Micro- and Ultramicroearthquake activities in the vicinity of the Fuji River around Suruga Bay, central Japan, have been investigated by the high sensitive seismographic network. In order to improve the accuracy of hypocenter determination, P- and S-wave velocity structures were examined by the explosion seismic observations carried out in and around our network, and checked using the Wadati diagrams. Station corrections were also made by arrival times of distant earthquakes.
Then, the detectability of hypocenters and origin times of microearthquakes were investigated by applying the method of prediction analysis. It is shown that all the events with M≥0 are detectable in the concerned area.
Finally the following results were obtained for the seismicity in this area during the period from June 1981 to October 1983;
(1) Most events are within the magnitude 0 to 1.5.
(2) In the inner most part of Suruga Bay and along the Fujigawa fault, seismic activities are very low.
(3) A belt shaped active area with about 15 km long, is recognized in the northern part of the seismic network, and composed of microearthquake clusters with 1-2 km linear dimension of surce region.
(4) Microearthquake activities in this region reflect on the subduction of the Philippine Sea Plate from the Suruga Trough.
