Niigata-Kobe Tectonic Zone Research Articles

Shallow structure and late quaternary slip rate of the Osaka Bay fault, western Japan

The Osaka Bay is situated at a seismically active region north of the Median Tectonic Line and east of Awaji Island in western Japan, known as part of the Kinki Triangle and the Niigata–Kobe Tectonic Zone. Dense distribution of active faults and high geodetic strain rates characterize the region, posing a major seismic hazard potential to the coastal and metropolitan areas of the Kansai region. To investigate the shallow structure and recent deformation history of active faults in the Osaka Bay, we acquired 15 high-resolution seismic profiles using a Mini-GI airgun and a Boomer as active sources, together with multi-beam bathymetry data across the Osaka Bay Fault. Our seismic sections image a ~ 0.1 to 3.7 km-wide asymmetric anticline forelimb above the Osaka Bay Fault at shallow depths, coupled with a ~ 2.6 km-wide syncline to the west, and a broad, ~ 11 km-wide syncline in the footwall to the east. The synclinal axial surface at shallow depths measured in this study ranges 75°–89°. We observe the vertical displacement of the Osaka Bay Fault increasing northwards along strike. The sediment thickness on the hanging wall, however, is variable, modified by non-tectonic processes such as by tidal currents, affecting the geometry of growth strata. The most recent deformation by the Osaka Bay Fault reaches to near the seafloor by active folding, with large vertical offsets of 8–14 m over the last ~ 11 ka, and 5–11 m over the last ~ 5 ka. By combining with previously reported borehole age data, the average uplift rate on the Osaka Bay Fault is estimated to be ~ 1.0 to 1.7 m/ka during the Latest Pleistocene to Holocene. The inferred slip of the Osaka Bay Fault during the Holocene is likely to account for > 5% of the regional geodetic strain accumulation within the Kinki Triangle. Further studies to evaluate the Holocene slip rates of regional faults are necessary to assess the seismic hazards and the internal strain budgets within the Kinki Triangle and the Niigata–Kobe Tectonic Zone.Graphical abstract

Time-independent forecast model for large crustal earthquakes in southwest Japan using GNSS data

In this study, we developed a regional likelihood model for crustal earthquakes using geodetic strain-rate data from southwest Japan. First, the smoothed strain-rate distributions were estimated from continuous Global Navigation Satellite System (GNSS) measurements. Second, we removed the elastic strain rate attributed to interplate coupling on the subducting plate boundary, including the observed strain rate, under the assumption that it is not attributed to permanent loading on crustal faults. We then converted the geodetic strain rates to seismic moment rates and calculated the 30-year probability for M ≥ 6 earthquakes in 0.2 × 0.2° cells, using a truncated Gutenberg–Richter law and time-independent Poisson process. Likelihood models developed using different conversion equations, seismogenic thicknesses, and rigidities were validated using the epicenters and moment distribution of historical earthquakes. The average seismic moment rate of crustal earthquakes recorded during 1586–2020 was only 13–20% of the seismic moment rate converted from the geodetic data, which suggests that the observed geodetic strain rate includes considerable inelastic strain. Therefore, we introduced an empirical coefficient to calibrate the moment rate converted from geodetic data with the moment rate of the earthquakes. Several statistical scores and the Molchan diagram showed all models could predict real earthquakes better than the reference model, in which earthquakes occur uniformly in space. Models using principal horizontal strain rates exhibited better predictive skill than those using the maximum horizontal shear strain rate. There were no significant differences in predictive skill between uniform and variable distributions for seismogenic thickness and rigidity. The preferred models suggested high 30-year probability in the Niigata–Kobe Tectonic Zone and central Kyushu, exceeding 1% in more than half of the analyzed region. The model predictive skill was also verified by a prospective test using earthquakes recorded during 2010–2020. This study suggests that the proposed forecast model based on geodetic data can improve the regional likelihood model for crustal earthquakes in Japan in combination with other forecast models based on active faults and seismicity.Graphical

Spatiotemporal Variation of Crustal Anisotropy in the Source Area of the 2004 Niigata, Japan Earthquake

