関東地域に発生した地震の卓越周期とマグニチュード

抄録

The relationship between the predominant period and the magnitude of the shallow earthquakes which occurred in and near the Kwanto district has been investigated by using the data recorded by HES-TL Seismometer at Tsukuba. The following results have been obtained for P- and S-waves of the earthquakes with 3.4≤M<6.
log T_p=-2.47+0.54M; for P-wave
log T₈=-2.40+0.58M; for S-wave
The regional variation of predominant period is found from the distribution of the softness value which indicates the deviation of the period from the standard value. The earthquakes which occurred in the northern part of the Kwanto district have the predominant period in the shorter range. On the other hand, the predominant period for the earthquakes in the southern part of the Kwanto district lies in the longer range. This result is consistent with the result for micro-earthquakes. The S-wave's predominant period (T₈) is longer than that of P-wave (T_p). The ratio of the predominant periods of P- and S-waves, T₈/T_p, tends to about 1.5 in the range of longer period. This result is not explained by the conventional dislocation model. The average stress drop for the earthquakes in the Kwanto district is calculated from the circular crack model as 60 bars.
The M-T relation for P-wave in a wider range of magnitude is obtained by adding the earthquakes which occurred in the Tohoku and Hokkaido regions. The result is
logT=-2.41+0.52M(3.4≤M≤7.2)
The data for larger earthquakes in Japan are also added, and the regression line for M-T relation is obtained as
logT=-2.14+0.46M(3.4≤M≤8.5)
These relations are not consistent with the M-T relation for micro-earthquakes. The discrepancy may be due to the difference of the definition of earthquake magnitude or due to the difference of source process, particularly to the difference of stress drop, between microearthquakes and larger earthquakes.