Seismic Intensity Distribution due to Fault Movement

Abstract

The strong ground motions show azimuthal dependence due to finitely-propagating rupture. Since we may consider that the extreme value of strong ground motions determines seismic intensity, the azimuthal dependence of seismic intensity is derived on the basis of short-period seismic directivity. Unilateral faulting forms an asymmetric intensity distribution like an egg shape. The largest intensity is found in the direction of rupture propagation and the smallest in the opposite along the equi-epicentral distance. Bilateral faulting forms a symmetric intensity distribution like an elliptical shape. The smallest intensity is in the normal direction of fault strike and the largest in both the directions of the fault trend along the equi-epicentral distance.
Seismic intensity distribution for the 1983 Japan Sea earthquake favors unilateral faulting, which matches with the rupture process estimated from the aftershock distribution. The distribution for the 1964 Niigata earthquake is consistent with bilateral faulting. The Niigata earthquake is known to be one of the largest bilateral faulting near Japan.