Toppling of PBRs Exposed to Ground Motions Estimated from the Composite Source Model of Earthquakes

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

Published August 15, 2016, SCEC Contribution #6745, 2016 SCEC Annual Meeting Poster #291

We have improved the description and documentation of precariously balanced rocks (PBRs) at Lovejoy Buttes 15 km NE of the San Andreas fault at the western edge of the Mojave Desert, and at sites within 20 km of the San Jacinto fault near the trans-tensional stepover between the Claremont and Casa Loma sections. We compare PBRs fragilities to ground motion simulations based on the composite kinematic source model of Anderson (SRL 2015), which includes effects of earth structure, attenuation, scattering, sub-event stress drops, and random distributions of slips. Along the San Jacinto we used three rupture models: a single rupture along the length of the two segments combined, and separate ruptures along each of the Claremont and Casa Loma segments. For each rupture we used models with both 20 and 60 bar sub-event stress drop.
Source models with the 20 bar sub-event stress drop are mostly consistent with the PBRs at Lovejoy and San Jacinto not having been knocked down, while several rocks at both locations are likely to be toppled by the 60 bar synthetics, thus potentially constraining the modeling parameter. If we were to reject 60 bars, then the 20 bar model would tell us that we cannot reject a continuous rupture through the stepover on the basis of the PBRs. For a given PBR, the probability of toppling appears to be sensitive to the distance to the nearest large sub-event in a simulation. The 20 bar synthetics tend to be comparable to or on the low side of GMPEs depending on the oscillator period.
Whether or not a particular PBR is toppled depends on using the actual 3d directionality (polarization) of the ground motion simulations as well as the correct shape and orientation. The ground motions that do not topple modeled rocks in their actual orientation are found to more often topple the same rocks if their simulated orientations are rotated by 90 degrees. Thus the actual orientation of the PBRs and the use of synthetics incorporating the physics of source radiation patterns and wave propagation is essential for somewhat finely-tuned limits on pre-historic near-source ground motions.

Key Words
Precariously balanced rocks PBR CSM Composite Source Model Kinematic Anderson

Citation
Brune, R. J., Anderson, J. G., Biasi, G. P., & Brune, J. N. (2016, 08). Toppling of PBRs Exposed to Ground Motions Estimated from the Composite Source Model of Earthquakes. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motion Simulation Validation (GMSV)