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Strain and tilt changes measured during a water injection experiment at the Nojima Fault zone, Japan
Author(s) -
Fujimori Kunio,
Ishii Hiroshi,
Mukai Atsushi,
Nakao Shigeru,
Matsumoto Shigeo,
Hirata Yasuhiro
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.1440-1738.2001.00320.x
Subject(s) - borehole , tiltmeter , geology , seismology , induced seismicity , fault (geology) , pore water pressure , tilt (camera) , geotechnical engineering , petrology , amplitude , mechanical engineering , physics , quantum mechanics , engineering
In order to make geophysical and geological investigations of the Nojima Fault on Awaji Island, Japan, three boreholes measuring 1800 m, 800 m and 500 m deep were drilled into the fault zone. The fault is one of the seismic source faults of the 1995 Hyogo‐ken Nanbu earthquake of M 7.2. A new multicomponent borehole instrument was installed at the bottom of the 800 m borehole and continuous observations of crustal strain and tilt have been made using this instrument since May 1996. A high‐pressure water injection experiment within the 1800 m borehole was done in February and March 1997 to study the geophysical response, behavior, permeability, and other aspects of the fault zone. The injection site was located approximately 140 m horizontally and 800 m vertically from the instrument. Associated with the water injection, contraction of approximately 0.7 × 10 −7 str (almost parallel to the fault) and tilt of approximately 1 × 10 ‐7 rad in the sense of upheaval toward the injection site were observed. In addition to these controlled experiments, the strainmeter and tiltmeter also recorded daily variations. We interpret strain and tilt changes to be related to groundwater discharge and increased ultra‐micro seismicity induced by the injected water.