Mikio Satomura

GPS meteorology project of Japan —Exploring frontiers of geodesy—

1Kyoto University, Uji, Kyoto 611-0011, Japan 2National Astronomical Observatory, Mizusawa, Iwate 023-0861, Japan 3Geographical Survey Institute, Tsukuba, Ibaraki 305-0811, Japan 4Meteorological Research Institute, Tsukuba, Ibaraki 305-0052, Japan 5Nagoya University, Nagoya 464-0814, Japan 6Kochi University, Kochi 780-8520, Japan 7Kyushu University, Fukuoka 812-0053, Japan 8Tsukuba University, Tsukuba, Ibaraki 305-0006, Japan 9Shizuoka University, Shizuoka 422-8017, Japan 10University of Tokyo, Tokyo 113-0032, Japan

Stress-induced spatiotemporal variations in anisotropic structures beneath Hakone volcano, Japan, detected by S wave splitting: A tool for volcanic activity monitoring

Hakone volcano, located at the northern tip of the Izu-Mariana volcanic arc, Japan, has a large caldera structure containing numerous volcanic hot springs. Earthquake swarms have occurred repeatedly within the caldera. The largest seismic swarm since the commencement of modern seismic observations (in 1968) occurred in 2001. We investigated the anisotropic structure of Hakone volcano based on S wave splitting analysis and found spatiotemporal changes in the splitting parameters accompanying the seismic swarm activity. Depth-dependent anisotropic structures are clearly observed. A highly anisotropic layer with a thickness of ~1.5 km is located beneath the Koziri (KZR) and Kozukayama (KZY) stations. The anisotropic intensity in the region reaches a maximum of 6–7% at a depth of 1 km and decreases markedly to less than 1% at a depth of 2 km. The anisotropic intensity beneath Komagatake station (KOM) decreases gradually from a maximum of 6% at the surface to 0% at a depth of 5 km but is still greater than 2.5% at a depth of 3 km. At KZY, the anisotropic intensity along a travel path of which the back azimuth was the south decreased noticeably after the 2001 seismic swarm activity. During the swarm activity, tilt meters and GPS recorded the crustal deformation. The observed decrease in anisotropic intensity is presumed to be caused by the closing of microcracks by stress changes accompanying crustal deformation near the travel path.

GPS measurements to investigate the reason why GPS is less accurate in mountain areas

It has been pointed out that GPS results in mountain areas are usually less accurate than those in flat areas, especially in the vertical component, when the height difference among observation stations is large. It is thought that the decrease of accuracy is caused by insufficient corrections for tropospheric delay of the GPS microwave.

Repeatable Measurements of Baseline Length by Global Positioning System in Central Japan

An accurate determination of baseline length is of basic importance in the interpretation of crustal motions in earth’s interior for the study of earthquake prediction. A complete understanding of the crustal motion of a particular region requires a dense set of geodetic measurements in space and time. During the past decade, Global positioning System (GPS) at the United State National Geodetic Service has been developed chiefly for instantaneous positioning and dynamic navigation. This system has developed further with the capability of measuring baseline length on the order of 1 to 2 ppm accuracy for distances of several ten to several hundred kilometers in length, by using phase interferometric method with the GPS carrier wave length (e.g,, Hothem and Fronczek, 1983).

Observation of Crustal Movements by Means of a Long Baseline Water-Tube Tiltmeter at Sagara, Shizuoka, Japan

It has been pointed out that a destructive earthquake will occur in the Tokai District, central Japan, by the interactive motions between the Philippine Sea Plate and the Eurasian Plate in the near future.