AbstractWe investigate the spatiotemporal pattern of crustal anisotropy in the source area of the 2004 Niigata earthquake (M 6.8) that occurred in the northern segment of the Niigata–Kobe tectonic zone, central Japan, by measuring shear‐wave splitting parameters from waveform data of local earthquakes. Our results show that the fast polarization directions in the upper crust have spatial variations across the region of the earthquake that are likely caused by both structural and stress field effects. The northwest–southeast direction near the northeastern end of the source zone (beneath station N.NGOH) and the east–west direction to the southwest (beneath station N.KWNH) are consistent with the spatial variation of the orientation of the maximum compression of the local stress field. Fast polarization directions at other stations tend to align in the directions of active faults and folds and thus are considered to be structure induced. These spatial patterns were unaffected by the earthquake. However, at two stations (N.NGOH and N.KWNH) we observe an increase in both the average and scatter of the normalized delay times (δt) during the aftershock period. In addition, two stations (HIROKA and N.YNTH) that are located in the strike‐normal direction east of the source area show an increase in the average of the normalized δt and a rotation of up to 90° of the fast direction immediately after the mainshock. We also notice that stations located very close to the source fault (DP.YMK and DP.OJK) show larger average delay times compared with stations farther away (HIROKA and N.YNTH) during the postseismic stage. To explain the temporal changes in the strength of the anisotropy, we speculate that spatiotemporal variations in microcrack development in and around the source area could be caused by static stress changes due to tectonic deformation and the earthquake rupture.

Persistent inelastic deformation in central Japan revealed by GPS observation before and after the Tohoku-oki earthquake

We analyze strain rate distribution in central-northeastern Japan based on continuous GPS data before and after the occurrence of the Great Tohoku earthquake (Mw 9.0) on March 11th, 2011, focusing on the far-field response in the Niigata Kobe Tectonic Zone (NKTZ), an E-W contraction region along the Japan seacoast of central Japan. Wavelength decomposition of the strain rate on the E-W direction, calculated from GPS data, indicates that the long-wavelength pattern changed from preseismic E-W contraction to postseismic E-W extension, representing elastic response to plate interaction at the Japan Trench. However, the short wavelength patterns reveal a persistent localized contraction zone around northern NKTZ before and after the Tohoku-oki earthquake. The persistent localized deformation pattern can be reproduced by an aseismic slip on a fault cutting the lower crust and a part of the upper crust. These results demonstrate that an inelastic process plays an essential role in the deformation of the Japan island arc.

Three-dimensional distribution of S wave reflectors in the northern Kinki district, southwestern Japan

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.

Excess strain in the Echigo Plain sedimentary basin, NE Japan: evidence from coseismic deformation of the 2011 Tohoku-oki earthquake

AbstractCoseismic deformation depends on both the source fault and on the elastic properties of the crust. Large coseismic deformation associated with the 2011 Mw 9.0 Tohoku-oki earthquake enabled us to investigate strain anomalies from crustal inhomogeneity. Concentrated contractional strain was observed in the Echigo Plain (Niigata-Kobe Tectonic Zone) before the Tohoku-oki earthquake, whereas continuous and campaign global navigation satellite system measurements show a widespread distribution of coseismic extensional strain in and around the plain. A 1-D displacement profile shows high strain (7.2 ± 0.7 microstrain) in a 17 km long section across the Echigo Plain and low strain (3.3 ± 0.4 microstrain) along a 15 km long section east of the plain, despite the latter being closer to the megathrust fault source. We performed numerical modelling of coseismic deformation using a heterogeneous subsurface structure and successfully reproduced excess extension in the plain, which is filled by low-rigidity sediments. This study demonstrates the importance of considering heterogeneous crust in deformation modelling.

Large heterogeneous structure beneath the Atotsugawa Fault, central Japan, revealed by seismic refraction and reflection experiments

A high-strain-rate zone termed the Niigata Kobe Tectonic Zone is located in central Japan and contains an active right-lateral fault called the Atotsugawa Fault. We present the results of an explosive-source seismic experiment that focused on identifying the formation mechanisms of the fault. The experiment used seven explosive sources and 1108 seismic stations, and was undertaken during October 2007 in the area of the Atotsugawa Fault. The seismic stations were set up on a survey line with a length of ~170km, oriented orthogonal to the strike of the fault. The linear array was used to identify the seismic structure of the fault zone and the deeper parts of the crust in the study area, yielding fine details of the seismic structure of the crust along the profile line, including lateral variations in P-wave velocity and the configuration of layers in the crust. A relatively low P-wave velocity reflective zone was detected beneath the fault at depths of 15–25km. This zone also has extremely low S-wave velocities, directly underlies three active faults, and contains three low-resistivity zones that underlie these faults. These reflective and low-resistivity zones are thought to represent fluid within the crust, and the data obtained during this study are consistent with a weak zone model for the formation of the active faults in the study area. This suggests that the reflective layer represents a weak zone in the lower crust that is responsible for the concentration of deformation within the upper crust. The presence of abundant fluids beneath the Atotsugawa Fault could also reduce the strength of the lower crust, again focusing deformation related to the regional stress regime in areas that overlie these fluid-containing zones. The results suggest that the reflective layer was an important factor in the formation of the Atotsugawa Fault.

Spatial variation in coda Q around the Nobi fault zone, central Japan: relation to S-wave velocity and seismicity

We investigate the spatial variation in coda Q around the Nobi fault zone in a high strain rate zone to assess the relation between coda Q, shear wave velocity, and seismicity. Waveform data were obtained from dense seismic observations. Low coda Q that follows the Niigata-Kobe Tectonic Zone in the high strain rate zone is distinct at the lowest frequency band of 1 to 2 Hz. However, at higher frequencies, such a spatial pattern in coda Q is unclear. A good positive correlation was found between coda Q at the 1- to 2-Hz frequency band and the S-wave velocity perturbation at 25-km depth, which suggests that the coda Q reflects the ductile deformation below the brittle-ductile transition zone. Furthermore, coda Q at the 1- to 2-Hz frequency band correlates negatively with seismicity at 10- to 15-km depth, which implies that there is a high stressing rate in the low coda Q area. These facts, together with results of previous studies, imply that a high deformation rate below the brittle-ductile transition zone produces the high strain rate observed by the Global Positioning System (GPS) on the surface in this region.

Source Scaling of Inland Crustal Earthquake Sequences in Japan Using the S-Wave Coda Spectral Ratio Method

We estimate corner frequencies and stress drops for 298 events ranging from M w 3.2–7.0 in 17 inland crustal earthquake sequences in Japan to investigate the source scaling and variation in stress drops. We obtain the source spectral ratio from observed records by the S-wave coda spectral ratio method. The advantage of using the S-wave coda is in obtaining much more stable source spectral ratios than using direct S-waves. We carefully examine the common shape of the decay of coda envelopes between event pair records. The corner frequency and stress drop are estimated by modeling the observed source spectral ratio with the omega-square source spectral model. We investigate the dependences of stress drops on some tectonic effects such as regionality, focal mechanism, and source depth. The principal findings are as follows: (1) a break in self-similar source scaling is found in our dataset. Events larger than M w 4.5 show larger stress drops than those of smaller events. (2) Stress drops of aftershocks are mostly smaller than those of mainshocks in each sequence. (3) There are no systematic differences between stress drops of events occurring inside and outside the Niigata-Kobe Tectonic Zone in Japan. (4) Clear dependence of the faulting type on stress drops cannot be seen. (5) Stress drops of aftershocks depend on their source depth. (6) The crack size obtained from the corner frequency corresponds to the total rupture area of heterogeneous slip models for large events.

Long‐term changes in the Coulomb failure function on inland active faults in southwest Japan due to east‐west compression and interplate earthquakes

AbstractInland earthquake activity in and around the Kinki region, southwest Japan, increases in the period from several decades before to about a decade after the occurrence of great interplate earthquakes along the Nankai Trough. To quantitatively investigate this relationship, we calculated long‐term changes in the Coulomb failure function (ΔCFF) on inland active faults in this region with viscoelastic slip response functions. As sources for the change in CFF, we investigated east‐west compression within the Niigata‐Kobe Tectonic Zone (NKTZ), historical interplate earthquakes and interseismic locking along the Nankai Trough subduction zone, and historical inland earthquakes in this region. Among these sources, the NKTZ east‐west compression is the primary cause of the long‐term changes in CFF. The changes in CFF due to interplate earthquakes are mostly negative on reverse faults and positive on strike‐slip faults. This result suggests that the inland reverse faulting activity mostly increases before interplate earthquakes and decreases after the earthquakes, whereas strike‐slip activity is mostly suppressed before interplate earthquakes and increases thereafter. This suggestion is supported by spatiotemporal pattern of historical inland earthquakes if focal mechanisms of historical earthquakes correspond to fault geometries in the region. The calculated changes in CFF are usually consistent with the occurrence of historical inland earthquakes. If we use the change in shear stress instead of the change in CFF, this consistency is enhanced, which suggests low apparent coefficients of friction in this region.

