Faulting Mechanism of the Gifu Earthquake of September 9, 1969, and Some Related Problems

The faulting process of a moderate-size shallow earthquake on inland Japan, the central Gifu earthquake of September 9, 1969(M=6.6), has been investigated by synthesizing all available seismic and geodetic data.The focal mechanism solution based on data from Japanese and WWSSN stations and the spatial distribution of aftershocks indicate that the earthquake was caused by left-lateral strike-slip motion along a vertical fault plane with dimensions of 20 by 10km striking in the N30°W direction. The strong-motion records from five JMA stations within a distance of 80km are compared with the synthetic seismograms computed from dynamic dislocation models. The comparison yields estimates for the seismic moment, average fault displacements, rise time, and rupture velocity. The pattern of vertical tectonic movements from pre- and post-earthquake leveling data along two nearby routes is generally consistent with that derived from the corresponding static models, but there is some possibility that displacements over the northwestern part of the fault might be considerably larger than the average. The strain steps that have been recorded during the earthquake on 23 strain meters at ten crustal movement observatories are not always consistent with theoretically expected values.In relation to the faulting process, pre-earthquake data from leveling and tiltmeter observations are also examined from the viewpoint of earthquake prediction, and tectonic implications of the earthquake are discussed.