San Andreas fault zone, North Coast section (Class A) No. 1b
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 1b, San Andreas fault zone, North Coast section, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, https://earthquakes.usgs.gov/hazards/qfaults, accessed 04/26/2024 05:27 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: This section extends from Point Area southeast to the Golden Gate and is similar to the segment designated by Sykes and Nishenko (1984 #5794). The southern boundary with the Peninsula section [1c], adjacent to the Golden Gate, is characterized by a 3-km-wide, right-releasing step that has probably persisted for at least 3 m.y. (based on presence of the Merced Formation), the splaying off of the San Gregorio fault [60], a reduced slip rate south of the Golden Gate, and a distinct drop in geodetically modeled slip associated with the 1906 earthquake (Thatcher and others, 1997 #5795). This section has been described by Wallace (1970 #1423) who carried it north to Cape Mendocino and south to Los Gatos. The North Coast segment was delineated by Working Group on California Earthquake Probabilities (1988 #5494; 1990 #549), who designated the southern boundary near Lower Crystal Springs Reservoir. The North Coast segment designated by the Working Group on Northern California Earthquake Probabilities (1996 #1216; 1999 #4946) and Petersen and others (1996 #4860) is similar to the earlier ones of the Working Group on California Earthquake Probabilities (1988 #5494; 1990 #549), but the southern boundary was considered to be at the Golden Gate. 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) |
MARIN COUNTY, CALIFORNIA MENDOCINO COUNTY, CALIFORNIA SONOMA COUNTY, CALIFORNIA |
| Physiographic province(s) |
PACIFIC BORDER |
| Reliability of location |
Poor Compiled at 1:750,000 scale. Comments: Location based on digital revisions to Jennings (1994 #2878) at 1:750,000 scale map using original mapping by Brown and Wolfe (1972 #4801), Clark and Brabb (1997 #4810), and Galloway (1977 #4845) at 1:24,000 scale; Wagner (1977 #5803) at 1:12,000 scale; and Blake and others (1971 #4797, 1974 #5272) at 1:62,500 scale.
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| 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).
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| Length (km) | This section is 244 km of a total fault length of 1082 km. |
| Average strike | N36°W |
| Sense of movement |
Right lateral
Comments: Well-defined geomorphic expression of dextral strike-slip fault (Brown and Wolfe, 1972 #4801), dextral offset of geologic units (for example, Prentice and Sieh, 1988 #5757; Prentice, 1989 #5754; Prentice and others, 1991 #5756; Baldwin, 1996 #4793; Baldwin and others, 2000 #4795; Prentice and others, 2000 #5758), dextral displacement associated with 1906 San Francisco earthquake (Lawson, 1908 #4969), and geodetically determined dextral slip (Thatcher and others, 1997 #5795).
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| Dip Direction |
V
Comments: Vertical dip based on linear geomorphic expression of fault, vertical to near vertical fault zone expressed in trench exposures by Prentice (1989 #5754), Noller and others (1996 #5748), Nimei and Hall (1992 #5747), and Hall and others (1986 #4952).
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| Paleoseismology studies |
There are eight detailed study sites for the North Coast section. Bodega Harbor (1-6) and Bolinas Lagoon (1-9) are drilling sites of Knudsen and others (2002 #5727). Interpretation of marsh stratigraphy at both sites indicates that coseismic subsidence has occurred and allows identification of paleoearthquakes including the most recent event Dogtown site (1-11). Hall and others (1986 #4952) excavated both fault parallel and fault normal trenches at the Dogtown site south of Olema. Offset late Holocene deposits allowed determination of late Holocene recurrence intervals and identification of the most recent event on the North Coast section. Subsequent work by Wells and others (1999 #5809) involved the excavation of two new trenches and re-excavation of two former trenches in order to better constrain the slip history of the North Coast section during the past 2 k.y. Scaramella Ranch site (1-15). Prentice (1989 #5754) excavated 17 trenches (13 fault normal and 4 fault parallel) into a Holocene alluvial fan at the Scaramella Ranch site near Point Arena. A buried Holocene stream channel has been dextrally offset a maximum of 64 m. Prentice (1989 #5754) identified at least 5 surface rupturing earthquakes in the past 2 k.y. at the Scaramella Ranch site. Vedanta Retreat site (1-23). Nimei (1992 #5746) and Nimei and Hall (1992 #5747) excavated 20 trenches (both fault normal and fault parallel) across traces of the