SCEC Award Number 13135 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Calibration of Rock Overturning Probabilities for CyberShake Waveforms
Investigator(s)
Name Organization
Glenn Biasi University of Nevada, Reno John Anderson University of Nevada, Reno
Other Participants none
SCEC Priorities 6, 4, 1 SCEC Groups CME, GMSV, Geology
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 Toppling probabilities were estimated for an ensemble of waveforms computed using CyberShake waveforms at the LBUT2 (Lovejoy Buttes) and BKBU (Black Butte) precariously balanced rock sites near the San Andreas fault. From over 200,000 waveforms for each site, samples of several hundred were drawn. To ensure sampling at relevant ground motions, SAF sources were binned into 0.1g groups by PGA, and 100 waveforms sampled from each bin. Legacy FORTRAN code rocking was translated to Matlab and used to evaluate toppling of blocks of different sizes predicted under CyberShake accelerograms. Toppling dynamics appear to be consistent with estimates from previous methods. For example, the vector combination of PGA and spectral acceleration at 1 Hz (SA(1)) better predict toppling probability than either separately. For the onset of toppling, SA(1) must be larger than PGA. East (~fault parallel) component waveforms appear to be slightly more favorable for toppling than are N-S components. New results can be used to estimate the conditional probabilities of overturning given CyberShake inputs, but require ground motion probabilities to develop estimates of rock survival likelihood.

We also estimated pedestal heights h as a proxy for PBR age for over 700 PBRs. Rocks with h > 50 cm are presumed to have required significant time to develop. Plots of ground motion limits versus distance from the SAF are much clearer among the h>50 cm set and show PBRs are consistent with median to +1σ GMPE predictions of AS-2008.
Intellectual Merit These results are the first to use actual CyberShake waveforms to evaluate their ability to topple precariously balanced rocks in southern California. Results are in the form of conditional probabilities given the input, and apply at present only to symmetric blocks. The methods developed under this project will enable a more complete evaluation of the likelihood of rock survival.
Broader Impacts Precariously balanced rocks are one of the few means to provide limiting ground motions near active southern California over several seismic cycles. Seismic hazard estimates used by the USGS and for site-specific studies rely on estimates of ground motion uncertainties. PBRs, especially whose exposure is a single fault source, provide a data basis for estimating path and source variability. Because of this, efforts to understand and interpret ground motion constraints from PBRs are a benefit to all social uses of seismic hazard estimates.
Exemplary Figure Figure 3. Transitions from survival to toppling for 100 CyberShake waveforms each of north (upper plot) and east components (lower plot) at LBUT2. Green symbols: rock survives at that PGA-SA(1) level. PGA and SA(1) are in cm/sec2. Slenderness angle is 0.30 radians; rocking arm length is 60 cm. Similar plots were developed for several waveform PGA bins, rocking arm lengths, and alphas. Figure was drawn by the authors.
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