Eiji Kurashimo

Multi-fault system of the 2004 Mid-Niigata Prefecture Earthquake and its aftershocks

A seismic network was deployed the day after the main shock of the 2004 Mid-Niigata Prefecture Earthquake to determine the major source faults responsible for the main shock and large aftershocks. Using the high-resolution seismic data for five days, three major source faults were identified: two parallel faults dipping steeply to the west located 5 km apart, and another dipping eastward and oriented perpendicular to the west-dipping faults. Strong lateral changes in the velocity of the source area resulted in the locations of the epicenters determined in this study being located approximately 4.3 km west-north-west of those reported by the JMA routine catalogue. The strong heterogeneity of the crust is related to the complex geological and tectonic evolution of the area and therefore the relatively large aftershocks followed around the main shock. This is considered to be responsible for the prominent aftershock activity following the 2004 Niigata event.

Seismic evidence for stretched continental crust in the Japan Sea

The Japan Sea is a major marginal sea in the western Pacific comprised of basins and bathymetric highs. Technically, these features were created by multiple-rifting and spreading processes, as exemplified in the creation of the Japan Basin and Yamato Basin. However, there are smaller scale features for which seismic crustal models have been inadequate to infer or verify their origin and evolution. Recent advances in marine seismological surveys enable us to obtain heterogeneous seismic crustal structures in sufficient detail to characterize these features. In September of 1992, a seismic experiment was conducted to obtain the detailed seismic crustal structure in the southwestern part of the Japan Sea, including the basin and bank areas. Seventeen ocean bottom seismometers (OBSs) were deployed on two lines. Explosives and airguns were fired as controlled seismic sources. The model of the crust obtained in the northern Tsushima Basin is about 13 km thick including a 1.5-km-thick sedimentary layer. Beneath the bank area the crust is 22 km thick and the upper crust is relatively homogeneous with a P-wave velocity of about 6 km/s. The bank is interpreted to be a stretched continental crust fragment.

Imaging heterogeneous velocity structures and complex aftershock distributions in the source region of the 2007 Niigataken Chuetsu-oki Earthquake by a dense seismic observation

The velocity structure and accurate aftershock distributions in the source region of the 2007 Niigataken Chuetsu-oki Earthquake (thrust type) are obtained by inverting the arrival times from 848 aftershocks observed by a dense seismic network deployed immediately after the mainshock (8 h later). Both the detailed velocity structure and the accurate aftershock distribution show lateral heterogeneity along the fault strike. In the northeast area, aftershocks are aligned along both the NW- and SE-dipping planes. These planes are conjugate to each other. The mainshock hypocenter is located close to the bottom of an approximately 50° NW-dipping plane, which indicates that the mainshock rupture could have initiated on the NW-dipping plane. The high-Vp body beneath this aftershock alignment shows a convex upward shape. In contrast, from the center to the southwest area, most of the aftershocks are aligned along SE-dipping planes. The high-Vp body beneath this aftershock alignment shows a convex downward shape. Based on these results, we suggest that the crustal structure in the source region is divided into two segments by a boundary zone situated between the northeast and southwest areas. It should be noted that this segment boundary zone is coincident with the complex aftershock zone where numerous conjugate fault planes exist. We propose that the mainshock rupture initiated near the bottom of the NW-dipping fault plane and ran to the southwest, then transferred at the segment boundary zone which has numerous conjugate fault planes to the SE-dipping plane.

