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OTHER FAULTS [c4, p89-90]

The rate of Quaternary strike slip along the northern section of the San Andreas fault south of San Francisco, about 1.5 cm/yr, is less than either the 5.1 cm/yr attributed to the plate boundary or the 2.5 to 3.5 cm/yr observed along more southerly segments of the fault. Much of this slip deficit can be accounted for by distributed slip on several other northern California faults that nearly parallel the main trace of the San Andreas.

One of these faults, the San Gregorio, is the northern-most of a 400-km-long set of coastal faults lying south-west of the main trace of the San Andreas. The San Gregorio fault extends northward from Monterey Bay, joining the San Andreas about 20 km northwest of San Francisco, near Bolinas Bay. Where it cuts the coastline near Point Ano Nuevo, the San Gregorio is a complex, 3-to 5-km-wide zone of near-vertical strike-slip and northeast-dipping reverse faults. Faults in this zone offset stream channels incised into marine and nonmarine strata that were deposited on a 105-ka-old wave-cut platform; the faults also offset the shoreline angle of this and at least one older wave-cut platform ( Figure. 4.7). Evidence from offset shoreline angles and faulted strata indicates a late Pleistocene, right-lateral slip rate of 0.6 to 1.1 cm/yr across the fault zone (Weber and Cotton, 1981, p. 16, 72-75); Hamilton and others (1979), however, argued for rates of only 0.1 cm/yr in the same area.

If the higher slip rate on the San Gregorio fault and the rates estimated for the San Andreas fault south of San Francisco are correct, Quaternary slip on the San Andreas north of its junction with the San Gregorio must be about 2.6 cm/yr-the sum of the rates on the two separate faults. This rate agrees closely with that previously cited for the San Andreas near Point Arena (Prentice, 1989).

Additional slip occurs along faults northeast of the San Andreas, as shown by geomorphic evidence, alignment of earthquake epicenters, and, on some faults, gradual fault creep amounting to as much as 1 cm/yr. Direct geologic evidence of Quaternary slip on such faults as the Hayward, Calaveras, and Green Valley is abundant, but the age and crossfault correlation of displaced stratigraphic markers are uncertain, and Quaternary slip rates are only weakly constrained. These rates, however, can be estimated from such other criteria as modern creep rates, geologic slip rates for displaced pre-Quaternary markers, and geomorphic expression relative to that along the San Andreas fault.

On such evidence, the most active fault trend, with Quaternary slip rates estimated to range from 0.5 to 1.0 cm/yr, is that defined by the southern section of the Calaveras fault and by the Hayward, Rodgers Creek-Healdsburg, and Maacama faults (Figure. 4.3). Together, these faults extend for 375 km as a series of chiefly right-stepping breaks that exhibit abundant geomorphic evidence of recent movement. The seismically active Green Valley fault, locally as well defined as the Rodgers Creek and 27 km farther east, offsets cultural features that record right-lateral creep of 0.5 cm/yr between 1922 and 1974 (Frizzell and Brown, 1976).

The east boundary of the northern section of the San Andreas fault system follows a series of faults that trend north-northwest, exhibit recognizable components of right-lateral strike slip, and approximately separate the Coast Ranges from the Great Valley. This discontinuous and somewhat diffuse belt of faulting includes, from north to south, the Stony Creek, Greenville, and Ortigalita faults. No obvious surface faulting bridges the 100-km interval between the Stony Creek and Greenville faults, but major earthquakes in 1892 at Vacaville (lat 38�12.5' N., long 121�35' W) and Winters (lat 38�18.5' N., long 121�34' W.) may signify fault continuity at depth on this trend.

The historically aseismic Stony Creek fault follows and locally truncates a major detachment surface (the Coast Range thrust) that has been rotated to vertical or overturned. It now exhibits a steep east-facing scarp along the front of the Coast Ranges and separates a mountainous Franciscan terrane on the west from less deformed rocks on the east. Stream-terrace and fan deposits of Quaternary age east of the fault terminate at its scarp and are gently tilted and dissected by erosion. Sparse geologic evidence of renewed Quaternary faulting favors dominantly vertical slip, up to the west, but a right-lateral strike-slip component is indicated by asymmetric cusps and bays in the mapped trace of the fault and by nearby southeasterly-plunging folds that deform rocks at least as young as late Pliocene (Fox, 1983, p. 22).

System-boundary faults to the south exhibit both local seismic activity and evidence of Quaternary slip. The Greenville fault, here (Figure. 4.3) combined with the Marsh Creek and Clayton faults, displaces Quaternary stream-terrace deposits and soils, as well as fold axes in middle to upper Tertiary marine deposits. Right-lateral slip rates on these faults range from 0.01 to 0.30 cm/yr, but most Quaternary rates are less than 0.1 cm/yr (Sweeney, 1982; Wright and others, 1982). These relatively low rates are consistent with weak geomorphic expression along this fault trend.