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Inversion tectonics, in which old normal faults act as reverse faults in current stress fields, are frequently observed in northeastern Japan (Tohoku District); however, the conditions that control these fault activities remain unclear. To improve the identification of faults that are more favorable to slip under current stress conditions, the regional fault mechanisms in the Tohoku District must be better understood. The stress field in the Tohoku District and the likelihood of fault activities were thus estimated using slip tendency (ST) analysis. The results show that in the eastern margin of the Japan Sea (EMJS), the reverse fault type is dominant in the stress field. The maximum horizontal direction changes clockwise from E–W to NW–SE from the northern to the southern regions and counterclockwise from NW–SE to E–W from the Japan Sea to the inland area. In the Tohoku inland area, the estimated direction of the maximum horizontal axis changed after the 2011 Tohoku-Oki earthquake, from E–W to WNW–ESE. ST values were calculated for seven events in the EMJS area. To avoid the influence of the Tohoku-Oki earthquake, only stress field data prior to the 2011 Tohoku-Oki earthquake were used to determine ST values for four of the events in the Tohoku inland area. The results showed eastward-dipping fault planes with low dip angles (approximately 30°–45°) and large ST values (approximately > 0.7). The large ST values indicate that the stress field fault is favorable to slip and the results were consistent with the actual fault plane in the EMJS area. However, in the Tohoku inland area and the southern part of fault model of the 1993 Hokkaido Nansei-Oki earthquake, fault planes with large ST values were found to be inconsistent with the slipped fault plane, thus indicating that slipping was unfavorable. The regional differences are consistent with the volcano distribution and thus, the fluid supply from volcanic activity may have helped the fault slip under difficult stress conditions. [ABSTRACT FROM AUTHOR]

The Aira caldera, located in southern Kyushu, Japan, originally formed 100 ka, and its current shape reflects the more recent 30 ka caldera-forming eruptions (hereafter, called the AT eruptions). This study aimed to delineate the detailed two-dimensional (2D) seismic velocity structure of the Aira caldera down to approximately 15 km, by means of the travel-time tomography analysis of the seismic profile across the caldera acquired in 2017 and 2018. A substantial structural difference in thickness in the subsurface low-velocity areas in the Aira caldera between the eastern and western sides, suggest that the Aira caldera comprises at least two calderas, identified as the AT and Wakamiko calderas. The most interesting feature of the caldera structure is the existence of a substantial high-velocity zone (HVZ) with a velocity of more than 6.8 km/s at depths of about 6–11 km beneath the central area of the AT caldera. Because no high ratio of P- to S-wave velocity zones in the depth range were detected from the previous three-dimensional velocity model beneath the AT caldera region, we infer that the HVZ is not an active magma reservoir but comprises a solidified and cool remnant. In addition, a poorly resolved low-velocity zone around 15 km in depth suggests the existence of a deep active magma reservoir. By superimposing the distribution of the known pressure sources derived from the observed ground inflation and the volcanic earthquake distribution onto the 2D velocity model, the magma transportation path in the crust was imaged. This image suggested that the HVZ plays an important role in magma transportation in the upper crust. Moreover, we estimated that the AT magma reservoir in the 30 ka Aira caldera-forming eruptions has the total volume of 490 km3 DRE and is distributed in a depth range of 4–11 km. [ABSTRACT FROM AUTHOR]

How are the sizes of the earthquakes determined? To solve this important problem, we analysed the data from a dense temporary seismic observation network installed in the aftershock area of the 2016 Mw 6.2 Central Tottori earthquake, which occurred in an intraplate region in Japan. We compared the stress field estimated from approximately 10   000 accurate focal mechanisms of aftershocks with the calculated post-earthquake stress field and found that the differential stress before the earthquake was very small near both horizontal edges. These results did not depend significantly on the modeled slip distribution and the orientation of the principal stress before the earthquake. Similar results were obtained for the 2000 Mw 6.7 Western Tottori earthquake, which also occurred in the same intraplate region in Japan. These results suggest that the fault size of large intraplate earthquakes can be determined by the region of small differential stress surrounding future earthquake faults. [ABSTRACT FROM AUTHOR]

Using data from high-density seismic observation networks installed in the western Nagano Prefecture region in Japan, we precisely determined focal mechanisms and estimated the high-resolution stress field at a scale of 1 km. Almost all differences between observed and calculated slip directions (misfit) were smaller than the errors in focal mechanisms at grid points away from the mainshock fault. This finding clearly indicates that the estimated uniform stress suitably explains focal mechanisms in each subregion apart from the mainshock fault. Misfits are relatively large at grid points near the mainshock fault, but many of these misfits are smaller than the errors in focal mechanisms, and stress is regarded as uniform for a greater portion within each subregion. However, we found that focal mechanisms and P-axes varied widely and differed from each other for a short focal distance of 100 m. These results clearly show that stress can be regarded as uniform, but that strength is heterogeneous. [ABSTRACT FROM AUTHOR]

