COMPARISON OF TRENCH-BASED AND LIDAR-BASED RUPTURE MODELS FOR PAST EARTHQUAKES ALONG THE 1857 RUPTURE OF THE SOUTHERN SAN ANDREAS FAULT

Decades of paleoseismic investigations on the Carrizo, Big Bend, and Mojave sections of the southern San Andreas Fault have yielded detailed records of ground-rupturing earthquakes along the 1857 rupture trace. These investigations, completed by trenching studies at five principal locations along 330 km of the fault show that on average, ruptures occur every 100-130 years, but that periods as short as ~40 or as long as 220 years can lapse between individual earthquakes. New dating from the Frazier Mountain paleoseismic site, located at the midpoint of the 1857 rupture, constrain the number of large (≥M7) earthquakes that could have occurred on this part of the fault since 1350 A.D. We find that 25% of the events ruptured with magnitudes equivalent to the 1857 rupture (Mw 7.7-7.9), the remaining events are smaller. We review the constraints on the possible slip history, highlighting sedimentologic records for improving correlations based on radiocarbon age dating alone. We then compare the rupture patterns permitted by the spatially sparse trenching data with lidar-based measurements of past slip distributions. Although the lidar-based measurements improve the spatial density of the measurements, proposed slip distributions that are based on connecting populations of similar offsets along strike are not compatible with the trenching-based slip history. This discrepancy suggests that smaller slip events and overlapping ruptures contribute but are not resolved by the geomorphic offsets, a result supported by complex slip distributions seen in modern earthquakes that are visually similar to the variable slip earthquake behavior models. Alternatively, the paleoseismic record may include a few moderate-sized ground-rupturing events. This possibility, however, is inconsistent with the historic seismicity on the fault (which has had no ground rupture since 1857) and slip-per-event estimated from trenching at Frazier Mountain.