Paleoseismologic Evidence for Holocene Activity on the Pinto Mountain Fault, Twentynine Palms, California
Ana Marisa Cadena
Excavations across the Pinto Mountain fault in Twentynine Palms, California exposed faulted strata across a 32-m wide zone. Trench wall exposures revealed clear evidence for five ground-rupturing events during the Holocene, and two additional events in the late Pleistocene. Optically stimulated luminescence ages from alluvial sediments suggest that the most recent event occurred between 1.7-2.9 ka B.P. and the penultimate event between 2.7-4.2 ka B.P.. Prior to the penultimate event, there were five ground-rupturing earthquakes on the eastern Pinto Mountain fault between 3.5-13.6 ka B.P.. The average recurrence interval since 13.3-13.6 ka B.P. is 1510-1680 years, and 1200-1500 years in the last 8.3-9.4 ka B.P.. Geomorphic evidence from offset streams southeast of Donnell Hill and offset gravel bars south and east of the Oasis of Mara suggests two to four meters of slip in the most recent event. Assuming three meters as characteristic displacement and seven events in 13.6 ka yields a Holocene slip rate of 1.59-1.80 mm yr-1. This rate greatly improves upon previously reported slip rate estimates inferred from the cumulative offset of crystalline bedrock contacts and structure.