Outline of the fault zone drilling project by NIED in the vicinity of the 1995 Hyogo-ken Nanbu earthquake, Japan

Ikeda, R

R. Ikeda, Outline of the fault zone drilling project by NIED in the vicinity of the 1995 Hyogo-ken Nanbu earthquake, Japan, ISL ARC, 10(3-4), 2001, pp. 199-205

16

INGLESE

Article

Earth Sciences

ISLAND ARC

1038-4871 → ACNP

10

3-4

2001

199 - 205

ISI

1038-4871(200109/12)10:3-4<199:OOTFZD>2.0.ZU;2-W

Three boreholes, 1001 m, 1313 m and 1838 m deep, were drilled by the Nation al Research Institute for Earth Science and Disaster Prevention (NIED) in t he vicinity of the epicenter of the 1995 Hyogo-ken Nanbu (Kobe) earthquake to investigate tectonic and material characteristics near and in active fau lts. Using these boreholes, an integrated study of the in situ stress, heat flow, and material properties of drill cores and crustal resistivity was c onducted. In particular, the Nojima-Hirabayashi borehole was drilled to a d epth of 1838 m and directly intersected the Nojima Fault, and three possibl e fault strands were detected at depths of 1140 m, 1313 m and 1800 m. Major results obtained from this study include the following: (i) shear stress a round the fault zone is very small, and the orientation of the maximum hori zontal compression is perpendicular to the surface trace of faults; (ii) fr om the results of a heat flow study, the lower cut-off depth of the aftersh ocks was estimated to be roughly 300 degreesC; (iii) cores were classified into five types of fault rocks, and an asymmetric distribution pattern of t hese fault rocks in the fracture zones was identified; (iv) country rock is characterized by a very low permeability and high strength; and (v) resist ivity structure can be explained by a model of a fault extending to greater depths but with low resistivity.