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An Mw 6.2 earthquake occurred in Suzu, northeastern Noto Peninsula, Japan, on 5 May 2023, followed by many aftershocks. Before this mainshock‐aftershock sequence, an intense earthquake swarm lasted in the vicinity for 2.5 years. Here, we estimated the rupture process of the Mw 6.2 mainshock and relocated >20,000 surrounding small earthquakes. The results show that systematic upward migration occurred via a complex network of faults in the preceding swarm period and that the mainshock rupture was initiated near the shallow end of the swarm earthquakes. The mainshock rupture propagated farther updip, followed by many aftershocks in the shallow extension. Upward fluid movement likely caused systematic upward earthquake migration from a depth of 18–5 km. The present results indicate the importance of monitoring swarm events since large (M > 6) and dangerous earthquakes can occur during such swarms. Plain Language Summary: Growing evidence suggests that fluid movements at depth can cause earthquake swarms in the overriding plates of subduction zones. However, there is less evidence of fluid involvement in the occurrence of large (M > 6) earthquakes, and it has been reported that the generation environments of large earthquakes and swarm earthquakes tend to be different. During intense swarm activity in the northeastern Noto Peninsula, Japan, starting at the end of 2020, an M6.2 earthquake occurred on 5 May 2023, followed by many aftershocks. To clarify the occurrence mechanism of the M6.2 mainshock, we relocated >20,000 small earthquakes and estimated the rupture process. The results showed a systematic upward migration of earthquakes in the preceding swarm period on the deep side of the mainshock fault (depth from 20 to 12 km). The mainshock rupture initiated near the shallow end of the swarm activity and propagated updip (depth from 12 to 9 km). Many aftershocks followed in shallower part of the same fault (depth from 10 to 5 km). Upward fluid migration likely caused the observed systematic upward earthquake migration throughout the sequence. The results indicate that earthquakes as large as M > 6 can be triggered by fluid migration during enhanced seismicity in swarms. Key Points: An Mw 6.2 event initiated near the shallow end of a fault where the preceding earthquake swarm showed systematic upward migrationThe Mw 6.2 rupture propagated farther updip, followed by many shallow aftershocks on the faultUpward fluid migration can trigger earthquakes as large as Mw > 6 [ABSTRACT FROM AUTHOR]

Characterizing shallow interplate slip is essential for modeling the potential generation of megathrust earthquakes and tsunamis. Large postseismic crustal deformation caused by the 2011 Tohoku‐Oki earthquake suggests the occurrence of shallow afterslip, but it is unclear what kind of slip is dominant at the shallow plate interface. Here we report episodic tremor activity south of the primary rupture area based on ocean bottom seismometer observations. Five tremor episodes with recurrence intervals of ~60 days show migration behavior and shear mechanisms consistent with plate subduction, indicating the occurrences of the episodic slow slip events in the vicinity of the afterslip area. The association of slow slip events and afterslip sheds new light on the frictional nature and stress state of shallow megathrusts and may help predict the possible rupture extent of future earthquakes. Plain Language Summary: Understanding the slip characteristics in the shallow part of subduction zone is essential to evaluate the potential risk of megathrust earthquakes and tsunamis. It remains unclear, however, what type of slip occurs at the shallow plate interface due to the difficulty in observing offshore seismic activity. In this paper, we report newly discovered tremor and slow slip activity in the afterslip region where coseimic rupture did not happen during the 2011 Tohoku‐Oki earthquake, based on a new campaign ocean bottom seismometer observation. We propose that the tremor and slow slip region illuminate the boundary between fast coseismic slip and stable aseismic slip that could limit the rupture extent of future megathrust earthquakes. The observed tremor and slow slip activities also suggest that the interplate stress state has been recovered to the level of that before the Tohoku earthquake. Key Points: We discover tectonic tremor activity in the Japan trench subduction zone based on a new campaign ocean bottom seismometer observationThe characteristics of tremor strongly suggest slow slip events in the vicinity of the afterslip area of the 2011 Tohoku earthquakeTremor and slow slip region may delineate the boundary between seismic and aseismic region along the plate interface [ABSTRACT FROM AUTHOR]

