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OTHER FAULTS [c4, p103-104]

The seismically active San Jacinto fault diverges from the San Andreas northwest of Cajon Canyon and follows a nearly straight course for 215 km, ending along the southwest side of the Salton Trough. On small-scale maps, it appears as a linear southward extension of the San Andreas segment bounding the San Gabriel Mountains-a simpler, more direct break than the arcuate, branching pattern of the nearly parallel San Andreas, 45 km to the northeast. Geomorphic and geologic evidence of Quaternary strike slip (Sharp, 1967, 1972, 1981) resembles that for the main San Andreas, but locally vertical motion, southwest side up, may account for as much as 10 percent of the net slip.

Sharp (1981) determined minimum slip rates for two active segments of the San Jacinto fault zone: the Clark fault near Anza Valley and the Coyote Creek fault, 80 km farther south. At the Anza Valley locality (lat 33�34' N., long 116�39' W.), the base of a distinctive gravel marker bed, displaced at least 5.7 km from its source terrane of schist, gneiss, and amphibolite, overlies 30 m of sediment which, in turn, overlie a rhyolite ash layer dated at 730 ka and correlated with the air-fall ash at the base of the Bishop Tuff. Geologic data require a slip rate of at least 0.8 cm/yr (Sharp, 1981, p. 1755-1757), and a higher rate, as high as 2.5 cm/yr, is possible (R.V. Sharp, in Clark and others, 1984). North of Anza, geologic relations durIng the past 30 ka yield evidence of average slip rates of 1.2 to 1.7 cm/yr (Merifield and others, 1987) for this fault. The minimum Holocene slip rate for the past 6 ka on the Coyote Creek fault - derived from a 14C-dated, displaced channel margin (lat 33�05.4' N., long 116�02.7' W.) - is 0.3 cm/yr, but a much higher rate is possible (R.V. Sharp, oral commun., 1986). Upper limits for the slip rate on the San Jacinto fault (Prentice and others, 1986; Rockwell and others, 1986; Merifield and others, 1987) thus range from 1.0 to 1.7 cm/yr, in close agreement with Sharp's (1981) estimate of 0.8 to 1.2 cm/yr for the late Quaternary.

The Elsinore fault and its northern branches, the Whittier and Chino faults, lie about 35 km to the southwest of the San Jacinto fault. Seismic activity, occurring chiefly as small earthquakes, increases southward, but the strongest evidence for Quaternary faulting is along the north half of the fault, where Kennedy (1977) and Ziony and Yerkes (1985, table 5) reported examples of offset stratigraphy and landforms. South of lat 33�30' N., evidence of recent movement on the fault is sparse and widely spaced (Clark, 1975), although faulted Holocene fan deposits within 25 km of the United States-Mexican border indicate a strike-slip rate of about 0.4+/-0.1 cm/yr (Pinault and Rockwell, 1984).

South of its junction with the northeast-dipping Whittier reverse fault, the northern section of the Elsinore fault displays consistent evidence of oblique, dominantly horizontal slip. The vertical component is up to the southwest (Jahns, 1954, p. 45-46; Kennedy, 1977; Heath and others, 1982), as it is along the San Jacinto fault. For 100 km, the northern section of the Elsinore fault follows the northeast front of the Santa Ana and Elsinore Mountains, juxtaposing a thick Quaternary sedimentary fill on the northeast against pre-Tertiary crystalline and sedimentary rocks in the mountain blocks. South of Lake Elsinore (lat 33�39' N., long 117�21' W.), a facies boundary between sandstone and conglomerate, dated at about 700 ka by a layer of the Bishop Tuff, is offset 5 km (Kennedy, 1977, p. 5, 9), indicating a strike-slip rate of 0.7 cm/yr. North of Lake Elsinore, Millman and Rockwell (1985) reported at least 1.6 km of strike-slip offset since the middle Quaternary, equivalent to a minimum slip rate of about 0.2 cm/yr. These observations and other geologic and geomorphic evidence along the fault suggest that the component of horizontal slip on the northern section of the Elsinore fault during the late Quaternary has been somewhat less than on the San Jacinto and probably does not exceed 0.7 cm/yr.

