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Propagation and resonance of SH waves in the Salt Lake Valley, Utah

GEOLOGY AND GEOPHYSICS DEPARTMENT UNIVERSITY OF UTAH, SALT LAKE CITY, UTAH 84112

Abstract

The Salt Lake Valley is an alluvium valley where seismic amplification measured at some sites can exceed a factor of 10 relative to hard-rock sites adjacent to the basin. It is important to understand the causes for these amplifications in order to predict which sites in the Salt Lake Valley are most prone to strong ground shaking.

Towards this end, finite-difference synthetic seismograms were used to successfully characterize key parameters controlling low-frequency SH-wave amplification in the Salt Lake Valley. Plane-wave source synthetic seismograms reveal that SH-wave amplification in the frequency band 0.2 to 2.0 Hz is primarily controlled by the two shallowest lithologic units, the 0.25 to 1.0 km thick unconsolidated alluvial Quaternary sediments (V_s = 0.98 km/sec), and the underlying 0.5 to 1.0 km thick semi-consolidated Tertiary sediments (V_s = 1.42 km/sec). The Quaternary sediments tend to increase the coda duration, while the semi-consolidated Tertiary sediments act as a wave guide to trap the low-frequency seismic energy.

It is also found that the basin can cast a seismic shadow over adjacent bedrock sites, inhibiting seismic wave propagation to sites adjacent to the basin. Calibration of valley site amplification to these bedrock sites may result in overestimation of basin site amplification. Finite-difference synthetic seismogram modeling can clearly demonstrate the significant effects of deep basin lithology and geometry on low-frequency site amplification observed across the Salt Lake Valley.

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