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

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

The Atlantic and Gulf coastal plains contain Cretaceous and Cenozoic sedimentary sequences of variable thickness. We investigate the differences 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 find that coastal plain sites experience amplification of low-frequency ground motions and attenuation of high-frequencies relative to average site conditions outside the coastal plain.

We use stacked coda and Lg spectra for sites outside the coastal plain as a reference. The spectral ratios at high-frequencies give estimates of the difference between kappa at coastal plain sites and the reference condition. Kappa values determined from the coda are strongly correlated with the thickness of the sediment section and agree well with previous estimates determined from Lg. Average estimates of kappa reach ~ 120 ms at Gulf coastal 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 2.86 Hz. For sediment thickness greater than approximately 0.5 km, the spectral amplitude ratio at frequencies greater than approximately 3 Hz is controlled by the value of kappa. Fourier amplitude spectral ratios at frequencies between ~ 1.0 and 0.1 Hz are strongly dependent on sediment thickness. For example, at 0.1 Hz, the mean Fourier amplitude ratio (coastal plain/reference) is approximately 2.7 for sediment thickness 12 km.

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.

Presenting Author: Zhen Guo

Authors