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OTHER FAULTS [c4, p95-97]

Other faults with known or probable Quaternary slip in the central Coast Ranges include the Rinconada, Big Pine, Ozena, South Cuyama, Morales, and a coastal fault defined by its aligned but separately named segments: Hosgri, San Simeon, Sur (lat 36�10.8' N., long 121�32' W.), Monterey Bay (lat 36�40' N., long 122�06' W.), and San Gregorio. For most of these faults, the evidence of Quaternary activity is sparse, discontinuous along strike, and in places arguable. It includes weakly expressed scarps and drainage offsets, displaced deposits of late Pliocene and Quaternary age, and historical seismicity along some fault trends. The geologic and geophysical evidence for Quaternary activity is somewhat more convincing for the Hosgri fault and its northern, colinear counterparts than for faults farther inland.

The Hosgri and similarly aligned coastal faults trend north-northwesterly from near Point Conception (Steritz and others, 1986) to the latitude of Monterey Bay (Silver and Normark, 1978), where they continue northward as the San Gregorio fault. For most of its length, this coastal fault zone is a complex, multistrand break a few kilometers off shore; it is recognized and mapped from juxtaposed stratal units interpreted from seismic-reflection profiles and truncated magnetic anomalies. Locally, seafloor scarps and displaced Holocene deposits, interpreted from the seismic-reflection profiles, confirm its Quaternary activity (Wagner, 1974; Leslie, 1981).

The San Simeon fault (Figure. 4.14), which may be an onshore segment of the Hosgri (Leslie, 1981), is also a multistrand break; it cuts and displaces late Pleistocene marine terraces and Holocene dune sands near San Simeon Point (Weber, 1983). Although faulted shoreline-angle geometry and the correlation of wave-cut platforms across this fault are subject to some uncertainty, Weber (1983, p. 56-59) and Hanson and others (1987) suggested a horizontal right-slip component of about 0.5 cm/yr. Earthquake focal mechanisms along the San Simeon fault indicate a dip of 55� E. and nearly equal components of reverse and strike slip (Eaton, 1984). Right-oblique slip at a rate of about 0.5 cm/yr on the Hosgri fault is indicated by earthquake focal mechanisms, sea-floor scarps, the distribution of late Quaternary wave-cut platforms, and faulted seismic-reflection horizons of late Cenozoic age. Much of this evidence is documented from investigations near the Diablo Canyon Powerplant (Pacific Gas and Electric Co., 1988), where it has been interpreted as pure strike slip.

In the western Transverse Ranges, faults with Quaternary displacement include some, like the San Andreas and San Gabriel, that exhibit chiefly strike slip and others, trending east-west or southwest, that exhibit reverse or oblique slip. The left-lateral Santa Ynez fault is an exception to the more general pattern of reverse and thrust faults. The reverse- and oblique-slip faults in the Transverse Ranges are complicated by splays, offsets, and changes in strike, but from north to south they define at least four major continuous, or nearly continuous, fault zones of different lengths: the Santa Ynez (130 km); the More Ranch-Mission Ridge-San Cayetano (110 km), from the coast near Santa Barbara, eastward; the Mission Hills-San Fernando-Sierra Madre (56 km), along the southwest side of the San Gabriel Mountains; and the South Frontal (250 km). Many of these faults exhibit evidence of Holocene movement, and the San Fernando fault produced the surface rupture and destructive M=6.6 earthquake of February 1971. Some of these faults, and others with east-westward trends, extend offshore into the Santa Barbara Channel (lat 34�09' N., long 119�33' W.), where they also cut Pleistocene or Holocene deposits (Clarke and others, 1985). Ziony and Yerkes (1985, p. 43-60) provided more detailed descriptions and maps of the late Quaternary faults in this and nearby areas; they also noted (p. 44) a profound increase in deformation rates in and near the Transverse Ranges during the past 750 ka. This pulse of orogenic activity-manifested by the growth of folds, uplift, and accelerated slip on reverse and thrust faults-coincides with middle Pleistocene and younger deformation observed to the south (Woodford and others, 1954, p. 77-78) in the Los Angeles Basin.

Maximum and minimum slip rates for many of the faults in the Transverse Ranges are deduced from offsets observed in drainages, fan surfaces, river terraces, and wave-cut platforms. Reported by numerous investigators (Clark and others, 1984), these rates of chiefly oblique slip cluster around a few tenths of a centimeter per year, but dip-slip components as great as 0.9 cm/yr are reported (Rockwell, 1988) for the San Cayetano fault. Observed slip rates may represent only a fraction of the total structural growth of the ranges because much of the late Quaternary deformation is by folding and uplift.

Although most Quaternary faulting south of the Gabilan Range is confined to structural blocks west and south of the San Andreas, two major faults, the Garlock and White Wolf, diverge northeastward from the San Andreas near the north boundary of the Transverse Ranges (Figure. 4.10). Neither of these faults exhibits a clearly defined junction with the San Andreas, but together they bound the horstlike block of Sierran basement that makes up the Tehachapi Mountains. The White Wolf, a southeast-dipping reverse fault that was the source of the Kern County M=7.7 earthquake of July 1952, exhibits geomorphic evidence of recent slip at the range front but is concealed to the west beneath the southern San JoaquIn Valley. The Garlock, devoid of major historical seismicity, exhibits clear evidence of left-lateral Quaternary slip (Clark, 1973) that increases eastward from 0.2 cm/yr at a point 55 km east of the San Andreas to 1.1 cm/yr at a point 75 km to the east (Clark and others, 1984).