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Earthquake size distribution is characterized by the "b-value" of the power law decay, which exhibits spatiotemporal variations. These variations are sometimes evident before a large earthquake. Understanding spatiotemporal variations is key to developing a model for large-earthquake generation. Previous studies have shown that changes in the b-value are caused by the tectonic stress regime. Furthermore, lab experiments have demonstrated the b-value dependency of acoustic emissions on the criticality of the failure condition. However, the factors controlling the b-values during natural seismic activity are unclear. In this study, changes in the b-value in small earthquake sequences are investigated, focusing on failure criticality. Based on our high-precision focal mechanism dataset, we conclude that the b-value decreases as it nears a critical failure condition, providing a physical explanation for the reduction in b-value before a major earthquake. Our findings elucidate fault failure models, facilitating improvement in earthquake alerts and disaster mitigation. The b-value in earthquake size distribution shows spatiotemporal variations, being key to consider large-earthquake genesis. By high-precision focal mechanism data, we show a physical reason for the reduction in b-value before a major earthquake. [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]

For understanding the physical condition of earthquake generation, we estimated the deviatoric stress fields and fault planes of the large earthquakes in the hypocentral area of the 2016 Kumamoto earthquake sequence, central Kyushu, Japan. We analyzed precise hypocenters and their focal mechanism tensors from 1993 to 2017 and found the complex fault planes of the largest foreshock and the mainshock during the sequence. The faults of large earthquakes could not be detected from the preceding high seismic activity in the area to the mainshock. The analyses of the deviatoric stress tensors obtained from pre‐ and post‐sequence seismic moment tensor data revealed the temporal change in the deviatoric stress field at the vicinity of the hypocenters of the largest foreshock and the mainshock. Because of this temporal change, we found that the stress level of the deviatoric stress field in this region is the same level as the co‐seismic stress change caused by the two large earthquakes. We attempted to estimate the differential stress at the region using a new method, which can be applied to the heterogeneous stress fields. The average value of the estimated differential stresses in the region was 15.4 ± 10 MPa. The rotation of the principal stress axes and the low differential stress are important information to understand the physical processes of the earthquake generation and the state of the crustal stress. Key Points: We investigated the characteristics of the hypocenters and their focal mechanism tensors before and after the 2016 Kumamoto earthquakeWe revealed the temporal change in the deviatoric stress field before and after the two large earthquakes around their faultsThe differential stress at the hypocentral area was 15.4 ± 10 MPa and did not significantly change after the events [ABSTRACT FROM AUTHOR]

Inelastic deformation due to seismic activity is an important signal that reflects fault evolution. In particular, aftershock sequences indicate the evolution of damage in a medium that has experienced a large earthquake. Herein, we discuss the inelastic strain rate surrounding the fault that produced the M 7.3 Western Tottori earthquake in 2000 using long-term aftershock analysis. To obtain high-resolution focal mechanisms 18 years after the earthquake occurrence, we conducted dense seismic observations in the focal area. The inelastic strain rate estimated from the aftershock seismic moment tensor data showed spatial variations within a range of 10−7–10−11 per year, 18 years after the main shock. By comparing the inelastic strain rates from immediately after the earthquake and 18 years later, we detected the increase in the spatial variations in the inelastic strain rate; the variations are as small as 102 (= 10−5/10−7) for the early stage but as large as 104 (= 10−7/10−11) for the later period. In addition, the decay of the rate during these two periods varied spatially from spatial bin to bin. Certain bins in the northern segment of the earthquake fault, southern edge of the fault, and surrounding the location of the preceding swarm activity to the M 7.3 event showed slower decay rates than the inverse of the lapse time since the occurrence of the M 7.3 earthquake. We modeled this decay rate change as the relaxation response of a power-law fluid to an elastic strain input from the large earthquake. Most parts of the fault can be explained by this model. However, the areas with low decay rates suggest the presence of a dragging mechanism, such as aseismic slip, at or around these locations. [ABSTRACT FROM AUTHOR]

