qfaults web comp As of January 12, 2017, the USGS maintains a limited number of metadata fields that characterize the Quaternary faults and folds of the United States. For the most up-to-date information, please refer to the interactive fault map.

San Andreas fault zone, San Bernardino Mountains section (Class A) No. 1i

Last Review Date: 2002-12-10

Compiled in cooperation with the California Geological Survey

citation for this record: Bryant, W.A., and Lundberg, M., compilers, 2002, Fault number 1i, San Andreas fault zone, San Bernardino Mountains section, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, https://earthquakes.usgs.gov/hazards/qfaults, accessed 05/26/2019 12:23 AM.

Synopsis General: The 1,100-km-long San Andreas fault zone is the principal element of the San Andreas fault system, a network of faults with predominantly dextral strike-slip displacement that collectively accommodates the majority of relative N-S motion between the North American and Pacific plates. Major elements of the San Andreas fault system include the Bartlett Springs [29], Maacama [30], Rodgers Creek [32], Green Valley [37], Calaveras [54], Hayward [55], San Gregorio [60], San Jacinto [125], Elsinore [126], and Imperial [132] fault zones. In this compilation, the San Andreas fault zone is considered to be the Holocene and historically active dextral strike-slip fault that extends along most of coastal California from its complex junction with the Mendocino fault zone [18] on the north, southeast to the northern Transverse Range and inland to the Salton Sea, where a well-defined zone of seismicity (the Brawley Seismic Zone [124]) transfers slip to the Imperial fault [132] along a right-releasing step. Two major surface-rupturing earthquakes have occurred in historic time: the 1857 Fort Tejon (Sieh, 1978 #5775) and 1906 San Francisco (Lawson, 1908 #4969) earthquakes. Additional historic surface rupturing earthquakes include the unnamed 1812 earthquake along the Mojave section [1h] (Jacoby and others, 1988 #4962; Sieh and others, 1989 #5779; Fumal and others, 2002 #5726) and the northern part of the San Bernardino Mountains section [1i] (Weldon and Sieh, 1985 #5806; Jacoby and others, 1987 #4961; 1988 #4962), and a large earthquake in the San Francisco Bay area that occurred in 1838 that was probably on the Peninsula section [1c] of the San Andreas fault (Toppozada and Borchardt, 1998 #5493; Bakun, 1999 #4790). Historic fault creep at rates as high as 32 mm/yr characterizes the 132-km-long Creeping section [1e] in central California (Burford and Harsh, 1980 #4806). The creep rate gradually tapers off to 0 mm/yr at the northwestern and southeastern ends of this section. The northern and southern ends of the Creeping section [1e] are transitional to the surface-rupture termination points of the 1906 earthquake to the north and 1857 earthquake to the south. Creep at rates as high as 4 mm/yr also has been measured on the Coachella section [1j] (Sieh and Williams, 1990 #5780). The San Andreas fault zone is the most extensively studied fault in California, and perhaps in the world. The fault zone first gained international scientific attention immediately following the great 1906 San Francisco earthquake. Lawson's 1908 report summarizing the investigation of the 1906 earthquake contained the first integrated description of the San Andreas fault, which was recognized as extending from Point Delgada in the north to Whitewater Canyon southeast of San Bernardino in the south, and formed the underlying basis for our modern studies of paleoseismology and earthquake geology (Prentice, 1999 #5755). More than 5,000 articles, maps, and publications describing various aspects of the San Andreas fault that have been produced since Lawson's pioneering work. In addition, there are about 1,000 site-specific fault rupture investigation reports (and maps) filed with the California Geological Survey in compliance with the Alquist-Priolo Earthquake Fault Zoning Act (Hart and Bryant, 1997 #4856). For this compilation, 51 detailed paleoseismic study sites along the fault zone are summarized. The fastest, generally accepted Holocene slip rate for the San Andreas fault is along the Cholame-Carrizo section [1g], which lies in the medial portion of the 1,100-km-long fault zone. Here, Sieh and Jahns (1984 #5778) reported a preferred late Holocene dextral slip rate of 33.9±2.9 mm/yr. In and south of the San Francisco Bay area, a significant portion of dextral slip is partitioned onto several faults of the San Andreas fault system, including the San Gregorio [60] on the west, and the Calaveras [54] and Hayward [55] faults on the