Project Abstract
|
The San Andreas Fault (SAF) is the primary structure accommodating motion between the Pacific and North American plates. The Coachella Valley segment of the southern SAF has not ruptured historically, and is considered overdue for an earthquake because it has exceeded its average recurrence interval. In the northwestern Coachella Valley, this fault splits into three additional fault strands: the Mission Creek strand, which strikes northwest in the San Bernardino Mountains, and the Banning and Garnet Hill strands, which continue west, transferring slip into San Gorgonio Pass. Determining how slip is partitioned between these faults is critical for southern California seismic hazard models. Recent work near the southern end of the Mission Creek strand at Biskra Palms yielded a slip rate of ~14-17 mm/yr since 50 ka, and new measurements from Pushawalla Canyon suggest a possible rate of ~20 mm/yr since 2.5 ka and 70 ka. Slip appears to transfer away from the Mission Creek strand and to the Banning and Garnet Hill strands within the Indio Hills, but the slip rate for the Garnet Hill strand is unknown and the 4-5 mm/yr slip rate for the Banning strand is applicable only since the mid Holocene. Additional constraints on the Holocene slip rate for the Mission Creek strand are critical for resolving the total slip rate for the southern SAF, and also for comparing slip rates on all three fault strands in the northern Coachella Valley over similar time scales. We have identified a new slip rate site at the southern end of the Mission Creek strand between Pushawalla and Biskra Palms. At this site, (the Three Palms Site), three alluvial fans sourced from three distinct catchments have been displaced approximately 80 meters by the Mission Creek Strand. Initial observations from an exploratory pit excavated into the central fan show soil development consistent with Holocene fan deposition and no evidence of soil profile disruption. To more precisely constrain the minimum depositional timing of the most well-defined alluvial fan, we processed samples for U-series dating on pedogenic carbonate. A single sample from which 3 fractions were dated gave a model-independent isochron age of 3.3 +/- 0.6 ka, and the 5 oldest fractions gave a weighted mean age of 3.49 +/- 0.92 ka (95%CI). This is entirely consistent with the low degree of soil development observed in the soil pits. This minimum depositional age combined with 80 m of offset yields a maximum slip rate of 23 +/- 6 mm/year. This preliminary slip rate overlaps within error the late Pleistocene rates of 12-22 mm/yr at Biskra Palms to the east and with ongoing work by Blisniuk et al. of 20-27 mm/yr toward the west at Pushawalla Canyon. |
