| Abstract |
The Geological Survey of Japan's microearthquake study program is intended to determine the geophysical properties of the fracture and fault systems which constitute known reservoir of hot water and/or steam and to systematize the study of microearthquakes as a geophysical investigating tool for geothermal area. Detailed studies of microearthquakes can reveal the spatial extent of the geothermal reservoir by establishing the epicenter distribution. Variations in seismic wave travel time may be related to anomalies in the spatial distribution of seismic veloelties. Low velocity regions may be due to the presence of highly fractured zones in geothermal areas. Spatial and temporal variations in induced earthquake epicenters allow an estimation of permeability around a injection well (Ito and Sugihara, 1988). The above studies require only the arrival time of first seismic motion as recorded by each seismometer. On the other hand Sugihara and Tosha (1988) identified microearthquake source processes in the geothermal area by an analysis of the shape of the seismic wave. The waveform itself is important in this later study. Shear wave splitting is observed when rocks under seismic stations are anisotropic in the propagation of shear wave. The anisotropy is caused by aligned microcracks (Crampin, 1978). This analysis requires a great dynamic range record because of the use of the first movement. An example of the shear wave splitting will be discussed later. Since December 1982, a digital acquisition system with seven 3-component seismometers has been operating in the Takinoue geothermal area (which contains the Kakkonda geothermal power plant) IwatePrefecture in Japan (Ito and Sugihara, 1987). The microearthquake data acquired by the system reveal the fundamental characteristics of the earthquake in the geothermal area. Design work is in progress on a portable version of this data acquisition system. In 1985, 1986, and 1988, we recorded numerous microearthquakes which took place in the Takinoue geothermal area. The seismic data are recorded not only in the seven permanent stations but also temporal stations. The field experiments were designed to provide improved hypocenter locations and compressional and shear wave velocity structures. Another objective was to identify any shortcomings in the portable acquisition system. After the experiment the system have been improved both in hardware and in software to overcome the shortcomings. |