Abstract

The diffusive approximation predicts that the ratio of S‐to‐P energy in earthquake coda depends only on the ratio of S and P wavespeeds. However, for seismic applications, the near‐surface geologic conditions can introduce complications to the original models of coda diffusion. Specifically, the free‐surface and near‐surface site effects can conspire to significantly affect the ratio of body‐wave energy when observed at the Earth’s surface. To address these effects, previous workers developed physics‐based models of coda diffusion to account for near‐surface effects such as body‐to‐surface‐wave mode conversions and Earth stratification. These models predict that the value of the S‐to‐P energy in coda will be strongly affected by near‐surface effects; however, there have been very few controlled experiments to verify this. In this article, we describe the results of an experiment that was designed to observe the effects of the near surface on the S‐to‐P strain energy ratio of the coda. Two arrays were deployed simultaneously at two different sites, each of which contained significantly different near‐surface geology. We observe that the energy ratio of the coda is significantly affected by the near‐surface geology and is highly frequency dependent.

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