SEISMIC ACTIVITY IN THE NORTHERN PART OF THE KINKI DISTRICT AND RELATED PROBLEMS (1) : EARTHQUAKE SWARM ACCOMPANYING THE WACHI EARTHQUAKE OF AUGUST 18, 1968

Abstract

An earthquake swarm accompanying the Wachi earthquake of August 18, 1968 was investigated in reference to the time-variation of seismic activity. About 9, 000 events observed from February 1968 to April 1969 were classified into three stages, namely, the foreshock sequence, and the first and second parts of the aftershock sequence, according to the mode of their occurrence. These sequences were eventually characterized by the respective portions with a linear, a logarithmic and an exponential relation in the curve of cumulative sum of their daily counts. As the result of analyses, significant differences in the focal distribution, the focal mechanism and the value of b in the Gutenberg-Richter's formula could be ascertained between the sequences. This fact may indicate that the property of the medium or the regional stress acting on the focal area varied significantly with respect to the sequences. Moreover, a detailed analysis concerning the foreshock sequence showed the interesting fact that the fault of the first major shock of February 14, 1968 lay just on a branch of the Mitoke Fault which was discovered recently by the use of geological methods, whereas during the later part of this sequence the foci of minor shocks gathered at the fault of the main shock, as if they had been prepared for the main rupture of the focal area, thus both of the major faults might form a conjugate set with an angle of intersection of about 75°.