1972 年 25 巻 4 号 p. 295-301
The relationships between the predominant period and the magnitude of the microearthquakes were studied. We assume the relationship in the following form. LogT=a+bM where, T is the predominant period in second and M is the earthquake magnitude. The following results were obtained, LogT=-1.50+0.50M (P wave, microearthquakes occurred in the vicinity of the Neo-Valley) LogT=-1.12+0.45M (S wave, microearthquakes occurred in the vicinity of the Neo-Valley) LogT=-1.38+0.44M (P wave, microearthquakes occurred in the vicinity of the Inuyama City) LogT=-2.03+0.65M (P wave, Matsushiro earthquake swarm) LogT=-1.45+0.40M (whole P wave). From the dislocation theory, the relation of T and M is derived as follows, LogT=-0.3-0.7 Logσ+0.5M. (1) where, σ is the stress drop in bar. Substituting σ=40 bars into (1), we get LogT=-1.4+0.5M. (2) This equation agrees well with the above mentioned results. But, when σ is not constant, the equation (1) suggests that the difference of the predominant period in the same earthquake magnitude is the differences of the stress drop. Indeed, the deviations of the data are so large that the stress drops seem to be not constant.
The predominant periods of the Matsushiro earthquake swarm are shoter than those of the other microearthquakes. This evidence may relate to the characteristics of the Matsushiro earthquake swarm.