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Fundamental Resonant Frequencies Derived From Shallow Sediment Properties for the Charleston, South Carolina Area

Session: Advances in Upper Crustal Geophysical Characterization II

Type: Oral

Date: 4/21/2021

Presentation Time: 03:00 PM Pacific

Description: 

For the Charleston, South Carolina area, predominant peaks in the acceleration response spectra match fundamental periods of many existing buildings. Spectral characteristics above one Hz are driven mostly by soil thickness and shallow shear wave velocities (Vs). We acquired 14 km of seismic land streamer data, in profile, to map faults related to the 1886 Charleston earthquake. From seismic field records, we pick fundamental surface wave dispersion curves to derive two-layer, two-dimensional Vs models. Here, Quaternary sediments are upwards of 30 m thick. These unconsolidated, mostly saturated sediments lie directly upon more competent carbonate Tertiary rock. Previous studies show little velocity-depth dependence within Quaternary strata, and our results are consistent with this observation. Using a grid search approach and rock physics relationships, we obtain Vs models, in profile, that minimize the misfit between observed and calculated dispersion. We derive seismic impedance and fundamental resonant frequencies (Fn) using the quarter wavelength criteria with Quaternary Vs estimates and layer thickness. We compare our >2700 Vs profiles to regional seismic cone penetrometer measurements, surficial geology, and detailed stratigraphic logs that identify a top of Tertiary boundary and provide additional constraints. We find consistent Vs and Quaternary layer thickness estimates between each dataset, allowing us to develop a regional relationship of Fn to soil thickness for the greater Charleston region. We compare this map to faults identified with seismic reflection data and liquefaction features documented from the 1886 earthquake. We suggest that our approach to estimating Fn may be applicable for other areas within the Atlantic Coastal Plain, when in the presence of a similar shallow high impedance boundary.

Presenting Author: William D. Schermerhorn

Student Presenter: Yes

Authors