Where's the slip? No known slip in ~40,000 years on northern-route strands of the San Andreas Fault from the northwest Indio Hills to central San Gorgonio Pass, southern California

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

Submitted September 11, 2022, SCEC Contribution #12162, 2022 SCEC Annual Meeting Poster #253

If any large earthquake on the southern San Andreas Fault (SAF) ruptures through San Gorgonio Pass (SGP) and the NW Coachella Valley (CV) it must follow one of two routes: a southern route involving the San Bernardino, Banning, and Garnet Hill strands of the SAF and the SGP Fault Zone, or a northern route involving the Mission Creek (MC) and Mill Creek strands of the SAF and/or associated structures. Southern-route faults account for <50% of the expected slip, thus raising questions about where the rest of the slip is accommodated. A recent study (Blisniuk et al., 2021, Science Advances) proposes that northern-route faults (1) account for all of the missing slip, (2) represent the primary SAF system moving at a dextral rate of ~20 mm/yr, (3) produce large earthquakes that rupture through SGP, and (4) represent the Quaternary plate boundary. In this scenario, northern-route faults NW of the Indio Hills would be expected to (1) generate paleoearthquake intervals of ~250 years, (2) dextrally displace early and middle Holocene landforms by as much as 100-200 m, and (3) demonstrably disrupt bedrock contacts. We have observed none of this evidence along this 50-km reach of the northern route. For example, at the beheaded “Dry Tributary”, a key location in the middle of the 50-km fault reach, a 45 m-deep canyon reveals a strath surface beneath 25 m of sedimentary fill. The MC fault zone does not break the strath surface or its overlying sedimentary fill. Applying conservative deposition and denudation rates constrains the last fault motion here to at least 40,000 years ago. The soil profile atop the alluvial fill also supports a latest Pleistocene age for fault shutdown. Although paleoseismic and slip-rate data in the southern Indio Hills suggest ~20 mm/yr (late Pleistocene) and 2-14 mm/yr (Holocene,) on the MC strand, the fault appears to have slowed or shut-off in the NW CV and SGP. Our analysis refutes the claim that the MC strand represents the primary SAF in southern California. In addition, the temporal and along-strike changes in northern-route slip-rate data can help revise recent geophysical models that produce results incompatible with our findings (e.g., Beyer et al., 2018, Geosphere). Although we have not identified where the missing slip is carried, perhaps other mechanisms like (1) slip transfer to the San Jacinto fault system and/or Eastern California Shear Zone, or (2) diffuse off-fault deformation in SGP can resolve the missing slip paradox.

Key Words
San Gorgonio Pass, Coachella Valley, Mission Creek strand San Andres Fault,

Citation
Yule, D., Matti, J. C., Kendrick, K. J., & Heermance, R. V. (2022, 09). Where's the slip? No known slip in ~40,000 years on northern-route strands of the San Andreas Fault from the northwest Indio Hills to central San Gorgonio Pass, southern California. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)