The Newport-Inglewood zone of deformation (Hill, 1971, p. 2958), 40 km southwest of the Elsinore fault, is a narrow (1-3 km wide) belt of echelon folds and left-stepping fault segments, characterized by oblique right slip, chiefly down to the west (Harding, 1973, Figure. 5). This zone contains six major oil fields, within which subsurface structural relations in upper Cenozoic strata are well defined by drilling. Quaternary folding and faulting is expressed as an aligned series of anticlinal hills and by deformed and faulted upper Pliocene and Pleistocene deposits (Poland and others, 1959; Barrows, 1974). The overall structural trend is commonly mapped as the Newport-Inglewood fault (Figure. 4.17), but individual segments are less continuous and less closely aligned than along other major strike-slip faults in the San Andreas system. Moreover, many anticlines along the zone retain their original ellipsoidal form even where they are cut by strike-slip faults (Harding, 1973, Figures. 2, 4, 7, 9-15).

The onshore, northern part of the Newport-Inglewood fault zone extends 65 km from the Santa Monica Mountains (lat 34�03' N., long 118�24' W.) at the southern margin of the Transverse Ranges southeastward to Newport Beach. Farther southeast, the offshore part of the zone continues for about 70 km, defined by folding and faulting in Tertiary and Quaternary deposits (Clarke and others, 1985, p. 359) beneath the sea floor. Whether this zone of deformation is continuous with aligned but more distant offshore faults, such as the Rose Canyon fault near San Diego, is unresolved; if the zone is continuous, its overall length is at least 240 km.

Aligned swales and discontinuous eroded scarps, a few meters to as much as 80 m high, delineate individual fault segments; many of these scarps cut folded Pleistocene deposits. At the Inglewood and Long Beach oil fields on the northern, onshore part of the fault, folding and faulting are nearly as great in upper Pliocene strata as in older rocks (California Division of Oil and Gas, 1961, p. 577, 595), indicating that most of the deformation is Quaternary in age. Historical faulting is evident from the 1933 Long Beach earthquake (M=6.3) on an offshore segment of the zone, and from faulted artificial fill near Newport Beach (lat 33�22' N., long 117�33' W.) (Guptill and Heath, 1981). The horizontal slip in upper Pliocene strata, estimated from offset fold axes and from structure-contour maps of producing zones in faulted anticlines (Poland and others, 1959, p. 75; California Division of Oil and Gas, 1961), is about 1.5 km, equivalent to a slip rate of less than 0.1 cm/yr. Vertical components of slip are about a fifth as large, and vertical separation of faulted basal Pleistocene deposits rarely exceeds 60 m (Yerkes and others, 1965, p. A48).

West of the Newport-Inglewood fault zone, northwest-trending faults that exhibit evidence of Quaternary slip (Clarke and others, 1985, Figure. 185) include the Palos Verdes Hills fault, and the offshore San Pedro Basin and Santa Cruz-Catalina Ridge faults. The direction and amount of Quaternary displacement on these faults are ill defined, but they probably have strike-slip components smaller than those on the San Jacinto and San Andreas faults.

Many northwest-trending faults end near the 34th parallel against a major structural boundary that extends westward 250 km from the San Jacinto fault to Santa Cruz Island. This boundary separates the east-west-trending structures of the Transverse Ranges, Santa Barbara Basin, and northern Channel Islands (lat 34� N., long 120� W.) from northwest-trending structures to the south; it also separates higher land and sea floor to the north from lower to the south. A north-dipping zone of aligned or echelon faults (Yerkes and Lee, 1987; Ziony and Jones, 1988), here named the "South Frontal fault zone" (of the Transverse Ranges block), follows the structural boundary and includes, from west to east, the Santa Cruz Island, Anacapa, Santa Monica, Hollywood, Raymond, Sierra Madre, and Cucamonga faults. All of these faults are down to the south, all exhibit evidence of Quaternary activity, and many have been active during the Holocene. The San Fernando fault, an element of the Sierra Madre trend, was the source of the damaging M=6.6 San Fernando earthquake in 1971.

Slip rates on the South Frontal fault zone are probably lower than 0.5 cm/yr but are difficult to measure because most of the best exposed range-front faults consist of several branches. Typically, the youngest branches are farthest from the range front (Bull and others, 1979; McFadden and others, 1982) and cut tectonically derived fan deposits.

The vertical component of slip on the Cucamonga fault, about 0.3 cm/yr (Matti and others, 1982), is derived from a progressive increase in scarp heights - all close multiples of 2 m - in successively older alluvial-fan deposits. These deposits range in age from middle Pleistocene (approx. 700 ka) to late Holocene. The ages are based on stratigraphic relations, the maturity of soils developed on the fan surface (McFadden and others, 1982), and correlation of these soils with others (Bull and others, 1979) on ancient stream terraces in the San Gabriel Mountains.