Abstract
The observational data (Tables 1 and 2) of earthquakes at Unzendake volcano are processed to make clear the features of the recent seismic activities, with the results by previous investigations.
There are several epicentral areas in and around this volcano (Fig.1).
The period since 1968 through 1974 is one of prominent seismic activitie s, judging from the frequent occurrence of earthquakes and felt shocks and the remarkable energy-release (Table 3, Figs.2,3 and 4). This active stage is peculiar in the past 53 years, because the high level of activities continued for seven years (Fig.4).
During the active period, most of the earthquakes showe d P-S time within 2.0 sec.. The epicentral area of such earthquakes is located around the Bay of Tiziwa on the west side of this volcano.
The estimation of the critical frequency of earthquakes to detect unusual seismic activity is also discussed by the method of the test of rejection proposed by Thompson on materials processed by the easy normalization test (Figs.5 and 7). As the result, the frequency of 18-19/day was calculated as the critical frequency of earthquakes during the period from 1967 through 1976 and 7-8/month in the case of felt shocks during the period from 1924 through 1976 (Figs.6 and 8). These unusual values are close to the lowermost frequency in the case of earthquake-swarms during the period from 1967 through 1976. It will be possible to use the above-mentioned frequency as the critical frequency of detection of an earthquake-swarm.
Remarkable change in Ishimoto-Iida 's Coefficients m in the amplitude-frequency distribution of earthquakes is not detected in and around the active period from 1968 through 1974 (Table 4 and Fig.9).
An increase of earthq u akes of P-S time over 3.1 sec. appears in 1967 and 1968, and the frequency lessened during the active period (Figs.10 and 11).
