PALEOSEISMOLOGY AND SLIP RATE OF THE SAN ANDREAS FAULT SYSTEM AT SAN GORGONIO PASS

Our understanding of the frequency and size of large earthquakes on the southern San Andreas Fault hinges upon deciphering the fault in its most structurally complex location, San Gorgonio Pass. We have excavated trenches and mapped terraces at several locations across the San Andreas Fault system here to constrain the timing of paleoearthquakes and fault slip rate. We present evidence from trenches for five ruptures. Preliminary radiocarbon dating of the exposed deposits constrains the five events to the past ~6000 yrs and the most recent event to 500-700 yrs ago. If the initial dates are maintained, the average interval for ruptures is ~1400 yrs, seven times longer than the average interval for neighboring sections of the southern San Andreas Fault. Vertical separation for each event measured in the trench was small, 30 to 60 cm. This contrasts with the nearby fault scarps that appear to have formed during the most recent event and are 1.5-2.5 m high. This discrepancy suggests that faults in the trenches record only 20-35% of the total vertical offset. Analyses of displaced landforms also help to constrain the slip rate. A displaced terrace riser estimated to be ~1500 yrs old shows 4.5 m of dextral and 2.5 m of vertical offset and a displaced late Pleistocene buttress unconformity in the Heights fanglomerate shows ~375 m of dextral and ~140 m of vertical offset. Both features show that the faults experience oblique slip where lateral exceeds vertical motion by 2-3:1. The slip vector is about 480 m directed S60°E nearly parallel to geodetic slip vector in the region. Assuming an oldest plausible age for the Heights surface of 100,000 yrs, this strand of the fault system accommodates a minimum of ~5 mm/yr. Despite this low rate, ~7 m of interseismic strain can accumulate given the ~1400 yr recurrence interval and generate an earthquake that probably will involve neighboring strike-slip sections of the San Andreas during magnitude 7.8 or greater earthquakes.