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New Evidence on the Slip Rate, Renewal Time, and Late Holocene Surface Displacement, Southernmost San Andreas Fault, Mecca Hills, California

6222 Kimberly Drive
Huntington Beach, California 92647
howshifftt{at}aol.com
(H.S.)

Camp, Dresser, and McKee Inc.
100 Pringle Ave., Suite 300
Walnut Creek, California 94596
(M.G.G.)

Department of Geological Sciences
California State University, Long Beach
1250 Bellflower Blvd.
Long Beach, California 90804-3903
(R.G.)

Department of Earth and Planetary Science and Geography
University of California
Berkeley, California 94720
(B.L.I.)

We estimate a renewal time for large earthquakes on the southernmost San Andreas fault (SAF) of 260 ± 100 years based on new paleoseismological data and a 34 millennia slip rate of 5–8 mm/yr. Our mean recurrence interval, 260 ± 100, is longer and its variance is smaller than the 160 + 240/–93 estimate for a M 7 earthquake used by the Working Group on California Earthquake Probabilities (WGCEP, 1995). A smaller creep rate than on contiguous 12-km fault segments is explained by Global Positioning System data that reveals 2–4 mm/yr of creep on the recently discovered Witbaard fault that bypasses the 12-km-long North Shore strand of the SAF. Geomorphic features near an archaeological site that cross the fault allow us to measure displacements of 155 ± 25 cm based on right-lateral gully offsets by the most recent earthquake (e.g., 1690). An Indian stone ring, straddling the fault trace, is currently offset 142 ± 12 cm. The ring lies on the 12-km-long North Shore segment of the SAF that has accumulated a minimum of 30 mm of triggered slip and aseismic creep since 1969. Contrasting slip rates of 23–35 mm/yr near Indio and 5–14 mm/yr on the southernmost SAF suggest that the Coachella Valley strand may be comprised of two distinct segments. Smaller long-term strain accumulation along the southernmost segment probably reduces earthquake potential for that area.