Global Positioning System Observation before and after the Hyogo-ken Nanbu Earthquake of January 17, 1995, Japan

Abstract

The Hyogo-ken Nanbu earthquake (M_JMA7.2), that took place on January 17th, 1995, in Kobe and Awaji area, caused tremendous disasters in the areas. After the earthquake, the Japanese University Consortium for GPS Research (JUNCO) deployed more than 30 GPS receivers around the hypocentral area to find co-seismic and to monitor post-seismic crustal deforma-tions related to the earthquake. Data have been archived first by on-site recordings and later by tele-communcations at the Disaster Prevention Research Institute, Kyoto Universsty. Temporary dense array terminated by the end of March, 1995. Further temporary occupa-tions were conducted in May, 1995, in November, 1995, and in March, 1996, as well as long term continuous monitorings at selected several sites. The monitorings ended in August, 1996. Co-seismic offsets were observed at several sites around the source area. The largest offsets amounted to 45cm at Iwaya site which is about 4 km east of the Nojima fault. These data were used for simultaneous inversion together with strong motion data to clarify slip dis-tribution on the buried rectangular faults. Post-seismic deformations were also found at all of sites. They mostly showed temporal decay and amounted to 2 to 3 cm. The largest one was observed at Iwaya site. The areal distri-bution of post-seismic displacement vectors seems to indicate afterslip on the fault planes, but not the areal visco-elastic readjustments. Relaxation processes at Iwaya and Kawaragi sites were fitted by a logarithmic curve. Application of a theory on the mechanics of afterslip based on the constitutive relationship of a fault surface indicated that the fitting give reasonable es-timate on the frictional rate parameter or the thickness of velocity-strengthning layer of the earth's surface. Assuming that the constitutive parameter is ranging from 0.001 to 0.005 based on the results of rock experiments, thicknesses of velocity-strengthning layer at Iwaya and Kawaragi were estimated to be ranging between 500m and 2 km, which is consistent with other seismological data. Monitoring of fault offsets using real-time kinematic GPS (RTK-GPS) were also conducted at two baselines crossing the Nojima fault and the Arima-Takatsuki tectonic line, respectively. Though significant deformation was not observed, it showed a potential capability of real time monitorings of ground deformations in a few centimeter accuracy.