Earthquake hazard assessment evaluated by fragile geologic features in coastal Central California

Anna H. Caklais, Dylan H. Rood, Mark W. Stirling, Christopher M. Madugo, Norman A. Abrahamson, Klaus Wilcken, Tania Gonzalez, Albert R. Kottke, & Alexander C. Whittaker

Published August 15, 2018, SCEC Contribution #8482, 2018 SCEC Annual Meeting Poster #232

Probabilistic seismic hazard (PSH) models typically provide estimates of ground motions for return periods that exceed historical observations. It is therefore important to develop methods to evaluate ground motion estimates for long return periods, especially in proximity to major earthquake sources where estimates can be very high. Here we provide constraints over 10,000s years on ground motions from onshore and offshore seismic sources in central California using the distribution, age and fragility (threshold ground motion for toppling) of fragile geologic features.

Fragilities are estimated for seven precariously balanced rocks (PBRs) formed on uplifted marine terrace paleo-seastacks. The site is <10 km from the Hosgri Fault, a major offshore fault considered part of the San Andreas Fault system. PBR 3D models were constructed using photogrammetry to estimate unexceeded PGA ground motions of ~0.2-0.6 g, which are consistent with Newmark analysis of a slid block. We additionally quantify the probability of toppling of the PBRs, using empirical relationships, as a vector ground motion (PGV/PGA).

We obtain fragility ages (time that each PBR achieved its current geometry) using Be-10 cosmogenic surface exposure dating. Extremely low Be-10 concentrations (~5000 at/g) in modern high-stand samples demonstrates minimal inheritance and reliability of chert age estimates. Additionally, the volume of colluvium surrounding the paleo-seastack outcrops, determined from LiDAR, combined with alluvial fan surface dating (using Be-10 and soil profile development indices) indicates low erosion rates (~10 mm/ky) and long-term stability. Exposure ages that bound the fragility age by approximating the removal of surrounding blocks range ~17-95 ky. The age distributions for the suite of features suggests that all PBRs share a common evolution, and we interpret ~21 ka as the most defensible fragility age estimate of all seven PBRs. Despite the lack of constraints on the recurrence behavior of the Hosgri Fault, the slip rate is such that the PBRs have almost certainly experienced multiple large-magnitude, near-field earthquakes, and therefore provide rare constraints on low frequency ground motions.

Ongoing work includes a rigorous probabilistic comparison of unexceeded ground motions to the site’s PSH model. We are developing methods to help reduce seismic hazard uncertainty for critical infrastructure, by the rejection of PSH logic tree branches inconsistent with the PBR observations.

Key Words
Precariously balanced rocks, fragile geologic features, ground motions, probabilistic seismic hazard analysis, cosmogenic isotopes

Caklais, A. H., Rood, D. H., Stirling, M. W., Madugo, C. M., Abrahamson, N. A., Wilcken, K., Gonzalez, T., Kottke, A. R., & Whittaker, A. C. (2018, 08). Earthquake hazard assessment evaluated by fragile geologic features in coastal Central California. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology