Contemporary crustal movements in California are concentrated within a plate-boundary-deformation zone that is typically 50 to 200 km wide, approximately centered on the San Andreas fault. Integrated right-lateral displacement rates across this zone range from 33 to 37 mm/ yr, representing about 75 percent of the Pacific-North American relative plate motion. Most or all of the rest may be taken up east of the San Andreas fault system in the Basin and Range province. Although aseismic fault slip (creep) is a locally important component of this relative plate motion, most of the geodetically measured deformation represents elastic strain on the crustal blocks adjacent to faults of the San Andreas system. Rates of secular (interseismic) shear strain are a maximum on the two currently locked segments of the San Andreas fault, sites of the great 1857 and 1906 earthquakes. Values range from 0.4 to 0.6 parts per million per year (ppm/yr) at the fault to 0.1 ppm/yr 30 to 80 km from it. Deformation occurring at the times of large strike-slip earthquakes (coseismic strain) is concentrated within a few tens of kilometers of the surface fault rupture, indicating that earthquake fault slip is largely confined to the upper 10 to 15 km of the crust. After major events, postseismic shear strain occurs at transiently high rates (more than 2 ppm/yr) that decay to background interseismic rates over a time scale of years to tens of years.
Observations of coseismic, postseismic, and interseIsmic movements define the earthquake deformation cycle and constrain models of strain accumulation and release for strike-slip plate boundaries. Observations are fitted equally well by two contrasting models. In the first model, the depth of coseismic faulting is much less than the thickness of the elastically strong lithosphere, and postseismic and interseismic deformation result from transient and steady aseismic slip on the downward extension of the earthquake fault plane. At the other extreme, if earthquake slippage extends through most or all of the elastic lithosphere, interearthquake deformation is due to transient or steady flow in the underlying weak substrate ("asthenosphere").