Title: Using precariously balanced rocks to test hazard models and estimate maximum ground shaking
Abstract: Pre-instrumental earthquake ground motions in the eastern U.S. are difficult to quantify because few ground motion proxies (e.g., paleoearthquake data, liquefaction, and written records) are available for the time period prior to recent, sparsely-distributed, instrumental records. One long-lived feature that can provide information on ground motions that have not been exceeded are precariously balanced rocks (PBRs), which are a form of fragile geologic features (FGFs). We have been systematically studying precariously balanced rocks in the eastern U.S., with a focus on rocks in formerly glaciated areas of New England, New York and New Jersey, where emplacement ages range from about 13,000 to 22,000 years based on the history of ice retreat. We make 3-dimensional models of the rocks using either lidar or photogrammetry to determine key geometric parameters such as the widths of the bases and the height of the centers of mass. We also install seismometers on the rocks and give them gentle pushes to determine their favored directions of rocking. From these measurements we can determine the ground motions likely to topple the rocks, can then map the maximum earthquake magnitudes that could have occurred in the nearby region, and we can assess whether the assumptions in the U.S. Geological Survey’s National Seismic Hazard Model are consistent with the survival of each of the PBRs.
Biography: Thomas Pratt earned a BA degree in Geology from Cornell University, and masters and Ph.D. degrees in Geophysics from Virginia Tech. He then returned to Cornell and worked for 6 years as a research associate, carrying out deep seismic reflection imaging studies of the continent. In 1991 he was hired by the USGS earthquake program in Golden, Colorado, to lead the project imaging active faults, and in 1994 transferred to the Seattle office. He has carried out seismic hazard studies in the U. S. Pacific Northwest, the central and eastern U. S., Alaska and Panama. In 2013 he transferred to the Reston, Virginia office of the USGS where he is now focusing on the earthquake hazards of the eastern U.S. He serves as the Central and Eastern Coordinator of the USGS Earthquake Hazards Program, and served for 5 years as editor-in-chief of the Bulleting of the Seismological Society of America. He is actively studying ground motions in the central and eastern U.S., including Puerto Rico, with a recent focus using precariously balanced rocks to look at maximum earthquake ground motions in the eastern U.S.
To join virtually: Zoom
Contact: tpratt@usgs.gov
Website: https://www.usgs.gov/staff-profiles/thomas-pratt#science
Recording: Zoom Recording (will be available within a week after the seminar)