Shear-Wave Velocity and Seismic Response Estimates From the Southern Isoseismal Region of the 1886 Charleston Earthquake: Results From a Seismic Land Streamer System

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

The 1886 M~7 earthquake damaged Charleston's infrastructure and killed more than 100 people. Today, ground amplification from a similar earthquake would devastate the region. For this fault mapping and site response study, we acquired 14 km of seismic data over five field days using a 72-channel, 90 m land streamer/accelerated weight drop system along city streets. The data collection was located within the southern isoseismal zone of the 1886 earthquake. We acquired shots every 2.5 m to obtain more than 5,000 dispersion curves and 360,000 first arrival picks. We estimate shear wave velocity (Vs) from phase velocity-frequency picks for the fundamental mode using a range of inversion approaches. We estimate p-wave velocity (Vp) by inverting first arrival picks. For our site response analysis, we assume a two-layer velocity model that represents Holocene sediments over older strata. Because Vp results are sensitive to water saturation that crosses lithologic boundaries, we rely on Vs to map Holocene layer thickness, to estimate resonant frequencies, and to estimate ground surface amplification. We compare our Vs results to mapped geology, layer thickness derived from auger holes, and to coincident seismic reflection results that we generated from the same dataset. We estimate Vs for Holocene sediments, late Pleistocene Wando Formation, middle Pleistocene Ten Mile Formation, and Tertiary Ashley Formation. We find the thickness of Holocene strata lies mostly at depths less than 30 m below the land surface. We suggest that this shallow boundary controls site amplification and that the seismic land streamer approach provides a rapid and effective high frequency site response tool for urban areas.

Presenting Author: Nicole L. Clizzie

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