SOUTHERN SECTION OF THE SAN ANDREAS FAULT SYSTEM

see http://plate-tectonic.narod.ru/califcrustseismphotoalbum.html

Southeast of the 1857 rupture, the San Andreas fault splays into several branches associated with intense seismicity in the Banning-San Gorgonio area. Like the Tejon Pass bend in the 1857 rupture zone, the San Gorgonio bend spawns a major left-lateral fault (the Pinto Mountain fault) conjugate to the San Andreas system. Unlike the situation at Tejon Pass, however, the San Andreas fault at San Gorgonio splays into a complex pattern of branching and intersecting fault segments. South of San Gorgonio, the San Andreas fault reconverges into a single strand and bends again to the more southeasterly trend that characterizes the southern section of the fault system.

This section of the fault system south of the Transverse Ranges is transitional from oblique spreading along the axis of the Gulf of California to the obliquely convergent strike-slip displacement that dominates deformation along the continental section of the San Andreas transform boundary to the north. Several major strike-slip faults run west of and subparallel to the main strand of the San Andreas fault south of the Transverse Ranges. These faults, which are considered part of the San Andreas system and include the Imperial, San Jacinto, and Elsinore faults, accommodate a significant proportion of the plate-boundary motion. The Imperial and San Jacinto faults, in particular, have produced more moderate earthquakes than any other segment within the fault system (Hanks and others, 1975).
SOUTHERN BRANCH OF THE SAN ANDREAS FAULT
The most intense seismicity along the main trace of the southern section of the San Andreas fault is associated with the San Gorgonio bend and is concentrated between the two principal branches of the San Andreas fault: (1) the Mission Creek fault, or northern branch of the San Andreas; and (2) the Banning fault, which runs nearly due west from the south end of the Mission Creek fault toward an ambiguous junction with the San Jacinto fault just south of San Bernardino (see fig. 5.3A and maps at front of book). Neither strand forms a continuous structure through the bend. This San Gorgonio seismicity cluster produced numerous M=5.0-6.5 earthquakes in the 1930''s and 1940''s, and in 1986 it produced the ML=5.6 North Palm Springs earthquake, which involved dextral strike-slip displacement on the north-dipping Banning fault (Jones and others, 1986). The background seismicity in this area is the highest in southern California, but it is distributed throughout a volume and cannot be clearly associated with any fault. To the west, seismicity associated with the Banning cluster abuts the dense lineation of epicenters coincident with the northernmost segment of the San Jacinto fault. Nicholson and others (1986) suggested that much of the seismicity within the upper 10 km of the crust in this cluster involves left-lateral slip on a series of northeast-striking faults; however, Jones (1988) pointed out that the evidence for northeast-trending lineations of epicenters within the Banning cluster is less than clear.
Diffuse seismicity extends northward from the Banning cluster into the San Bernardino Mountains and eastward into the Pinto Mountains, with no clear lineations along the sinistral Pinto Mountain fault. Indeed, a diffuse, north-south-trending lineation of epicenters seems to cut directly across the Pinto Mountain fault from the west-central Pinto Mountains. Two M=5.2 earthquakes (see events 75, 76, fig. 5.11A) with right-lateral strike-slip planes parallel to this trend occurred at the north end of this zone in 1975 and 1979. Somewhat farther south, however, a broad, east-west-trending lineation appears to coincide with the Blue Cut fault. Even farther south, a second broad lineation extends eastward from near the junction of the Banning and Mission Creek branches, although this lineation does not coincide with a mapped fault.

The southernmost section of the San Andreas fault, the Indio segment, which extends from the junction of the Banning and Mission Creek branches southward to the end of the San Andreas at the Salton Sea, has been almost completely aseismic in historical time. At the north end of this segment, periodic swarms of small (M<4) earth- quakes a few kilometers east of the San Andreas appear to occur on small northeast-trending structures (for example, Norris and others, 1986; Jones, 1988). The sparse background seismicity is also offset a few kilometers to the east from the surface trace of the San Andreas. Although the possibility of systematic offsets related to P-wave-velocity contrasts across the fault has not been investigated in detail, the observed offset seems too large to be explained entirely by lateral velocity contrasts.

