Microearthquake Activity in the Southernmost Part of Yamanashi Prefecture, Central Japan (1)

The location of the Fujigawa Crustal Movement Observatory is quite near to the Itoigawa-Shizuoka tectonic line, Mt. Fuji, Mt. Akashi and the Suruga Bay. Despite its geographical significance, microearthquake observation has been carried out only at one station so far, from which we could not know more than S-P distribution about the seismicity of this region. To remedy this situation, tripartite observation was begun in June, 1978. Prior to the start of observation, all IC type pre-main-power amplifiers and crystal clocks which can be corrected with time signals from a radio receiver were newly designed and constructed. As a first step, by assuming semiinfinite medium, focal locations were decided geometrically with P-times of three stations and S-P times of one station, UMJ. The following features can be pointed out from obtained epicentral distribution. (1) A seismic zone which has a NE-SW trend is clearly seen. Although this trend does not coincide with the geological one, it may harmonize with a general trend of seismicity in a wider area around the tripartite. (2) Along the Itoigawa-Shizuoka tectonic line, can be seen rather high seismicity although it is considerably scattered and obscure. (3) The seismicity near Mt. Fuji and Mt. Akaishi seems to be low, and also in the region of the mouth of the Fuji River, which is the innermost part of the Suruga Bay. As to the seismicity along the southern part of the Itoigawa-Shizuoka tectonic line, one interesting fact was found by use of JMA data. The seismic activity in this area was quite silent within the interval from 1969 to 1973. Each of these two nodal years, 1969 and 1973 corresponds to an epoch marking a fact that the crustal movement in the central part of Japan has notably changed its general tendency