We estimated the coseismic slip distribution associated with the Mw 7.2 and 6.5 foreshocks of the 2011 Tohoku-Oki earthquake based on analysis of the tsunami waveform records obtained just above their focal areas. The results show that the main rupture areas of each of the foreshocks do not overlap with each other, and show a distribution that is complementary to the postseismic slip area of the first Mw 7.2 foreshock as well as to the epicenters of smaller earthquakes during foreshock activity. After the second largest foreshock, seismicity increased in the area between the rupture area of the second largest foreshock and the mainshock epicenter, suggesting propagation of aseismic slip towards the mainshock epicenter. The calculated stress drop of the second largest foreshock was smaller than the largest one, implying strength reduction during the postseismic period of the largest foreshock. Based on a comparison of coastal tsunami records, it is suggested that the asperity ruptured in the M 7.0 earthquake in 1981 ruptured again during the largest foreshock in 2011, but it expanded to the updip side of the 1981 rupture area and became larger in magnitude, exemplifying the irregularity of earthquake recurrence in the area. [ABSTRACT FROM AUTHOR]

Abstract: We describe two transient slow slip events that occurred before the 2011 Tohoku-Oki earthquake. The first transient crustal deformation, which occurred over a period of a week in November 2008, was recorded simultaneously using ocean-bottom pressure gauges and an on-shore volumetric strainmeter; this deformation has been interpreted as being an M6.8 episodic slow slip event. The second had a duration exceeding 1month and was observed in February 2011, just before the 2011 Tohoku-Oki earthquake; the moment magnitude of this event reached 7.0. The two events preceded interplate earthquakes of magnitudes M6.1 (December 2008) and M7.3 (March 9, 2011), respectively; the latter is the largest foreshock of the 2011 Tohoku-Oki earthquake. Our findings indicate that these slow slip events induced increases in shear stress, which in turn triggered the interplate earthquakes. The slow slip event source area on the fault is also located within the downdip portion of the huge-coseismic-slip area of the 2011 earthquake. This demonstrates episodic slow slip and seismic behavior occurring on the same portions of the megathrust fault, suggesting that the faults undergo slip in slow slip events can also rupture seismically. [Copyright &y& Elsevier]

To study the physical properties along the subducting plate boundary at the Japan Trench, a seismic study was carried out using ocean bottom seismometers (OBSs) and artificial sources was carried out. The 250 km long survey line with an almost N–S strike crosses the two major focal zones of the 1968 Tokachi-Oki earthquake and the 1994 Sanriku-Haruka-Oki earthquake. Ray tracing, and non-linear inversion were used to analyze the data obtained by the OBSs. P-wave velocity structure down to the island-arc-Moho was obtained.The result shows distinct heterogeneities along the plate boundary at 40°10′N, which are located exactly at the southern boundary of major moment release regions of the above two earthquakes. P-wave velocities for the crust north of 40°10′N are approximately 7% slower than those for the south. The depths of the island-arc-Moho vary from 15 km in the south to 21 km in the north below the sea surface. Three explanations of the velocity heterogeneities are presented in terms of the migration of water. [Copyright &y& Elsevier]

An unprecedentedly extensive seismic refraction and wide-angle reflection survey using 65 ocean bottom seismographs revealed detailed crustal structure around the eastern Nankai Trough. A previously published crustal model shows an abrupt offset of the Moho at the south of the Zenisu Ridge, a prominent topographic high along the oceanward slope of the Nankai Trough. Our crustal model indicates that this offset of the Moho extends southwestward continuously to 138°E, decreasing its gap. The survey area experienced the last two great earthquakes in 1854 and 1944. However, the northeastern part of the survey area seems to have remained unruptured since the 1854 event. Factors controlling the size of the rupture area for great earthquakes are still a matter of debate. There are several candidates for these factors in the survey area: hypothetical tectonic boundaries that may or may not be oceanward prolongation of major on-land tectonic lines, estimated locations of slab disruption, and the extent of Moho offset along the strike of the Zenisu Ridge. The main purpose of this survey is to clarify the relation between the crustal structure and these geophysical and geological features bounding the rupture area. Our crustal model from the trough axis to the continental slope is characterized by a well-developed sedimentary wedge bounded by island arc crustal blocks, consisting of upper and lower crust, to the northwest. Furthermore, the subducting oceanic crust, which can be traced down to 25 km depth, shows that the down-dip angle steepens at 55 km landward from the trough axis. On the basis of compilation of our crustal model with previously published models around the eastern Nankai Trough, we derived an image of the entire subducting plate geometry for depths shallower than 20 km, which is still poorly constrained by the land observation of microearthquakes. Significant lateral variations of the crustal structure and the slab geometry are recognized along one prominent canyon, and the offset of the Moho at the south of the Zenisu Ridge disappears to the southwest of the canyon. Moreover, it seems that the slab disruption recognized at a depth greater than 20 km is connected to this canyon. Therefore, the lateral variation of the crustal structure along the canyon may be one of the causes to stop rupture propagation of great earthquakes. Furthermore, the crustal variation may also form a tectonic boundary that distinguishes the subduction pattern of the Philippine Sea plate, including the influence of the Izu–Ogasawara collision, in the eastern Nankai Trough from the simple subduction pattern of the western Nankai Trough. [Copyright &y& Elsevier]