Deep seismic reflection experiment using a dense receiver and sparse shot technique for imaging the deep structure of the Median Tectonic Line (MTL) in east Shikoku, Japan

A seismic experiment was carried out in east Shikoku, Japan, to detect deep reflections across the Median Tectonic Line (MTL), which juxtaposes low-P/T metamorphic rocks with high-P/T metamorphic rocks. Our experiment employed an unconventional technique: sparse shot spacing, a strong energy source (dynamite) and a dense array of seismometers. The above specifications produce only single fold coverage without common midpoint (CMP) stacking. Nevertheless, the reflection profile provides essential information on the deep structure of the MTL, of other major faults, and of the Moho in east Shikoku. On the MTL, this profile is the first to delineate the MTL from the surface to about 12 km depth. The following three factors were essential to the success of our experiment. First, the receiver interval was sufficiently small to provide horizontal resolution that was able to detect deep reflectors. Second, the simple crustal structure does not require CMP stacking to enhance data quality. Third, a thin weathering layer at the surface reduced the attenuation of seismic waves and minimized the generation of the surface waves that often obscure deep reflectors. In these conditions, the technique can be an effective means of probing the deep crust while substantially reducing survey costs.

Three-dimensional velocity structure in the source region of the Noto Hanto Earthquake in 2007 imaged by a dense seismic observation

The velocity structure and accurate aftershock distributions of the Noto Hanto Earthquake in 2007 (thrust type) are elucidated by inverting the arrival times from 917 aftershocks using double-difference tomography. P-wave velocity (Vp) of the hanging wall in the southeast appears to be higher than that of the footwall in the northwest, and the high-Vp body of the hanging wall has a relatively high Vp/Vs ratio. Conversely, the low-Vp body in the footwall appears to have a low Vp/Vs ratio at depths greater than 3 km. Aftershocks associated with the mainshock fault are roughly distributed along this velocity boundary between the hanging wall and footwall. Near-surface thin layers with significantly low Vp and high Vp/Vs are imaged in a northwest direction from the mainshock epicenter. A likely explanation is that the mainshock fault plane was reactivated as a reverse fault in terms of the inversion tectonics due to the crustal shortening which initiated from the late Miocene. Both the mainshock hypocenter and the vertical alignment of aftershocks beneath it are located in the low-Vp and low-Vp/Vs zones, indicating the potential presence of water-filled pores. Crustal stretching and shortening in and around the Noto Peninsula have created complex structures, including weak high-dip angle faults, almost vertical faults, and low velocity zones, which can potentially affect the seismic activities around the source region.

Highly resolved distribution of aftershocks of the 2007 Noto Hanto Earthquake by a dense seismic observation

The 2007 Noto Hanto Earthquake occurred on March 25, 2007, in the Noto Peninsula, central Japan. A half day after the main shock, we started installing temporary seismic stations in order to determine the precise locations of its aftershocks. Ten universities and two research institutes deployed 88 temporary seismic stations in and around the source area. The observation lasted for about 2 months. We relocated 1318 aftershocks with arrival time corrections at each station. The relocated hypocenters show relatively small errors—less than 0.2 km in the horizontal direction and less than 0.4 km depth. Most of the relocated hypocenters are about 2.0 km shallower than those determined by JMA. The distribution of the aftershocks forms a southeast-dipping plane. The main shock is located at the bottom part of their distribution. The precise aftershock distribution extends into a shallower area than the original, and it coincides with sea floor-ward extension of the active faults previously known from a sonic reflection survey. Heterogeneous distribution of the aftershocks on the fault plane shows low seismicity just above the main shock hypocenter, and middle-size aftershocks are distributed on the periphery of the main shock. A precursory event (M 4.4) that occurred 0.6 s before the main rupture is located close to the M 2.2 foreshock that occurred 12 min before it.

