UMD Geology: Pratt on Coastal Plain strata in earthquakes

2017 Geology Colloquium Series

Friday, February 3rd 2017 at 3:00 pm
in PLS 1140

Tom Pratt
USGS-Reston

The influence of eastern U.S. Atlantic Coastal Plain strata on earthquake ground motions, and damage in Washington, DC, during the 2011 Mineral, Virginia, Earthquake

During the 2011 Mw5.8 Mineral, VA earthquake, many buildings in Washington, DC, including national landmarks like the Washington National Cathedral, the Smithsonian “Castle,” and the Washington Monument, sustained damage despite being 130 km from the epicenter. The surprisingly large amount of damage from weak ground motions raises questions of how much the southeast-thickening sedimentary strata of the Atlantic Coastal Plain (ACP) strata beneath the city amplify and trap seismic energy. Partially consolidated ACP marine sedimentary strata overlie crystalline or indurated sedimentary rocks throughout coastal regions of the eastern U.S., extending more than 200 km inland from the coast. The strata taper landward from as much as 1 km near the coast to pinching out in the Washington, DC area. Shallow sedimentary strata are known to amplify earthquake ground motions due to low seismic impedance and strong reverberations. Between November 2! 014 and August 2015, we used 27 seismometers to measure ground motions across Washington, DC, using four sites on crystalline rocks as reference sites. We also used data from continental-scale seismic experiments that span the ACP to examine the influence of the broader ACP strata on earthquake ground motions. Recordings of teleseisms and regional earthquakes provided data with sufficiently high signal-to-noise for computing amplitude ratios relative to the bedrock sites. Amplifications of 10 or greater are found in the Washington, DC area due to the ACP strata, with the peak amplifications occurring near the estimated resonant frequencies of buildings throughout the city. Amplitudes decrease as the strata thicken, but even coastal sites on 600 m of ACP strata show amplification factors as great as 5. We use the frequency of the resonance peaks to invert for an average velocity function within the ACP strata. This work indicates that amplification of short-period ground mot! ions by thin ACP strata contributed to the damage in Washington, DC, d uring the 2011 earthquake, and documents longer-period amplifications that could affect larger structures beneath coastal regions of the eastern U.S. during earthquakes.

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