東北日本弧一日本海溝系の地震テクトニクス
1985 年 31 巻 1 号 p. 124-146
詳細
1985 年 31 巻 1 号 p. 124-146
The most recent data obtained from the high gain seismograph network and the exact leveling in Tohoku District, northeastern Honshu, Japan, are used to investigate seismic activities and tectonics in the northeastern Japan Island Arc. The location of the plate boundary between the descending lithospheric slab and the mantle above it was clearly determined from the difference in arrival times between the ScS and ScSp waves. Accurate determinations of microearthquake hypocenters have led to conclusive evidence that the boundary is located on the upper plane of the double planed deep seismic zone. The Miyagiken-oki Earthquake with M7.4 occurred on June 12, 1978 off the coast of Miyagi Prefecture. The data obtained from the seismograph network of Tohoku University are made the best use of an investigation on seismic activity before and after the main shock. The followings became clear after the detailed analyses on the spatial distribution of main shock and aftershocks. The main shock and almost all the aftershocks occurred at the boundary between the oceanic slab and the overriding continental lithosphere. Focal-mechanism solutions of the main shock and aftershock indicate that these earthquakes are characterized by the low angle reverse faulting. We apply a two-dimensional finite element method to estimate the crustal movement connected with plate movement, by using a proposed model in which the lower crust and the upper mantle are assumed to be viscoelastic materials. It is revealed in the model above mentioned that mutual movement at the boundary between descending oceanic slab and the continental lithosphere above it, is more fast at the region from the trench to the a-seismic front compared with that of other boundary, as the result of the calculation on the crustal movement on land caused by the subducting plate. We may conclude from the results of numerical computations that the distribution of crustal stress is clearly related to the change of crustal structure, and lead to the recognition that the precise determination of the crustal structure is of very importance.
