Elastic property of damaged zone inferred from in-situ stresses and its role on the shear strength of faults

Abstract

The Nojima fault in Hyogo prefecture, Japan, ruptured during the 1995 Hyogo-ken Nanbu earthquake (MJMA = 7.3). The stress measurements at sites close to this fault have revealed that the direction of the largest horizontal stress is almost perpendicular to the strike of this sub-vertical fault and that, in the zone within about 100 m from the fault core axis, the ratio of the largest shear stress to the normal stress is significantly small compared with that of the outside. It is thus the logical consequence that the principal stress outside the zone tends to direct perpendicularly to the fault plane. A model called the fracture process model is introduced for the relationship between fracture strength and elastic property of rocks. Making use of this model on the assumption that the observed shear stress equilibrates to the shear strength of damaged zone, it is found that the elastic wave velocities estimated from the stress well explain the observed velocities of damaged zone. This model suggests further that the friction coefficient of fault can be smaller than 0.15 due to the characteristic deformation of damaged zone and that the pressurized fluid is not essential for the formation of weak faults. Copyright © The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences.