MECHANICS OF DEFORMATION
[c7, p195]

The observations described in the previous section point to a range of mechanical behavior for the faults comprised by the San Andreas system, from freely sliding with only minor accompanying seismicity, to completely locked from the surface to seismogenic depths except for abrupt slip during infrequent great earthquakes.

On the 160-km-long central section of the San Andreas fault, virtually all fault slip occurs aseismically. Slip rates measured at or near the fault are close to the average rate for the entire San Andreas boundary zone (Fig. 7.5), no strain is detectable in the crustal blocks adjacent to the fault, and historical earthquakes of M>~5-1/2 have not occurred. Abundant minor seismicity (see Fig. 4.10) contributes only negligibly to the slip budget, and except for a few small patches of fault surface that are in frictional contact between these small earthquakes, the first-order steady-state model for this segment involves rigid translation of adjacent fault blocks across the freely sliding plane of the San Andreas.

A transitional behavior applies to those fault segments where steady-state fault creep is observed at the surface but historical or prehistoric earthquakes of M>=6 have been documented. Examples include the Parkfield and Coachella Valley segments of the San Andreas fault, the Hayward fault, and the Imperial fault. On these segments, during the interseismic phase of the earthquake cycle, the fault is inferred to be freely slipping in its upper few kilometers, in locked frictional contact at seismogenic depths (approx 3-10 km), and once again freely slipping at greater depths (Fig. 7.8A). The result of this slip distribution is interseismic creep at the surface fault trace and elastic-strain accumulation in the adjacent blocks (Figs. 7.8B, 7.8C).