Assessing evidence for late Quaternary right-lateral slip along the Mission-Mill Creek fault in the Eastern San Bernardino Mountains, Southern California

Miranda Owen, Sally F. McGill, & Burakhan Gedik

Submitted August 30, 2026, SCEC Contribution #15425, 2026 SCEC Annual Meeting Poster #097

Published fault slip rates for the Mission-Mill Creek strand of the San Andreas fault in Southern California range from 0 to 30 mm/yr, obscuring our understanding of the fault activity, slip transfer, and earthquake hazards in this complex tectonic region. This study focuses on previously recognized rills that have been disrupted by earthquakes on the Mission-Mill Creek strand in the easternmost San Bernardino Mountains. We have produced new, high-resolution optical drone photography of the offset rills, and have constructed from this a digital elevation model with a 10 cm pixel size. This facilitates the correlation of individual rills across the fault, allowing measurements of their right-lateral offset. Several rills are right-laterally offset ~ 3 m, which could represent slip in the most recent earthquake on this strand. Other rills are offset larger amounts, potentially up to ~ 30 m. Clasts on the colluvial surface that is incised by the rills have weak to moderate rubification on their undersides. Very preliminary comparison of the degree of clast rubification suggests that the rills are incised into a colluvial surface that is most likely younger than Owen et al.’s (2014, Quaternary Science Reviews) Fm4a alluvial fan surface (at the mouth of Mill Creek), which has a mean age of 64.20 ± 9.56 ka or 59.86 ± 9.09 ka, based on Be-10 dating of five surficial boulders, using the Lal(1991)/Stone(2000) time-independent or time-dependent production rates, respectively. Combined with a ~ 30 m offset, this maximum age for the offset rills implies a minimum late Quaternary slip rate of ~ 0.5 mm/yr for the Mission-Mill Creek fault in the eastern San Bernardino Mountains but does not preclude a substantially higher rate. Ongoing work aims to more tightly constrain the age of the colluvial surface by comparing its clast rubification to younger dated surfaces and by direct IRSL dating of the colluvial sediments. A tighter constraint on the age of the surface will still only provide a minimum slip rate, but we expect to be able to provide a tighter minimum bound on the slip rate.

Key Words
San Andreas, Mission Creek, slip, slip rate

Citation
Owen, M., McGill, S. F., & Gedik, B. (2026, 08). Assessing evidence for late Quaternary right-lateral slip along the Mission-Mill Creek fault in the Eastern San Bernardino Mountains, Southern California. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology