断層破壊伝播速度と被害地震の震度分布

―浅発地震の断層不均質性と短周期波動のドップラー効果―

抄録

Isoseismals of seismic intensity distribution are shown not only by the size of earthquakes but also by the faulting mode and the rupture velocity of fault propagations. Unilateral faulting forms an egg-shaped isoseismal, meanwhile bilateral faulting forms an elliptical one. It is also shown that the ratio of major and minor axes of the isoseismals is sensitive to the variation of rupture velocities. Rupture velocities, faulting modes, fault trends of the 1964 Niigata and the 1983 Japan Sea earthquakes in Japan and the 1975 Haicheng and the 1976 Tangshan earthquakes in China have been determined from the seismic intensity maps by matching the theoretical isoseismals to those observed. Rupture velocities thus estimated are mostly 70 to 90% of shear wave velocity. They are a little larger than those obtained from long-period seismic waves. This difference would be considered as follows: Short-waves which determine the seismic intensity are strongly dependent on the rupture of small-scale fault heterogeneities and on the jerky-onsets and abrupt terminations of local rupture propagations. On the other hand, the rupture velocities from the long-waves represent the average rupture propagation along the whole length of the fault. Faulting-modes and fault trends estimated from the seismic intensity maps match with the independent results of the earthquake faultings. This suggests that the present method would be applicable to infer the faulting mode, rupture velocity, and rupture direction of some historical earthquakes.