North Coast section at the Vedanta Retreat site near Olema. The site is along the alluviated valley of Gravely Creek. Here, a wind gap cut across a medial ridge subsequently has been blocked by dextral offset along the North Coast section. Buried late Holocene stream channel deposits identified in trenches are dextrally offset. Evidence for 2 to 5 pre-1906 surface rupturing earthquakes was reported. Alder Creek site (1-32). Baldwin (1996 #4793) excavated 3 fault normal and 3 fault parallel trenches across traces of the North Coast section at Alder Creek near Point Arena. A late Holocene terrace riser at Alder Creek is dextrally offset 9.5 m. Baldwin (1996 #4793) identified at least three earthquakes: the 1906 earthquake, event P (thought to have occurred between 680 A. D. and 1603 A. D.), and a pre-Holocene earthquake. Baldwin and others (2000 #4795) conducted ground-penetrating radar surveys and excavated two additional trenches at the Alder Creek site. Baldwin and others (2000 #4795) reported a stream channel dextrally offset a minimum of 8.5±0.5 m and a maximum of 9.0±0.5 m. The age of channel deposits is 400±100 cal yr BP, allowing estimate of 3.1-4.6 m dextral offset for the penultimate event. Archae Camp site (1-33). Noller and others (1996 #5748) excavated 4 trenches across the North Coast section at the Archae Camp site near Fort Ross. Here the fault is expressed as a single trace delineated by a southwest-facing scarp, linear troughs, and closed depressions. Three trenches extended across an archeological site. Several test pits and borings were used to delineate the margins of the archeological site. Noller and others (1996 #5748) identified 3 to 5 late Holocene surface faulting events, based on upward fault terminations, truncated colluvium and scarp-derived colluvial wedges, fissure fills, and cross-cutting stratigraphic relationships. Mill Gulch site (1-42). Langridge and Prentice (2000 #4966) excavated one trench near Fort Ross that exposed an abandoned and filled channel of proto-Mill Gulch. Charcoal from the top of gravel deposits yielded an age of 4,530±40 14C yr BP.
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| Geomorphic expression |
The North Coast section is marked by geomorphic features characteristic of Holocene dextral offset, such as dextrally offset stream channels, aligned linear valleys, closed depressions, shutter ridges, linear troughs and trenches in alluvium, linear scarps on alluvium, aligned benches and saddles, sidehill troughs, and linear vegetation contrasts (Brown and Wolfe, 1972 #4801; Wallace, 1990 #5804).
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| Age of faulted surficial deposits | Fault offsets late Holocene and historic fluvial and terrace deposits in the Point Arena area as determined from detailed studies including radiocarbon dating by Baldwin (1996 #4793) and Prentice (1989 #5754). Noller and others (1996 #5748) reported that anthropic deposits are displaced near Fort Ross. Nimei (1992 #5746) and Nimei and Hall (1992 #5747) identified late Holocene (1,800±78 yr BP) and historic fluvial deposits that are offset at the Vedanta paleoseismic site. |
| Historic earthquake | San Francisco earthquake 1906 |
| Most recent prehistoric deformation |
latest Quaternary (<15 ka)
Comments: The most recent event occurred in 1906. For the penultimate event (most recent paleoevent), Prentice (1989 #5754) reported that event R/V at the Scaramella Ranch near Point Arena occurred between AD 1530 A. D. and 1906 A. D., with a preferred time after 1635 A. D., but before 1850, based on a historical record indicating no major earthquakes between 1850 and 1906. Noller and others (1996 #5748) reported that the most recent paleoevent at the Archae Camp occurred between 1170 A. D. and 1650, based on radiocarbon dating of charcoal sampled from a scarp-derived colluvial wedge. The most recent paleoevent at this site, based on dendrochronology, occurred between AD 1400 and AD 1648. Knudsen and others (2002 #5727) reported that the penultimate event occurred after AD 1600 at the Bodega Harbor. Nimei (1992 #5746) reported that the most recent paleoearthquake at the Vedanta Retreat site occurred during or after 1591 A. D. to 1661 A. D. Hall and others (1986 #4952) reported that the most recent paleoearthquake at the Dogtown site occurred between AD 1521 and AD 1688. Wells and others (1999 #5809) identified deformation from the 1906 event but reported that extensive bioturbation in a paleosol obscured evidence of multiple pre-1906 earthquakes. They identified a series of faults that died out in the paleosol, representing at least one paleoearthquake between 300 14C yr BP and 1300 14C yr BP.
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| Recurrence interval |
200–400 yr (<2 k.y.)
Comments: Various studies of recurrence intervals for the past 2 k.y. range from 200 to 400 yr for this section. Prentice (1989 #5754) interpreted a minimum of 5 earthquakes in the past 2 k.y., based on trench exposures at the Scaramella Ranch site. Assuming a uniform recurrence model, Prentice concluded that an average recurrence interval of 200–400 yr best fits the data collected at the Scaramella Ranch site. Noller and others (1996 #5748) reported an average recurrence interval of 300–350 yr based on reconstruction of event chronology interpreted from trenches at the Archae Camp site near Fort Ross. Nimei (1992 #5746) recognized three earthquakes at the Vedanta Retreat site that yield an average recurrence interval of 240–318 yr. Nimei also reported a preferred average recurrence interval of 221±40 yr on the basis of the slip rate calculated for the Vedanta Retreat site and the assumption that the 4.9–5.5 m dextral slip reported for the 1906 earthquake is characteristic of previous events. Hall and others (1986 #4952) reported an average recurrence interval of 230–350 yr at the Dogtown site, based on event chronology. |
| Slip-rate category |
Greater than 5.0 mm/yr
Comments: Reported late Holocene slip rates for the North Coast section 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). Minimum slip rate. The minimum slip rate value reported by Noller and others (1996 #5748 from Archae Camp site) is based on a dextrally offset margin of an archaeological site. Anthropic deposits were dated using radiocarbon, obsidian-hydration age estimates, and local archaeological chronology. The slip-rate value should be considered a minimum value because the location of the southwest margin of the archaeological site was not well constrained due to erosion. Maximum slip rate. A maximum slip rate value of 25.5 mm/yr reported by Prentice (1989 #5754 from Scaramella Ranc site) is based on 64±2 m maximum dextral offset of a 2.36- to 2.71-ka buried stream channel based on 14C dating of detrital charcoal. A late Pleistocene slip rate of less than 39 mm/yr was reported by Prentice on the basis of 1.7 km dextral offset of landslide deposits dated as older than 43 ka. Correlation of marine terrace risers across the North Coast section indicates 1.5–2.5 km of dextral displacement. U-series age of solitary coral and correlation with global sea-level high stands suggest ages between 83 ka and 133 ka for the terrace surfaces, which indicates a 18–19 mm/yr slip rate. Prentice and others (2000 #5758) reported a revised slip rate estimate of 16–24 mm/yr. Prentice also reported a Pliocene slip rate of 12–20 mm/yr based on tentative correlation of the Pliocene Ohlson Ranch Formation in northwest Sonoma County with deposits 50 km to the northwest near Point Arena. A fission-track age of 3.3±0.8 Ma determined from zircons in tuff unit within rocks of the Ohlson Ranch Formation limits the maximum time for the 50 km of displacement. Holocene slip rates. Baldwin (1996 #4793 from Alder Creek site) reported a preferred late Holocene slip rate of 25±6 mm/yr based on 9.6±3.5 m dextral offset of a terrace riser. Timing of the terrace riser offset is based on the time that paleo-Alder Creek abandoned cutting of the riser. Overbank fines deposited over channel gravels are assumed to mark abandonment of the terrace riser. The overbank fines are dated at 400±90 14C yr BP from detrital charcoal recovered from the overbank deposits. A late Holocene dextral slip rate of 18±3 mm/yr (best estimate) was reported by Prentice and others (2001 #5759) based on 80–100 m dextral offset of abandoned channel of Mill Gulch site that has an age of 4,290–5,290 cal yr BP. Nimei (1992 #5746) and Nimei and Hall (1992 #5747), from Vedanta Retreat site reported a minimum late Holocene slip rate of 24±3 mm/yr based on 42.5±3.5 m dextral offset of stream channel deposits dated at 1,800±78 14C yr BP. The slip rate is considered a minimum because the detrital organic matter recovered in the stream channel yields a maximum age for the channel deposits and additional distributed deformation may not be accounted for at the site. |
| Date and Compiler(s) |
2002
William A. Bryant, California Geological Survey Matthew Lundberg, California Geological Survey |
| References |
#4787 Allen, C.R., 1957, San Andreas fault zone in San Gorgonio Pass, southern California: Geological Society of America Bulletin, v. 68, no. 3, p. 315-350. #4788 Allen, C.R., 1968, The tectonic environments of seismically active and inactive areas along the San Andreas fault system, in Dickinson, W.R., and Grantz, A., eds., Proceedings of conference on geologic problems of San Andreas fault system: Palo Alto, California, Stanford University Publications, Geological Sciences, v. XI, p. 70-82. #4789 Anderson, F.M., 1899, The geology of Point Reyes Peninsula: Berkeley, California, University of California Publications in Geological Sciences, v. 2, p. 119-153. #4790 Bakun, W.H., 1999, Seismic activity of the San Francisco Bay region: Bulletin of the Seismological Society of America, v. 89, no. 3, p. 764-784. #4793 Baldwin, J.N., Jr., 1996, Paleoseismic investigation of the San Andreas fault on the North Coast segment, near Manchester, California: San Jose, California, San Jose State University, unpublished M.S. thesis, 127 p., 4 pls. #4795 Baldwin, J.N., Knudsen, K.L., Lee, A., Prentice, C.S., and Gross, R., 2000, Preliminary estimate of coseismic displacement of the penultimate earthquake on the northern San Andreas fault, Pt. 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