[ghsc-seminars] GHSC Seminar—Thomas Pratt, USGS, Tuesday, March 1st, 2022, 10–11am MST

[Boyd, Oliver S]

Looking at eastern U.S. ground motions: Shallow sediments, deep seismic experiments, and balanced rocks

Speaker: Thomas Pratt
                  Research Geophysicist
                  USGS
Location: online/virtual
Date/Time: Tuesday, March 1st, 2022, 10–11am MST

Abstract: The central and eastern U.S. can have strong ground motions during earthquakes because of the efficient transmission of energy in the crust and the presence of widespread, unconsolidated sediments deposited on crystalline basement rocks. The first of these results in levels of ground motion extending for much greater distances than in the western U.S. The latter results in a large material contrast at the base of sediments that can cause strong amplifications, which can raise moderate ground motions to damaging levels even at large distances. These factors were demonstrated by damages to some buildings in Washington, DC, during the 2011 Mineral, Virginia, earthquake despite the moderate size of the earthquake and the large distance from the city. In particular, the widespread Atlantic Coastal Plain sediments beneath the city caused strong amplifications, as seen subsequently by a site response study in the city. I will describe recent efforts to characterize the amplifications caused by the Coastal Plain sediments, making use of existing data from the USARRAY sites and several large seismic experiments designed for imaging the crust and mantle. We have also used these data to estimate shear-wave velocities from resonant frequencies, and by inversions of dispersion curves and spectral ratios from the Eastern North America Margin (ENAM) deep seismic experiment. Finally, I will discuss initial efforts to determine whether fragile geologic features, namely balanced and perched rocks, can be used to estimate the maximum ground motions, and from that maximum earthquake magnitudes, that could have occurred in the eastern U.S. in the Holocene.

[cid:6543fe5f-7aaf-46fe-89bb-362d43b529f7]

Bio: Thomas Pratt is a research geophysicist with the USGS in Reston, Virginia, and serves as Central and Eastern Regional Coordinator of the Earthquake Hazards program. After obtaining degrees from Cornell University and Virginia Tech, he returned to Cornell to work as a research associate before taking a position with the USGS. He joined the USGS in the Golden office, then moved to Seattle for 20 years, and in 2013 moved to Reston. His research includes seismic imaging and modeling of active faults, as well as ground motion studies. He is just completing a 5-year term as Editor-in-Chief of the Bulletin of the Seismological Society of America.


GHSC Seminar Committee: Oliver Boyd <olboyd at usgs.gov>, Josh Rigler <erigler at usgs.gov>, Francis Rengers <frengers at usgs.gov>

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