Ground-Motion Correction Factors for the Atlantic Coastal Plain Strata of the Southeastern U.S.: Preliminary Results

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

The Atlantic Coastal Plain (ACP) strata are a landward-thinning sequence of unconsolidated or semi-consolidated sedimentary strata that cover areas of the southeastern United States to as much as 300 km inland. These strata have a strong effect on ground motions, causing amplification in the Washington, D.C. area contributing to the damage during the 2011 M5.8, Mineral, Virginia earthquake. Correction factors derived for sedimentary basins (z1.0 and z2.5) may not apply to the ACP strata, which dip at about 1 degree or less and do not have sloping basin edges to efficiently generate local surface waves. Corrections based on the fundamental frequency in thin strata have not been explored for the thick ACP strata. This study examines ground motion amplifications by ACP strata, and assesses frequency-dependent correction factors for ground motion estimates. Data from teleseisms recorded on crustal-scale seismic experiments are used to compute horizontal spectral ratios relative to bedrock sites on Piedmont rocks west of the ACP strata. The ratios were computed using 100-sec time windows encompassing the strongest shaking, with the two horizontal-component spectra being combined using the vector sum at each frequency. Results show the primary resonance peak caused by reverberations of vertically-traveling shear waves reflecting between the free surface and the strong reflector at the base of the ACP strata. At frequencies below the primary resonance peak the ACP strata do not amplify shear waves significantly, while peak amplifications at higher frequencies can average factors of 2 to 4. These results suggest that for a given thickness of ACP strata a frequency and amplitude of the primary resonance peak can be predicted. From these, correction factors specifying amplification versus frequency can be developed for the ACP strata. Such correction factors have the potential to improve ground motion predictions in deterministic and probabilistic seismic hazard analyses.

Presenting Author: Thomas L. Pratt

Authors