Multicomponent observation of crustal activity in the DPRI 800 m borehole close to the Nojima Fault

An 800 m borehole was drilled near the Nojima Fault, on which a strike–slip larger then 1 m occurred during the 1995 Hyogo‐ken Nanbu earthquake ( M = 7.2). Crustal activity near the fault has been observed since May 1996 using a multicomponent instrument installed at the bottom of the borehole. Data of three components of strain, two components of tilt and temperature observed from May 1996 to December 1998 were analyzed. Long‐term changes of strain and tilt show a north‐east–south‐west extension and southwards subsidence. As for the Earth tides and atmospheric effect, orientation of the principal axis of strain was mainly east‐west and orientation of the maximum subsidence was mainly north‐south. The observational data of strain had variations corresponding to a change in temperature at a depth of 800 m. The thermal expansion coefficient of the crust was calculated to be approximately 2.0 × 10 −6 /°K.