HR: 0804h
AN: NS11E-0831    [Abstracts]
TI: Natural Paleoseismometers: Cosmogenic Nuclide Dating of Precariously Balanced Rocks (PBRs) - Integral Constraints on Maximum Ground Accelerations
AU: * Perg, L A
EM: lperg@umn.edu
AF: University of Minnesota - Twin Cities, Geology and Geophysics, Minneapolis, MN 55455, United States
AU: Ludwig, L G
EM: lgrant@uci.edu
AF: University of California - Irvine, Environmental Health, Science, and Policy, Irvine, CAP 92697,
AU: Kendrick, K
EM: kendrick@usgs.gov
AF: U.S. Geological Survey, Western Region Earthquake Hazards Team, Pasadena, CA 91106, United States
AU: Brune, J
EM: brune@seismo.unr.edu
AF: University of Nevada - Reno, Geological Sciences and Engineering, Reno, NV 89557, United States
AU: Purvance, M
EM: mdp@seismo.unr.edu
AF: University of Nevada - Reno, Geological Sciences and Engineering, Reno, NV 89557, United States
AU: Anooshehpoor, R
EM: rasool@unr.edu
AF: University of Nevada - Reno, Geological Sciences and Engineering, Reno, NV 89557, United States
AU: Akciz, S
EM: sakciz@uci.edu
AF: University of California - Irvine, Environmental Health, Science, and Policy, Irvine, CAP 92697,
AB: Precariously balanced rocks (PBRs) act as natural seismometers constraining maximum ground acceleration over the surface exposure history of the PBR. These key paleoseismic indicators have the potential to validate ground motions on the timescale necessary to test earthquake rupture forecasts and Seismic Hazard Assessment estimates, and are an active topic of research to validate CyberShake results and constrain National Seismic Hazard Maps. This research focuses on examining the post-exhumation history of PBRs using in-situ terrestrial cosmogenic nuclides (TCNs). TCNs provide a record of near-surface exposure history. The measured concentrations are a function of the residence time in the upper ~20 m of the subsurface (inherited concentration), the timing and rate of exhumation, and post-exhumation surface spalling and chemical erosion. Our goal in the project is to provide reasonable constraints on the post-exhumation history, specifically the age of the PBRs and evolution of precariousness: we should be able to constrain whether the rocks were of similar precariousness 2.5 ka, 5 ka, and 10 ka ago. These specific targets will provide important constraints on time since exceedance for the CyberShake models. We developed our sampling strategy to address subsurface inheritance, exhumation rate and timing, and post- exhumation spalling and chemical erosion. PBRs were selected to meet a variety of considerations. These rocks constrain ground motions from large earthquakes on the San Jacinto and Elsinore faults, in Southern California. Inherited concentrations lead to an age estimate that is too old; we are investigating inherited concentrations though sampling a rock quarry near Perris CA, with shielded samples at greater than 15 m depth. We also have partially shielded samples from the interior of rocks toppled to measure their stability, and through vandalism. To determine exhumation age and rate, our sampling strategy is to collect 5-6 samples per PBR: 1 on top, 3 on the sides, 1 on the pedestal (on pedestal samples), and 1 on the ground surface to determine modern landscape lowering rate. Our hope is that PBRs experienced rapid exhumation, and that 1 sample near the top will provide a good estimate of the exhumation age for future sampling campaigns. Lateral erosion is most important around the pivot point where the balanced rock rests on the pedestal, and the lower portion of the balanced rock, which can increase precariousness. 2-3 samples from were collected from this region, and selected to determine the effects of post-exhumation jointing and grüssification. Concentrations in this region that are significantly lower than the concentrations on the top and on the lower pedestal would suggest lateral erosion. We also have two samples for in situ 14C analysis; if the sample was exposed for 10 kyr with no erosion, the concentration should be close to secular equilibrium. Lower concentrations would indicate younger exposure ages, erosion, or a combination both.
DE: 1105 Quaternary geochronology
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 7212 Earthquake ground motions and engineering seismology
DE: 7221 Paleoseismology (8036)
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting