SCEC Award Number 16221 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Spatially Extensive Evaluation of the UCERF3-Based 2014 National Seismic Hazard Map Using Precarious Rock Ground Motion Constraints
Investigator(s)
Name Organization
Glenn Biasi University of Nevada, Reno John Anderson University of Nevada, Reno
Other Participants none
SCEC Priorities 6b, 6e SCEC Groups GMSV, GMP, Geology
Report Due Date 03/15/2017 Date Report Submitted 05/22/2017
Project Abstract
 We have developed a spatially extensive test of the consistency between hazards from the 2014 National Seismic Hazard Map (NSHM) and dynamic toppling acceleration (DTA) estimates from precariously balanced rocks (PBRs) in southern California. Previous similar tests have been confined to small zones such as the line between the Elsinore and San Jacinto faults. We first updated the database of precarious rocks, improving the locations of over 300 rocks and adding results of over 20 new photomodeled rocks. New code was developed to extract NSHM hazard estimates at PBR locations from gridded data files. We find that the NSHM peak ground accelerations (PGAs) at a 475 yr return time are generally consistent with DTAs. DTAs for one direction are more sensitive than the 475 yr PGA, but only 7 are inconsistent in both toppling directions. These results show that rock asymmetry must be considered when comparing to the NSHM. In map view PBRs vulnerable to 475 yr PGA occur outside a “halo” along the major faults, inside of which PBRs presumably have been toppled. For NSHM PGA at a 2475 yr return period, many 10’s of PBRs are inconsistent. These PBRs occur beside the San Jacinto, Elsinore, Pinto Mountain, and San Andreas faults. There are too many to be easily dismissed as data accidents. PBR results could be explained if the 2475 yr PGA ground motion variability (sigma) is consistently too large. This has been suggested from other lines of investigation.
Intellectual Merit This research provides the first southern California-wide comparison of the predictions of the National Seismic Hazard Map with dynamic toppling ground accelerations of precariously balanced rocks (PBRs). Earlier comparisons have considered smaller areas such as the area between the Elsinore and San Jacinto faults. Smaller comparisons might more easily be criticized as special cases. PBR dynamic toppling accelerations compare well with peak ground acceleration (PGA) at 475 yr return periods except for asymmetric rocks that are fragile in one direction, but not in another. At 2475 yr return periods, the NSHM PGAs exceed PBR dynamic toppling estimates near many active faults. These results could be explained if most PBRs are recent erosional remnants, but are more likely due to fault ground motions having smaller variability from earthquake to earthquake.
Broader Impacts This research benefits society by providing a long-term estimate of peak ground acceleration near southern California faults. Instrumental records span less than a century. PBRs indicate that ground motions from SoCal faults are more narrowly distributed around the median than would be implied by lognormal relationships of the ground motion prediction equations.
Exemplary Figure Figure 4. Map view of PBRs with dynamic estimated toppling accelerations smaller than the predicted NSHM PGA. (upper) 475 yr return. Red circles indicate where both directions are exceeded; blue “+” are one axis only. Dynamic toppling is estimated at 1.3*static toppling acceleration. (lower) 2475 yr hazard. The southern lineation is between the Elsinore and San Jacinto faults. See text for further discussion.