小地震の前震余震分布

抄録

A modified master event method has been used for the pricise determination of hypocenters of small earthquake sequences that occurred within the telemetered station network of the Abuyama Seismological Observatory in the northern Kinki district, central Japan. By use of the accurate P wave arrival times of telemetered stations, this method has decreased the uncertainty of relative location of each event to its master event to be about 0.1km. And the detailed spatial distributions have been obtained for ten earthquake sequences of which magnitudes are from 2.5 to 4.0. The characteristics of the distributions are as follows; (1) The distribution of aftershocks and foreshocks of small earthquake, as well as that of large earthquake, defines a plane which seems to indicate a fault plane. (2) The trend of the aftershock area agrees with the trend of one of the nodal planes of main shock determined from the first motion of P waves. (3) Most of main shocks have been located on one end of aftershock areas and foreshocks have been located in the vicinity of the main shock. (4) The length of aftershock area is about 0.8km for M=4.0 earthquake sequence and 0.5km for M=3.0 sequence. These values roughly agree with the expolated values of UTSU's empirical equation between linear dimension of aftershock area and magnitude. The area of earthquake swarm, however, is two or three times larger than that of ususl main shock-aftershock sequence. (5) The aftershock area enlarged slightly during the activity.