極微小地震の規模別頻度分布について

抄録

Frequency distribution of magnitude of the ultramicro-earthquakes was studied by using the data of the Matsushiro earthquake swarm observed in December 1966.
As a result of analyses, the Gutenberg-Richter's formula concerning the frequency distribution of magnitude was found to hold well for the magnitude range down to about -0.9. The number of shocks then decreased very sharply with decreasing magnitude and no shocks of magnitude less than -1.3 were registered, though still within the limit of detection capability of the sensors. The seismograms of smaller events, if any, should be identified as earthquake motions, because the signal to noise ratio for a minimum shock was yet about 3, so that smaller events could not be masked by disturbances.
A crude estimation showed that the linear dimension of the focal region for the minimum shock was of the order of a couple of meters. The size of focus for the shock of magnitude -0.85, where a peak in the frequency distribution occurred, amounted roughly to double the size for the minimum one.