Ground Motion Prediction Constraints and Hazard Curve Evaluation in Southern California Using Precariously Balanced Rocks

Glenn P. Biasi, John G. Anderson, James N. Brune, & Richard J. Brune

Published August 15, 2019, SCEC Contribution #9853, 2019 SCEC Annual Meeting Poster #289

Precariously balanced rocks (PBRs) provide approximate bounds on unexceeded ground motions for the period of their current configuration. For many PBRs, this can be thousands of years, which is much longer than any instrumental records. We use estimates of dynamic toppling accelerations for rocks in southern California to compare with ground motion prediction equations and to bound inter-event and single-station contributions to total uncertainty. When plotted by distance to the San Andreas fault, PBRs are consistent with median ground motions on hard rock for characteristic Mw 7.8 to 8.0 earthquakes. By assuming a representative earthquake recurrence rate, event-to-event and single-station contributions to sigma can be estimated. Both are low relative to total sigma in GMPEs. Small “single-station” sigma has been suggested by researchers on other lines of argument, including geologic assessments and evidence from repeat strong-motion recordings; the PBR data give strong support. We estimate that the event sigma contribution to GMPE variability for SAF sources is less than about 0.2 in natural log units. PBRs can also be used individually in comparisons to USGS hazard curves. We find that most rocks are not inconsistent with median predicted ground motions at a 475-yr return period, but are often inconsistent with the hazard curve at a 2475-yr return. The Uniform California Earthquake Rupture Forecast v.3 (UCERF3) magnitude predictions for probable earthquakes on the northern San Jacinto fault are found to be inconsistent with PBR survival nearby. SCEC hosts the Precarious Rock Archive, with rock locations and available dynamic toppling acceleration estimates, for researchers interested in exploring further.

Key Words
precarously balanced rocks, ground motion prediction equation, hazard map

Citation
Biasi, G. P., Anderson, J. G., Brune, J. N., & Brune, R. J. (2019, 08). Ground Motion Prediction Constraints and Hazard Curve Evaluation in Southern California Using Precariously Balanced Rocks. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions