Quantifying Site Response of the Atlantic Coastal Plain Strata in the Eastern United States Using Horizontal to Vertical Spectral Ratios

Date: 4/26/2019

Time: 09:00 AM

Room: Pine

Ground motion intensities measured during the 2011 magnitude 5.7 Virginia earthquake exceeded expected ground motions in Washington, DC, leading to surprising damage to the Washington Monument and National Cathedral. The larger-than-estimated site response caused by the unconsolidated sediments of the Atlantic Coastal Plain (ACP) during the Virginia earthquake highlights the need to improve earthquake site response estimates in the eastern United States. The horizontal-to-vertical spectral ratio (HVSR) method, using either earthquake signals or ambient noise as input, offers an appealing method for measuring site response because it uses only a single seismometer rather than requiring two or more seismometers traditionally used to compute a horizontal sediment-to-bedrock spectral ratio (SBSR). We compared site response estimates on the ACP strata using the HVSR and SBSR methods. We find a close match in the frequencies of the first primary resonance peaks (f_peak) with both the SBSR and HVSR methods using teleseismic signals recorded by regional arrays. The HVSR method estimates the amplitude of f_peak within a factor of 2 for ACP strata up to about 200 m thick, but consistently underestimates by a factor of ~5 the amplitude for ACP strata with thicknesses above about 200 m. Although previous studies show mixed results when comparing HVSR with SBSR methods, many of these studies were carried out on confined sedimentary basins that can generate substantial 3D basin surface wave effects. The strong bedrock reflector underlying ACP strata causes 1D resonance effects to dominate the site response estimates, with the amplitudes of the resonance peaks determined largely by the reflection coefficient at the base of the sediments. These preliminary results suggest that the HVSR method may successfully estimate regional site response amplifications from the ACP, or similar geologic environments, if appropriate amplitude correction factors are used. [This abstract represents the views of the authors and does not necessarily represent the views of the DNFSB.]

Presenting Author: Lisa S. Schleicher

Authors