Latest Quaternary slip rates of the San Bernardino Strand of the San Andreas Fault, southern California, from Cajon Creek to Badger Canyon

Sally F. McGill, Lewis A. Owen, Ray J. Weldon, Katherine J. Kendrick, & Reed J. Burgette

Submitted December 18, 2019, SCEC Contribution #10001

Four new latest Pleistocene slip rate estimates from two sites along the northwestern half of the San Bernardino strand of the San Andreas fault suggest the slip rate decreases southeastward as slip transfers from the Mojave section of the San Andreas fault onto the northern San Jacinto fault zone. At Badger Canyon, offset estimates coupled with radiocarbon and optically stimulated luminescence (OSL) ages provide three independent slip rate estimates (with 95% confidence intervals): (1) the apex of the oldest dated fan (~30–28 ka) is right-laterally offset about 300–400 m yielding a slip rate of 13.5 +2.2/-2.5 mm/yr; (2) a terrace riser incised into the northwestern side of this alluvial fan is offset about 280-290 m and was abandoned ~23 ka, yielding a slip rate of 11.9 +0.9/-1.2 mm/yr; and (3) a younger alluvial fan (13-15 ka) has been offset 120–200 m from the same source canyon, yielding a slip rate of 11.8 +4.2/-3.5 mm/yr. These rates are all consistent with each other and result in a preferred, time-averaged for the past ~28 ka of 12.8 +5.3/-4.7 mm/yr (95% confidence interval), with an 84% confidence interval of 10-16 mm/yr. At Matthews Ranch, in Pitman Canyon, about 13 km northwest of Badger Canyon, a landslide offset ~ 650 m with a 10Be age of ~47 ka yields a slip rate of 14.5 +9.9/-6.2 mm/yr (95% confidence interval). These results suggest that ~12 mm/yr of slip transfers from the Mojave section of the San Andreas fault to the northern San Jacinto fault zone (and other faults) between Cajon Creek and Badger Canyon, with most (and potentially all) of this slip transfer happening between Cajon Creek and Pitman Canyon. This has been a consistent behavior of the fault for at least the past 47 ka.

Key Words
slip rate, San Andreas fault, latest Pleistocene, Late Quaternary

Citation
McGill, S. F., Owen, L. A., Weldon, R. J., Kendrick, K. J., & Burgette, R. J. (2019). Latest Quaternary slip rates of the San Bernardino Strand of the San Andreas Fault, southern California, from Cajon Creek to Badger Canyon. Geosphere, (submitted).