Сейсмическая активизация в районе северо-восточного побережья острова Хонсю (ноябрь–декабрь 2025 г.) и ее возможная связь с геодеформационными процессами на о. Шикотан (Малая Курильская гряда)

Summary Two ocean bottom seismometer (OBS) arrays were deployed to study microearthquake activity in the southernmost Kuril Trench area; at a junction of the Kuril and Japan Trenches. The first OBS observation was carried out from 1982 June to 1982 July and the second was from 1983 July to 1983 August. Each array consisted of 10 OBSs covering adjacent areas with an overlap. During the observation periods each OBS recorded about 50 earthquakes a day in average, a tenth of which were located in and near the arrays. A detailed picture of seismic activity associated with subduction and/or bending of the Pacific plate was obtained at the very point where it started to subduct. Spatial distribution of microseismicity in the southern Kuril Trench area is, in general, similar to that found in the Japan Trench area off Sanriku. A prominent microseismicity was found both landward and seaward of the axis of the Kuril Trench. The seaward activity had a sharp seaward boundary; along the edge of the seaward trench wall, in the Pacific Basin beyond which no detectable event with magnitude of greater than 2 was observed. Although seismicity was high beneath a continental slope between the Kuril Trench and Hokkaido, there was a seismicity gap beneath the inner trench wall. The gap coincided with an area of between 3 and 5 km in sea depth. The depth of the events beneath the seaward trench wall ranged from 0 to 30 km and just beneath the trench axis it was slightly deeper (0–50 km). A detailed comparison of seismicity between the Kuril and Japan Trenches showed a variation in spatial distribution along the trench axis: microseismicity near the trench axis is not uniform. The activity beneath the continental slope formed a seismic zone dipping landward with a low dip angle of less than 10°, which seemed to be a shallower extension of the lower plane of the double structured seismic zone in the Kuril Trench area. It shows a contrast to the Japan Trench area where the seismicity corresponds to the upper plane.

Universal classification schemes have been proposed for the vertical zonation of sessile benthos on intertidal rocky shores; however, generalized patterns among deep-sea meiofauna of different regions remain rare. We conducted a preliminary, family-level study of deep-sea harpacticoid diversity and community composition off Tosa Bay, Shikoku island, Japan, along a transect on a landward continental slope of the Nankai Trough. We then compared these data to those of other subduction zones along the Japanese island arc, including the Kuril, Japan, and Ryukyu trenches. Common bathymetric trends in harpacticoid composition were observed among these subduction zones from upper bathyal to abyssal depths (400–5800 m). Ectinosomatidae was the most abundant family within this depth interval; however, its frequency decreased with increasing depth, whereas the abundance of Pseudotachidiidae was highest at abyssal depths. Harpacticoid composition differed significantly between landward slopes of the Ryukyu Trench and those of the Japan and Kuril trenches. However, harpacticoid composition on the landward slope of the Nankai Trough, between the Ryukyu and Japan trenches, was similar to that around the neighboring subduction zones. These findings imply that patterns of deep-sea harpacticoid community composition are similar along the continental slope of the Japanese island arc, and that the Izu–Ogasawara arc, which subdivides the subduction zones, has not functioned as a physical barrier to harpacticoid expansion between zones.

I relocated the hypocenters of the 2018 M6.7 Hokkaido Eastern Iburi earthquake and its surrounding area, using a three-dimensional seismic structure, the double-difference relocation method, and the JMA earthquake catalog. After relocation, the focal depth of the mainshock became 35.4 km. As previous studies show, in south-central Hokkaido, the Hidaka collision zone is formed, and anomalous deep and thickened forearc crust material is subducting at depths of less than 70 km. The mainshock and its aftershocks are located at depths of approximately 10 to 40 km within the lower crust of the anomalous deep and thickened curst near the uppermost mantle material intrusions in the northwestern edge of this Hidaka collision zone. Like the two previous large events, the aftershocks of this event incline steeply eastward and appear to be distributed in the deeper extension of the Ishikari-teichi-toen fault zone. The highly inclined fault in the present study is consistent with a fault model by a geodetic analysis with InSAR. The aftershocks at depths of 10 to 20 km are located at the western edge of the high-attenuation (low-Qp) zone. These kinds of relationships between hypocenters and materials are the same as the 1970 and 1982 events in the Hidaka collision zone. The anomalous large focal depths of these large events compared with the average depth limit of inland earthquakes in Japan could be caused by the locally lower temperature in south-central Hokkaido. This event is one of the approximately M7 large inland earthquakes that occurred repeatedly at a recurrence interval of approximately 40 years and is important in the collision process in the Hidaka collision zone.

Spatio-temporal Changes in Stress Field and Occurrence of the 2003 Tokachi Oki Earthquake in Hokkaido, Northern Japan

The split Hopkinson bar was used to determine the stress, strain and strain rate response of woven graphite-epoxy laminates during high shear strain rate deformation. The specimens were oriented and tested in both interlaminar shear and in transverse shear at strain rates ranging from 6000 to 18000 sec-1. The results show that the strain rate effect in interlaminar shear is small. The fracture surface is characterized primarily by interfacial failure between the fibers and the matrix. In transverse shear, however, the strain rate effect is significant due to the greater role played by the resin matrix in the deformation process. In both cases, at the high strain rates imposed in this investigation, sparks were produced by the specimens during deformation. These are believed to be caused by the combustion of fragments resulting from the high rate deformation.

Large intermediate-depth earthquakes and the subduction process

We analyze permanent Global Positioning System (GPS) data obtained over Japan between 1995 and 1997 to estimate the instantaneous interseismic coupling ratio of the seismogenic zones due to the subduction of the Pacific and Philippine Sea plates below the Japanese islands. We first derive the GPS strain rate fields that characterize the crustal deformation of southern and northern Japan and invert them to determine the effective subduction velocity along the central Nankai trough on one side and the Japan‐west Kurile trench on the other. These “reference free” velocities are close to those predicted by plate motion models with respect to Eurasia. We conclude that the Eurasian reference frame gives a good approximation to the subduction motion and that to first order, both subduction zones were fully locked during the period of measurements. We then test whether the coupling ratio shows local variations within the seismogenic zones. To do this, we divide the subduction interface into 35 km×30 km elements that we model by point source groups, and we invert the GPS velocity field referenced to Eurasia to derive the coupling ratio (between 0 and 1) on each fault element. The results are coherent over the 3 years and confirm that both the central Nankai and the Japan‐west Kurile seismogenic zones are homogeneously fully locked. Most of the coupling ratios are close to 1 and a few are close to 0; intermediate values are rare. The zones of decoupling correspond either to strong postseismic afterslip associated with the 1994 Sanriku‐Oki interplate earthquake (Japan trench) or to a small overestimation of the actual lower limit of the locked zone. We conclude that within the resolution of the GPS data and the model, (1) partial coupling did not exist during these 3 years along the Nankai and Japan‐west Kurile trenches; (2) the small seismic coupling ratio previously derived from earthquakes analysis for the Japan and Kurile trenches may indicate that a significant part of the elastic energy is dissipated silently through slow earthquakes and postseismic afterslip; and (3) the heterogeneous coseismic slip pattern observed for the large and great earthquakes that rupture both subduction zones is in great contrast to the homogeneous loading. Finally, we discuss the nonelastic residual deformation within the frame of the long‐term deformation of the Japanese islands.

Petrogenesis and chemogenesis of oceanic and continental orogens in <scp>A</scp>sia: Current topics, <scp>Part I</scp>

Historical seismicity data since the year 1850in Tohoku and Hokkaido, northern Japan, show a spatial and temporal correlation between intraplate seismicity in the land area and occurrence of large interplate earthquakes along the Japan and Kuril trenches. In the continental lithosphere adjacent landward to the rupture zones of recent large interplate earthquakes, almost all the earthquakes of magnitude 6.0 and above occurred during the periods from the 50 years before to the 10 years after the occurrence of the large interplate events. A simple statistical test shows that this correlation is significant at a 99.5% confidence level. During the period for the last 20 years, a group of intraplate earthquakes occurred in middle Tohoku with no large interplate earthquake seaward of their epicentral locations; this suggests a possibility of a large interplate event off the southern Sanriku coast in the near future. We examine other geophysical data to rate the possibility. All the examined data, i.e., historic and recent seismicity off the Sanriku, and crustal deformation revealed by the geodetic work, support the possibility of occurrence of a large event off the southern Sanriku in the near future. Source parameters are estimated for this expected event; they are conformable to those of recent large interplate events along the Japan and Kuril trenches. We can use the hypothesis on the correlation between intraplate seismicity and large interplate earthquakes as a criterion for forecasting the land area of high seismic risk for intraplate events as well as a criterion for forecasting a rupture zone of an impending large interplate earthquake. This may be of significance for instrumentation to detect various kind of phenomena associated with imminent intraplate and/or interplate earthquakes.

