THE PALEOSEISMIC RECORD AT BURRO FLATS: EVIDENCE FOR A 300-YEAR AVERAGE RECURRENCE FOR LARGE EARTHQUAKES ON THE SAN ANDREAS FAULT IN SAN GORGONIO PASS, SOUTHERN CALIFORNIA

The San Andreas fault at San Gorgonio Pass, with its high degree of structural complexity and no historic record of large earthquakes, has remained enigmatic to geoscientists for many years. The complexity of the San Andreas fault system here may act as a barrier to throughgoing rupture. Under this scenario relatively short stretches of the fault would rupture in relatively frequent M7.5 to M7.7 events. Alternatively, the structural complexity at San Gorgonio Pass may only prolong throughgoing rupture. Under this scenario the fault may rupture from the Coachella Valley to the Mojave in relatively infrequent M7.8 to M8.0 events. To test these scenarios an extensive network of trenches has been excavated within San Gorgonio Pass at the Burro Flats site located at a 75-m-wide transtensional stepover in the San Bernardino strand of the San Andreas fault. Fault terminations and growth fold relations exposed in the trenches define five distinct events since about A.D. 500. No hiatus in the sediment record has been recognized and the full width of the stepover basin has been excavated, therefore this five-event record is considered complete. The precise timing of these events has not been established, but preliminary age data constrain the five events to1500-1850, 1400-1550, 1300-1450, 700-1100, and 450-800 (ranges given in calibrated calendar years A.D.). These data suggest irregular recurrences of between 100 and 475 years or an average recurrence interval of about 300 years, similar to the average interval at the Indio site (Sieh, 1984). Furthermore, it is possible to correlate events at Burro Flats with each of the events at the Indio site and with about ½ of the events at sites along the Mojave segment, including Pitman Canyon, Wrightwood, and Pallett Creek (Seitz, et.al., 1997; Fumal, et.al., 1993; and Sieh, et.al., 1989). The San Andreas fault system in San Gorgonio Pass therefore appears to support the second scenario, with relatively infrequent large earthquakes that may rupture the Coachella Valley, San Bernardino, and Mojave segments of the fault.