To clarify the relationship between earthquake occurrence and fluid, we analyzed data from a dense aftershock-observation network with 69 high-gain short-period seismographs installed immediately after the mainshock occurrence (October 21) in the aftershock area of the 2016 Central Tottori Prefecture earthquake. We determined the hypocenters and focal mechanisms of the aftershocks very precisely in the period from October 22 to December 15. We then investigated the temporal changes in the spatial distributions of hypocenters and T-axis azimuths of focal mechanisms. The distributions of aftershock hypocenters and T-axis azimuths are basically temporally stable, except those in limited portions in the shallow layer near the western edge of the aftershock area, where rapid decrease of aftershocks with T-axis azimuths of WSW to west was observed. If fluid rises from the lower crust due to fault rupture, the locations of aftershocks and focal mechanisms may change over time, especially in the deepest part of the aftershock region. However, the temporal change in these parameters was not apparent at depth. These observations suggest that the aftershock activity of the Central Tottori Prefecture earthquake was controlled mainly by stress concentration rather than strength reduction due to high fluid pressure. [ABSTRACT FROM AUTHOR]

Earthquakes with non‐double‐couple (NDC) components are indicators of geometric fault complexity, anisotropy, tensile failure, and fluid flow effects. NDC earthquakes have been observed in volcanic zones and interpreted as faulting related to fluid effects in the hypocentral region. In this study, we provide evidence of the occurrence of microearthquakes with NDC components in a tectonic zone. Aftershocks of the 2000 Western Tottori earthquake revealed definite NDC components based on a polarity analysis of the first P wave motion data from a hyperdense seismic observation. We modeled these events using both shear faulting with tensile failure and multiple shear ruptures. Six of the eight NDC events were well modeled by shear faults accompanied with an opening tensile crack. These results suggest that pressurized fluid or a weak tensile crack may exist along the rupture zone of the Tottori earthquake. Plain Language Summary: An earthquake is caused by a relative slip between two rock faces that breaks rocks around them. The characteristics of the source process can inform us about how the rock was deformed, which is important to deepen our understanding of earthquake generation. We studied small earthquakes in the area where a large earthquake, the 2000 Western Tottori Earthquake in Japan, occurred, using hyperdense seismic observation data. In the observations, 1,000 sensors were placed in the area, 1 km from each other. We found that small earthquakes did not only slip along the fault surfaces, but also induced an opening of tensile cracks. The opening can be explained by a local high‐pressure fluid or/and an ease of crack opening in the area. According to previous studies, both possibilities could be related to the large earthquake occurrence. Therefore, we conclude that the rock was damaged by fluid infiltration and dynamic shaking. Key Points: Microearthquakes with non‐double‐couple components in the tectonic earthquake hypocentral area were detected using P wave motion dataSix of eight non‐double‐couple events were modeled by shear fault with tensile crack, and two remaining events by multiple shear ruptures [ABSTRACT FROM AUTHOR]

In order to clarify the origin of aftershocks, we precisely analyze the hypocenters and focal mechanisms of the aftershocks following the 2000 Western Tottori Earthquake, which occurred in the western part of Japan, using data from dense seismic observations. We investigate whether aftershocks occur on the mainshock fault plane on which coseismic slip occurred or they represent the rupture of fractures surrounding the mainshock fault plane. Based on the hypocenter distribution of the aftershocks, the subsurface fault structure of the mainshock is estimated using principal component analysis. As a result, we can obtain the detail fault structure composed of 8 best-fit planes. We demonstrate that the aftershocks around the mainshock fault are distributed within zones of 1.0-1.5 km in thicknesses, and their focal mechanisms are significantly diverse. This result suggests that most of the aftershocks represent the rupture of fractures surrounding the mainshock fault rather than the rerupture of the mainshock fault. The aftershocks have a much wider zone compared with the exhumed fault zone in field observations, suggesting that many aftershocks occur outside the fault damage zone. We find that most aftershocks except in and around the large-slip region are well explained by coseismic stress changes. These results suggest that the thickness of the aftershock distribution may be controlled by the stress changes caused by the heterogeneous slip distribution during the mainshock. The aftershock is also distributed within a much wider zone than the hypocenter distribution observed in swarm activity in the geothermal region, which is thought to be caused by the migration of hydrothermal fluid. This result implies a difference in generation processes: Stress changes due to the mainshock contribute primarily to the occurrence of aftershocks, whereas earthquake swarms in the geothermal region are caused by fluid migration within the localized zone. [ABSTRACT FROM AUTHOR]

