Association of Environmental & Engineering Geologists
D.C. – Maryland – Virginia Chapter
Notice of Meeting
Thursday, October 19, 2017
Topic: Amplification of earthquake ground motions by Atlantic Coastal Plain sediments: Implications for Central and Eastern U.S. seismic hazards
Presenter: Thomas Pratt, PhD
Research Geophysicist, U.S. Geological Survey
Damage in Washington, D.C., during the 2011 Mw5.8 Mineral, Va., earthquake was surprisingly high for an epicenter 130 km away, and “Did-You-Feel-It” felt reports suggest that ground motions in the city were amplified by Atlantic Coastal Plain and other unconsolidated deposits. We measure this potential ground amplification relative to bedrock sites in the city using teleseismic and regional earthquake signals recorded on a temporary seismometer array. The resulting spectral ratios show amplification in the 0.7 to 4 Hz frequency range, which overlaps resonant frequencies of buildings in the city as inferred from their heights, suggesting amplification at frequencies to which many buildings are vulnerable to damage. The 2011 earthquake thus emphasizes the importance of local ground motion amplification in stable continental regions, where low attenuation extends shaking levels over wide areas and unconsolidated, shallow deposits on crystalline or igneous bedrock can create strong contrasts in near-surface material properties. Thicker Atlantic Coastal Plain and Mississippi Embayment strata throughout the central and eastern U.S. produce strong fundamental resonance peaks in the 0.2 to 4 Hz frequency range on spectral ratios computed from crustal-scale seismic experiments. These spectral ratios can be converted from frequency to depth, resulting in depth-converted spectral ratios across the array that produce an image of the strata causing the resonances. The data sets thus provide an average velocity function for the sedimentary sequence, the frequencies and amplitudes of the major resonance peaks, and a subsurface image of the major reflectors producing resonance peaks, and show that teleseismic signals can be used to characterize sedimentary strata in the upper km.
Dr. Thomas Pratt is a research geophysicist at the U.S. Geological Survey’s Geologic Hazards Science Center within the Earthquake Hazards program. His research interests are in seismic imaging of fault systems beneath the surface, computer modeling of geologic structures, studying the tectonic settings of active faults, and understanding ground motions during earthquakes. His past research has focused on active faults throughout the continental U.S. and Alaska, as well as Japan and Panama, and ground motion studies in the Pacific Northwest and the eastern U.S. Dr. Pratt was based in Seattle, WA, for twenty years but recently moved to Reston, VA, where the primary focus of his research is earthquake hazards in the Central and Eastern United States. Dr. Pratt serves as Editor-in-Chief of the Bulletin of the Seismological Society of America, which is one of the premier scientific journals for earthquake science. He received his bachelor’s degree in geology at Cornell University in 1980, and his master’s (1982) and doctorate (1986) degrees in geophysics at Virginia Tech.
Thursday, October 19, 2017 Amphora Restaurant
5:00 PM to 7:30 PM 377 Maple Ave W
Vienna, VA 22180
Cost (dinner and meeting): Agenda:
Members $40 5:00-5:30 PM Social & Check-in
Non-members $45 5:30-6:15 PM Dinner
Discounts Available: 6:15-6:30 PM Section Announcements &
Students save $20 Sponsor Presentation
Retirees save $10 6:30-7:30 PM Presentation followed by Q&A
A special thank you to our meeting sponsor:
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AEG DMV Officers (2017-2019)
Chair: Drew Thomas, CPG
ECS Mid-Atlantic, LLC
Vice Chair: Katelyn Foster, PG
Treasurer: John Garber
ECS Mid-Atlantic, LLC
Secretary: Cheryl Gannon, CPG