Strain accumulation along the San Andreas Fault

The patterns and rates at which strain is accumulating along the San Andreas fault in California are analyzed from geodetic data. Geodimeter and triangulation data indicate displacements near the fault that are a result of homogeneous strain to an approximation within the accuracy of the data. Both sets of data indicate that nearly pure shear with a maximum shear plane nearly parallel to the fault is accumulating at a rate of about 0.7×10−6 yr−1 from San Francisco Bay to the Transverse Ranges. The nearly east-west trending segment of the fault in the Transverse Ranges is accumulating nearly uniaxial compression normal to the fault at a rate of 1.1×10−6 yr−1, indicating a ‘locking’ mechanism. At no point, with the possible exception of the Cholame Valley, do fault creep and small earthquakes substantially release the accumulating strain. A more complete set of data in the Imperial Valley suggests that the strains accumulating there are such as would be released by fault slippage at a rate of 8.5 cm yr−1 to a depth of 25 to 40 km. Slip on the Imperial fault due to small earthquakes and creep, however, occurs at a rate of only 1.3 cm yr−1, and earthquakes occur only to a depth of approximately 10 km. Strains at greater depths must be relieved by aseismic slip or inelastic deformation.