Distinct reflected waves (S × S) are observed in the northern Kinki district, southwestern Japan. We conducted a high-resolution reflection analysis by using data from 128 seismic stations with an average spacing of about 5 km. We used a stacking method to obtain three-dimensional distributions of relative reflection strengths of S waves and found a thin planar zone of high reflection strengths at depths of 25-30 km, which we call a S wave reflector. We also found that the zone of high reflection strengths is dipping to the north and that low-frequency earthquakes (LFEs) occurred near the edge of the zone at depths around the Moho discontinuity. It is inferred from these results that fluid is concentrated in this zone of high reflection strengths. It is likely that the zone of high reflection strengths is a path of fluid upwelling from the mantle, together with the hypocentral region of LFEs, that is located near the lower edge of the zone. The northern Kinki district is thought to be part of the Niigata-Kobe Tectonic Zone (NKTZ) high strain rates region. The high reflection strengths zone may be associated with high strain rates in the NKTZ. [ABSTRACT FROM AUTHOR]

We conducted a three-dimensional traveltime tomographic reconstruction in and around the source region of the 1984 western Nagano Prefecture earthquake to investigate the generation process for the main shock and associated swarm activity. Up to 220 000 high-resolution traveltime records (2 ms error) were compiled from a dense seismic network. From these records, we performed accurate, high-resolution calculations to estimate hypocentre distributions and three-dimensional velocity structure. Most hypocentres aligned along the same path or within the same plane, rather than in three-dimensional clusters. Hypocentres in the swarm region are located in regions with low Vp/Vs ratios, while few earthquakes occurred in regions with high or normal Vp/Vs ratios. We suggest that differences in the number of small fractures and fluid content between these two regions influenced the seismic activity. Rupture propagation associated with the main shock appears to be confined by relatively higher Vp/Vs surroundings, and a low-velocity region which limits its vertical extent. [ABSTRACT FROM AUTHOR]

We use an inversion method based on stopping phases (Imanishi and Takeo, 2002) to estimate the source dimension, ellipticity, and rupture velocity of microearthquakes and investigate the scaling relationships between source parameters. We studied 25 earthquakes, ranging in size from M 1.3 to M 2.7, that occurred between May and August 1999 at the western Nagano prefecture, Japan, which is characterized by a high rate of shallow earthquakes. The data consist of seismograms recorded in an 800-m borehole and at 46 surface and 2 shallow borehole seismic stations whose spacing is a few kilometers. These data were recorded with a sampling frequency of 10 kHz. In particular, the 800-m-borehole data provide a wide frequency bandwidth with greatly reduced ground noise and coda wave amplitudes compared with surface recordings. High-frequency stopping phases appear in the body waves in Hilbert transform pairs and are readily detected on seismograms recorded in the 800-m borehole. After correcting both borehole and surface data for attenuation, we also measure the rise time, which is defined as the interval from the arrival time of the direct wave to the timing of the maximum amplitude in the displacement pulse. The differential time of the stopping phases and the rise times were used to obtain source parameters. We found that several microearthquakes propagated unilaterally, suggesting that all microearthquakes cannot be modeled as a simple circular crack model. Static stress drops range from approximately 0.1 to 2 MPa and do not vary with seismic moment. It seems that the breakdown in stress drop scaling seen in previous studies using surface data is simply an artifact of attenuation in the crust. The average value of rupture velocity does not depend on earthquake size and is similar to those reported for moderate and large earthquakes. It is likely that earthquakes are self-similar over a wide range of earthquake size and that the dynamics of small and large earthquakes... [ABSTRACT FROM AUTHOR]

We compared the cutoff depth of seismicity in and around the Nojima fault broken by the 1995 Kobe earthquake occurring in intraplate Japan with the brittle–ductile transition depth of the widely accepted strength profile model of the crust. We successfully determined the temperature profile from borehole measurements, since almost the same geothermal gradients were observed at two boreholes located about 4 km apart from each other, and the thermal conductivity and heat production were also measured by taking numerous core samples. We found that the cutoff depth was much deeper than the transition depth under the assumption that wet granite is deformed at a strain rate of 3×10−15 s−1. This small strain rate implies, however, that plastic flow is uniformly distributed below the seismogenic region. When the strain rate is assumed to be greater than 10−13 s−1, the cutoff depth can be attributed to the transition depth. This suggests that deformation is localized in a narrow fault zone below the seismogenic region, even in the intraplate region. [Copyright &y& Elsevier]

The nature and origin of the concentrated deformation zone along the Japan Sea coast (NKTZ: Niigata–Kobe tectonic zone) was investigated by carefully analyzing the GPS data and qualitatively modeling the lower crust in NKTZ. It was concluded that this deformation zone is not a plate boundary between the Amurian plate (AMU) and the North America plate but is rather an internal deformation zone near the eastern margin of AMU. The data previously obtained on the conductivity anomalies in the lower crust and the 3He/4He ratios suggest that the concentrated deformation in NKTZ results from the lower crust in NKTZ being weakened by a high water content. The high water content is thought to result from the dehydration of subducting slabs. NKTZ has a higher water content in the lower crust than other regions do because there is no Philippine Sea plate (PHS) seismic slab beneath NKTZ. In other regions, it is estimated that the mantle wedge above the seismic Philippine Sea slab prevents the water dehydrated from the slab from rising to the lower crust, and that the lithosphere within PHS itself prevents the water dehydrated from the Pacific plate from rising up through it. [Copyright &y& Elsevier]

In order to examine the relationship between anomalous crustal deformations and subsequent large earthquakes, we compiled and examined data on preseismic anomalous crustal deformations for two earthquakes that occurred in and around the Japanese Islands in recent years. We found that the slip on a downward extension of a seismogenic fault accelerates before the main shock and that the average slip velocity is inversely proportional to the time remaining until the main shock occurs. [Copyright &y& Elsevier]

Abstract The 1995 Hyogo-ken Nanbu (Kobe) earthquake, M7.2, occurred along the north-east–south-west trending Rokko–Awaji Fault system. Three boreholes of 1001 m, 1313 m and 1838 m deep were drilled in the vicinity of the epicenter of the earthquake. Each borehole is located at characteristic sites in relation to active faults and the aftershock distribution. In particular, the Nojima–Hirabayashi borehole [Hirabayashi National Research Institute for Earth Science and Disaster Prevention (NIED) drilling] in Awaji Island was drilled to a depth of 1838 m, approximately 320 m southeast from the surface rupture of the Nojima Fault, and it crosses fracture zones below a depth of 1140 m. In situ stress measurements by the hydraulic fracturing method were conducted in these boreholes within 1.5 years after the earthquake. Measurement results suggest the following: (i) Differential stress values are very small, approximately 10 MPa at a depth of 1000 m at each site; (ii) the orientation of maximum horizontal compression is almost the same in the boreholes, perpendicular to the surface trace of the faults, north-west–south-east; (iii) fault types estimated from the state of stress differ among these sites; and (iv) the differential stress value just beneath the fault fracture zone decreases abruptly to one-half of that above the fault zone in the Hirabayashi NIED drilling. These features support the idea that the shear stress along the Rokko–Awaji Fault system decreased to a low level just after the earthquake. [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]

