Constraints on Ground Accelerations Inferred from Unfractured Hoodoos near the Garlock Fault, California

Rasool Anooshehpoor, James N. Brune, Jaak Daemen, & Matthew D. Purvance

Published February 2013, SCEC Contribution #1533

In the absence of long‐term instrumental data, the presence of fragile geologic features near active faults can provide physical limits on the level of ground shaking that could potentially have significant implications for seismic hazards. This paper introduces a multidisciplinary investigation that uses unfractured hoodoos in seismically active regions to constrain the level of ground accelerations at those locations. Although there is a large uncertainty associated with the age of the hoodoos because of their rapidly eroding nature, they can still be useful in providing physical limits on ground motions associated with recent large events. Here, we consider the fragilities of two hoodoos in the Red Rock Canyon region within a few kilometers of the Garlock fault, which is an active strike‐slip fault in a transtensional region with at least a few large earthquakes in the Holocene. The hoodoos at these sites could be evidence of median or relatively low ground motions associated with large transtensional strike‐slip earthquakes. Results of our field and laboratory tests on two hoodoos provide constraints on peak ground accelerations (PGAs) of 0.36+/-0.06 g and 0.59+/-0.14 g. Using the U.S. Geological Survey’s (USGS) probabilistic seismic hazard (PSH) deaggregation, the dominant earthquake contributing to the hazard at the site of the hoodoos for the recurrence intervals of 475, 975, and 2475 years is located at a distance of 4.8 km and has a magnitude of 7.63, consistent with the observed paleoearthquake evidence on the Garlock fault. The PGAs corresponding to these three return periods are 0.26, 0.40, and 0.61 g, respectively. Therefore, the survival of the more fragile hoodoo during a presumably large event on the Garlock fault in the past 550 years would be consistent with the 2008 seismic hazard level if the ground motions during that event were below the median value.

Anooshehpoor, R., Brune, J. N., Daemen, J., & Purvance, M. D. (2013). Constraints on Ground Accelerations Inferred from Unfractured Hoodoos near the Garlock Fault, California. Bulletin of the Seismological Society of America, 103(1), 99-106. doi: 10.1785/0120110246 .