east. Hall and others (1999 #4954) reported a late Holocene slip rate of 17±4 mm/yr for the Peninsula section [1c]. North of the Golden Gate, dextral slip from the San Gregorio fault zone [60] may be transferred to the North Coast section [1b] along a right-releasing step. Reported late Holocene slip rates for the North Coast section [1b] range from a minimum value of 16–18 mm/yr reported by Noller and others (1996 #5748) to a maximum value of 25.5±2.5 mm/yr reported by Prentice (1989 #5754). To the south, the San Andreas fault zone is delineated by an extremely complex zone of dextral strike-slip, reverse-oblique, and thrust faults in the southeastern Transverse Ranges. Fault nomenclature in the San Gorgonio Pass area is complex and different workers have assigned faults different names. West-northwest of San Gorgonio Pass Dibblee (1964 #1340; 1968 #4817; 1982 #4841) termed the principal active strand of the San Andreas fault located along the foot of the San Bernardino Mountains the South Branch San Andreas fault, which is referred to as the San Andreas fault by Allen (1957 #4787) and San Bernardino strand San Andreas fault by Matti and others (1992 #5735). For this compilation, this strand will be referred to as the San Andreas fault (South Branch). A fault that strikes sub-parallel located to the north was called the North Branch San Andreas fault by Dibblee (1964 #1340; 1968 #4817) and is referred to as the Mill Creek fault by Allen (1957 #4787), Matti and others (1992 #5735), and Jennings (1994 #2878). This strand will be referred to as the Mill Creek fault in this compilation. East-southeast of San Gorgonio Pass two principal dextral strike-slip faults comprise the Holocene active San Andreas fault zone. The southern trace has been referred to as the South Branch San Andreas fault by Dibblee (1967 #1345; 1981 #4840) and Jennings (1994 #2878); Matti and others (1992 #5735) refer to this trace as the Coachella Valley segment, Banning fault. This branch will be referred to as the South Branch San Andreas fault (Banning strand) in this compilation. The northern trace is referred to as the North Branch San Andreas fault by Dibblee (1967 #1345; 1981 #4840) and Jennings (1994 #2878); Mission Creek fault by Allen (1957 #4787); Matti and others (1992 #5735) named this trace the Coachella Valley segment, San Andreas fault and will be referred to as the North Branch San Andreas fault (Coachella strand) in this compilation. Refer to Matti and others (1992 #5735) for a detailed discussion of San Andreas fault nomenclature for the Mojave [1h], San Bernardino [1i], and Coachella [1j] sections. Weldon and Sieh (1985 #5806) reported a Holocene slip rate of 24±4 mm/yr at the northern end of the San Bernardino Mountains section [1i]. Harden and Matti (1989 #4955) reported a preferred Holocene slip rate of 14 mm/yr to 25 mm/yr near Yucaipa along the San Andreas fault (South Branch). Keller and others (1982 #4964) reported a preferred late Quaternary slip rate of 23 mm/yr to 35 mm/yr for the Coachella section [1j] near Biskra Palms. Surface-exposure age constraints (10Be-26Al) of the offset alluvial fan complex at Biskra Palms yields a better constrained late Quaternary dextral slip rate of 23.3±3.5 mm/yr (van der Woerd and others, 2001 #5800). Several average values of recurrence have been reported for the fault zone; in general they range from a little more than 100 to as much as 450 yr. The North Coast section [1b] ranges from 180–260 yr (Niemi and Hall, 1992 #5747) to 200≠400 yr for the past 2 k.y. (Prentice, 1989 #5754). The Santa Cruz Mountains section [1d] is 247-266 yr (Schwartz and others, 1998 #5771) and the Cholame-Carrizo section [1g] is 160–450 yr (Sieh and Jahns, 1984 #5778; Grant and Sieh, 1994 #4950; Sims, 1994 #5787; Stone and others, 2002 #5792). Recurrence intervals for the Mojave section [1h] are well-constrained based on paleoseismic studies by Sieh and others (1989 #5779), Biasi and others (2002 #5724) and Fumal and others (1993 #624; 2002 #5725). Sieh and others (1989 #5779) reported an average recurrence interval of 132 yr for the time interval AD 734 to 1857 at Pallett Creek, whereas Biasi and others (2002 #5724) refined the average recurrence interval at 135 yr. Fumal and others (2002 #5725) reported an average recurrence interval of 105 yr for the past 500 yr at Wrightwood. An average recurrence interval of 150–275 yr has been reported for the northern San Bernardino Mountains section by Weldon and Sieh (1985 #5806), Seitz and Weldon (1994 #5772), and Yule and others (2001 #4948). The Coachella section [1j] averages large earthquakes about 207–233 yr based on Sieh (1986 #5777).

Sections: This fault has 10 sections. From north to south they are the Shelter Cove [1a], North Coast [1b], Peninsula [1c], Santa Cruz Mountains [1d], Creeping [1e], Parkfield [1f], Cholame-Carrizo [1g], Mojave [1h], San Bernardino Mountains [1i], and Coachella [1j] sections. Different behavior patterns along different parts of the San Andreas fault where first noticed when Steinbrugge and Zacher (1960 #5791) documented creep along the fault in central California. Since that time, other workers have proposed various segmentation models for the San Andreas fault including five segments by Allen (1968 #4788), eight segments by Wallace (1970 #1423), 12 segments by Sykes and Nishenko (1984 #5794), Petersen and others (1996 #4860), the Working Group on California Earthquake Probabilities (1988 #5494; 1995 #4945; 1999 #4946), and the Working Group on Northern California Earthquake Probabilities (1996 #1216). Some segment boundaries are well documented or constrained for the San Andreas fault zone, whereas others are not. For this compilation, boundaries generally are similar to those described in models adopted by the Working Group on California Earthquake Probabilities (1988 #5494; 1990 #549; 1995 #4945; 1999 #4946), the Working Group on Northern California Earthquake Probabilities (1996 #1216), and Petersen and others (1996 #4860).
Name comments General: Traces of the San Andreas fault were first mapped in northern California by Lawson (1893 #4967) and were first named the San Andreas rift by Lawson (1895 #4968) after the type locality of the fault in the San Andreas Valley (San Mateo County, California). North of San Francisco, Anderson (1899 #4789) mapped traces of the fault on the Point Reyes Peninsula, but did not name the fault. Schuyler (1896–1897 #5769) described parts of the fault zone in southern California for a 200-mi (about 320-km) length through Kern, Los Angeles, and San Bernardino Counties and referred to the fault not as the San Andreas but as the "great earthquake crack", referring to surface fault ruptures associated with the 1857 Fort Tejon earthquake. The significance and extent of the San Andreas fault was not recognized until after the 1906 San Francisco earthquake. J.C. Branner and S. Tabor proposed the name Portola-Tomales for the fault zone, but A.C. Lawson (1908 #4969) preferred the term "San Andreas fault" (Hill, 1981 #4958). For this compilation, we use San Andreas fault zone owing to the complex nature and multiple strands (or faults) that comprise the structure.

Section: The San Bernardino Mountains section extends from a few kilometers northwest of Cajon Creek southeast to the area between Thousand Palms and Myoma. This section was originally designated as the San Bernardino Mountains segment by the Working Group on California Earthquake Probabilities (1988 #5494; 1995 #4945) and was adopted by Petersen and others (1996 #4860). The San Bernardino Mountains section is characterized by a large left-restraining step between the Mojave section [1h] to the northwest and the Coachella section [1j] to the southeast. The San Andreas fault zone is very complex in this restraining step, consisting of dextral strike-slip, thrust, and oblique slip faults. Allen (1957 #4787) reported that various structural complications in the San Gorgonio Pass area make continuity of a through-going dextral fault doubtful. The principal late Quaternary and Holocene faults comprising the San Bernardino Mountains section include the San Andreas fault (South Branch), Mill Creek fault (also named North Branch San Andreas fault), Mission Creek fault, Garnet Hill fault (Allen, 1957 #4787; Hope, 1969 #4960), Gandy Ranch fault (Allen, 1954 #4786; 1957 #4787), and Banning fault (Vaughn, 1922 #5801; Hill, 1928 #4959; Allen, 1957 #4787; Matti and others, 1992 #5735). Matti and Morton (1982 #5733) further divided the Banning fault into the Banning Strand A and Banning Strand B. The San Jacinto fault zone [125] branches from the San Andreas fault zone in the vicinity of Cajon Pass. The north-dipping San Gorgonio Pass fault zone [250] is located in this left-restraining bend, but is be considered separately for this compilation. Fault nomenclature in the San Gorgonio Pass area is complex and different workers have assigned faults different names. West-northwest of San Gorgonio Pass, Dibblee (1964 #1340; 1968 #4817; 1982 #4841) called the principal active strand of the San Andreas fault located along the foot of the San Bernardino Mountains the "South Branch San Andreas fault," which is referred to as the San Andreas fault by Allen (1957 #4787) and San Bernardino strand San Andreas fault by Matti and others (1992 #5735). For this compilation this strand will be referred to as the San Andreas fault (South Branch). A fault that strikes sub-parallel located to the north was called the North Branch San Andreas fault by Dibblee (1964 #1340; 1968 #4817) and is referred to as the Mill Creek fault by Allen (1957 #4787), Matti and others (1992 #5735), and Jennings (1994 #2878). This strand will be referred to as the Mill Creek fault in this compilation. East-southeast of San Gorgonio Pass, two principal dextral strike-slip faults comprise the Holocene (active) San Andreas fault zone. The southern trace has been referred to as the South Branch San Andreas fault by Dibblee (1967 #1345; 1981 #4840) and Jennings (1994 #2878); Matti and others (1992 #5735) refer to this trace as the Coachella Valley segment, Banning fault. This branch will be referred to as the South Branch San Andreas fault (Banning strand) in this compilation. The northern trace is referred to as the North Branch San Andreas fault by Dibblee (1967 #1345; 1981 #4840) and Jennings (1994 #2878); Mission Creek fault by Allen (1957 #4787); Matti and others (1992 #5735) named this trace the Coachella Valley segment, San Andreas fault and will be referred to as the North Branch San Andreas fault (Coachella strand) in this compilation. Matti and others (1992 #5735) have a detailed discussion of San Andreas fault nomenclature for the Mojave [1h], San Bernardino [1i], and Coachella [1j] sections.

Fault ID: Refers to Jennings (1994 #2878) numbers 87 (San Andreas fault (SAF) Shelter Cove), 116 (SAF splays), 119 (SAF Fort Ross to Manchester), 145 (SAF offshore), 147 (SAF offshore Bolinas), 162 (SAF boundary faults), 194 (SAF San Francisco to Watsonville), 217 (SAF 1989 ground fractures), 234 (SAF San Juan Bautista to Priest Valley), 240 (SAF historic creep), 278 (SAF Priest Valley to Cuyama), 311 (SAF Cuyama to Palmdale), 358 (SAF Palmdale to Cajon Canyon), 360 (SAF 1812 rupture), 427 (Mill Creek), 427A (SAF Cajon Canyon to Burro Flats), 452 (SAF South Branch), 453 (SAF North Branch), 472 (SAF Indio to Salton Sea), 477 (SAF Bombay Beach and vicinity), 452 (SAF South Branch), 449 (Banning fault western part), and 450 (Mission Creek fault), and numbers A1 (SAF 1906 rupture), A2 (SAF Peninsula), A3 (SAF Santa Cruz Mountains), and A7 (SAF creeping section) of the Working Group on Northern California Earthquake potential (1996 #1216).
County(s) and State(s) SAN BERNARDINO COUNTY, CALIFORNIA
RIVERSIDE COUNTY, CALIFORNIA
Physiographic province(s) PACIFIC BORDER
BASIN AND RANGE
Reliability of location Good
Compiled at 1:62,500 scale.

Comments: Location based on digital revisions to Jennings (1994 #2878) 1:750,00-scale map using original mapping by Hope (1969 #4960), Matti and others (1982 #5734; 1992 #5736), Miller (1979 #5742), Morton (1978 #5743), Morton and Matti (1991 #5744), and Treiman (1994 #5798) at 1:24,000; mapping by Dibblee (1964 #1340, 1965 #4816, 1974 #6605, 1981 #4839, 1981 #4840) at 1:62,500 scale; and mapping by Allen (1957 #4787) at 1: 63,360 scale.

Geologic setting The San Andreas fault zone is a major dextral strike-slip fault zone that extends for about 1,100 km along the western side of California. It is near the coast in northern California, but stays entirely inland to the south of San Francisco, extending all the way to the northern Gulf of California in Mexico. The San Andreas fault zone is the principal element of a network of dextral strike-slip faults that constitute the San Andreas fault system that collectively accommodates the majority of relative N-S motion between the Pacific and North American plates (Wallace, 1990 #5804). Wilson (1965 #4947) first proposed that the San Andreas fault was a transform fault connecting two spreading oceanic ridges between the Pacific and North American plates. The San Andreas fault zone extends from the Salton Trough near Bombay Beach northwest to its complex junction with the Mendocino fault zone [18] near Punta Gorda. At the southern end of the fault zone near Bombay Beach, dextral slip is transferred to the Imperial fault [132] along a right-releasing step-over delineated by a zone of seismicity referred to as the Brawley Seismic Zone [124]. The San Andreas fault traverses the length of the Coast Ranges geomorphic subprovince and forms the boundary between the Transverse Range and Mojave Desert geomorphic subprovinces as well as the boundary between the Salton Trough and Mojave Desert geomorphic subprovinces. Noble (1926 #1592) was the first to suggest a large amount of dextral slip (38 km) on the San Andreas fault. Hill and Dibblee (1953 #923) postulated that as much as 560 km of dextral slip has occurred on the basis of proposed correlation of Mesozoic basement rocks. Post-early Miocene cumulative dextral slip is approximately 315 km, based on correlation of the Neenach Volcanic Formation (22.5–24.1 Ma minimum K-Ar age reported in Sims, 1993 #5786) on the east side of the fault zone with early Miocene Pinnacles Formation (24.2±0.5 Ma average K-Ar age reported in Sims, 1993 #5786) on the west side of the fault (Matthews, 1976 #931). Stanley (1987 #5790) reported 325–330 km of post late Oligocene dextral slip and 320–325 km of post-early Miocene dextral slip. Further discussions of the displacement history the San Andreas fault zone are included in Powell (1993 #5753), Weldon and others (1993 #5807), and Matti and Morton (1993 #5737).