The Hokkaido Eastern Iburi earthquake (MJMA = 6.7) occurred on September 6, 2018, in the Hokkaido corner region where the Kurile and northeastern Japan island arcs meet. We relocated aftershocks of this intraplate earthquake immediately after the main shock by using data from a permanent local seismic network and found that aftershock depths were concentrated from 20 to 40 km, which is extraordinarily deep compared with other shallow intraplate earthquakes in the inland area of Honshu and Kyushu, Japan. Further, we found that the aftershock area consists of three segments. The first segment is located in the northern part of the aftershock area, the second segment lies in the southern part, and the third segment forms a stepover between the other two segments. The hypocenter of the main shock, from which the rupture initiated, is located on the stepover segment. The centroid moment tensor solution for the main shock indicates a reverse faulting, whereas the focal mechanism solution determined by using the first-motion polarity of the P wave indicates strike-slip faulting. To explain this discrepancy qualitatively, we present a model in which the rupture started as a small strike-slip fault in the stepover segment of the aftershock area, followed by two large reverse faulting ruptures in the northern and southern segments. [ABSTRACT FROM AUTHOR]

We conducted high-resolution reflection analysis of data from 168 seismic stations with an average spacing of about 5 km, in northern Kinki district, southwestern Japan. Reflection analysis has previously been conducted in this region, assuming a homogeneous horizontal structure, resulting in an inclined planar zone of high relative reflection strengths (S-wave reflector). However, if the reflector is actually inclined, the location of the S-wave reflector differs from that of an assumed homogeneous horizontal structure. Hence, this study conducted high-resolution reflection analysis to determine the accurate location of the S-wave reflector. We confirm the previously reported S-wave reflector (reflector W). Furthermore, we detected the accurate location of the S-wave reflector and obtained more detailed results that revealed a second S-wave reflection structure (reflector E) to the east of reflector W, in an area that has not be imaged by previous studies. The northern edges of reflector E and reflector W are located near different hypocentral areas of deep low-frequency earthquakes (DLFs). Reflector W exists along the Kyoto Nishiyama fault zone, and its position appears to change along the fault zone as it deepens. Similarly, reflector E exists along the Hanaore and Biwako Seigan fault zones and its position appears to change along these fault zones. The reflector W and reflector E are imaged as separate S-wave reflectors in deeper regions, but they coalesce in shallower regions. According to previous studies, crustal fluid by dehydration from the Philippine Sea plate exists near these epicenters and we infer that this crustal fluid causes DLFs and forms S-wave reflectors. [ABSTRACT FROM AUTHOR]

Abstract: We examined the spatial relationship between seismicity and upper crustal structure in the Wakayama region, northwestern Kii Peninsula, Japan, by investigating microearthquake focal mechanisms and the local stress field. The focal mechanisms of most events studied fall into three categories: (1) normal faulting with N–S-oriented T-axes mainly occurring at shallow depths, (2) reverse faulting with E–W-oriented P-axes dominating at intermediate depths, and (3) strike-slip faulting with N–S-oriented T-axes and E–W-oriented P-axes mainly seen at greater depths. The stress field varies with depth: the shallow part is characterized by a strike-slip-type stress regime with N–S tension and E–W compression, while the deep part is characterized by an E–W compressional stress regime consistent with reverse faulting. The depth-dependent stress regime can be explained by thermal stress caused by a heat source, as expected from geophysical observations. Geologic faults, acting as weak planes, might contribute to generate shallow normal fault-type and deeper strike-slip fault-type microearthquakes.Graphical Abstract: [ABSTRACT FROM AUTHOR]

We estimated 275 focal mechanisms from P-wave first-motion polarities of small earthquakes obtained in an extensive seismic survey during 2004–2008 in and around the Atotsugawa fault, central Japan, where ongoing dextral shear strain concentration has been observed. Along the fault trace, the azimuth direction of P-axes is oriented WNW–ESE, which agrees well with previous studies. The regional stress disturbance is detected by stress inversion analysis. The azimuth of the maximum principal stress axis systematically rotates counterclockwise as the distance from the fault trace decreases. The regional stress disturbance is explained by a cumulative slip deficit in the shallower portion of the Atotsugawa fault relative to the surrounding fault surface (i.e., the eastern, western, and deeper extensions of the fault plane). [ABSTRACT FROM AUTHOR]

In this paper, using data from dense seismic observations in and around the seismic belt of the San'in district, Japan, we describe our analysis of focal mechanisms of stationary seismic activity, estimated stress states, and pore fluid pressures. We found these focal mechanisms to be described by the estimated stress field, suggesting that the stress field can be treated as uniform in each sub-region of the study area. We also found that events occurred even on unfavorably oriented fault planes with small shear stress. Further, we inferred that pore fluid pressures of approximately 20% of the faults analyzed are greater than the magnitude of minimum principal stress, when we assume μ = 0.6. A possible explanation is localized high pore fluid pressure anomalies; another is the coefficient of friction of at least a part of faults analyzed in the study area being substantially smaller than 0.6, which we view as more plausible, since it is difficult to maintain pore fluid pressures higher than the magnitude of minimum principal stress for long periods of time. [ABSTRACT FROM AUTHOR]

Abstract: A seismic experiment with five explosive sources and 396 seismic stations was conducted in August 2005 in the Atotsugawa fault area, central Japan. The 396 seismic stations were located on a survey line with a length of 47 km. The profile line was located along the Atotsugawa fault. The average spacing of the seismic stations was about 119 m. The spatially high-density linear-array was used to reveal the seismic structure of the fault zone. The fine structure along the Atotsugawa fault was obtained at depths shallower than 6 km. A large lateral variation of P-wave velocity, which was consistent with the geological structure, was found at the shallowest layer. The heterogeneous variation with a velocity range of 5.7 km/s–6.2 km/s was obtained at the second layer. The second layer was divided into three parts. The western part of the research area was determined to be high velocity. The low-velocity area was detected at the central part. At the east end of the profile line, the P-wave velocity was higher than that of the central part. The low-velocity area coincided with a low-seismicity area. The depth section of the reflection profile showed a boundary located at a depth of around 13 km. The boundary was roughly consistent with the cut-off depth of the seismicity of the microearthquakes. The high-seismic activity area was characterized to be reflective. [Copyright &y& Elsevier]

Abstract: The 2004 Mid-Niigata prefecture Earthquake (Mjma 6.8) occurred in the region of large strain rates (>0.1ppm/y contraction) in the intraplate region in Japan. The mainshock was followed by four major aftershocks with Mjma>=6.0. The hypocenters of the mainshock and two large aftershocks that occurred in the central part of the aftershock region were located near the lower limit of the earthquake distribution, while hypocenters of the other two aftershocks near both ends, are located near its upper limit. Furthermore, the fault planes of the latter two aftershocks were confined within the upper half of the upper crust. Also, the lower limit of the aftershock distribution is deepest in the central part and becomes shallower toward the NNE and SSW ends. These data can be explained by the hypothesis that a localized stress concentration occurred near the bottom of the seismogenic region only in the central part. The stress concentration may be generated by the deformation in the very weak region of low strength in the lower crust beneath the central part of the aftershock region. [Copyright &y& Elsevier]

The model called "water-weakened lower crust model" has been proposed to explain the mechanism of stress concentration on faults causing inland earthquakes (Iio et al. 2002). According to this model, the viscosity of the deeper part of faults in the lower crust decreases by water, and stress concentrates on faults in the upper crust due to the localized deformation in the deeper part. Although this model has been reported to be qualitatively reasonable, we have not obtained a result indicating that the model is quantitatively reasonable. This is because we do not resolve a heterogeneous structure due to fluid in the deeper part of faults in the lower crust. In order to solve this problem, we try to image the heterogeneous structure in the lower crust in detail.As a method for imaging the heterogeneous structure in the lower crust, there is an S-wave reflection analysis (Aoki et al. 2016, Katoh et al. 2018). If the heterogeneous structure is related to fluid, seismic waves can be reflected there. Therefore, the heterogeneous structure due to fluid can be determined as a reflector. In this analysis, the resolution is extremely high, 1.5 km in the horizontal direction and 1 km in the vertical direction. Katoh et al. (2018) indicated that the reflector reflects that fluid exists in fractures generated by faulting from the positional relationship between the reflector, DLFs, and a fault zone, and they proposed the reflector was a fluid pathway. In the S-wave reflection analysis, it is difficult to determine the structure deeper than the reflector, therefore it is not resolved whether the reflector exists as a thin layer of the fluid pathway. For that reason, we estimated the structure beneath the reflector by calculating receiver functions using seismic waves incident on the reflector from below. In order to compare the result of the receiver function analysis with the result of S-wave reflection analysis, we used a same velocity structure in their analysis. As a result of the S-wave reflection analysis and the receiver function analysis, it is found that the reflector only exists on the north side of the Arima-Takatsuki fault zone (ATFZ), and it is found that reflector is a thin layer with low S-wave velocity. As the origin of the thin layer with low S-wave velocity, it is possible to consider fluid dehydrated from the Philippine Sea slab subducting beneath southwestern Japan and the ATFZ. In the vicinity of the ATFZ, it is known that fluid that seems to originate in the mantle spring out (Kazahaya et al. 2014). In the lower crust, pores between minerals exist independently (Yoshino et al. 2002), and fractures caused by faulting is thought to be necessary for fluid movement (Yoshino et al. 2002). Furthermore, since the spatial distribution of the reflector is related to the ATFZ, we consider that the reflector in the lower crust indicates that fluid exists in fractures caused by faulting. This result supports the water-weakened lower crust model. [ABSTRACT FROM AUTHOR]