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The Noto-Hanto-Oki earthquake (MJMA=6.6) occurred NE off Noto Peninsula on February 7, 1993. According to JMA (Japan Meteorological Agency), the focal depth of this event was 25 km, which was deeper than most inland shallow events in Japan. There were distinct differences among the focal mechanisms obtained from the data of the P-wave initial motions, the long-period seismic waves, the short-period strong ground motions, and the tsunami waves. However, most of them indicated that the earthquake fault of this event was of the reverse fault type. We evaluated the source depth and the focal mechanism of the event through parametric simulation studies in the period range from 4 to 20 seconds of long period strong ground motions observed at 4 local stations. These stations were located from 46 to 133 km from the epicenter. First, the subsurface structure from the source to each station was estimated as a multi-layered model by an inversion analysis of the group velocities from the observed Rayleigh waves. The structure obtained for each station was consistent with the distribution of gravity anomaly. It was concluded from the best fit of the simulation and the location of aftershock region that the fault plane of the event dipped 45° toward N140°E with a slip angle of 70°. We found that the fault plane simultaneously dipped 45° toward N330°E with a slip angle of 70°. However, this case was inconsistent with the deformation of the sea bottom by active faults. The depth of the equivalent point source, whose location corresponds to the center of the fault plane, was 10 km in the best case of the simulation. The depth was significantly shallower than the focal depth reported by JMA and was consistent with other inland shallow earthquakes in Japan. All the results were obtained from the simulation using the point source model. However, it was also found that the conclusion did not change for the results from the model of finite moving source of this event.

jishin. dai1shuu/jishin/jishin. dai2shuu

Source Depth and Focal Mechanism of the 1993 Noto-Hanto-Oki Earthquake and Subsurface Structures in and around Noto Peninsula Derived from Rayleigh Waves Observed at Local Stations