COMPLEX CORRELATION OF SEISMIC AMPLIFICATION WITH GEOLOGICAL SITE CONDITIONS IN CHARLESTON, SOUTH CAROLINA

During 2010 we collected ambient seismic noise (microtremor) at 75 sites across the greater Charleston, South Carolina region. We processed this data to produce horizontal-to-vertical spectral ratios (HVSR) to estimate the frequency and relative amplitude of seismic amplification. We compare these seismic amplification estimates with mapped surface geology, depth to the Quaternary/Tertiary (Q/T) interface and shear wave velocity (V_S) from geotechnical boreholes to better understand the role geological site conditions play in producing earthquake damage, both in the 1886 earthquake and in future events. We find that HVSR have peaks at ~0.2 Hz and 1-2 Hz across most of the surveyed region. Modeling of HVSR shows that the 0.2 Hz peak is consistent with the ~900 meters of post-Jurassic sediment (average V_S ~ 700 m/sec) beneath the Charleston region. The 1-2 Hz amplification peak appears to be a result of “piling on” of amplifications set up by impedance contrasts at ~63 meters and ~20 meters together with a higher mode amplification produced by the entire sedimentary column. The amplitude of this 1-2 Hz peak is strongly correlated with the age of the geologic surface. Amplitudes > 8 occur almost exclusively on artificial fill sites, while older surfaces have lower 1-2 Hz amplitudes. Surprisingly, geotechnical estimates of shallow V_S do not correlate well with the amplitude of this 1-2 Hz peak; i.e., higher amplifications are generally expected on lower velocity materials. The estimated depth of Q/T interface does however correlate with the amplitude of the 1-2 Hz peak, with higher amplifications occurring on deeper Quaternary sections. We will reexamine the spatial distribution 1886 earthquake damage in light of these new results.