SEISMIC INTENSITY DISTRIBUTION OF SHALLOW EARTHQUAKES DUE TO RUPTURE VELOCITIES AND FAULTING MODES

Isoseismals not only represent seismic intensity distributions but also represent earthquake source size, faulting mode, and rupture velocity of fault propagation. Unilateral faulting forms egg-shaped isoseismals, while bilateral faulting forms elliptical ones. It is found that the ratio of major to minor axes of isoseismals is sensitive to rupture velocity. Rupture velocity, faulting mode, and fault trend have been determined from the seismic intensity maps of the 1964 Niigata and the 1983 Japan Sea earthquakes in Japan and the 1975 Haicheng and the 1976 Tangshan earthquakes in China by matching theoretical isoseismals. Rupture velocities thus estimated are mostly 70 to 90% of shear wave velocity They are a little higher 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 terminations of local rupture propagations. On the other hand, rupture velocity from long-waves represents an average rupture propagation along the whole fault length. Faulting mode and fault trend estimated from seismic intensity maps match with the earthquake faulting independently determined. This suggests that the present method could be applicable to some historical earthquakes with known seismic intensity distribution to obtain detailed information on the faulting process.