Frictional Sliding and Rocking of Precariously Balanced Rocks Under Earthquake Excitation

California Institute Of Technology, Pasadena CA

Investigators

Abstract

There are several precariously balanced rocks (PBRs) in the western US located on hill-sides and cliffs. Many of these rocks have evolved naturally and have been in their present configuration for thousands of years. Cosmogenic dating of the pedestal, rock and other surfaces on the surrounding landscape has been conducted to estimate the amount of time these rocks have remained precarious. Through analysis, if we can establish the nature of ground shaking that would cause these rocks to topple, we can confidently state that such motion could not have occurred during the "precarious age" of the rock. Thus, these rocks are effectively low-resolution strong motion seismoscopes that provide strong statistical constraints on ground motion from historic earthquakes in the region. Such information is critical for the proper siting and designing of nuclear/chemical power plants and waste storage facilities to protect them against plausible ground shaking from long-term future earthquakes. The goals of this research are: (i) to develop a fundamental understanding of the mechanics of rocking and sliding of precariously balanced rocks (PBRs) under earthquake excitation; (ii) to characterize the sensitivity of the overturning of PBRs to ground motion features; (iii) to use PBRs to help establish the veracity of ground motion simulations; and (iv) to estimate the upper bounds of seismic shaking intensity in a given region or a specific site, perhaps where hazardous materials are handled (e.g., nuclear or chemical waste storage facilities or plants). We plan to achieve the project goals by combining recently developed 3-D imaging (Terrestrial Laser Scanning) and image processing techniques with a recently developed rigid body dynamics algorithm to create high-fidelity 3-D models of existing precariously balanced rocks and the supporting pedestals. The models will strive to mimic the physical condition of contact to credibly capture the interplay of sliding and rocking when the pedestal is shaken by 3-component ground excitation. The investigators propose to validate the analysis algorithm using data from shake table tests on regular blocks and irregular rocks. They intend to conduct parametric analyses of several PBRs subjected to idealized waveform excitation characterized by frequency, amplitude, and number of cycles. This will help establish the toppling sensitivity of the rocks to these ground motion features. Using these toppling analyses, we plan to compute statistical bounds on the values of various ground motion features from historical earthquakes that have occurred at the sites of the PBRs. These results will not only help inform future editions of the National Seismic Hazard maps but also help guide the siting and design of critical nuclear and chemical facilities. In addition, they plan to excite the PBRs with 3-component acceleration histories predicted for these sites by seismic wave propagation simulations of past earthquakes. If the synthetic excitation is found to overturn the PBR models, it would point to shortcomings in the ground motion simulation methodology. Alternately, the absence of overturning would lend credibility to seismic wave propagation simulations.