Holocene slip rates along the San Andreas Fault System in the San Gorgonio Pass and implications for large earthquakes in southern California.

Abstract:

Since the late 17th century the San Andreas Fault (SAF) in southern California has produced three large (M7.3-7.8) earthquakes that did not break through San Gorgonio Pass (SGP). This pass-as-a-barrier behavior can be explained by the complexity of the SAF system. Here the fault splays and rotates into a series of en-echelon, oblique-dextral thrusts. In the center of SGP two sub-parallel thrusts are 1.5 km apart (the northern (NF) and southern (SF) splays), and form well-preserved fault scarps (up to 15 m high) in at least 3 alluvial fan terraces formed on the Millard Canyon fan. New, ¹⁰Be cosmogenic depth profiles and boulder exposure ages constrain the ages of the two oldest Holocene fan surfaces (Qt4, Qt3) to 8900 (range: 7300-11600) and 8300 (range: 6800-10300) y.b.p.. Radiocarbon ages constrain the younger surface (Qt2) at ~1500 y.b.p.. Qt4 contains a 10.5-15.1 m high scarp along the SF and has an uplift rate of 0.9-2.1 mm/yr. Qt3 is inset 1.5 m within Qt4, and contains a 3.6-6.2 m high scarp on the SF and 2.9-5.3 m high scarp on the NF with 0.4-0.9 mm/yr and 0.3-0.8 mm/yr uplift rates, respectively. Qt2 is inset ~4 m within Qt3, and contains a 1.0-2.9 m high scarp on the NF with a 0.9-2.4 mm/yr uplift rate. Using measured fault dips of 45 (NF) and 20-30 (SF) degrees combined with a slip vector inferred from regional GPS data and 2:1 lateral to thrust offset markers, we calculate the net oblique slip rate on these faults at Millard Canyon at 2.1-4.8 mm/yr for Qt4 and 1.0-2.7 mm/yr for Qt3 (SF), and 0.8-2.2 mm/yr for Qt3 and 1.9-6.8 mm/yr for Qt2 (NF). The cumulative slip rate across Millard Canyon (SF+NF) is therefore 1.8-11.6 mm/yr, or more narrowly constrained at 4.0-5.9 mm/yr by using only the overlapping slip rates along each fault strand. An average slip rate of ~5 mm/yr is ~30-70% of the slip rates outside of SGP. This ‘slip deficit’ may reflect some combination of slip carried by faults within in the San Bernardino Mountains to the north, transferred onto the Eastern California Shear Zone or San Jacinto Fault, or accommodated by off-fault deformation through the SGP. Nonetheless, the slip at Millard Canyon, combined with evidence for sporadic slip events from nearby fault trenches, suggests that displacement on these faults occurs synchronously with some, but not all, outside-of-the-pass SAF earthquakes.