MAPPING OFFSET FEATURES USING HIGH-RESOLUTION LIDAR DATA AND FIELD OBSERVATIONS ALONG THE SAN ANDREAS FAULT IN THE SAN BERNARDINO/ CAJON PASS AREA, SOUTHERN CALIFORNIA

One way to estimate paleo-earthquake magnitudes along a fault is to determine the lateral extent and slip-per-event of a rupture. This can be done by mapping out the geomorphic offsets along the fault that are the result of the last few ground-rupturing earthquakes on the fault. We are trying to do this along the southern section of the San Andreas fault.

We have created a new tectonogeomorphic strip map along the San Andreas fault from Wrightwood 47 km south to Highland using B4 light detection and ranging (LiDAR) imagery and field observations. The primary purpose of this mapping was to look for small (1-5 m) offsets that would allow us to map out the extent and lateral slip distribution of the most recent ruptures along this section of the San Andreas fault. For the entire project area, field mapping coupled with high-resolution LiDAR imagery and aerial photos analysis was used to map the fault zone and to identify geomorphic offset features.

Our mapping shows that there are no offset features smaller than about 6 m. Calculated offsets range from 6 m to 48.6 m, with most above 10 m. However, we found abundant tectonic geomorphic offset features indicating recent fault slip along the fault zone including offset and deflected stream channels, shutter and pressure ridges, linear valleys and troughs, benches, aligned notches, saddles, sag ponds, and scarps. The general lack of offset (or deflected) streams less than 6 m suggests that smaller offsets from the last one or two ground- rupturing earthquakes were not preserved. The new mapping will be used to catalog and evaluate the distribution of offset features as well as to look for previously unexploited slip-rate sites