Although it has not ruptured with a major earthquake in historical time, the aseismic Indio segment of the San Andreas fault seems to have much in common with the 1857 and 1906 rupture zones. Sieh (1986) presented geologic evidence for at least four major ruptures along the Indio segment since A.D. 1000; the last occurred approximately 300 yr ago. Unlike the two major locked sections, however, the south end of the Indio segment adjacent to the Salton Sea shows minor aseismic creep (Louie and others, 1985) and has shown episodes of sympathetic slip accompanying M��6 earthquakes on the Imperial fault and the southern section of the San Jacinto fault (Sieh, 1982). Not only is the Indio segment aseismic, but also the entire Coachella block extending from the San Andreas fault on the northeast to the crest of the San Jacinto Mountains on the southwest.

A cross section of hypocenters along the southern branch of the San Andreas fault (M-M'') shows that the earthquakes associated with the bend at San Gorgonio are among the deepest in southern California, with maximum focal depths approaching 25 km. The maximum focal depths deepen southward along the San Andreas fault from about 12 km beneath the Mojave segment to 25 km beneath the San Gorgonio fault. At the south end of the San Gorgonio area, however, maximum focal depths abruptly decrease to 10 km. This shallowing of seismicity is associated with a shift in the most concentrated seismicity from between the two segments (Mission Creek and Banning) of the San Andreas to east of the Mission Creek fault. The sparse seismicity of the Indio segment is limited to depths of 5 km or less.
ASSOCIATED FAULTS
Although the southernmost section of the San Andreas fault is almost completely aseismic, associated subparallel faults are extremely active. These faults are marked by the three bold north-south- to northwest-trending alignments of epicenters that dominate the seismicity pattern within the San Andreas fault system south of the Transverse Ranges (fig. 5.10A), from east to west: (1) the Brawley seismic zone (Johnson, 1979), defined by a dense, spindle-shaped cluster of epicenters connecting the north end of the Imperial fault and the south end of the Indio segment of the San Andreas fault; (2) the northwestward alignment of densely clustered epicenters along the San Jacinto fault zone, which appears to branch from the northern section of the Imperial fault; and (3) the northwestward alignment of more diffusely clustered epicenters along the Elsinore fault, which appears to branch from somewhere near the south end of the Imperial fault
The Brawley seismic zone and the cluster of epicenters at the south end of the Imperial fault (coincident with the Cerro Prieto volcanic-geothermal field in Mexico) represent the two northernmost in the series of small spreading centers offset by right-lateral transform faults that characterize oblique spreading in the Gulf of California (Lomnitz and others, 1970; Johnson and Hill, 1982). The Imperial fault itself, which is marked by a scattered alignment of epicenters, serves as the transform fault between these two small spreading centers. The M=7.1 El Centro earthquake ruptured the entire length of the Imperial fault in 1940, and the M=6.6 Imperial Valley earthquake of 1979 ruptured the north two-thirds of the fault; intensity data suggest that moderate earthquakes (5.5<M<6.3) in 1906, 1915, 1917, and 1927 may also have been located on the Imperial fault (Johnson and Hill, 1982). Most of the aftershocks associated with the 1979 Imperial Valley earthquake were concentrated in the south half of the Brawley seismic zone, which was first recognized because of the many earthquake swarms it produced from 1973 through mid-1979 (Hill and others, 1975; Johnson 1979; Johnson and Hutton, 1982). Many of the individual swarm sequences, as well as individual clusters of events in the aftershock sequence, defined lineations transverse to the strike of the Imperial fault and the long axis of the Brawley seismic zone. Most earthquakes within the Brawley seismic zone have strike-slip focal mechanisms; thus, kinematically, these transverse lineations represent conjugate structures to the dominant north-northwestward trend of the Imperial-Brawley fault system.

Irregular clusters of epicenters mark the San Jacinto fault zone, which runs along the southwest base of the Santa Rosa and San Jacinto Mountains. These clusters tend to be concentrated near bends and junctions within the complex set of multiple fault strands that form the surface expression of this fault zone. In several places, particularly within the southern and northern sections of the fault zone, epicenters define linear concentrations that tend to be closely aligned with mapped fault traces. The San Jacinto fault zone has produced at least 10 earthquakes of M=6.0-6.6 since 1890, the most recent of which were the M=6.2 earthquake of 1954, the M=6.6 Borrego Mountain earthquake of 1968, and the M=6.6 Superstition Hills earthquake of 1987. Thatcher and others (1975) pointed out that this series of historical M>6 earthquakes along the San Jacinto fault zone has left two seismic gaps: one along the northern 40 km of the fault, and the other along a 20-km-Iong stretch of the central section of the fault zone (the Anza gap). The Anza gap shows up in figure 5.10A as a relatively quiescent stretch of the fault zone between two dense clusters, with a third cluster located off the fault zone some 20 km southwest of the gap (see Fletcher and others, 1987; Sanders and Kanamori, 1984).

The Elsinore fault zone is defined not so much by a coincident alignment of epicenters as by the loci of western end points for clusters of epicenters elongate northeastward between the Elsinore and San Jacinto fault zones. This pattern is most pronounced along the southeast half of the fault; the northwest half, which defines the northeast scarp of the Elsinore Mountains, is marked by scattered clusters of epicenters. As the Elsinore fault enters the Los Angeles Basin to the north, it splays into the Whittier and Chino faults. Historical seismicity levels are considerably lower along the Elsinore fault than either the San Jacinto fault zone or the Imperial fault/Brawley seismic zone. The largest historical earthquake on the Elsinore fault was an M=6 event in 1910 in the central section. The Whittier Narrows earthquake (ML =5.9) of 1987, which caused over $300 million in damage, was located at the north end of the Elsinore-Whittier fault. Because its mechanism was thrust faulting on an east-west-striking plane with a shallow dip, however, it does not appear to be simply related to the Elsinore system.

Seismicity in the relatively quiescent southwestern corner of California between the Elsinore fault and the coast shows up in figure 5.10A as small, sparsely scattered clusters of epicenters. Activity picks up again, however, in the vicinity of the major northwest-striking faults along the coast (the Rose Canyon fault through San Diego and the Newport- Inglewood and Palos Verdes faults along the western margin of the Los Angeles Basin). Except for weak alignments along the Newport-Inglewood fault, which ruptured with an M=6.3 earthquake in 1933 (Richter, 1958), the seismicity patterns associated with these faults show little tendency to align along mapped fault traces.

Moving northwestward along the San Jacinto fault zone", the base of the seismogenic crust deepens systematically to a maximum of 20 km beneath the stretch adjacent to San Jacinto Mountain (which at 3,293 m, is the second highest point in southern California) midway along the fault zone (cross sec. P-P'', fig. 5.10B). The base of the seismogenic crust maintains this 20-km depth farther northwestward along the fault zone to its junction with the Banning fault just south of San Bernardino (fig. 5.10A), beyond which it begins to shallow again. Note, in particular, that earthquakes tend to be concentrated between 10- and 20-km depth beneath the San Jacinto fault zone, leaving the upper 10 km of the crust relatively quiescent along the middle stretch of the fault zone. The dense knot of hypocenters in the upper 5 km of the crust midway along cross section P-P'' corresponds to the cluster of epicenters 15 km southwest of the fault zone near the Anza gap (fig. 5.10A). The Anza gap itself shows up between ��=120 and 140 km in cross section P-P'' as a quiescent zone below and southeast of the shallow cluster of hypocenters (Fletcher and others, 1987; Sanders, 1987). The distribution of hypocenters beneath the Elsinore fault zone (cross sec. R-R'', fig. 5.10B) is in many ways similar to that beneath the San Jacinto fault zone. Maximum focal depths increase northwestward from 12-15 km at the southeast end of the fault near the United States-Mexican border to about 20 km midway along the fault zone (generally coincident with the highest topography in this section of the Peninsular Ranges) and then gradually decrease farther northwestward toward the Los Angeles Basin. Maximum focal depths show evidence of increasing again at the northwest end of the fault as it approaches the Transverse Ranges and branches into the Whittier and Chino faults. The hypocenters along the south half of the Elsinore fault also tend to concentrate in the lower 10 km of the seismogenic crust, although this pattern is not as well defined in the diffuse seismicity of the Elsinore fault zone as in the dense clustering along the San Jacinto fault zone.