Poster #087, San Andreas Fault System (SAFS)

Where's the slip? No slip in at least 40,000 years on northern-route strands of the San Andreas fault northwest of the Indio Hills, southern California

Doug Yule, Jonathan C. Matti, Katherine J. Kendrick, & Richard V. Heermance
Poster Image: 

Poster Presentation

2021 SCEC Annual Meeting, Poster #087, SCEC Contribution #11275 VIEW PDF
Large throughgoing earthquakes linking Coachella Valley and Mojave Desert segments of the San Andreas fault (SAF) must navigate the complex San Gorgonio Pass (SGP) region. Here, the fault follows two routes. Southern-route (SR) faults have youthful expressions, but account for only ~50% of the expected slip and earthquake history. Recent work by others claims that northern-route (NR) faults are the primary SAF system, carry ~20 mm/yr of slip, and produce large earthquakes that rupture through the Pass on the Mission Creek SAF strand. In contrast to these studies, our evidence from field, lidar, and air photo mapping suggests that faults along a 50 km-long reach of the NR shut down during the... late Pleistocene. Specifically, Holocene deposits between Forest Falls and the northern Indio Hills show no clear expression of surface disruption. Dry Tributary (DT), midway along the NR, is a key locality for understanding the late Pleistocene activity on the Mission Creek strand. The fault extends along the upper 2 km reach of DT valley but lacks any surface expression in coalesced alluvial fans that now fill the valley. At its NW extent, DT is beheaded and incised 45 m at a tributary of Catclaw Flat valley. Here, the tributary canyon walls expose a bedrock strath overlain by 25 m of sediment capped in turn by a yellowish-brown soil. These features are unbroken and continuous across the Mission Creek fault zone. The Catclaw Flat valley was once a main trunk of the Whitewater River (WR) drainage but is now beheaded and incised 20 m by the modern WR. Using the documented incision and deposition rates from the region of 0.5-2.0 mm/yr gives an age estimate for latest motion on the Mission Creek strand at DT to be prior to 42-170 ka, consistent with the degree of soil-profile development. Thus, it is detrimental to include and promote the NR as the primary pathway for SAF motion, not only because doing so disregards the data presented in this and previous studies, but if included as the primary system in seismic hazard models, it may result in erroneous SAF earthquake scenarios and risk assessment. Though the question “Where’s the slip?” remains a conundrum, we emphasize that NR faults do not carry the SR’s missing slip. Furthermore, off-fault deformation in the SGP region and/or slip transfer to other fault systems, including the San Jacinto fault and Eastern California Shear Zone, must play a significant role to balance the SAF slip rates across the SGP region.