Numerical simulation of crustal deformation using a three-dimensional viscoelastic crustal structure model for the Japanese islands under east-west compression

The three-dimensional viscoelastic crustal structure beneath Japanese islands was modeled to simulate their crustal deformation by using a finite-element method and applying boundary conditions of the east-west horizontal compression. The result shows that there are relatively narrow zones of high strain rate at shallow depths, whose pattern is similar to that of the Niigata-Kobe tectonic zone revealed by GPS. High strain rates are not necessarily concentrated in regions where the elastic layer is relatively thin, but rather where its thickness changes abruptly.

Spatial variations in 3He/4He ratios along a high strain rate zone, central Japan

A linear zone with high strain rates along the Japan Sea coast, the Niigata-Kobe Tectonic Zone (NKTZ), is considered to be associated with rheological heterogeneities in the lower crust and/or upper mantle. Helium isotope variations along the NKTZ reveal a close association with the geophysical evidence for rheological heterogeneities in the crust and mantle. In the southern NKTZ, the 3He/4He ratios lower than 3.4Ra (Ra denotes the atmospheric 3He/4He ratio of 1.4×10−6) could be interpreted as a two-component mixture of helium stored in aqueous fluids driven off the subducting oceanic crust and radiogenic crustal helium. Higher 3He/4He ratios are observed in the central NKTZ where Quaternary volcanoes and high-temperature hot springs are concentrated, suggesting that the 3He emanation manifest in the central NKTZ results from the effective transfer of mantle helium by intrusion and degassing of mantle-derived magma in the crust. In the northern NKTZ where two large inland earthquakes occurred recently, there appears to be many samples with 3He/4He ratios significantly higher than those observed in the fore-arc side of northeast Japan. A plausible source of mantle helium could be attributed to upward mobilization of aqueous fluids generated by dehydration of the subducting Pacific Plate slab.

Strain anomalies induced by the 2011 Tohoku Earthquake (M w 9.0) as observed by a dense GPS network in northeastern Japan

We have evaluated an anomalous crustal strain in the Tohoku region, northeastern Japan associated with a step-like stress change induced by the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0). Because the source area of the event was extremely large, the gradient of the observed eastward coseismic displacements that accompanied uniform stress change had a relatively uniform EW extension in northeastern Japan. Accordingly, the deformation anomaly, which is determined by subtracting the predicted displacement in a half-space elastic media from the observed displacement, should reflect the inhomogeneity of the rheology, or stiffness, of the crust. The difference of the EW extension anomaly between the forearc and backarc regions possibly indicates a dissimilarity of stiffness, depending on the crustal structure of the Tohoku region. The Ou-backbone range—a strain concentration zone in the interseismic period—shows an extension deficit compared with predictions. A low viscosity in the lower crust probably induced a relatively small extension. Meanwhile, the northern part of the Niigata-Kobe tectonic zone, another strain concentration zone, indicates an excess of extensional field. This is probably caused by a low elastic moduli of the thick sedimentation layer. The detection of strain anomalies in the coseismic period enables a new interpretation of the deformation process at strain concentration zones.

新潟－神戸ひずみ集中帯を横断する測地観測による越後平野周辺の地殻変動

We examined GEONET GPS data to clarify the contemporary deformation in and around the Echigo plain, Niigata prefecture where located in the Niigata-Kobe tectonic zone and the deformation zone of the eastern margin of the Japan Sea. Compressional strain in a direction of ESE-WNW is concentrated in a narrow zone where strain rate is about 0.2ppm/yr along the coast of the Japan Sea. This strain concentration zone which is about 25km wide accommodates more than 5mm/yr of compression and geographically corresponds to the Echigo plain. The zone significantly subsided whereas surrounding regions was uplifted. These characteristics of the deformation are concordant with that measured by conventional geodetic surveys spanned more than several decades. The strain rate observed by GPS does not change significantly during more than a decade in the strain concentration zone except for coseismic periods, which is contrast with a large temporal change of a strain rate in the eastern region affected by a subduction of the Pacific plate. We proposed a simple model to explain the characteristics of the deformation. The model consists of stable sliding on a deep part of reverse faults which are extended to the east and west rims of the Echigo plain. Fault geometry penetrating the crust and calculation with gravitational viscoelastic asthenosphere is essential to reproduce the observed subsidence in the strain concentration zone.

