An integrated geophysical research for Atotsugawa fault system (AF), Central Japan - Relation between fault structure and surrounding crustal inhomogeneity -

HR: 16:15h
AN: S34C-02 [Abstracts]
TI: An integrated geophysical research for Atotsugawa fault system (AF), Central Japan - Relation between fault structure and surrounding crustal inhomogeneity -
AU: * Iwasaki, T
EM: iwasaki@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, the University of Tokyo, Yayoi 1-1-1, Bunkuo-ku, Tokyo, 113- 0032, Japan
AU: for the Niigata-Kobe Tectonic Zone, t
EM: iidaka@eri.u-tokyo.ac.jp
AB: An integrated geophysical observations in and around the Atotsugawa fault system (AGF), central Japan, delineated the clear relationship of the fault characteristics and the surrounding inhomogeneous crustal structure. The AGF, located within a zone of high strain rate (the Niigata-Kobe Tectonic Zone) running in the northern part of central Japan with ENE-WSW direction, is one of the prominent active faults in central Japan, and responsible for the 1858 Hietsu earthquake of M7.0. This observation project, which started from 2004, involves dense seismic observation, magnetotelluric survey, GPS measurement and refraction/wide-angle reflection experiment. Major finding so far obtained is a very low velocity anomaly (5 percents) located in the lower crustal part beneath the AGF. The upper crustal structure around the AGF is characterized by high velocity (6-6.3 km/s) patches with less seismic activity. They are 10乣20 km in size and correlated with damaged area of the Hietsu event. These results strongly indicate that the high velocity patches represent asperities of this earthquake. Both edges of the AGF are bounded by low velocity areas probably representing the present volcanic activities. Probably, the anelasticity associated with the volcanism may determine the size of this fault. The low velocity body in the lower crust extends upward to a boundary part of the high velocity patches. This upwelling portion shows low resistivity, indicating the existence of fluid. The GPS measurement indicates almost the entire part of the AGF is locked although some ambiguity remains outside of our array. Present results suggest that the prominent lower crustal heterogeneity controls the stress loading process to the AGF and the stress concentration at the boundaries of asperity with aid of fluids.
DE: 7209 Earthquake dynamics (1242)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7270 Tomography (6982, 8180)
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Seismology [S]
MN: 2007 Fall Meeting