Postseismic deformation following the 1906 San Francisco earthquake

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Abstract
We have reevaluated triangulation data from northern California following the 1906 San Francisco earthquake, thereby increasing the temporal and spatial resolution of postseismic deformation following that event. We have calculated uniform shear strain rates and average station velocities at Point Arena using data from 1906–1907, 1929–1930, and 1973–1975 and for the Point Reyes‐Petaluma arc using data from 1929–1930, 1938–1939, and 1960–1961. With the addition of recent geodetic data we infer an effective relaxation time for long‐term, postseismic deformation following the 1906 earthquake of 36±16 years. The Point Arena data are satisfactorily fit with accelerated afterslip at depth along the San Andreas fault plane. For a 10‐km‐deep, 25‐km‐wide afterslip zone the average slip rate between 1906 and 1930 is 11.2 cm/yr. Between 1929 and 1975 it is 4.6 cm/yr. Deformation in the Point Reyes‐Petaluma arc is clearly asymmetric with respect to the trace of the San Andreas fault, especially between 1929 and 1939. After inverting for the range of acceptable accelerated deep afterslip and horizontal detachment models, a detailed analysis using geologically reasonable geometries revealed that both model types have trouble explaining the spatial variations in the deformation field through time. In particular, accelerated deep afterslip models cannot reproduce the breadth of the observed deformation field to the northeast of the San Andreas fault. As a result, more complex, time‐varying deformation mechanisms are required to explain the Point Reyes‐Petaluma arc observations.