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New Methods for Analyzing Precariously Balanced Rocks in the Eastern U.S.

Description: 

We are systematically studying precariously balanced rocks (PBRs) in the eastern U.S. to estimate maximum ground motions since the rocks became precarious. We have now analyzed 47 PBRs in the northeastern U.S. that were set in place during the ~21,800 ka to ~13 ka retreat of the continental ice sheet. The keys for PBR analysis are to locate the center of mass from a 3D model made using lidar or photogrammetry, and to estimate the width of the base to determine the slenderness angles and radii. The ground motions that will topple the PBR can then be computed and compared to those predicted in hazard curves using the method of Baker et al. (2013). For our PBR analyses we have developed and refined a number of novel methods. The simplest of these is wrapping a wire around the base to measure the circumference, which allows for more accurate interpretation of the contact surface. We also place seismometers on top of the PBRs and give them gentle pushes to measure the frequency of rocking and the direction of rocking, the latter of which is especially useful for determining the directions of the slenderness angles. The decay in the amplitude of rocking with time also allows for computing the slenderness angles, as outlined in Anooshehpoor et al. (2004), with preliminary results showing close matches with measured angles. The slenderness angles determined from seismometer records may be more accurate than measurements made using other methods. In addition to comparing the ground motions to hazard curves from the National Seismic Hazard Model (NSHM), we determine the probability of PBR survival by using the magnitude-distance pairs in the NSHM disaggregation files as scenario earthquakes. This latter estimate of survival probability provides an independent check on the comparison with the NSHM hazard curves. A final method we have developed is to map maximum magnitudes of earthquakes that could have occurred near the PBR without toppling it. Using this mapping method for the 47 PBRs we have studied provides a contour map of maximum earthquake magnitudes in the northeastern U.S. since ice retreat.

Session: Earthquake Shaking and the Geologic Record: Triggered Phenomena and Preserved Fragile Geologic Features [Poster]

Type: Poster

Date: 4/17/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Thomas

Student Presenter: No

Invited Presentation: 

Poster Number: 26

Authors