Little Salmon fault zone (Class A) No. 15
Last Review Date: 1999-03-24
Compiled in cooperation with the California Geological Survey
citation for this record: Hart, E.W., compiler, 1999, Fault number 15, Little Salmon fault zone, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, https://earthquakes.usgs.gov/hazards/qfaults, accessed 04/07/2020 06:17 PM.
Initially named and mapped by Ogle (1953 #4929), who also named and mapped the connecting Yager fault. Kelsey and Allwardt (1984 #4928) applied the name Little Salmon to the southeast extension of both faults, and subsequent workers generally applied the name Little Salmon fault or fault zone to all of the closely related faults in the zone. Other names applied locally (e.g., Bay Entrance, Buhne Point faults).
Fault ID: Refers to number 47 (Little Salmon fault) (onshore) of Jennings (1994 #2878); also includes his number 46 (east trace of Little Salmon), 41 (Bay Entrance), 45 (North Spit), 51 (Yager), and 52 (Goose Lake), as well as Buhne Point of Woodward-Clyde Consultants (1980 #4934). Does not include number 37 (Little Salmon offshore) of Jennings (1994 #2878) or the Table Bluff-Tompkins Hill faults of McCrory (1996 #1217).
|County(s) and State(s)||
HUMBOLDT COUNTY, CALIFORNIA
|Reliability of location||
Compiled at 1:62,500 scale.
Comments: Locations based on digital revisions to Jennings (1994 #2878) using original mapping by Ogle (1953 #4929) at 1:62,500 scale, Smith (1982 #4930; 1982 #4931) and Wills (1990 #4933) at 1:24,000 scale.
The Little Salmon fault zone is part of a broad, compressional thrust/fold belt developed in the accretionary wedge above the Cascadia subduction zone  (Carver, 1992 #4919). The 40-km-long onshore fault zone may extend another 40 km into the offshore as discontinuous thrust faults and folds (Clarke, 1990 #4143; 1992 #4919; McCrory, 1996 #1217). The zone is a northeast-dipping imbricate thrust zone that offsets all late Cenozoic sedimentary units of the Eel River basin. Faulting is associated with folding, most of which occurred in Quaternary time and continues to the present. Estimates of total dip-slip displacement vary and may be as much as 7 km with a late Quaternary slip rate as much as 10 or 12 mm/yr (Carver, 1987 #4918; Clarke, 1992 #4092; 1992 #4919; McCrory, 1996 #1217).
|Length (km)||91 km.|
|Sense of movement||
Comments: Ogle (1953 #4929), Woodward-Clyde Consultants (1980 #4934), Carver and Burke (1988 #4926), McCrory (1996 #1217).
Comments: Woodward-Clyde Consultants (1980 #4934), Carver and Burke (1988 #4926), McCrory (1996 #1217).
Eight trenches and two auger holes were dug across a rounded scarp on the western trace at the Little Salmon Creek paleoseismic site (15-1) revealing a deformed zone 75 m wide and a shallow NE-dipping thrust that offsets a sequence of late Holocene soils and floodplain deposits (Carver and Burke, 1988 #4926; Clarke and Carver, 1992 #4091). Based on radiocarbon age determinations, at least three events occurred in late Holocene time, the last during the past 400–500 years: a slip rate of 5.5 mm/yr was determined from this site 15-1. Dip-slip per event was estimated to be 3.3–4.5 m for the western trace (Carver and Burke, 1988 #4926). The eastern trace of the Little Salmon fault was also trenched nearby and similar but smaller surface displacements were interpreted "that may be synchronous" with the three events on the main trace (Clarke and Carver, 1992 #4091). Total slip per event for both traces was estimated to be 4.5–7 m (Carver, 1992 #4919). Based on the same data, McCrory (1996 #1217) recalculated the offsets per event and made more conservative slip-rate estimates.
Another paleoseismic site [15-2] on the Goose Lake fault trace indicated at least two displacements of lakebeds dated at 16,100±110 yrs B.P. and a possible slip rate of 0.3 to 1.0 mm/yr (Woodward-Clyde Consultants, 1980 #4934). A sharp step in the soil shown on the log of trench T-1 suggests late (?) Holocene displacement.
Based on four late Holocene subsidence events in the nearby Freshwater syncline (Vick, 1988 #4932), additional late Holocene events have been identified that may be related to the Little Salmon fault (Carver and others, 1989 #4927), however, McCrory (1996 #1217) considers this relationship as unconfirmed.
Other trench and boring investigations by Woodward-Clyde Consultants (1980 #4934) near the College of the Redwoods and Humboldt Bay revealed the location and geometry of several late Quaternary thrust faults and other structures related to the Little Salmon fault. Their work is summarized in some detail by McCrory (1996 #1217).
Locally defined by linear scarps, benches and breaks in slope along southwest-facing escarpment, but mostly not well defined geomorphically. Some traces concealed by water, young alluvium or landslides (Smith, 1982 #4930; Carver and Burke, 1988 #4926; Wills, 1990 #4933).
|Age of faulted surficial deposits||All faults offset late Pleistocene units with the exception of the Yager fault, which offsets sediment of the Eel River/Rio Dell Formation of Pliocene-Pleistocene age (Ogle, 1953 #4929; Woodward-Clyde Consultants, 1980 #4934; McCrory, 1996 #1217). The west and east traces of the Little Salmon fault offset Holocene alluvium and soils radiocarbon dated at 430±70 yrs B.P. (Carver and Burke, 1988 #4926) and the Goose Lake trace offsets lakebeds dated as 16,100±110 yrs B.P.|
|Most recent prehistoric deformation||
latest Quaternary (<15 ka)
Comments: Carver and Burke (1988 #4926) date the last event as occurring in the past 430±70 yrs, based on a radiocarbon date of the uppermost faulted soil.
400 to 800 yrs (<1,700 yrs)
Comments: Carver and Burke (1988 #4926) interpret three events in the past 1,700 yrs. McCrory (1996 #1217) reviewed the data and estimated a recurrence of 400–1,000 years.
Greater than 5.0 mm/yr
Comments: Clarke and Carver (1992 #4091) estimated a combined late Holocene deformation rate of 6–12 mm/yr across the western (main) and eastern traces. McCrory (1996 #1217) has a more conservative estimate of 3.3–4.3 mm/yr on the western (main) trace but does not suggest a slip rate for the eastern trace. Her estimate for the Yager fault is 3.6–5.4 mm/yr during the past 1.0–1.5 m.y.
|Date and Compiler(s)||
Earl W. Hart, California Geological Survey
#4924 California Division of Mines and Geology, 1991, Official and Revised Official Earthquake Fault Zone Maps of the Fields Landing, Fortuna and Hydesville quadrangles, scale 1:24,000: .
#4918 Carver, G.A., 1987, Late Cenozoic tectonics of the Eel River basin region, coastal northern California, in Schymiczek, H., and Suchsland, R., eds., Tectonics, sedimentation and evolution of the Eel River and associated coastal basins of northern California: San Joaquin Geological Society Miscellaneous Publication 37, p. 61-71.
#4925 Carver, G.A., 1989, Maps of Little Salmon fault including east trace, data: scale 1:16,000.
#4919 Carver, G.A., 1992, Late Cenozoic tectonics of coastal northern California, in Carver, G.A., and Aalto, K.R., eds., Field guide to the late Cenozoic subduction tectonics and sedimentation of northern coastal California: American Association of Petroleum Geologists, v. GB-71, p. 1-9.
#4926 Carver, G.A., and Burke, R.M., 1988, Trenching investigations of northwestern California faults, Humboldt Bay region: Technical report to U.S. Geological Survey, Reston, Virginia, under Contract 14-08-0001-G1082, 53 p.
#4927 Carver, G.A., Vick, G.S., and Burke, R.M., 1989, Late Holocene paleoseismicity of the Gorda segment of the Cascadia subduction zone: Geological Society of America Abstracts with Programs, Cordilleran Section, Spokane, WA, v. 21, no. 5, p. 64.
#4143 Clarke, S.H., Jr., 1990, Map showing geologic structures of the northern California continental margin: U.S. Geological Survey Miscellaneous Field Studies Map MF-2130, 1 sheet, scale 1:250,000.
#4092 Clarke, S.H., Jr., 1992, Geology of the Eel River Basin and adjacent region—Implications for Late Cenozoic tectonics of the southern Cascadian subduction zone and Mendocino Triple Junction: The America Association of Petroleum Geologists Bulletin, v. 76, no. 2, p. 199–224.
#4091 Clarke, S.H., Jr., and Carver, G.A., 1992, Late Holocene tectonics and paleoseismicity, southern Cascadia subduction zone: Science, v. 255, p. 188-192.
#2878 Jennings, C.W., 1994, Fault activity map of California and adjacent areas, with locations of recent volcanic eruptions: California Division of Mines and Geology Geologic Data Map 6, 92 p., 2 pls., scale 1:750,000.
#4928 Kelsey, H.M., and Allwardt, A.D., 1984, Geology of the Iaqua Buttes 15-minute quadrangle, Humboldt County, California: California Department of Conservation, Division of Mines and Geology Open-File Report 87-6, scale 1:62,500.
#1217 McCrory, P.A., 1996, Evaluation of fault hazards, northern coastal California: U.S. Geological Survey Open-File Report 96-656, 87 p., 2 pls.
#4929 Ogle, B.A., 1953, Geology of the Eel River Valley area, Humboldt County, California: California Department of Conservation, Division of Mines and Geology Bulletin 164, 128 p., 6, scale 1:62,500.
#4860 Petersen, M.D., Bryant, W.A., Cramer, C.H., Cao, T., Reichle, M.S., Frankel, A.D., Lienkaemper, J.J., McCrory, P.A., and Schwartz, D.P., 1996, Probabilistic seismic hazard assessment for the State of California: California Department of Conservation, Division of Mines and Geology Open-File Report 96-08 (also U.S. Geological Open-File Report 96-706), 33 p.
#4930 Smith, T.C., 1982, Little Salmon and Yager faults, Humboldt County: California Division of Mines and Geology Fault Evaluation Report 142, 5 p.
#4931 Smith, T.C., 1982, Goose Lake and related faults, Humboldt County: California Division of Mines and Geology Fault Evaluation Report 130, 15 p.
#4932 Vick, G.S., 1988, Late Holocene paleoseismicity and relative sea level changes of the Mad River Slough, northern Humboldt Bay, California: Humboldt State University, unpublished M.S. thesis, 87 p.
#4933 Wills, C.J., 1990, Little Salmon and related faults, Humboldt County, California: California Division of Mines and Geology Fault Evaluation Report 215, 15 p.
#4934 Woodward-Clyde Consultants, 1980, Evaluation of the potential for resolving the geologic and seismic issues at Humboldt Bay Power Plant Unit Number 3, Appendices A, B and C: Technical report to Pacific Gas and Electric Company.