Edward H. Field

Spectral amplification in a sediment-filled Valley exhibiting clear basin-edge-induced waves

  • Geochemistry and Petrology
  • Geophysics

Abstract Various site-response estimates are presented for a linear array deployed in the Coachella Valley, California, during the 1992 Landers/Big Bear aftershock sequence. This systematic comparison is unique in that the response of the site is clearly dominated by basin-edge-induced waves. Average sediment to bedrock spectral ratios for long S-wave windows, which include the basin-edge-induced waves, exhibit amplification factors as high as ∼ 18, below 7 Hz. The deep basin structure, which gives rise to the obvious multi-dimensional effects, produces a fundamental resonant peak that shifts between basin sites (from 0.23 to 0.48 Hz) as the depth to bedrock changes. Above 0.6 Hz, where the largest amplifications occur, the response is remarkably similar between sites and appears to be dominated by a near-surface layer that is relatively uniform across the valley. The apparent fundamental resonant frequency of this layer is between 0.8 and 1 Hz. Sediment to bedrock spectral ratios computed using shorter windows that exclude the basin-edge-induced waves imply that the multi-dimensional effects are significant only below ∼ 4 Hz, where they increase amplifications by an approximate factor of 2. Spectral ratios computed using coda windows, taken at twice the S-wave travel time, exhibit amplifications that are an average factor of 1.7 greater, between 1 and 4 Hz, than those of the S-wave estimates. This discrepancy does not improve by taking coda windows later at four times the S-wave travel time. Horizontal- to vertical-component S-wave spectral ratios do not agree with the sediment to bedrock ratios. However, they do exhibit a clear peak at the fundamental resonant frequency of the deep basin structure. Sediment to bedrock spectral ratios of ambient seismic noise are also inconsistent with the S-wave estimates. However, horizontal to vertical noise ratios exhibit clear peaks near the fundamental resonant frequencies of both the deep basin structure (below 0.6 Hz) and the suspected near-surface layer (between 0.8 and 1 Hz). Therefore, ambient-noise data appear to provide valuable constraints on the basin structure. Ongoing efforts involve multi-dimensional modeling of the observed basin-edge-induced phases and resonant frequencies.

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