One approach that can be used to evaluate the potential for an earthquake occurrence is the detection of the stress concentration at an earthquake fault. While the stress fields for pre- and post-seismic event stages differ, this change cannot provide information regarding the potential for an earthquake. Here, we propose a detection method for states of stress that uses focal mechanism data from microearthquakes. The state of stress can be defined both by the background stress and by a moment tensor equivalent to the stress concentration. We applied this method to actual focal mechanism data from the 2013 Awaji Island earthquake (M6.3), Japan, and the results showed the presence of stress concentration around the earthquake fault before the mainshock. In addition, the regional differential stress was shown to be about 13 MPa. The magnitude of the obtained stress concentration in the focal area and the high dip angle of the mainshock fault imply that the faulting occurred in the crust where it was overpressurized to a level near the lithostatic pressure. [ABSTRACT FROM AUTHOR]

The M 7.8 Kaikoura earthquake occurred in the northern South Island of New Zealand on 3 Nov., 2016, involving the rupture of >20 faults. To understand the complexity of the Kaikoura earthquake, details of the fault geometry, seismic velocity distribution, and stress field are necessary. We have undertaken seismic tomography along the c. 200 km length of the rupture zone. Data from both 51 temporary stations and 22 permanent (GeoNet) stations were collected from March 2011 to December 2018. The hypocenter of the Kaikoura earthquake and aftershocks near the Kekerengu fault locate along lineaments where seismic velocity changes laterally in the epicentral region. In the uppermost crust, lower velocities occur beneath the Emu Plain and Cape Campbell. A higher velocity region near Kaikoura may have acted as a barrier that prevented eastward rupture from the hypocenter and led to the complex fault distribution in this area. These complexities in the seismic velocity structure may relate to the multi-segment rupture character of the Kaikoura earthquake. Spatial correlations between rupture areas and high Vp/Vs suggest the involvement of overpressured fluid in the nucleation and propagation of rupture segments, which is also supported by the reactivation of unfavourably oriented strike-slip ruptures, many lying at c.70° to the regional maximum compressive stress trajectories. [Display omitted] • The aftershock alignments are consistent with most of the corresponding sub-faults. • High seismic velocity body may have acted as a barrier during the rupture process. • High Vp/Vs is present along the aftershock area. • Overpressured fluid likely promoted the multi-fault rupture of the Kaikoura earthquake. [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]

To understand the stress controls on the occurrence of a multi-fault rupture, we estimated the crustal stress between April 2013 to December 2018, i.e., before and after the Mw7.8 Kaikōura earthquake that occurred in New Zealand on 13 November 2016. We used both the focal mechanism solutions from the temporary seismic networks and the GeoNet moment tensor solutions and selected the solutions that differed significantly from the mainshock fault planes and rakes. Then, we performed stress tensor inversions for the selected focal mechanism solutions. Using the stress tensor inversion results, we also calculated the slip tendency. Prior to the Kaikōura earthquake, the stress regime was the strike-slip type, and the maximum eigenvalue of the stress tensor (σ 1) was oriented WNW–ESE. The stress field orientation did not change significantly after the earthquake. This suggests that the stress change during the Kaikōura earthquake was too small to alter the stress orientations, implying that there may have been large differential stress prior to the Kaikōura earthquake. However, the average stress ratio in different clusters changed in two different patterns after the earthquake, suggesting possible changes in the magnitude of different components of the stress tensor, or of pore pressure in different regions. A high slip tendency was observed at the hypocentre, while a low slip tendency was observed at the northern end of the Kaikōura earthquake faults. This may suggest that the stress orientation and the stress ratio controlled the initiation and the end of the multi-fault rupture. These results corroborate previous fault propagation models. • Little or no coseismic change in stress orientation occurred due to the 2016 Kaikōura earthquake. • A large differential stress value was observed prior to the Kaikōura earthquake. • A high slip tendency prior to the earthquake was observed along the epicentral fault • A low slip tendency prior to the earthquake was observed at the northern end of the rupture zone. [ABSTRACT FROM AUTHOR]