HR: 08:20h
AN: S51D-02 INVITED     [Abstracts]
TI: Characterization of Seismogenic Faults of Central Japan by Geophysical Survey and Drilling
AU: * Ikeda, R
EM: ikeryu@ep.sci.hokudai.ac.jp
AF: Graduate School of Science, Hokkaido University, N10-W8, Kita-ku,, Sapporo, 060-0810 Japan
AU: Omura, K
EM: omura@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, 305-0006 Japan
AU: Matsuda, T
EM: mtatsuo@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, 305-0006 Japan
AB: Integrated investigations on seismogenic faults by geophysical survey and drilling are indispensable to better understand deep structure and physical properties of a fault fracture zone. In central Japan, three large active faults, Neodani, Atotsugawa and Atera faults, exist and are remarkable for research because of the potentiality of a scale of magnitude 7 to 8 class earthquake and the different characteristics of the seismogenic activities in these faults. Each individual fault shows its own characteristic features, which may reflect different stages in an earthquake cycle. High seismicity is concentrated with a clear lineation on and around the Atotsugawa fault, which is recognized as aftershocks from the latest event of the 1858 Hida earthquake (M=7.0). On the other hand, extremely low seismicity is found around the Atera fault, of which some parts seemed to be dislocated by the 1586 Tensyo earthquake (M=7.9). As an example of the results of study at the Atera fault, we obtained a wide variety of fault structures, composed materials, states of crustal stress and strengths of the fault from the geophysical survey (resistivity and gravity) and in-situ borehole experiments. Our findings are as follows: (1) The fracture zone around the Atera fault shows a very wide and complex fracture structure, from approximately 1 km to 4 km wide. (2) The average slip rate was estimated to be 5.3 m /1000 yr by the distribution of basalt in the age of 1.5 Ma as determined by radioactive dating. We inferred that the Atera fault has been repeatedly active in recent geologic time; however, it is in a very weak state at present. (3) Stress magnitude decreases in the area closer to the center of the fracture zone. These are important results to evaluate fault activity. Recent in-situ downhole measurements and coring through active faults have provided us with new insights into the physical properties of fault zones. In the vicinity of the epicenter of the 1995 Hyogo-ken Nanbu (Kobe) earthquake, we have conducted an integrated study by using 1,000 m to 1,800 m deep drilling wells. In particular, the Nojima-Hirabayashi borehole was drilled to a depth of 1,838 m and directly intersected the Nojima fault. Three possible fault strands were detected at depths of 1,140 m, 1,313 m and 1,800 m. Major results obtained from this study include the following: (1) Shear stress around the fault zone is very small, and the orientation of the maximum horizontal compression is perpendicular to the surface trace of faults. (2) From the results of a heat flow study, the lower cut-off depth of the aftershocks was estimated to be roughly 300 _E#8249;C. (3) Cores were classified into several types of fault rocks, and an asymmetric distribution pattern of these fault rocks in the fracture zones was identified. (4) Country rock is characterized by very low permeability and high strength. (5) Resistivity structure can be explained by a model of a fault extending to greater depths but with low resistivity. The integrated study by geophysical survey, drilling and core analyses, downhole measurements and long-term monitoring directly within these fault zones, provide us with characteristic features and dynamics of active faults.
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
DE: 8010 Fractures and faults
DE: 8123 Dynamics, seismotectonics
DE: 8164 Stresses--crust and lithosphere
SC: Seismology [S]
MN: 2004 AGU Fall Meeting