Low Vp and Vp/Vs zone beneath the northern Fossa Magna basin, central Japan, derived from a dense array observation

The northern Fossa Magna (NFM) basin is a Miocene rift system formed in the final stage of the opening of the Japan Sea. The northern part of the Itoigawa-Shizuoka Tectonic Line (ISTL) bounds the western part of the northern Fossa Magna. In order to understand the active tectonics in these areas, it is essential to explain the seismic velocity structures, deep structures of active faults, and microseismicity near the active faults. In the autumn of 2002, we conducted a seismic array observation across the northern part of the ISTL and the NFM to obtain a structural image beneath the NFM. Arrival times of local earthquakes and explosive shots were used in a joint inversion for earthquake locations and 3-D Vp and Vp/Vs structures. P- and S-wave arrival time data were obtained from 73 events including 4 explosive shots, and 3809 P- and 2659 S-wave arrival times were used for the inversion analysis. We obtained a seismic velocity model revealing good correlations with the surface geology along the profile. In particular, we found thick low-velocity zones beneath the NFM and the Komoro basin and a high-velocity zone beneath the Central Uplift Zone. Beneath the NFM, a low-velocity zone with low-to-moderate Vp/Vs extends to a depth of approximately 10 km. The low-velocity suggests the existence of aqueous fluid-filled pores with high aspect ratios.

Short-term spatiotemporal variations in the aftershock sequence of the 2004 mid-Niigata prefecture earthquake

We deployed 56 temporary seismic stations within approximately a month after the occurrence of the 2004 mid-Niigata prefecture earthquake. Using manually-picked arrival data obtained from the temporary and surrounding permanent seismic stations, 1056 aftershocks have been relocated. Based on the spatiotemporal variations in the relocated aftershocks, the cluster activities associated with the mainshock and some large aftershock events are identified. The aftershocks associated with the mainshock, the largest occurred on the two steep west-dipping planes at an angle of 60° and approximately 5 km away. In contrast, the aftershocks following the event on Oct. 27 are aligned on east-dipping plane at a low angle of 25°. It is further observed that the aftershock area extended in both northeastward and southwestward directions at a later stage. The triggered seismicity around the northeast edge was more significant than that around the southwest edge. This difference could be understood by the discrepancy in the shear stress level accumulated at the dynamic shear rupture due to the mainshock.

Seismological and geological characterization of the crust in the southern part of northern Fossa Magna, central Japan

The northern Fossa Magna (NMF) is a Miocene rift basin formed in the final stages of the opening of the Sea of Japan. The northern part of Itoigawa-Shizuoka Tectonic Line (ISTL) bounds the western part of the NMF and forms an active fault system that displays one of the largest slip rates in the Japanese islands. Reflection and refraction/wide-angle reflection profiling and earthquake observations by a dense array were undertaken across the northern part of ISTL in order to delineate structures in the crust, and deep geometry of the active fault systems. The ISTL active fault system at depth (ca. 2 km) shows east-dipping low-angle in Omachi and Matsumoto and is extended beneath the Central Uplift Zone and Komoro basin keeping the same dip-angle down to ca. 15 km. The upper part of the crust beneath the Central Uplift Zone is marked by the high Vp and high resistivity zone. Beneath the folded zone of the NMF, the middle to lower crust shows low Vp, low resistivity and more reflective features. The balanced geologic cross-section based on the reflection profiles suggests that the shortening deformation since the late Neogene was produced by the basin inversion of the Miocene low-angle normal fault.

Spatial variation in coda Q and stressing rate around the Atotsugawa fault zone in a high strain rate zone, central Japan

We investigated a detailed spatial distribution of coda Q around the Atotsugawa fault zone in a high strain rate zone, central Japan, using waveform data from dense seismic observations. Low coda Q at lower frequencies is localized along the fault zone, showing a good spatial correlation with a low velocity zone in the lower crust. On the other hand, we find no characteristic spatial pattern of coda Q at higher frequencies. The spatial correlation between the low coda Q at the lower frequencies, and the low velocity zone, suggests that ductile deformations below the brittle-ductile transition zone in the crust contribute to the variation in coda Q at lower frequencies. We estimated a spatial variation in the stressing rate of 15–18 kPa/year in the crust from that of coda Q in the analyzed region. This value is greater than that estimated from GPS data. We conclude, therefore, that a high deformation rate below the brittle-ductile transition zone causes the high stressing rate, which results in the high strain rate along the fault zone observed by GPS.