RELATION OF GEOLOGIC STRUCTURE TO SEISMIC BEHAVIOR [c3, p68-69]

From the preceding descriptions, the San Andreas fault evidently cuts through many different types of rocks and regional structures along its traverse from the Transverse Ranges to the Mendocino triple junction, and the patterns of seismicity differ strikingly along various segments of the fault. These differences in seismic behavior coincide so closely with certain geologic situations along the San Andreas fault in central California as to suggest a causal relation.

Certain segments of the San Andreas fault in central California are characterized by frequent small-magnitude earthquakes and aseismic slippage (creep); creep also occurs along the San Andreas fault in Coachella Valley, the Imperial fault, and the Superstition Hills fault. Other segments, said to be "locked," are characterized by infrequent earthquakes, some of which have been historically of large magnitude, and by an absence of creep. The occurrence of creep has been described by Allen (1968), Wallace (1970), Nason and Tocher (1970), and Thatcher (see chap. 7), among others. The creep is mainly on faults along the west side of the Diablo antiform. The creeping segment of the San Andreas fault extends from Cholame to near San Juan Bautista. Other faults known to creep include segments of the Calaveras, Hayward, Concord, Green Valley, and Sargent faults ( Figure 3.7). The locked segments of the San Andreas fault in central California extend northward from near San Juan Bautista and southward from Cholame. Though of no recognized significance, the creeping segment of the San Andreas fault terminates near the north end of the Barrett Ridge slice of the Salinian block.

The creeping segments in central California occur where the faults regionally cut the upper plate of the Coast Range thrust (Figure 3..shg} and Figure 3.7; Irwin and Barnes, 1975). This position accounts for the common presence of serpentinite along the creeping segments of the San Andreas fault, because the serpentiniferous Coast Range ophiolite in the upper plate of the thrust is at the fault interface along these segments. The lower-plate Franciscan rocks form a geochemical province characterized by carbon dioxide-rich springs. Where the upper plate of the thrust is present, it may act as a hydraulic cap that helps to maintain high pore pressure caused by carbon dioxide in the underlying Franciscan rocks, and to direct fluid flow into the fault (Irwin and Barnes, 1975). The importance of pore pressure in relation to creep is its ability to reduce the frictional strength of rocks by lowering the effective confining pressure, as demonstrated by Byerlee and Brace (1972). The tectonic relations between carbon dioxide springs and seismicity were described by Irwin and Barnes (1980).