COSMOGENIC 3HE PALEO-SEISMOLOGY OF AN ACTIVE NORMAL FAULT, NORTHERN CALIFORNIA

Over 100,000 earthquakes are felt each year around the world, 100 of which can cause significant damage to people; the human and financial cost of earthquakes underscores the need for improved understanding of how earthquakes on normal fault systems work. The Hat Creek Fault in northern California is a world class example of a segmented normal fault that cuts through a 24 ka basalt with an estimated slip rate of 2.2-3.6 mm/yr (Blakeslee and Kattenhorn, 2012). It is part of a system of normal faults that threatens local infrastructure, including a hydroelectric dam on the Pit River. Very little is known about its seismic hazard, although estimates based on the characteristic earthquake model have predicted a ~Mw 6.7 event with a recurrence interval of ~700 years (Blakeslee and Kattenhorn, 2012). A better understanding of its paleo-seismic history will inform regional seismic hazard models, while also providing a test of common assumptions used when applying the characteristic earthquake model to normal faults. One major assumption is that a fault will rupture at a consistent magnitude and recurrence interval over time. Another assumption, used by previous seismic analyses, is that all sections rupture in tandem, which would lead to over-estimates of earthquake magnitude. This study develops a paleo-seismic history by using cosmogenic 3He in pyroxene and olivine to date exposure ages of co-seismically toppled blocks and corresponding scarp faces to determine the timing of major seismic events. Three of the eight fault sections were sampled in this study to assess whether the sections rupture in tandem or separately. Preliminary results suggest that different segments may have ruptured in distinct events, an emerging chronology that will be clarified by forthcoming results.