Site amplification of coda waves from local earthquakes in central California

Abstract The goals of this study are two-fold: (1) to examine the site effect of coda waves, believed to be the average site effect of shear waves, in order to understand its spatial and frequency-dependent behavior, and (2) to learn what we can about the processes that generate the coda itself. We use digital data from over 90 earthquakes, each recorded by a subset of some 150 stations in the Coast Ranges of central California between San Francisco and San Luis Obispo. Results from the band 1.5 to 24 Hz indicate that site amplification depends strongly on surface geology and frequency. At low frequencies (1.5 to 3.0 Hz), changes in amplification up to a factor of 20 are observed; amplification generally varies inversely with deposit age at sediment sites and is lowest at granite and Franciscan (basement) sites. At high frequencies (6 to 24 Hz), the pattern changes. Many granite sites, the majority in the Gabilan range, exhibit increasing amplification with frequency relative to the average station. This behavior differs strikingly from that observed at sediment and Franciscan sites which roll-off at an intermediate rate, and many sediment sites adjacent to the San Andreas fault zone which decay away even more rapidly with frequency. These results can be explained qualitatively by appealing to variable near-site impedance and attenuation, however, complicating phenomena such as mode excitation in valley sediments must exist at low frequencies. Peaked behavior at a few hard rock sites at high frequency (12 Hz) currently lacks explanation. At a few sediment sites, the low-frequency excitation is so strong that the coda envelopes begin to take on different shapes. This contradicts the assumption based on previous observations that the coda shape is independent of source-receiver location, thus, some site measurements may be artificially high. The working model of coda waves as body-wave energy backscattered from randomly distributed inhomogeneities must be modified to include the possibility of efficient near-site resonance excited by incident direct and coda wave energy.