing processes that are the result of ocean current–tidal interactions along an irregular seafloor boundary.

Although earthquakes are known to recur in approximately the same areas, their recurrence patterns and final sizes can vary considerably. To understand this variability, we analyzed a prominent sequence of repeating earthquakes from the latest catalog in Japan, activated following the 2011 earthquake (Mw 9.0). Waveform analysis of 53 events (2003–2023) revealed variations in magnitude from Mw 3.2 to 4.0. Their moment‐rate functions suggest they were not mere repetitions, but rather represented a more generalized form of recurrence. Their ruptures were consistently initiated in the same patch, and most events exhibited double ruptures after the Tohoku earthquake, transitioning to single ruptures over time, in a surprisingly systematic manner. A comparison with 40 smaller events (Mw 2–2.5) showed no obvious relationship between initial momentum and final magnitude. Our results suggest earthquake rupture patterns respond remarkably systematically to changes in the background loading rate, reflecting in situ frictional characteristics.

We develop a methodology to compile an objective tremor catalog by utilizing distinctive event features that differentiate tectonic tremors from non‐tremor events, and combining the envelope cross‐correlation method with clustering technique and neural network. This approach enables tremor extraction without subjective criteria, allowing for the detection of previously overlooked short‐duration tremors. The event features employed to distinguish tremors and non‐tremor events are depth, the mean amplitudes at high and low frequencies, the ratio of these two amplitudes, and event duration. The duration is defined as the minimum period that contains 50% of the seismic energy. The application of this method to western Japan detects 1.7 times more tremors than the previous studies, with the durations of 0.3–∼100 s. The events with short durations are considered low‐frequency earthquakes. The relationship between seismic moment and duration of the detected tremors is consistent with the scaling law of slow earthquakes. Plain Language Summary Slow earthquakes are characterized by very slow underground deformation compared with regular (fast) earthquakes and are important for understanding the preparation period prior to large earthquakes. Tectonic tremors, which are a type of slow earthquakes, radiate tiny seismic waves with frequencies of several Hz, occur episodically and densely in space and time, and may last for long durations of up to several hundred seconds, which is much longer than the durations of fast earthquakes of equivalent magnitude. In this study, we detect and differentiate tectonic tremors from fast earthquakes and anthropogenic events. We do this using a set of event features, without relying on subjective criteria. The durations of the detected tremors range from 0.3 to ∼100 s, and they appear consistent with a previously proposed scaling relationship for slow earthquakes. This result suggests that fast earthquakes and slow earthquakes have different physical mechanisms. Key Points We compile a more complete tectonic tremor catalog for western Japan using a clustering method based on event features Event duration, newly defined using energy radiation, clearly separates tectonic tremors from fast earthquakes Tectonic tremors, ranging in duration from 0.3 to 100 s, are consistent with the scaling law of slow earthquakes

Taiwan offers a distinctive tectonic setting as a collisional orogen, ideal for studying tectonic tremors and the slow deformation process in the mountain‐building process. Using continuous seismic data at many stations, which have become available recently, and employing the envelope correlation method, we detected ∼7,000 tremor events from 2012 to 2022, with waveform characteristics similar to tectonic tremors worldwide. Beyond the previously known tremor zone beneath the southern Central Range, where newly detected tremors align along a low‐angle thrust plane, we identified several new tremor “hotspots” spanning 200 km along the mountain belt. These hotspots are situated at the termination of the subducting slabs and around the deep (25–50 km) extension of the Central Range fault, where repeating earthquakes occur at a depth of 10–25 km. Our findings suggest a strong linkage between the tremor generation mechanism and the mountain‐building process, potentially influenced by underground fluid and temperature anomalies. Plain Language Summary Since around 2000, tectonic tremors have been discovered worldwide as geophysical phenomena strongly related to slow deformation and fluid movement inside the Earth. While a small cluster of tremors has been identified in Taiwan, where a rapid mountain‐building process occurs, the comprehensive distribution of tremors has remained unknown. Utilizing recently released seismic network records for all of Taiwan, we have clarified a broader pattern of tremor activity, previously unknown, in the region. The tremors are distributed planarly, corresponding to the mountain‐building processes and deep underground deformation in Taiwan. Key Points Using newly available continuous seismograms, we identified ∼7,000 tectonic tremors in Taiwan Several tremor hotspots spanning 200 km were newly identified along the mountain belt Tremor distribution suggests planer structures that are contributing to the rapid mountain‐building process

Recent studies of slow earthquakes along plate boundaries have shown that tectonic tremor, low-frequency earthquakes, very-low-frequency events (VLFEs), and slow-slip events (SSEs) often accompany each other and appear to share common source faults. However, the source processes of slow events occurring in the shallow part of plate boundaries are not well known because seismic observations have been limited to land-based stations, which offer poor resolution beneath offshore plate boundaries. Here we use data obtained from seafloor observation networks in the Nankai trough, southwest of Japan, to investigate shallow VLFEs in detail. Coincident with the VLFE activity, signals indicative of shallow SSEs were detected by geodetic observations at seafloor borehole observatories in the same region. We find that the shallow VLFEs and SSEs share common source regions and almost identical time histories of moment release. We conclude that these slow events arise from the same fault slip and that VLFEs represent relatively high-frequency fluctuations of slip during SSEs. Slow earthquakes are now increasingly recognised to occur at plate boundaries globally. Here, the authors examine seafloor observational data from the Nankai trough and find that very-low-frequency events and slow-slip events frequently occur together and share the same common source fault.

Some of the most devastating earthquakes are generated in subduction zones. Analysis of the stress state of subduction zones worldwide suggests that large earthquakes are generated more frequently where a young, buoyant plate subducts. The occurrence of subduction zone earthquakes is primarily controlled by the state of stress on the interface between the subducting and overriding plates. This stress state is influenced by tectonic properties, such as the age of the subducting plate and the rate of plate motion 1 , 2 , 3 , 4 . It is difficult to directly measure stress on a plate interface. However, the stress state can be inferred using the Gutenberg–Richter relationship’s 5 b -value, which characterizes the relative number of small compared to large earthquakes and correlates negatively with differential stress 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 . That is, a subduction zone characterized by relatively frequent large earthquakes has a low b -value and a high stress state. The b -value for subduction zones worldwide varies significantly 14 , 15 , but the source of this variance is unclear. Here we use the Advanced National Seismic System earthquake catalogue to estimate b -values for 88 sections in different subduction zones globally and compare the b -values with the age of the subducting plate and plate motions. The b -value correlates positively with subducting plate age, so that large earthquakes occur more frequently in subduction zones with younger slabs, but there is no correlation between b -value and plate motion. Given that younger slabs are warmer and more buoyant, we suggest that slab buoyancy is the primary control on the stress state and earthquake size distribution in subduction zones.