Date of this Version
Wittich, C.E., Hutchinson, T.C., Sandwell, D., and DeSanto, J. (2018). 3-D reconstructions and numerical simulations of precarious rocks in southern California. 11th National Conference on Earthquake Engineering, Los Angeles, CA, June 25-29. 10 pp.
Reliable estimates of seismic hazard are essential for the development of resilient communities; however, estimates of rare, yet high intensity earthquakes are highly uncertain due to a lack of observations and recordings. Lacking this data, seismic hazard analyses may be based on extrapolations from earthquakes with more moderate return periods, which can lead to physically unrealistic earthquake scenarios. However, the existence of certain precariously balanced rocks (PBRs) has been identified as an indicator of an upper bound ground motion, which precludes toppling of the balanced rock, over its lifetime. To this end, a survey of PBRs was conducted in proximity to the Elsinore fault east of San Diego, CA. Each identified PBR is modeled using point clouds derived from ground-based laser scanning and images from an unmanned aerial vehicle. The resultant geometric reconstructions are then used in a probabilistic overturning analysis and compared to the anticipated seismic hazard at the site. Accounting for an estimated age range and 50% probability of overturning for the PBRs, approximately half of the surveyed PBRs indicate a potential overestimation of seismic hazard at the site.