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<i>In situ</i> Stresses Near the Nojima Fault Estimated by Deformation Rate Analysis

2003; Seismological Society of Japan; Volume: 56; Issue: 2 Linguagem: Inglês

10.4294/zisin1948.56.2_157

2186-599X

Namiko Sato, Yasuo Yabe, Kiyohiko Yamamoto, Hisao Ito,

Seismic Waves and Analysis

The Nojima fault in Awaji Island, Hyogo prefecture, Japan was ruptured during the 1995 Hyogo-ken Nanbu earthquake (MJMA=7.3). Drillings were performed at sites close to the fault about one year after the earthquake and rock core samples were retrieved from the boreholes. We measured In-situ stresses by Deformation Rate Analysis (DRA) using oriented core samples of three sites. The stresses measured by DRA are expected to reflect the stress state before the earthquake, because the method is based on the rock property of long-term memory of stresses. Two of the borehole sites locate along the fault segment where surface break is observed. The other is near the southern end of the estimated buried fault, which is at a distance of about 3km extended southwest from the surface break. The stresses have been determined at depths between 310m and 415m for the two sites on the surface break segment and at depths from 351m to 720m for the site on the buried fault segment. The measured stresses show that all the sites are in the strike-slip regime. The maximum horizontal stress lies in NW-SE direction at all the depths except for shallower part in the buried fault segment. This direction is almost perpendicular to the fault plane that is nearly vertical. It is also obtained that the r-value defined as the maximum shear stress divided by the normal stress on the maximum shear plane is relatively small within the zone of about 100m distance from the estimated fault trace. The zone is called the damaged zone. These results suggest that the observed stress orientation is the consequence of the small shear stress of the damaged zone. Our data and the long-term crustal strain of the area adjacent to the fault suggested that the Nojima fault is considered to be weak, even before the earthquake.