Seismic Directivity on Maximum Accelerations and RMS Accelerations

Abstract

Azimuth dependence of strong motion amplitudes has been studied. The component waves of short period strong motion are characterized to be random phases, since they are generated by random fractures of small-scale fault heterogeneities (fault patches) and propagating real media through multiple reflection, refraction and scattering. Such the waves donot show coherent azimuth dependence like as one due to a point source radiation pattern. However, they show short period seismic directivity effect. The effect comes from azimuthal variation of energy radiation due to the finite-moving source. Azimuth dependence of maximum acceleration and rms acceleration has been derived for a unilateral faulting as
α·(mLr²)^1/2(c/v-cos θ)^-1/2
where α denotes the whole path attenuation, m is a number density of fault patches per unit fault area, L is fault length, and r² is the average seismic energy radiated from one fault patch. c and v stand for the velocity of shear waves and average rupture propagation, and θ is station azimuth measured from the direction of the rupture propagation. Strong motion accelerations due to the 1968 Tokachi-oki earthquake and the 1983 Japan Sea earthquake do show such the azimuth dependence for maximum and rms accelerations.