| Co-Investigator(Kenkyū-buntansha) |
IWASAKI Takuya The University of Tokyo, Earthquake Research Institute, Professor, 地震研究所, 教授 (70151719)
IKEDA Yasutaka The University of Tokyo, Depr.of Earth and Planetary Science, Associate Professor, 大学院・理学系研究所, 助教授 (70134442)
IMAIZUMI Toshifumi Tohoku University, Dept.of Geography, Professor, 大学院・理学系研究科, 教授 (50117694)
YOSHIDA Takeyoshi Tohoku University, Institute of Mineralogy, Petrology and Economic Geology, Professor, 大学院・理学系研究科, 教授 (80004505)
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| Research Abstract |
The geometry of source fault, which generate devastative crustal earthquakes, is crucial for seismic hazard mitigation and evaluating the risk of active faults. In order to reveal the relationship between an active fault and a source fault, including their connectivity, seismic reflection profiles, surface geofogy and drill hole data were examined. In 2002, as an add on project to the deep seismic experiments across the SW Japan, we deployed 800 Texan recorders across the Shikoku island and recorded the explosive seismic signals by very dense interval (ca.120 m). The low-fold seismic sections portray the subducting slab and detailed crustal structure. The deeper extension of the Median Tectonic line is clearly traced by northward dipping reflections at 30 degrees. The oblique slip on the gently dipping fault plane is the dominant movements of the MTL.
The geometry of the deeper extension of the Itoigawa-Shizuoka tectonic line(ISTL) active fault system, was also examined by seismic refle
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ction profiles and surface geology across the northern part of this fault system. The deep seismic reflection profiles suggests the low-angle geometry of the fault. Using the balanced cross section, kinematic basin evolution model has been examined. Based on this model, the northern part of the ISTL was a basin bounding normal fault and reactivated as a thrust, since the Pliocene.
In 2003, the seismic reflection profiling was performed across the epicentral area of the 2003 Northern Miyagi earthquake (Mj 6.4) to correlate the geologic structure with seismic source fault. Seismic reflection profile indicated the reactivation of Miocene normal fault as reverse fault. The fault system corresponds to the eastern margin of the back-arc rift system. The deep geometry of source fault estimated from the distribution of aftershocks shows good agreement with the geometry estimated by balanced cross-section, indicating a whole upper crustal inversion of Miocene normal fault as a reverse fault.
In 2004, similar rift system and basin inversion processes were examined along the Kitakami river valley, using the industry seismic reflection data and data of surface geology and microseismicity. According to the observation of microseismicity in the epicentral area of the 1962 Northern Miyagi earthquake. The array of hypocenters shows the west dipping, Istric geometry and accords well to the geometry of source fault of the 2003 earthquake. In Mizusawa area, a series of west dipping Miocene normal faults are well imaged by seismic reflection profiling. The active faults, such as the Dedana fault, are clearly demonstrated that they were produced by basin inversion processes. In case of such active faults, the dip angle of source fault is higher (ca.50℃) than that of simple reverse fault (ca.30℃).
Though this research, it is well understood that in the crustal deformation processes, reactivation of pre-existing faults is very common. Thus, simple geophysical estimation based on rock mechanics, is not effective. Because the fault itself is produced in geological time when the stress condition was not same as in late Quaternary. Thus, the reconstruction of geologic processes using the balanced geologic cross sections, is very effective to estimate the deep geometry of the active fault system (source fault). Less
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