Shallow structure effects on broadband teleseismic P waveforms

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Abstract
The first 5 sec of source-equalized P-wave receiver functions from an array of broadband portable instruments located about 50 km from the coast in western Washington are strongly influenced by very shallow structure (depths less than 3 km). These data display behavior associated with a dipping, high-velocity contrast interface at shallow depths. The direct P wave and the P-to-S converted phase vary strongly with backazimuth producing an apparent delay in first arriving energy at western backazimuths. These patterns are attributed to a shallow contact between low-velocity sandstones at the surface and an underlying high-velocity volcanic unit. This shallow structure influences even the original unprocessed teleseismic P waves, suggesting that such shallow structure could affect other studies, such as seismic source analysis, utilizing broadband data.