Abstract
Although seismometers have been placed on natural arches and pillars, and on bedrock to examine site amplifications at precariously balanced rock (PBR) sites, we are unaware of seismometers being used on PBRs to discern parameters directly relevant to toppling the rocks. To help use PBRs to constrain eastern U.S. ground motions over tens of thousands of years, we have been placing 3-component, 5-Hz seismometers on PBRs and on nearby bedrock surfaces to record both environmental noise and gentle pushes of the PBRs. Spectral ratios between the rock and bedrock recordings show one or more prominent peaks that define the dominant frequencies of rocking during weak, ambient ground motions. Because the frequency of rocking decreases with increasing amplitude of rocking, the dominant frequencies during weak motions likely reveal the highest frequencies relevant to toppling the rocks during strong ground motions. After giving gentle pushes to perpendicular sides of the rocks, particle motion diagrams from the seismometers on the rocks show the dominant directions of rocking. Increases in frequency with decreasing amplitude after the pushes confirm that rocking is occurring, which also is confirmed by visible motions of some rocks. The directions of rocking are useful for interpreting the orientations of alpha angles most important for estimating fragility by established methods. The decay in the amplitude of rocking per cycle allows for determination of the coefficients of restitution, which previous workers show can be directly related to the alpha angles and, if the heights of the centers of mass are known, the widths of the contact surfaces with the underlying rocks. We have also been measuring the locations and strengths of the input pushes in preparation for calibrating future 3D modeling of the rock responses. These measurements have been made with extremely small angles of perturbation, and stronger rocking may change the direction and frequency of rocking due to uneven bases to the rocks, but the weak motion responses help characterize the responses of the PBRs during initiation of rocking.
Oral presentation.