Comparison of Earthquake Mechanism Solutions Obtained from First Motion Analysis with those from Waveform Analysis

Abstract

To examine availability of moment tensor data for local earthquakes obtained from broadband seismic observation, we made a comparison of earthquake mechanisms between moment tensor solutions and first motion solutions for earthquakes occurring beneath the Kanto-Chubu district, central Japan. Agreement of these two solutions is good as a whole. Large difference between the first motion and moment tensor solutions is, however, observed for a considerable number of earthquakes.
No systematic relationship is found between the quality of the solution and the difference between them. The difference thus cannot be explained by uncertainty in determination of the mechanism solutions. Difference of first motion and moment tensor solutions seems to become weak for deeper earthquakes. The direction of principal axes (P- and T-axes) of first motion solutions is much scattered than that of moment tensor solutions among neighboring events. It is also recognized that earthquakes with large difference in mechanism solutions are occurring in the two clustered regions; off Ibaraki prefecture, and near the Miyake-jima island. There would be a number of small faults in a well-ruptured region such as the Miyake-jima island region. It is possible that the orientation of these small faults is considerably scattered because of the local heterogeneity of strength in the crust. Suppose that a slip along one of them triggers an earthquake occurrence along a different main fault. The first motion solution of this event must be dominated by the orientation of the starting small fault, but the moment tensor solution represents the main fault, which is dominated by the surrounding tectonic stress. In this case the first motion solutions are different from event to event. In contrast the moment tensor solutions are almost the same among neighboring events. From the present study it is concluded that the moment tensor solutions are more suitable to discussion of local seismotectonics.