On the Mechanisms of the Earthquakes of Southeastern Part of Akita Prefecture in 1970

The focal mechanisms of the earthquake of southeastern part of Akita Prefecture which occurred on October 16, 1970 and its aftershocks are studied by using the initial motions of the P- and S-waves. All available data are obtained from WWSSN, JMA, microearthquake observatories of Tohoku Univ., and temporary stations of Tokyo Univ.The results are as follows:1) The mechanism solution of the main shock based on the data of the initial motions of the P and S-waves indicates reverse faulting. The orientation of the tension axis is nearly vertical and that of the pressusre axis is nearly horizontal in the WSW-ENE direction. The fault plane is not distingushed from the auxiliary plane by the distribution of the aftershocks and other data of the field observations.2) The mechanism solutions for four larger aftershocks are obtained by the use of the data of the initial motions of the P-waves. The mechanism solutions for the other small aftershocks are inferred from the contributions of the regionally superposed initial motions of the P-waves. They indicate that there are two types of the orientation of the earthquake generating stresses in the aftershock area. The orientation of the earthquake generating stress for the north-west region of the aftershock area is different from that of the main shock and the orientation for the rest part of the aftershock area is consistent with that of the main shock.