The San Simeon/Hosgri Fault System
Published:January 01, 1994
The San Simeon fault zone disrupts a flight of emergent marine terraces and offsets a series of drainages near San Simeon Point along the coast of south-central California. Detailed studies of the offset marine terraces and drainages have provided data that we have used to estimate the late Pleistocene slip rate for this fault zone. In this study, we mapped four and five marine terraces to the northeast and southwest, respectively, of the southern onshore reach of the San Simeon fault zone. These terraces correlate with sea-level highstands at ∼60 or 80, ∼80 or 105, ∼120, ∼210, and ∼330 ka. The marine terrace strandlines are displaced by the San Simeon fault zone along two or possibly three primary fault traces within a zone of shearing and warping up to 500 m wide. Ratios of horizontal to vertical slip are 8:1 to greater than 50:1, demonstrating that the fault is predominantly a right-lateral strike-slip fault. Estimated slip rates based on the present locations of strandlines for the San Simeon (80 or 105 ka), Tripod (120 ka), and Oso (210 ka) terraces, and paleogeographic reconstructions of the shoreline configurations during their development, range from about 0.4 to 11 mm/yr, with the best constrained values ranging from 1 to 3 mm/yr. Slip rates based on deflections and apparent offset of drainages across the primary active traces of the San Simeon fault zone are in agreement with the 1-t o 3-mm/yr values estimated from the marine terrace study.
The San Simeon fault zone, therefore, accommodates a significant amount of transpressional strain along the North America-Pacific plate margin. The fault zone is part of the larger San Gregorio-San Simeon-Hosgri system of near-coastal faults. The geologically determined slip rate of 1 to 3 mm/yr is comparable to geodetically modeled estimates of fault-parallel shear west of the San Andreas fault.