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Probabilistic Ground-motion Assessment of Balanced Rocks in the Mojave Desert

Pengcheng Liu
Bureau of Reclamation

The first 20% of the full text of this article appears below.

	INTRODUCTION

 
Brune (1996, 1999) proposed using balanced rocks located at a variety of distances from faults, some with relatively high slip rates (e.g., the San Andreas fault), as proxy paleo-ground-motion indicators of peak acceleration. Shi et al. (1996) derived simplified relationships between peak horizontal accelerations (PHA) and toppling probabilities to convert pseudostatic field measurements of rock pedestal stability to limits on PHA consistent with rock stability and persistence. Anderson and Brune (1999) concluded that the persistence of balanced rocks 10-30 km from the San Andreas fault was inconsistent with the ergodic assumption implicit in probabilistic seismic hazard analyses (PSHA) derived using the approach of Cornell (1968). The existence of a modest population of balanced rocks on the Mojave side of the San Andreas fault affords an opportunity to test the hypothesis of Anderson and Brune (1999) that the persistence of these balanced rocks is inconsistent with site-specific PSHA and the ergodic assumption. Through Monte Carlo simulations of balanced-rock life cycles we show that the existing empirical ground-motion relation that explicitly accounts for site velocity (Boore et al. 1997) is consistent with PSHA assumptions (Cornell 1968) and the persistence of balanced rocks east of the San Andreas fault in the Mojave Desert (Brune 1996, 1999).

In this paper, discussion is not limited to balanced rocks, because a wide variety of balanced objects can provide useful information on ground motions. For example, various types of monuments have the advantage that the duration that the monument has remained standing is documented in the historical record, and the fragility of the monument can be well-established. The duration of balanced-object persistence is a necessary quantity to make statistical inferences about the rate of exceedance or nonexceedance . . . [Full Text of this Article]

William Lettis and Associates, Inc.
433 Park Point Drive, Suite 250
Golden, Colorado 80401 USA
oconnell@lettis.com
(D.R.H.O.)
laforge@lettis.com
(R.L.)

Bureau of Reclamation
P.O. Box 25007 86-68330
Denver, Colorado 80225-0007 USA
pliu@do.usbr.gov
(P.L.)