Length (km) This section is 128 km of a total fault length of 1082 km.
Average strike N79°W
Sense of movement Right lateral

Comments: The northwest and southeastern ends of the San Bernardino Mountains section are characterized by dextral strike-slip displacement. Faults in the San Gorgonio Pass area strike more east-west, resulting in significant compressional (N-S) slip. Traces of the Banning fault in the San Gorgonio Pass area are characterized by high-angle strike-slip (Banning Strand A) and north-dipping reverse displacement (Banning Strand B) (Matti and others, 1992 #5735; Treiman, 1994 #5798). East of San Gorgonio Pass, the North Branch San Andreas fault (Coachella strand) and South Branch San Andreas fault (Banning strand) are the principal strike-slip faults of the San Andreas fault zone.

Dip 90–35° N.

Comments: Steeply dipping to vertical strike-slip faults include the San Andreas fault (South Branch), Banning fault east and west of San Gorgonio Pass, Gandy Ranch, and Garnet Hill faults (Matti and others, 1992 #5735; Treiman, 1994 #5798). The Banning fault in San Gorgonio Pass dips between 35? and 65? N. (Allen, 1957 #4787; Rasmussen and Associates, 1990 #5762; Sieh and Matti, 1992 #5781). Reverse and thrust faults in the San Gorgonio Pass area (Banning fault) dip to the north (Allen, 1957 #4787; Matti and others, 1992 #5735; Treiman, 1994 #5798).

Paleoseismology studies There are eight detailed study sites along the San Bernardino Mountains section. In addition, there are over 180 site-specific fault rupture hazard investigations along the section involving trenching done in compliance with the Alquist-Priolo Earthquake Fault Zoning Act of 1972 (Hart and Bryant, 1997 #4856).

Lost Lake site (1-10). Weldon and Sieh (1985 #5806) excavated one fault-normal trench across traces of the San Andreas fault (South Branch) at the southeast end of Lost Lake near Cajon Pass. The trench exposed late Holocene colluvium, peat, lacustrine clay, and fluvial sediments. Detailed mapping and radiocarbon dating of Cajon Creek and Lone Pine Creek terrace deposits provided evidence allowing construction of late Holocene fluvial history and dextral slip measurements resulting in estimation of late Holocene slip rates. A total of 14 radiocarbon ages from peat, fluvial terrace, and lacustrine deposits provided age control in order to determine Holocene slip rates and recurrence intervals for large earthquakes on the San Bernardino Mountains section. Weldon and Sieh concluded that rupture associated with the 1857 Fort Tejon earthquake did not extend as far south as Cajon Creek.

Wilson Creek site (1-19). Studies by Harden and Matti (1989 #4955) combined detailed geologic mapping and assessments of soil-profile development to estimate Holocene and late Quaternary slip rates for the San Andreas fault (South Branch) at the Wilson Creek site near Yucaipa. Wilson Creek drains an area containing distinctive red-purple sedimentary rocks of the Miocene (?) Potato Sandstone. These sedimentary clasts were deposited in alluvial fans that cross the San Andreas fault. Harden and Matti described 16 soil profiles on these dextrally offset alluvial fans and estimated ages by comparing them with dated soils from Cajon Pass (Weldon and Sieh, 1985 #5806) and the Central Valley of California using a statistical method of maximum likelihood.

Pitman Canyon site (1-29). Studies by Seitz and Weldon (1994 #5772) and Seitz and others (1997 #5773) involved the excavation of 10 trenches (both fault normal and fault parallel) at Pitman Canyon. The Pitman Canyon site is characterized by 2 strands of the San Andreas fault, the Mill Creek fault and San Andreas fault (South Branch). The San Andreas fault (South Branch) is the principal active trace of the San Andreas fault zone and is delineated here by a downhill-facing scarp, shutter ridge, and groundwater barrier. Trenches exposed late Holocene debris flow and fluvial deposits interbedded with peat layers. Evidence of seven and possibly eight earthquakes during the past 1,150 yr was reported. Event horizons were delineated by fissure fills, upward termination of faults, folding, and incremental down-section increases in separation. About 4 m of dextral slip occurred in each of the past two earthquakes.

City Creek site (1-30). Sieh and others (1994 #5782) identified a suite of fluvial terraces that are dextrally offset by the Mill Creek fault at City Creek. Ages of terrace surfaces were approximated using soil profile development and dextral displacement was measured in order to constrain the late Quaternary slip rate of the Mill Creek fault.

City Creek site (1-36). McGill and others (1998 #5739) excavated a trench across the San Andreas fault (South Branch) within the flood control channel of City Creek. One event horizon was stratigraphically bracketed by fluvial deposits containing detrital charcoal dated at ASD 1020–1260. Because the dated samples were detrital, it is not known to what extent they may overestimate the time of deposition of the layers. Older faulting events may also have been represented within the trench, but the evidence was less clear. There were no visible faults that ruptured higher in the stratigraphic section. However, fault scarps visible on air photos project into a portion of the trench in which massive, bouldery alluvium has filled a large channel that incised into the faulted fluvial deposits that were found farther south in the trench. Thus, the one event horizon that was documented probably was on a secondary splay of the fault zone. Given the dates of prehistoric earthquakes at other sites along the San Bernardino Mountains section, it seems likely that younger events must have occurred on the main strand of the fault, but that the evidence for those younger events was destroyed when the bouldery channel was incised. Alternatively, faults may be present but not visible within the massive, bouldery channel fill.

Plunge Creek site (1-37). Studies by McGill and others (1998 #5739; 2002 #5741) and Dergham and McGill (2000 #4815) included several trenches across traces of the San Andreas fault (South Branch) at the Plunge Creek site. In some trenches the fault juxtaposed early Holocene and latest Pleistocene alluvium and colluvium on the northeast side against late Holocene alluvium on the southwest side. In other trenches the fault zone was located entirely within late Holocene deposits. In all cases, the fault strands were found several to a few tens of meters southwest of the topographic break in slope at the mountain front. McGill and others (1998 #5739) examined five trench logs prepared by Suitt (1992 #5793) in compliance with the Alquist-Priolo Earthquake Fault Zoning Act of 1972, and found relationships suggestive of between two and five faulting events since AD 900–1215. They also excavated or re-excavated five trenches of their own, in which the timing of the most recent earthquake was bracketed by sedimentary layers containing detrital charcoal dating to AD 1510–1730 (dendro-calibrated). They did not find evidence for an the 1812 earthquake or one about AD 1700, although these younger events (especially the AD 1700 event) cannot be conclusively ruled out because the only age control is from radiocarbon dates on detrital charcoal, which could overestimate the ages of the layers from which the samples were collected. Weak evidence for at least one and possibly more older events since AD 1220 was found in two of the trenches. City Creek site (1-41). McGill and others (1999 #5740) trenched across the Mill Creek fault about 1.6 km west of City Creek. Within the trench, latest Pleistocene sediments on the north side of the fault were juxtaposed against a subsurface ridge of severely fractured plutonic and metamorphic basement rock on the south side, along a 10- to 60-cm-wide zone of fault gouge. The zone of gouge strikes N. 78° W., dips 60–70° N. and extends up to the ground surface. A second fault zone was also found about 20 m farther north, with a strike of N. 73° W. and an apparent dip of 77° SW. Stratigraphic and structural relationships exposed in the trench imply that multiple earthquakes have occurred on the Mill Creek fault. Radiocarbon dates on detrital charcoal indicate that the fault has been active within the past 14,160±80 14C yr BP. No deposits of known Holocene age were encountered in the trench.

Burro Flats site (1-43). Yule and others (2001 #4948) excavated 10 trenches across the San Andreas fault (South Branch) at the Burro Flats site. A 7.5-m-thick section of well stratified distal-fan, pond, peat, and peaty-soil deposits were exposed. Trenches exposed two fault zones and associated folding that provide evidence of four rupture events and perhaps three additional events. AMS radiocarbon dates of the exposed stratigraphic section range from AD 1900 near the top to 1600 BC at 1 m above the base.