Abstract: The March 2011 9.0 Tohoku-Oki earthquake activated shallow aftershocks with normal faulting focal mechanisms near the trench axis. To investigate stress state in the shallow subduction zone near the trench axis, we used ocean bottom seismographs (OBSs) to record earthquakes from August to October 2011. We estimated hypocenter locations and focal mechanisms of the recorded earthquakes by using a grid-search method in a 2-D velocity model based on an active seismic survey. The results show that most of the earthquakes occurred within both the overriding and subducting plates, with very few on the plate interface and none in the most seaward 45 km of the overriding plate (aseismic wedge). The low seismicity along the plate interface is consistent with the nearly complete stress drop on the megathrust fault in the 9.0 earthquake. Focal mechanisms indicate that trench-normal tension is pre-dominant in both plates and extends to a depth of about 30 km at least. On the other hand, several trench-normal compressional earthquakes were located at the landward end of the aseismic wedge. These earthquakes suggest horizontal compression of the overriding prism that may be caused by a strengthening of the shallowest part of the megathrust. [Copyright &y& Elsevier]

Abstract: We obtained ambient seismic noise interferograms as seismic reflection images using autocorrelation functions (ACFs) in the northeastern Japan subduction zone. The ACFs with a time window length of 120s were calculated from the continuous seismic records obtained at each seismic station over an analysis period of 300days. These ACFs show some distinct signals with relatively large amplitude without any significant temporal variations during the analysis period. The signals, which are stable, appear at both small lag times of less than 10s and large lag times of 20–50s during the analysis period. The lag time of 10s corresponds to the travel time of the PP reflection arrival from the continental Mohorovičić discontinuity. The signals with the large lag times between 30 and 50s correspond to the back-scattered signals from the mantle wedge or the plate boundary; these signals are identified clearly at the stations located on the back-arc side. In the ACFs calculated from the records obtained from the fore-arc side stations, weak signals (interpreted as the reflection from the plate boundary) with a lag time range of 20 to 30s are observed. We constructed depth-migrated images using the ACFs to obtain the reflectivity profile by assuming that the ACFs represent Green''s functions composed of a random wavefield excited by stochastic sources or scatterers distributed in the vertical or near-vertical direction from the stations. Further, we assumed that the ACFs can be treated as zero-offset seismic traces recorded at each of the stations. The depth-migrated images show a relatively seismically transparent structure within the subducting Pacific slab and a reflective structure within the mantle wedge; this reflective structure is characterized by low-velocity zones corresponding to the wedge flow imaged by 3-D seismic velocity tomography. [Copyright &y& Elsevier]

Abstract: The Woodlark Basin, located south of the Solomon Islands arc region, is a young (∼5 Ma) oceanic basin that subducts beneath the New Britain Trench. This region is one of only a few subduction zones in the world where it is possible to study a young plate subduction of several Ma. To obtain the image of the subducting slab at the western side of the Woodlark Basin, a 40-day Ocean Bottom Seismometer (OBS) survey was conducted in 1998 to detect the micro-seismic activity. It was the first time such a survey had been performed in this location and over 600 hypocenters were located. The seismic activity is concentrated at the 10–60 km depth range along the plate boundary. The upper limit just about coincides with the leading edge of the accretionary wedge. The upper limit boundary was identified as the up-dip limit of the seismogenic zone, whereas the down-dip limit of the seismogenic zone was difficult to define. The dip angle of the plate at the high seismicity zone was found to average about 30°. Using the Cascadia subduction zone for comparison, which is a typical example of a young plate subduction, suggests that the subduction of the Woodlark Basin was differentiated by a high dip angle and rather landward location of the seismic front from the trench axis (30 km landward from the trench axis). Furthermore, as pointed out by previous researchers, the convergent margin of the Solomon Islands region is imposed with a high stress state, probably due to the collision of the Ontong Java Plateau and a rather rapid convergence rate (∼10 cm/year). The results of the high angle plate subduction and inner crust earthquakes beneath the Shortland Basin strongly support the high stress state. The collision of the Ontong Java Plateau, the relatively rapid convergence rate, and moderately cold slab as evidenced by low heat flow, rather than the plate age, may be dominantly responsible for the geometry of the seismogenic zone in the western part of the Woodlark Basin subduction zone. [Copyright &y& Elsevier]