REINVESTIGATING THE MISSION CREEK FAULT: HOLOCENE SLIP RATES IN THE NORTHERN COACHELLA VALLEY AND IMPLICATIONS FOR SOUTHERN CALIFORNIA EARTHQUAKE HAZARD ASSESSMENT

Near San Gorgonio Pass the San Andreas fault zone encounters a structural knot which causes strain to be distributed regionally onto the San Jacinto fault and Eastern California Shear Zone and locally onto a series of evolving fault strands. Each strand was activated and subsequently abandoned as it became locked; current interpretations show that the Mission Creek fault was the dominant strand in the early Pleistocene before being abandoned in favor of the presently active Banning fault. Recent slip rate investigations along the Mission Creek fault have challenged this interpretation, however, and motivate new studies into strain distribution through San Gorgonio Pass and mechanisms of strain transfer to the Eastern California Shear Zone. It is therefore essential to establish an accurate Holocene slip rate on the Mission Creek fault and revisit current interpretations of San Andreas fault zone kinematics.

In support of this goal, detailed fault and quaternary unit mapping was conducted in two field areas along the Mission Creek fault in the northern Coachella Valley. Separated by ~3 km, the two field areas allow for characterization of along-strike changes in Mission Creek fault behavior and interaction with regional faults. Nineteen samples were collected from dextrally offset landforms for Terrestrial Cosmogenic Nuclide (TCN) dating. TCN dating measures the total concentration of in situ produced ¹⁰Be, which is proportional to exposure age of the surface. TCN surface dates therefore provide the age constraint for accurate Holocene-Late Quaternary slip rate analysis.

Dated surfaces within Big Morongo Canyon field area yield preliminary TCN ages of 8 ka and 20 ka in locations that record 88-97 m and 31-37 m of dextral displacement, respectively. Based on the calculated dates and measured offsets, local slip rates are calculated to be 11.4-14.0 mm/yr, which is significantly faster than previously estimated rates on the Mission Creek fault in the northern Coachella Valley. Constraining active slip on the Mission Creek fault has significant implications for southern California fault modeling and earthquake hazard assessment, and allows quantification of maximum strain transfer in the Coachella Valley from the Mission Creek fault to the Eastern California Shear Zone (~9 mm/yr).