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The Hyogo-ken Nanbu earthquake (MJMA7.2), 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 Data have been archived first by on-site recordings and later by tele-communcations at the Disaster Prevention Research Institute, Kyoto Universsty.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.

Temporary observation of aftershocks of the 1995 Hyogo-ken Nanbu earthquakes, occurred January 17, 1995, was conducted from the next day of the main shock during about two weeks. Nine individual stations were set by Kyoto University and one by Tokai University on Awaji island and to the north of the Kobe City. The observation was followed by the University Group Joint Observation with a telemetered observation network started onJanuary 26. From the frequency distributions of the S-P times, it is found that aftershocks were very few along the Nojima fault on Awaji island, of which surface dislocation is as large as 1-2m in right-lateral strike slip motion. This feature of the epicenter distribution is certified from the precise determination of hypocenters by the telemetered network stations. Thus, the aftershock area was defined in the early stage of the earthquake sequence within half to one day, though the area spread gradually. The fact that few events occur along the earthquake surface fault is very important to interpret the relationship between the active fault and the dislocation and /or the rupture process of the earthquake. Besides, the temporary observation of aftershocks is necessary for the reduction of disaster just after the main shock and prediction of the large aftershocks as well as the study of the earthquake itself. To achieve this purpose, temporary network with radio telemeters is effective at immediately after the main shock. A telemetered network with telephone lines or satellite communication is also useful some time after the main shock, when some parts of disaster are recovered.

An experiment for OPS strain seismometer, which possibly records largeamplitude near-field seismic motions with flat frequency responses from zero to several 0.1 Hz, has been conducted. Assigning displacement motions with the double amplitude of 15 cm and with periods of 25-300 seconds to a GPS rover antenna, we have made 3-hour GPS observations also at 2 other base sites 160 m and 160 km apart from the rover site, with a sampling interval of 1 second. Post-processing kinematic analyses determine the rover site motion every second from two base sites. Comparison of post-processing results determined from two base sites 60 m and 160 km apart from the rover site show that kinematic GPS can resolve horizontal displacement motions with an accuracy of 1-2 cm every second even from a site 160 km apart, though there exist drift components up to 10 cm during a period of 3 hours. These drift components are, however, mainly due to the use of broadcast ephemerides, and these components will be removed if using precise ephemerides. Thus, our experiment shows a possibility of GPS strain seismometer, which records large-amplitude nearfield ground displacement motion with flat frequency responses from zero to several 0.1 Hz., 文部省科学研究費補助金・総合研究 (A) 「測地学におけるGPSの高度利用に関する総合的研究」, 研究代表者: 加藤照之, 課題番号: 04302019, p.73と74の間に追加1頁(著者原稿)あり