Geomorphic expression The San Andreas fault (South Branch) is delineated by well-defined geomorphic features indicative of Holocene active dextral slip, including dextrally deflected drainages, shutter ridges, linear ridges, linear scarps and vegetation contrasts on Holocene alluvium, linear troughs, closed depressions, and sidehill benches (Hope, 1969 #4960; Treiman, 1994 #5798). Reverse and thrust faults in the San Gorgonio Pass area are delineated by sinuous scarps on late Pleistocene and Holocene alluvial fans, faceted spurs, and vertically offset and deflected drainages (Treiman, 1994 #5798). Scarp-slope angles range from 11? to 25? (Treiman, 1994 #5798). Compound scarp slopes along some traces of the Banning fault (Banning Stand B) indicate multiple Holocene events (Treiman, 1994 #5798). Geomorphic evidence of Holocene dextral offset is less obvious for the Mill Creek fault (Matti and others, 1992 #5735).

Age of faulted surficial deposits Weldon and Sieh (1985 #5806) reported that Holocene colluvium, peat, lacustrine clay, and fluvial deposits are offset. Ages of faulted deposits at the Lost Lake site in the Cajon Creek area range in age from 8,150±550 yr BP to 580±80 yr BP based on dendro-corrected radiocarbon dates. Faulted peat deposits at the Pitman Canyon site range in age from 1,170±60 yr BP to 200±50 yr BP (corrected ages) (Seitz and Weldon, 1994 #5772). Faulted deposits at the Plunge Creek site range in age from AD 900–1640 (dendro-corrected ages of detrital charcoal) (McGill and others, 1998 #5739; Dergham and McGill, 2000 #4815; 2002 #5741).
Historic earthquake
Most recent prehistoric deformation latest Quaternary (<15 ka)

Comments: The 1812 (unnamed) earthquake is the most recent event on the northern part of the San Bernardino Mountains section of the San Andreas fault (Weldon and Sieh, 1985 #5806; Jacoby and others, 1987 #4961; 1988 #4962). The penultimate event recognized at the Pitman Canyon site occurred between AD 1649 and AD 1753 (Seitz and Weldon, 1994 #5772). McGill and others (1998 #5739; 2002 #5741) reported that the most recent event recognized at the Plunge Creek site occurred between AD 1510 and AD 1730, and preferred a date of AD 1630. McGill and others (2002 #5741) did not find evidence for either the AD 1812 or 1700 event, although it is not known to what extent the detrital charcoal might overestimate the depositional ages of the layers that bracket the youngest event at Plunge Creek. Yule and others (2001 #4948) identified the most recent paleoevent (their event I) at the Burro Flats site as occurring between AD 1650–1850. Yule and others (2001 #4948) reported that the timing constraints for event I currently are not sufficient to correlate with either the AD 1680 event or the 1812 event.

Recurrence interval 150–275 yr (<1 k.y.)

Comments: Weldon and Sieh (1985 #5806) reported an average late Holocene recurrence interval of about 150 to 200 yr at the Cajon Creek site. They recognized two and as many as four earthquakes between AD 1290 and AD 1805 and perhaps as many as six events in the past 1 k.y. Seitz and Weldon (1994 #5772) reported an average late Holocene recurrence interval of about 147 yr at the Pitman Canyon site. Seitz and Weldon reported that seven and as many as eight events occurred in the past 1,150 yr based on radiocarbon dates of offset fluvial and peat deposits exposed at the Pitman Canyon site. They inferred that the most recent event recorded at Pitman Canyon was the 1812 earthquake. Earlier events identified at the Pitman Canyon site include: Event 2: AD 1649–1753 Event 3: AD 1495–1530 Event 4: AD 1490–1502 Event 5: AD 1267–1326 Event 6: AD 1099–1171 Yule and others (2001 #4948) identified five events at the Burro Flat site. Preliminary event chronology based on 36 AMS radiocarbon dates from 16 different peat layers suggest irregular recurrences of between 100 and 475 yr, with an average recurrence interval of about 275 yr. At the Plunge Creek site, McGill and others (2002 #5741) observed evidence for a surface-rupturing earthquake that occurred between AD 1510 and AD 1730 and an event (event R), which postdates AD 1220, with a preferred date of about AD 1450. McGill and others (2002 #5741) suggest that their event R probably correlates with event 4 (AD 1490–1502) at the Pitman Canyon site.
Slip-rate category Greater than 5.0 mm/yr

