Abstract
Kinugawa area (N36.0°-36.3°, E139.7°-140.0°), the southwestern part of the Ibaraki prefecture, is one of earthquake clustering regions at depths from 40km to 60km in the Kanto district, central Japan. A seismic moment of an earthquake of August 4, 1974, which is the largest event (M5.8) in the past two decades in the Kinugawa area, is obtained by the least square's method for P wave amplitude data from WWSSN long period records. The seismic moment obtained is 0.77×1025dyne·cm with a focal mechanism of low-angle thrusting to the NNW direction.
There were 2 events of a magnitude larger than 5.4, excluding the largest event, in a period of 15 years from 1966 to 1980. The seismic moments of the events are estimated by the Rayleigh wave amplitudes from the WWSSN long period records of SHK relative to that of the largest event. Thus, the total amount of the seismic moment released in the Kinugawa area (120km2) during the 15 year period is 0.98×1025dyne·cm. This estimation is about 20-30% of an amount that is predicted by a simple scheme on the basis of relative movements of the Philippine Sea and Eurasian plates in the Kinugawa area.
