The hydraulic fracturing technique is a method to measure the stress states in the rocks based on the change of the water pressure observed when the induced fracture in the borehole wall generated by water pressure reopens or closes. A new standard of hydraulic fracturing method in Japan will have two important observation parameters as shown below. One parameter is the water pressure Ps (Shut-in pressure) at the time when the tip of the fracture begins to close after shut-in operation at stopping pressurization in a test interval. The other parameter is the water pressure Pr (Reopening pressure) at the time when the mouth of the fracture begins to open when the test interval is re-pressurized. The new hydraulic fracturing technique highlights that a compliance of the measuring system was adequate for correctly measuring Pr. The small compliance means that the capacity of the water supply system is extremely small and the water supply system has the high stiffness. Small compliance is synonymous with high stiffness, and an ideal measuring system is required to have smaller compliance. We got an opportunity to measure the crustal stress by the hydraulic fracturing technique around Nojima fault which appeared on the surface at the 1995 Southern Hyogo Prefecture Earthquake. The hydraulic fracturing test was conducted at the foot side of Asano fault which is derived from Nojima fault. The measurement depth is about 800 m. The magnitude of the measured maximum principal stress is smaller than the overburden stress, and its principal stress direction does not match the fault movement and the direction of the compression axis is greatly deviated. Therefore, the current stress state around the fault is considered to represent the stress relaxation state after the fault activity.

1 Introduction

Nojima fault in Awaji Island was active at the 1995 Southern Hyogo Prefecture Earthquake (hereinafter referred to as "the 1995 earthquake"). In the crustal stress measurement by the hydraulic fracturing test around this measurement point carried out immediately after this earthquake, it is reported that the small stress values and the NW-SE direction in maximum principal stress are measured (Tsukahara et al., 1998). Here, it was measured at a depth of about 1500 m at the upper side of Nojima fault, but no induced longitudinal fracture due to hydraulic fracturing was confirmed, and the principal stress direction was estimated from the borehole breakout phenomenon observed in the same borehole.

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