Source region of the Matsushiro earthquakes



Teleseismic and near-field data of the Matsushiro earthquakes were studied collectively in order to clarify the focal mechanisms of the swarm activity. The author first applied the Aki and Brune techniques to the Love waves recorded at Gifu, 180 km southwest of Matsushiro, and discovered that the seismic moment of major swarm shocks is about 0.7 ˜ 3.5×1022 dyne cm for the magnitude range of 4.6 ˜ 5.1. Stress drop for these shocks is then estimated at 0.6 ˜ 7.0 bars, properly assuming several other source parameters.

Comparision of the retriangulation data with theoretical fault models proved that horizontal ground movements in the epicentral area are attributed fairly well to a buried strike-slip fault, as formerly suggested by geologists on the basis of surface evidence. Geometrical dimensions of the buried faults were thus estimated at 3 km (deep) ×7 km (long), with surface cover of 0.5 km (thick).