Development of Seismometers Sensor Network for Observation on Sea Floor ——P goes to Oceans——

Because large earthquakes have repeatedly occurred in subduction zones, it is important to observe seismic activities on the sea floor. An ocean bottom cabled seismometers (OBCS) system is the most suitable tool for this purpose since data can be obtained in real-time. Although the existing OBCS systems are useful for the study of seismic activities, the number of stations is limited due to their cost. Therefore, lower cost in both production and installation is desired. We have developed a new OBCS system utilizing IP technologies. IP technologies yield the new OBCS system that are more compact and less expensive, while a large amount of complex hardware is used in the existing OBCS system. System reliability is ensured by using IP network technologies that provide redundancy. The new OBCS system was first installed to observe the NiigataKobe tectonic zone in the Japan Sea on September 2010. Although this first OBCS system has a total length of 25 km, it has been proven that seismic data can be successfully obtained and that the new OBCS system is effective and useful for the dense observation of seismogenic activities on the sea floor around Japan. key words: OBCS, earthquake

Static strain and stress changes in eastern Japan due to the 2011 off the Pacific coast of Tohoku Earthquake, as derived from GPS data

The 2011 M 9.0 Tohoku earthquake induced regional crustal deformation not only in the Japanese Islands but also in north-eastern Asia. Strain release due to mainshock faulting should cause strain redistribution in the overriding plates. The dense GPS network in Japan enables us to calculate co-seismic strain and stress changes from observed data. Strain is a more objective indicator than displacement because no reference frame is required. The co-seismic strain field clearly indicates island-scale strain redistribution. Huge extensional strain changes were concentrated in the southern Iwate and northern Miyagi regions, with a maximum value of 45 × 10−6, which might correspond to approximately 225 to 450 years of strain accumulation. This implies relatively large strain accumulation and release in these regions. Small strain decay was observed in the northernmost NiigataKobe tectonic zone and a possible anomalous Coulomb failure stress change was observed in the Mt. Fuji region. Earthquakes triggered in the above regions might be associated with these anomalies, and/or these non-uniform crustal deformations may reflect crustal heterogeneity.

Strain accumulation process around the Atotsugawa fault system in the Niigata-Kobe Tectonic Zone, central Japan

SUMMARY In order to investigate the strain accumulation process around the Niigata-Kobe Tectonic Zone in central Japan, we estimate a precise GPS velocity field around a representative fault area, the Atotsugawa fault system. The velocity profiles obtained from two dense GPS arrays crossing the Atotsugawa fault demonstrate the existence of a laterally heterogeneous deformation pattern along the fault strike. The West Array suggests the possibility of surface fault creep, which disagrees with inferences gleaned from previous EDM measurements. High shear strain rate, which is calculated from wider velocity field, distributes around not only the Atotsugawa fault system, but also the Takayama-Oppara fault zone, which is located 50 km south from the Atotsugawa fault. In order to consider the effect of interaction between those active faults, we conduct a block-fault model analysis. Our block model consists of nine blocks and 33 fault segments. Relative motion between two blocks at the northern and southern ends of the fault system is calculated to be 9.9 mm yr−1. The intervening region between these two blocks corresponds to the Niigata-Kobe Tectonic Zone with a strain rate of 0.2 ppm yr−1. Slip deficit rates at the Atotsugawa, Ushikubi, and Takayama-Oppara fault zones are estimated to be 2.2, 3.9 and 2.3 mm yr−1, respectively. Although the ratio of slip deficit rate (2.2 : 3.9 : 2.3 ≈ 1 : 2 : 1) of these three faults is similar to that of geological long-term slip rates (1 : 2 : 0.7), the magnitude of geodetic slip deficits is 30 per cent to 50 per cent larger than long-term slip rates. This systematic difference may be due to inelastic deformation of the crust in this region. Since the total slip deficit rate between the Ushikubi and the Takayama-Oppara fault zone is 8.4 mm yr−1, about 85 per cent of the relative block motion, the deformation is mostly accommodated by elastic strain accumulation on these three faults.

