Causes of low‐frequency ground motion amplification in the Salt Lake Basin: the case of the vertically incident P wave

Abstract

We use simulations of 1‐D, 2‐D and 3‐D wave propagation to identify the major causes of low‐frequency (0.2–1.2 Hz) seismic amplification in the Salt Lake Basin. For a simple two‐layer basin model and a vertically incident P wave, we examine how amplification is influenced by mode conversion, surface‐wave generation, impedance effects at the sediment‐bedrock boundary, resonance, and 2‐D and 3‐D focusing and scattering. Results show the following. (1) Approximately 30 per cent of the total cumulative kinetic energy at the Salt Lake Valley floor consists of shear‐wave energy generated by P‐to‐S converted waves and surface waves. The surface waves appear to be generated primarily along the edges of the basin, and the instantaneous S/P energy ratio in the sedimentary layer is as large as 3. (2) The largest peak particle velocity at the free surface is due to the direct P wave. The value is roughly predicted by the transmission coefficient of 1.46 at the sediment‐bedrock interface, i.e. a normally incident P wave in the stiff bedrock will be magnified in amplitude by 1.46 times as it enters the softer sediments. (3) The low‐frequency elastic response of the two‐layer Salt Lake Basin model is characterized by surface‐wave propagation and resonance from vertically interfering waves. (4) The peak particle velocities, cumulative kinetic energies, and mean spectral magnitudes computed from the 2‐D (1‐D) synthetics underestimate the values computed from the 3‐D synthetics by up to 40 per cent (48 per cent) along a profile above the deepest part of the basin model. The 2‐D and 1‐D signal duration times underestimate the 3‐D values by up to 59 and 94 per cent, respectively. Our results suggest that 2‐D basin modelling may yield good approximations to the 3‐D ground motion amplification above the deepest part of the Salt Lake Basin. Our results show that several mechanisms contribute significantly to low‐frequency seismic amplification in the semi‐consolidated sediments of the Salt Lake Basin—P‐to‐S wave conversion, surface‐wave generation, impedance effects at the sediment‐bedrock boundary, and resonance. Future attempts to estimate ground motion amplification in the Salt Lake Basin should therefore account for the amplification effects of all these mechanisms. Copyright © 1995, Wiley Blackwell. All rights reserved