Tectonic (or non-volcanic) tremors have been extensively documented at subduction zones and are considered as the signature of transport processes of dehydration-related fluids in subduction zones, often recorded in close association with geodetically observed shear induced slow-slip events. However, to the best of our knowledge, they have not yet been reproduced in the laboratory at subduction zones P&amp;#8211;T conditions in such way that their first-order controlling mechanisms remain enigmatic. &amp;#160;This work investigates the mechanism of these seismic events by performing dehydration-deformation experiments combined with detailed investigations of mineral reactions and acoustic emissions. Experiments were carried out on chlorite-peridotite powders (Balmuccia peridotite with synthetically added chlorite, a mineral that is typically found in subduction zone lithologies), following a subduction zone geothermal gradient using a high-pressure apparatus (Griggs-type). The experiments were conducted from ambient conditions to maximum pressures of 1.5-3.0 GPa and temperatures of 750-800 &amp;#176;C. Experiments were executed under hydrostatic conditions and an additional one with deformation. An ultrasonic transducer (0.5-10MHz dynamic range) was employed to monitor and detect the micro-seismic events. High-resolution electron beam techniques (EMPA, SEM and TEM) have been applied for analyzing the sample material.Dehydration of ~15 vol.% of the initial chlorite suffices to trigger acoustic emissions, which display waveforms reminiscent of those of tectonic tremors. The moment distribution statistics of these laboratory tremor-like signals follows the Gutenberg-Richter relationship and a scaling between moment vs. event duration. Finally, we observe a match between the ratios of size and typical frequency of natural over laboratory tremors. Microstructural observations document metamorphic olivine and pyroxene growth in the decomposing chlorite and demonstrate that an almost isochemical dehydration of the chlorite took place. Accordingly, the appearance of the tremor-like acoustic emissions after crossing a temperature of 600 &amp;#176;C can be linked to a dehydration process related to the chlorite breakdown in the sample. Thermodynamic calculations show that a small amount of released fluids (breakdown of ~1.5 vol.% of a hydrous phase) is enough to trigger seismic signals analogues to tremors. The experiment with additional deformation produced no tremor-like acoustic emission suggesting that the large macroscopic shear stress suppressed the development of the processes that lead to acoustic emissions. We conclude that fluid release during dehydration is the cause of tectonic tremors, whereas shear-stress seems to counteract their development with no occurrence of tremors at high rates of deformation. According to the results from this study, the triggering mechanism can be tentatively interpreted as a fluid propagation front resulting in the vibration of grain boundaries.

I. Source Analysis of Large Earthquakes in Mexico. II. Study of Intermediate-Depth Earthquakes and Interplate Seismic Coupling

Abstract. Maximum shear stress along an active deformation zone marking the subduction plate interface is important for understanding earthquake phenomena and is an important input parameter in subduction zone thermomechanical modeling. However, such maximum shear stress is difficult to measure directly at depths more than a few kilometers and is generally estimated by simulation using a range of input parameters with large associated uncertainties. In addition, estimated values generally represent maximum shear stress conditions over short observation timescales, which may not be directly applicable to long-timescale subduction zone modeling. Rocks originally located deep in subduction zones can record information about deformation processes, including maximum shear stress conditions, occurring in regions that cannot be directly accessed. The estimated maximum shear stress is likely to be representative of maximum shear stress experienced over geological timescales and be suitable to use in subduction zone modeling over timescales of millions to tens of millions of years. In this study, we estimated maximum shear stress along a subduction plate interface by using samples from the Sanbagawa metamorphic belt of southwestern (SW) Japan, in which slivers of mantle-wedge-derived serpentinite are widely distributed and in direct contact with metasedimentary rocks derived from the subducted oceanic plate. These areas can be related to the zone of active deformation along the subduction plate interface. To obtain estimates of maximum shear stress at the subduction interface, we focused on the microstructure of quartz-rich metamorphic rocks – quartz is the main component of the rocks we collected and its deformation stress is assumed to be roughly representative of the stress experienced by the surrounding rock and plate interface deformation zone. Maximum shear stress was calculated by applying deformation temperatures estimated by the crystallographic orientation of quartz (the quartz c-axis fabric opening-angle thermometer) and the apparent grain size of dynamically recrystallized quartz in a thin section to an appropriate piezometer. Combined with information on sample deformation depth, estimated from the P–T (pressure–temperature) path and deformation temperatures, it is suggested that there was nearly constant maximum shear stress of 15–41 MPa in the depth range of about 15–30 km, assuming plane stress conditions even when uncertainties related to the measurement direction of thin section and piezometer differences are included. The Sanbagawa belt formed in a warm subduction zone. Deep slow earthquakes are commonly observed in modern-day warm subduction zones such as SW Japan, which has a similar thermal structure to the Sanbagawa belt. In addition, deep slow earthquakes are commonly observed to be concentrated in a domain under the shallow part of the mantle wedge. Samples showed the depth conditions near the mantle wedge, suggesting that these samples were formed in a region with features similar to the deep slow earthquake domain. Estimated maximum shear stress may not only be useful for long-timescale subduction zone modeling but also represent the initial conditions from which slow earthquakes in the same domain nucleated.

Seismic studies are helping scientists learn more about the structure of subducting oceanic plates. Using an air gun array and 80 ocean bottom seismometers spaced along a 500‐kilometer profile, Fujie et al. conducted a seismic reflection and refraction survey at the Kuril trench in the northwestern Pacific margin, where part of the Pacific plate is subducting beneath the Okhotsk plate. They estimated the water content of the subducting plate by measuring the velocity of seismic waves—both P waves and S waves—through the plate. The ratio of seismic wave velocities (Vp/Vs) is an indicator of the lithology, porosity, and presence of fluid in the plate. Their findings showed that the water content in the plate increased toward the trench, along with greater bending and fracturing, suggesting that water enters the plate through the fractures. The authors conclude that the bending and fracturing of the plate as it subducts play an important role in the water cycle in subduction zones. (Geophysical Research Letters, doi:10.1029/2012GL054340, 2013)

Crustal structure and tectonics of the Hidaka Collision Zone, Hokkaido (Japan), revealed by vibroseis seismic reflection and gravity surveys

Upper and middle crustal deformation of an arc–arc collision across Hokkaido, Japan, inferred from seismic refraction/wide-angle reflection experiments