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SAN ANDREAS FAULT [c4, p98-103]

South of the Transverse Ranges, the San Andreas fault system (Figure. 4.17) extends to the latitude of the Salton Sea as a broad belt of northwest-trending strike-slip faults bounded on the northeast by the San Andreas and on the southwest by the offshore Santa Cruz-Catalina Ridge fault zone. Between the Salton Sea and the Gulf of California, the San Andreas fault system merges with a complex pattern of active ridge segments and transform faults that continues beneath the gulf for more than 1,200 km (see chap. 3). Alluvium of the Colorado River, lake deposits, and the waters of the gulf obscure the transition from the San Andreas pattern of deformation, dominated by northwest-trending strike-slip faults, to the ridge-transform pattern of the gulf, but current practice (see chap. 3; Crowell, 1981, p. 596-597) places the south boundary of the fault system near the latitude of the Salton Sea. Thus defined, the southern section of the San Andreas fault system is about 200 km long by 150 to 175 km wide.

Late Quaternary faulting in this region follows several major breaks, of which the San Andreas is the most easterly and most active. The dominant role of the San Andreas fades south of Cajon Canyon (lat 34�18' N., long 117�28' W.), where the San Jacinto fault diverges from it, and farther south along the south front of the San Bernardino Mountains, where it merges into the Banning and Mission Creek reverse faults (lat 33�56' N., long 116�47' W., and lat 34�00' N., long 116�33' W.). Despite such complexities, continuity of Quaternary faulting on the main San Andreas can be traced at least as far south as the Salton Sea.

Large increments of Quaternary strike slip on the segment of the San Andreas fault along the San Gabriel Range front near Palmdale (lat 34�34' N., long 118�07' W.) were first recognized by systematic right-lateral stream offsets and by Pleistocene fan and terrace deposits displaced 2 to 8 km from their source regions across the fault (Wallace, 1949, p. 799-802; Noble, 1954, p. 46). Underlain by distinctive bodies of plutonic and metamorphic rocks, the ranges have contributed eroded debris to fans that spread across the range-front faults. Subsequent strike slip along these faults has displaced the distal parts of the fans laterally, separating them from their source rocks and opposing them against unlike rock types in nearby parts of the range front. Recent mapping (Barrows and others, 1985, p. 195-197) along the fault segment first described by Wallace (1949) and Noble (1954) constrains the slip rate there to values of 1.5 to 3.0 cm/yr ( Figure. 4.18).

Most historical slip on the San Andreas fault has accompanied earthquakes of M>=6. A remarkable record of earlier earthquake-related faulting has been interpreted (Sieh, 1978, 1984; Sieh and others, 1989) from natural and manmade exposures across the fault at Pallett Creek, about 55 km northeast of Los Angeles, within the eastern part of the segment mapped by Barrows and others (1985). At Pallett Creek, Sieh described sand-blows and other liquefaction structures, buried scarps, and truncated fault strands in dated peat and alluvium that record ( Figure. 4.19) a history of 12 earthquakes during the past 1,700 yr. The latest 10 episodes of faulting establish an average recurrence interval of about 132 yr (Sieh, 1989) but show an irregular, clustered distribution over time for large earthquakes on this section of the fault.

Slip rates along the San Andreas south of Pallett Creek are lower than those in the central section of the fault system; in part, this difference is accounted for by slip on the San Jacinto and other, more westerly faults.

A site on the San Andreas fault in Cajon Canyon (lat 34�16.4' N., long 117�27.9' W.) about 100 km east of Los Angeles, establishes a consistent slip rate (Weldon and Sieh, 1985) for the late Pleistocene and Holocene time. Along Cajon and Lone Pine Creeks, the fault displaces terrace risers, buried and active stream channels, and landslides. The alluvial and swamp deposits have yielded 14 14C ages. These dated fault displacements, in combination with a reconstructed fluvial history of the site, provide four independent measurements of fault slip that document an average slip rate for the past 14.4 ka of 2.5+/-0.4 cm/yr (Figure. 4.20; Weldon and Sieh, 1985).

Harden and Matti (1989) reported slip rates that are less well constrained and possibly more variable for the San Andreas fault near Yucaipa, 45 km southeast of Cajon Canyon. There, displaced alluvial fans yielded average slip rates of 1.4 to 2.5 cm/yr for the past 14 ka, 2.2 to 3.4 cm/yr for the past 30 ka, and 1.2 to 1.6 cm/yr for the past 65 or 90 ka. These rates imply an accelerating rate of latest Quaternary slip, but uncertainties in measurements of displacement and age permit rates that are constant or even diminishing over time.

Paired stream offsets along the southern branch of the San Andreas fault (lat 34�07.5' N., long 117�10.0' W.), between Yucaipa and Cajon Canyon and in San Bernardino (lat 34�06' N., long 117�17' W.), indicate a maximum slip rate of 2.5 cm/yr (Rasmussen, 1982, p. 112) based on estimated ages of 30 and 50 ka for faulted alluvial units. If, as Rasmussen suggested, these are minimum ages, the actual slip rate may be somewhat lower.

On the San Andreas fault and about 30 km north of the Salton Sea (lat 33�46.9' N., long 116�14.4' W.), Keller and others (1982) mapped a total offset of 700 m where two strands of the fault cut an alluvial fan. Soils developed on the fan surface indicate an age of 30 to 20 ka, bracketing the slip rate between 2.3 and 3.5 cm/yr. At a nearby locality, a rate of only a few tenths of a centimeter per year can be derived from displaced (1 m) sedimentary deposits associated with the latest highstands of former Lake Cahuilla, 700-300 yr B.P. (Sieh, 1981). These deposits, however, may be too young to record slip accompanying large earthquakes with recurrence times longer than 700 yr.

The San Andreas follows the east side of the Salton Sea southward for about 30 km; its trace is marked by offset Holocene features and at least two historical slip events (Sharp, 1982, p. 9). Farther south, in the Imperial Valley, evidence of surface faulting disappears, and a 40-km gap separates the south end of the San Andreas from the north end of the Imperial fault. This gap, nearby geothermal and volcanic activity, and the broad structural trough occupied by the Salton Sea and the Imperial Valley indicate a major change in deformation processes and a transition from the San Andreas fault system to the ridge-transform system of the Gulf of California.