GPS Observations of Post-Seismic Crustal Movements in the Focal Region of the 1995 Hyogo-ken Nanbu Earthquake. Static and Real-Time Kinematic GPS Observations

Post-seismic deformations in the focal region of the 1995 Hyogo-ken Nanbu earthquake (M=7.2), in southwest Japan, which occurred on January 17 (JST), were observed with GPS observations. We deployed more than 30 GPS stations in and around the area of aftershock activity within several days after the occurrence of the earthquake. First the data were stored and manually collected at the observation sites, but by the middle of February the data at most of sites were started to be telemetered through public phone lines to data centers. GPS observations continued until the end of March, except for 4 sites left active for monitoring long-term deformations. We used two types of GPS observations, usual static and real-time kinematic GPS observations. Static day-by-day analyses, which are based on 24-h data sampled every 30 s, show the deformation associated with post-seismic readjustment with a relaxation time of about 70 days and with an amplitude of 1-2 cm in the area close to the epicenter. Also, rapid movements were observed just after the occurrence of the earthquake. However, since our analyses are only preliminary, further analyses are needed to clarify these observations and to investigate models of post-seismic deformations. Real-time kinematic GPS observations, which is a new type of GPS observations, were made every 1 s from February 9 to March 22 for detecting possible rapid relative movements between two sites across the surface fault trace on Awaji Island and across the other fault, the Arima-Takatsuki Tectonic Fault on the Kobe side. Present real-time kinematic GPS observations show scatters with the amplitude of 0.5-3.0 cm depending on components and baseline lengths. Any motions with amplitudes larger than these noise levels could not be detected during the observation period.