Abstract

We compare various site-response estimation techniques using after-shock data of the 1989 Loma Prieta earthquake collected in Oakland, California. Because of recent interest in comparing results from weak and strong motion (to infer any nonlinearity) and between direct S and coda waves, we pay particular attention to the uncertainties. First, sediment to bedrock spectral-ratio estimates between pairs of sites are compared with those obtained from various generalized-inversion approaches where the source and site effects of multiply recorded events are solved for simultaneously. We find that the site amplification factors are very similar among these approaches, but that the uncertainties can be significantly different depending on how the data are weighted.

We also examine and test three site-response estimation techniques that do not rely on a reference site to estimate source and path effects. The first involves a parameterized source- and path-effects inversion. Even when the bedrock data are excluded from consideration, this approach is found to reveal the frequency dependence of site response at each of the sediment sites. The second technique involves taking horizontal- to vertical-component spectral ratios (receiver-function-type estimates) of shear-wave aftershock data. These are also found to reveal the frequency dependence of site response at the sediment sites, and the results for the bedrock site are relatively flat and near unity. the third estimate is formed by taking horizontal-to vertical-component ratios of ambient seismic noise, and these are shown to reveal the fundamental resonant frequency of the sediment sites. Unfortunately, discrepancies exist among all of the site-response estimates (and with one-dimensional predictions) with respect to a frequency-independent scaling factor. Nevertheless, the highly frequency-dependent character of site response is well constrained, and the fact that non-reference-site-dependent methods are capable of revealing this is promising for site-specific hazard assessments in regions that lack adequate reference sites.

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