レシーバ関数解析から推定された日本列島の地殻構造

Recently, several researchers have elucidated crustal structures over a large area of the Japanese Islands from travel time inversion analyses. However, very few studies have paid attention to velocity discontinuities due to the limitations of spatial resolution. In this study, we apply a receiver function analysis to estimate seismic velocity structure and seismic velocity discontinuities of the crust in the Japanese Islands. We search for the best-correlated velocity structure model between an observed receiver function at each station and synthetic ones using a grid search method. Synthetic receiver functions are calculated from many assumed simple velocity structures that consist of a sediment layer to compensate for the effects of the low-velocity sediment layer and two velocity discontinuities. As a result, we clarified the spatial distributions of the crustal S-wave velocities and the tops of mantle depths. Several plain and basin areas are covered with thick low-velocity sediment layers. There are low-velocity layers corresponding to volcanoes in the upper crust (5-15 km deep). In the lower crust (15-25 km deep), our results show low-velocity structures in the eastern part of the Niigata-Kobe Tectonic Zone (NKTZ) and the Median Tectonic Line. Around the crust-mantle boundary, we see clear low-velocity zones beneath volcanoes, the western part of the NKTZ, and in the occurrence regions of the non-volcanic low-frequency tremor. High velocities near the southern coastline of the Japanese Islands correspond to the crust-mantle discontinuity of the subducting Philippine Sea plate. The crustal structure beneath the Itoigawa-Shizuoka Tectonic Line (ISTL) shows relatively low velocities in the shallower part and high velocities in the deeper part compared to neighborhood areas. The ISTL is also the boundary of the velocity structure of the upper crust in the Japanese Islands. The northeastern Japan region has heterogeneities of velocity perturbations, whereas the southwestern Japan region has the relatively stable high-velocity zones. The tops of mantle depths tend to increase in mountain regions with some undulations. The discontinuity of the subducting Philippine Sea plate at a depth of more than 40 km is extracted in several areas from the Kanto district to the Kyushu district. This suggests that the velocity discontinuity of the subducting Philippine Sea plate is larger than that of the overriding plate.

Spatial variation in shear wave splitting of the upper crust in the zone of inland high strain rate, central Japan

We investigate a detailed spatial variation in shear wave splitting in the zone of inland high strain rate, called the Niigata-Kobe Tectonic Zone (NKTZ), central Japan. Most observations show stress induced anisotropy, that is, the orientation of the faster polarized shear wave is parallel to the axis of the maximum horizontal compressional strain rate estimated from GPS data. Others show structure induced anisotropy, that is, the orientation is parallel to the strike of active faults. For the stress induced anisotropy, time delays normalized by the path length in the anisotropic upper crust is proportional to the differential strain rate. We estimate a spatial variation in stressing rate of the upper crust beneath the high strain rate zone based on a response of the normalized time delay to a step-wise stress change caused by a moderate-sized earthquake. The variation in the stressing rate of 3 kPa/year estimated from shear wave splitting is coincident with that from GPS data. We conclude, together with other seismological features in the NKTZ reported previously, that the high strain rate in the NKTZ is attributed to the high deformation rate below the brittle-ductile transition zone in the crust.

Structural Heterogeneity in Northeast Japan and Its Implications for the Genesis of the 2004 and 2007 Niigata Earthquakes

Abstract We utilized 188,766 P -wave and 179,328 S -wave high-quality arrival-time data from 14,666 local earthquakes to determine the three-dimensional (3D) seismic velocity ( V P , V S ), V P / V S ratio, crack density ( e ), and bulk velocity ( V ϕ ) structures in and around the source areas of the 2004 and 2007 Niigata earthquakes (Japan Meteorological Agency magnitude [ M J ] 6.8) in Japan in order to understand the generation mechanisms of the Niigata mainshocks. The aftershock hypocenters were relocated by applying the double-difference location method simultaneously with the tomographic inversion, indicating a complicated fault system that includes not only two parallel dominant faults dipping to the northeast but also some conjugate faults crossing the Niigata mainshock hypocenters. In general, high velocity ( V P , V S ) with low e at 11 km depth is visible in the southeastern part of this area in contrast to anomalous low velocity with high crack density in the northwestern part along the Niigata-Kobe tectonic zone (NKTZ). These characteristics of the velocity and crack density are consistent with the spatial distribution of active faults along the NKTZ. The surface geological features that indurated pre-Neogene rock outcrops were observed in the southeastern area, whereas deep sedimentary basins were observed on the opposite side. Low V P and low V S zones with high V P / V S anomalies were imaged in and/or under the source areas, showing agreement with those revealed by previous studies. All the features of the velocity and V P / V S structures, together with the images of the crack density and bulk velocity, suggest the presence of fluids under the source areas. Such fluids might have weakened the mechanical strength of the fault zone of the source areas, thus triggering the 2004 and 2007 Niigata earthquakes. Our present study, together with other recent tomographic images, reveals that fluids penetrated into the source rocks from the lower crust and uppermost mantle as a result of dehydration of the subducting Pacific slab and thus may play a key role in the genesis of these large crustal earthquakes.