An Examination of Amplification and Attenuation Effects in the Atlantic and Gulf Coastal Plain Using Spectral Ratios

Abstract

The Atlantic and Gulf Coastal Plain in the southern and southeastern United States contains extensive Cretaceous and Cenozoic sedimentary sequences of variable thickness. We investigated the difference in response of sites in the Coastal Plain relative to sites outside that region using Fourier spectral ratios from 17 regional earthquakes occurring in 2010–2018 recorded by the Earthscope transportable array and other stations. We used mean coda and Lg spectra for sites outside the Coastal Plain as a reference. We found that Coastal Plain sites experience amplification of low‐frequency ground motions and attenuation at high‐frequencies relative to average site conditions outside the Coastal Plain. The spectral ratios at high frequencies gave estimates of the difference between kappa at Coastal Plain sites and the reference condition. Differential kappa values determined from the coda are correlated with the thickness of the sediment section and agree with previous estimates determined from Lg waves. Averaged estimates of kappa reach ∼120 ms at Gulf coast stations overlying ∼12 km of sediments. Relations between Lg spectral ratio amplitudes versus sediment thickness in successive frequency bins exhibit consistent patterns, which were modeled using piecewise linear functions at frequencies ranging from 0.1 to 2.8 Hz. For sediment thickness greater than ∼0.5 km, the spectral amplitude ratio at frequencies higher than approximately ∼3 Hz is controlled by the value of kappa. The peak frequency and maximum relative amplification at frequencies less than ∼1.0 Hz depend on sediment thickness. At 0.1 Hz, the mean Fourier amplitude ratio (Coastal Plain/reference) is about 2.7 for sediment of 12 km thickness. Analysis of residuals between observed and predicted ground motions suggests that incorporating the amplification and attenuation as functions of sediment thickness may improve ground‐motion prediction models for the Coastal Plain region.

U.S. Geological Survey coastal plain amplification virtual workshop

“…We used mean coda and Lg spectra for sites outside the Coastal Plain as a reference. We found that Coastal Plain sites experience amplification of low-frequency ground motions and attenuation at high-frequencies relative to average site conditions outside the Coastal Plain (Guo and Chapman, 2019). The spectral ratios at high frequencies gave estimates of the difference between kappa, a measure of ground motion attenuation, at Coastal Plain sites and the reference condition.…”

“…By Oliver S. Boyd (USGS) and Sarah Mills (Colorado School of Mines) Earthquake ground motions along the Atlantic and Gulf Coastal Plains show significant frequency-dependent deviations relative to other areas in the Central and Eastern United States (Guo and Chapman, 2019;Pratt and Schleicher, 2021). Ground motions have been shown to be strongly correlated with sediment thickness, where sediment is generally defined as being of Cretaceous Period or younger age.…”

“…Ground motions have been shown to be strongly correlated with sediment thickness, where sediment is generally defined as being of Cretaceous Period or younger age. Studies have found that greater sediment thickness leads to higher attenuation and lower ground motion amplitudes at short periods, but higher ground motion amplitudes at longer periods (Guo and Chapman, 2019;Pratt and Schleicher, 2021). With the recent implementation of basin depths for select areas in seismic hazard analyses for the western United States, a USGS working group has been formed to consider whether a sediment thickness model can be constructed for the Atlantic and Gulf Coastal Plains and successfully implemented in Central and Eastern United States seismic hazard analyses.…”

Regional attenuation models in Central and Eastern North America using the NGA-East database

“…Previous studies of regional CENA attenuation models are numerous; recent works include Cramer (2017), Cramer and Al Noman (2016), Bisrat et al (2014), Yassminh et al (2020), Chapman and Conn (2016), Guo and Chapman (2019), Pasyanos (2013), Gallegos et al (2014), and NGA-East (PEER, 2015). The NGA-East project was a multi-disciplinary research project managed by Christine Goulet with the objective to develop a new ground motion characterization (GMC) model for CENA.…”

“…Guo and Chapman (2019) evaluated the difference between site response inside and outside of the Coastal Plain (Atlantic and Gulf) using Lg-wave spectral ratios and assuming an average Q common to all stations and sources. This method incorporates any variations in crustal attenuation into the site term which is based on sediment thickness in the coastal plains.…”