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In situ stress measurements in NIED boreholes in and around the fault zone near the 1995 Hyogo‐ken Nanbu earthquake, Japan
Author(s) -
Ikeda Ryuji,
Iio Yoshihisa,
Omura Kentaro
Publication year - 2001
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1111/j.1440-1738.2001.00323.x
Subject(s) - borehole , geology , seismology , fault (geology) , aftershock , drilling , differential stress , scientific drilling , active fault , geotechnical engineering , oceanography , mechanical engineering , deformation (meteorology) , engineering
The 1995 Hyogo‐ken Nanbu (Kobe) earthquake, M 7.2, occurred along the north‐east–south‐west trending Rokko–Awaji Fault system. Three boreholes of 1001 m, 1313 m and 1838 m deep were drilled in the vicinity of the epicenter of the earthquake. Each borehole is located at characteristic sites in relation to active faults and the aftershock distribution. In particular, the Nojima–Hirabayashi borehole [Hirabayashi National Research Institute for Earth Science and Disaster Prevention (NIED) drilling] in Awaji Island was drilled to a depth of 1838 m, approximately 320 m southeast from the surface rupture of the Nojima Fault, and it crosses fracture zones below a depth of 1140 m. In situ stress measurements by the hydraulic fracturing method were conducted in these boreholes within 1.5 years after the earthquake. Measurement results suggest the following: (i) Differential stress values are very small, approximately 10 MPa at a depth of 1000 m at each site; (ii) the orientation of maximum horizontal compression is almost the same in the boreholes, perpendicular to the surface trace of the faults, north‐west–south‐east; (iii) fault types estimated from the state of stress differ among these sites; and (iv) the differential stress value just beneath the fault fracture zone decreases abruptly to one‐half of that above the fault zone in the Hirabayashi NIED drilling. These features support the idea that the shear stress along the Rokko–Awaji Fault system decreased to a low level just after the earthquake.