Effects of fault finiteness on near-source ground motion

Abstract

Near-source ground motion at four azimuths but constant epicentral range is computed from a buried circular strike-slip fault in a half-space. Particle acceleration, velocity, and displacement at each station on the free surface is computed in the frequency band 0.0 to 5.0 Hz. The assumed dislocation is derived from the Kostrov (1964) displacement function for a continuously propagating stress relaxation. The azimuthal variations in the amplitudes and waveforms directly result from spatially varying slip on the fault, spatially varying radiation pattern over the fault, and the magnitude and direction of the rupture velocity. The near-source ground motions are dominated by the rupture in the direction of the receiver. Using a 100-bar effective stress (initial stress minus sliding friction) in a Poisson solid with β = 3.0 km/sec the shear wave speed, and shear modulus μ = 3.0 × 10¹¹ dyne/cm², the simulated earthquake has a moment M_o = 4.5 × 10²⁵ dyne-cm. Using a rupture velocity of 0.9β, the peak acceleration is 1195 cm/sec² and velocity 104 cm/sec for the receiver directly on strike. For a receiver 30° off strike, the maximum acceleration 236 cm/sec² occurs on the vertical component.