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Correcting Horizontal-to-Vertical Spectral Ratio Fundamental Resonance Amplitudes on Flat Sediments: An Example From the Atlantic Coastal Plain, Eastern United States

Session: Advances in Upper Crustal Geophysical Characterization II

Type: Oral

Date: 4/21/2021

Presentation Time: 02:45 PM Pacific

Description: 

Damaging ground motions from the 2011 M_w5.8 Virginia earthquake were likely increased due to site amplification from the unconsolidated sediments of the Atlantic Coastal Plain (ACP), highlighting the need to understand site responses on these widespread strata in the coastal regions of the eastern U.S. The horizontal-to-vertical spectral ratio (HVSR) method, using earthquakes or ambient noise, offers an appealing method for measuring site response because it requires a single seismometer rather than the two or more seismometers necessary to compute a horizontal sediment-to-bedrock spectral ratio (SBSR). Although previous studies show mixed results when comparing these methods, the majority of these studies investigated site responses in confined sedimentary basins that can generate substantial three-dimensional effects. In contrast, the flat ACP strata and the underlying bedrock reflector cause 1D resonance effects to dominate site response, with the amplification of the fundamental resonance peaks controlled by the impedance contrast between the base of the sediments and the underlying bedrock. We compare site response estimates for the ACP strata derived using the HVSR and SBSR methods from teleseismic signals recorded by regional arrays and observe a close match in the frequencies of the fundamental resonance peak (f₀) determined by both methods. We find that the HVSR method yields an amplification at f₀ within a factor of 2 of the SBSR for ACP strata up to 100 m, but for sites with thicknesses between 100 to 1800 m, correcting the HVSR amplitude using source term information from a bedrock site and multiplying the peak by a factor of 1.2 results in amplitude peaks that on average match SBSR results. Therefore, the HVSR method may successfully estimate regional linear site response amplifications from the ACP, or similar geologic environments, when appropriate amplitude ratio adjustment factors are used.

Presenting Author: Lisa S. Schleicher

Student Presenter: No

Authors