Precise geoidal height by GPS was detected in and around Sakurajima volcano. First, in this research, a temporal change ∂N in the geoidal height, which is, in some cases, induced by a large ejection from magma reservoir, is introduced by developing a new conversion formula without gravity data and Stokes' integral. The temporal change in the geoidal height is determined from ∂h-∂H, where, ∂h and ∂H are temporal changes of the ellipsoidal and orthometric heights, respectively. ∂h and ∂H are detected by GPS and levelling or triangulation surveys carried out at the same time and on the same period, respectively. Second, the undulation pattern of the geoidal height which was related to the past magma ejection was precisely detected by GPS using the triangulation and the leveling points data in and around Sakurajima Volcano considering ∂N =0. This result agrees well with the profile of the volcanic underground structure determined from gravity anomaly. Including this, geoidal undulationgap was detected between the eastern part of Kagoshima City and Sakurajima Volcano, which suggests a graben-like normal fault pattern having a steep slope structure of the basement rocks. Third, recent time-variation of the geoidal height in Sakurajima Volcano was estimated, comparing with that in the case of the 1914 great eruption of Sakurajima from the standpoint of long term prediction of the volcanic eruption, including the final recovery phase.

View full abstract

Based on the analysis of water vapor radiometer data taken at Shionomisaki, daily fluctuation and azimuthal non uniformity of tropospheric wet delay are investigated. It is shown that wet delay has its maximum value at night then decreases till it reachesits minimum value around noon with an amplitude of about 1.0 cm in the zenith direction. It is also revealed that wet delay is not symmetric in the north-south direction with maximum difference of 1-2 cm at the elevation angles of 50-15 degrees. Besides Chao's and Niell's mapping functions are also evaluated using the radiometer observation. The two mapping functions give no significant difference in calculated slant delays and always overestimate the wet delay at Shionomisaki compared to water vapor radiometer data. This overestimation error reaches about 30 cm at 15° elevation angle.

View full abstract

Changes in the horizontal crustal strain in Korea have been derived from the survey data of new precise geodetic networks, and those of the old triangulation made in 1919-1915. The mean rate of the maximum shear strain is 0.12 μ/yr. This rate is about one third of that in Japan, and its spatial distribution is consistent with the seismicity in Korea. The mean direction of principal axes of the compression is about N 80°E. This direction is well accordant with the P-axis distribution of the focal mechanisms of earthquakes in this region. Dilatation has not been calculated because of the scale error between the two surveys.

View full 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.

View full abstract