USE OF FAULT STRIATIONS AND DISLOCATION MODELS TO INFER TECTONIC SHEAR-STRESS DURING THE 1995 HYOGO-KEN-NANBU (KOBE) EARTHQUAKE

Dislocation models of the 1995 Hyogo-ken Nanbu (Kobe) earthquake deriv ed by Yoshida et al. (1996) show substantial changes in direction of s lip with time at specific points on the Nojima and Rokko fault systems , as do striations we observed on exposures of the Nojima fault surfac e on Awaji Island. Spudich (1992) showed that the initial stress, that is, the shear traction on the fault before the earthquake origin time , can be derived at points on the fault where the slip rake rotates wi th time if slip velocity and stress change are known at these points. From Yoshida's slip model, we calculated dynamic stress changes on the ruptured fault surfaces. To estimate errors, we compared the slip vel ocities and dynamic stress changes of several published models of the earthquake. The differences between these models had an exponential di stribution, not gaussian. We developed a Bayesian method to estimate t he probability density function (PDF) of initial stress from the stria tions and from Yoshida's slip model. Striations near Toshima and Hirab ayashi give initial stresses of about 13 and 7 MPa, respectively. We o btained initial stresses of about 7 to 17 MPa at depths of 2 to 10 km on a subset of points on the Nojima and Rokko fault systems. Our initi al stresses and coseismic stress changes agree well with postearthquak e stresses measured by hydrofracturing in deep boreholes near Hirabaya shi and Ogura on Awaji Island. Our results indicate that the Nojima fa ult slipped at very low shear stress, and fractional stress drop was c omplete near the surface and about 32% below depths of 2 km. Our resul ts at depth depend on the accuracy of the rake rotations in Yoshida's model, which are probably correct on the Nojima fault but debatable on the Rokko fault. Our results imply that curved or cross-cutting fault striations can be formed in a single earthquake, contradicting a comm on assumption of structural geology.