Comments: Weldon and Sieh (1985 #5806) mapped and correlated dextrally offset terraces in Cajon Creek and Lone Pine Creek (Lost Lake site) using radiocarbon dates on detrital charcoal, and fluvial history reconstruction to calculate slip rates for the northern San Bernardino Mountains section. The longest-term slip rate is based on 350±30 m dextral offset of the eastern channel edge of Cajon Creek (channel in which Qoa-c unit was deposited). Weldon and Sieh's interpretation of the fluvial history of Cajon Creek indicates that Cajon Creek aggraded above its current level at the San Andreas fault at 14±1 ka, preserving the channel edge. The post-14 ka slip rate calculated from these data is 24±4 mm/yr. Post Qoa-c incised streams are dextrally deflected about 300 m. Three stream channels that constitute Prospect Creek, just southeast of Cajon Creek, are dextrally deflected 290±10 m. Weldon and Sieh suggested that the deflections at Prospect Creek can be correlated with the post Qoa-c stream incision, which they show to be sometime after 12.4±1 ka. This age is based on the rate of fill indicated by dendro-corrected radiocarbon ages of samples CC-3 and CC-4, extrapolated to the surface of Qoa-c. The post-12 ka slip rate calculated from these data is 23 mm/yr (+5 mm/yr, -2 mm/yr) based on the dextrally deflected drainages at Prospect Creek divided by the estimated age of the incision of fluvial unit Qoa-c. Weldon and Sieh calculated a mid to late Holocene slip rate of 25 mm/yr (+6 mm/yr, -4 mm/yr) based on a 145±5 m dextral offset of the inner channel of Pink River and dextral offset of their Qt-r terrace riser on the northwest side of Cajon Creek. Timing of the displacement is indicated by the change in character of lake clay deposits in Lost Lake at 5.9±0.9 ka. Rasmussen (1982 #5761) reported a slip rate of 25 mm/yr for the San Andreas fault (South Branch). The rate is based on offsets (amounts not reported) of Elder Gulch and Bledsoe Gulch that are incised into Pleistocene alluvial-fan deposits. The ages are crude estimates based on clast weathering and clay enrichment of the soil horizon. Rasmussen considered the age estimates to be minimum values and, thus, the reported slip rate a maximum rate. Seitz and Weldon (1994 #5772) reported an estimated late Holocene slip rate of about 24 mm/yr for the Pitman Canyon site by assuming that a total of 7-8 m of dextral slip occurred AD 1665–1812. The amount of dextral slip was estimated form apparent vertical separation, accounting for a minor reverse-slip component, and a consistent dip component of bedding. Sieh and others (1994 #5782) reported a preferred late Quaternary slip rate of 2.1±0.5 mm/yr for the Mill Creek fault (North Branch San Andreas fault) at City Creek. Sieh and others identified a suite of fluvial terraces that are dextrally offset along the Mill Creek fault (North Branch San Andreas fault). Ages of terraces were approximated using soil profile development. Harden and Matti's (1989 #4955) best estimates for slip rates along the San Andreas fault (South Branch) at Yucaipa site range from 14 to 25 mm/yr for the past 14 k.y., 22 mm/yr for the past 30 k.y., and 12–16 mm/yr for the past 90 k.y. Their preferred maximum slip rate for the past 14 k.y. is 35 mm/yr, although the data allow a maximum slip rate of 63 mm/yr for the past 90 ka. Harden and Matti (1989 #4955) estimated long-term late Pleistocene to Holocene slip rates near Yucaipa by restoring the dextrally offset medial axis of Wilson Creek alluvial fans. Maximum displacements were also obtained by restoring the westernmost extent of each of the dextrally displaced Wilson Creek alluvial fans. Timing of displacement was constrained by comparing soil profile development of offset alluvial fan surfaces with dated soils from Cajon Pass and the Central Valley of California. They used a statistical method of maximum likelihood and Monte Carlo simulations of larger data populations to account for soil variability, uncertainty of calibration dates, and the limited number of soils sampled and described. Harden and Matti mapped and estimated ages for three displaced alluvial fans (Qyf1, 11–35 ka; Qof2, 14≠49 ka; Qof1, 20–130 ka). Estimated dextral slip is 250–350 m for Qyf1, 670 m for Qof2, and 1,040 m for Qof1.
Date and Compiler(s) 2002
William A. Bryant, California Geological Survey
Matthew Lundberg, California Geological Survey
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