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Analyses of Balanced Rocks to Constrain Ground Motions in the Eastern U.S.

Description: 

Although analyses of fragile geologic features, and specifically Precariously Balanced Rocks (PBRs), have been used to constrain the strength of ground shaking in the western U.S. and elsewhere, the method has not been used in the eastern U.S. However, the latter region is especially amenable to such studies because: 1) there are many glacial erratics placed in precarious positions when the continental ice sheet melted in the northeastern U.S., and there are a number of PBRs in the Appalachians that were formed by erosion; 2) PBRs in the northeastern states have the advantage of reliable age estimates because of placement following the last glacial maximum, with glacial striations beneath some of the PBRs confirming that they were deposited after the glaciers scoured the landscape; and 3) relatively low seismic attenuation means that PBRs are sensitive to earthquakes over larger areas than in the western U.S. We are beginning studies of eastern U.S. PBRs, with initial efforts in New England, the Adirondack Mountains of New York, and the Blue Ridge Mountains of Virginia. Studies so far have focused on refinement of methodologies, including comparisons of 3-D models made from photogrammetry (structure-from-motion) and terrestrial lidar, with each showing some advantages over the other. We also installed temporary seismometers on top of the rocks, and then pushed them gently to characterize the rocking after small perturbations. We compare the likely toppling accelerations computed using several different methods to accelerations in hazard curves at the PBR sites, and we use a ground motion model to estimate the magnitudes of nearby earthquakes that likely would topple the PBRs. Preliminary results show promise for using PBRs to provide insights to past ground motions in the region.

Session: Tectonics and Seismicity of Stable Continental Interiors

Type: Oral

Date: 4/19/2023

Presentation Time: 03:00 PM (local time)

Presenting Author: Thomas L. Pratt

Student Presenter: No

Invited Presentation: 

Authors