San Gregorio fault zone, Sur Region section (Class A) No. 60b
Last Review Date: 1999-01-16
Compiled in cooperation with the California Geological Survey
citation for this record: Bryant, W.A., and Cluett, S.E., compilers, 1999, Fault number 60b, San Gregorio fault zone, Sur Region section, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, https://earthquakes.usgs.gov/hazards/qfaults, accessed 05/18/2024 08:27 PM.
| Synopsis |
General:
Holocene active, structurally complex transpressional fault zone as much as 5 km wide. The fault zone is mainly located offshore, west of San Francisco Bay and Monterey Bay, with onshore locations at promontories, such as Moss Beach, Pillar Point, Pescadero Point, and Point A?o Nuevo. Cumulative dextral displacement may total about 155 km (Dickinson, 1996 #5397), but a component of west-vergent reverse displacement also characterizes the fault zone (Lewis, 1994 #5405). Simpson and others (1998 #5414) reported a late Pleistocene dextral slip rate of 3.5-4.5 mm/yr along the Seal Cove fault [60a, San Gregorio section], based on displaced paleochannel deposits at Moss Beach. This is a partial slip rate because faults offshore to the west are also considered active and may contribute an unknown amount of dextral slip. Weber (1994 #5420) calculated a late Quaternary slip-rate of about 10 mm/yr, based on correlation of dextrally offset marine terrace deposits at Point A?o Nuevo. Conversely, dextrally offset stream channels near Point A?o Nuevo indicate a late Pleistocene slip-rate of 4-10 mm/yr (Weber, 1994 #5420). Alternatively, Sedlock, (1999 #5411) argues that piercing points are not well constrained across dextrally offset stream channels and suggests that a lesser dextral slip rate of 1-3 mm/yr better characterizes the San Gregorio fault zone. Clark and Rosenberg (1999 #5394) estimated late Quaternary and Holocene dextral slip rates of between 0.4 and 3.5 mm/yr, based on offset streams, shoreline angles, and colluvial deposits. The most recent earthquake along the San Gregorio fault zone occurred after 1270 AD to 1400 AD, but prior to the arrival of Spanish missionaries in 1775 AD (Simpson and others, 1997 #5413). Sections: This fault has 2 sections. There is insufficient data to document seismogenic segments. Petersen and others (1996 #4860) and Working Group on Northern California Earthquake Potential (1996 #1216) modeled the fault zone with two segments. Their segment boundary was placed in the north-central part of Monterey Bay. Fault segments designated by Petersen and others and Working Group on Northern California Earthquake Potential herein are considered as sections. |
| Name comments |
General: The San Gregorio fault zone was first mapped and named by Lawson (1908 #4969) for the on-land portion from Pescadero Point to A?o Nuevo Point, although Graham and Dickinson (1978 #5398) erroneously reported that Branner and others (1909 #5381) first named the fault. The San Gregorio is a complex fault zone that consists of several named faults, including the Seal Cove, Frijoles, Coastways, Greyhound Rock, Carmel Canyon, Denniston Creek, and A?o Nuevo faults. The fault zone extends from Bolinas Lagoon south to the Point Sur region. The Sur and Palo Colorado fault zones (first named by (Trask, 1926 #5416) are herein considered a part of the southern section of the San Gregorio fault zone. Greene and others (1973 #1323) and McCulloch and Greene (1990 #5406) used the name Palo Colorado-San Gregorio fault zone for the fault where it is offshore west and south of Monterey Bay. Jennings (1994 #2878) modified this nomenclature and named the offshore fault south of Monterey Canyon the Palo Colorado fault. Section: Section named by Petersen and others (1996 #4860) and Working Group on Northern California Earthquake Potential (1996 #1216). This section is located almost entirely offshore and extends from approximately Monterey Canyon (subsurface feature in Monterey Bay) south to the vicinity of Point Sur. The southern extent of this section of the San Gregorio fault zone is conjectural and structurally complex. Greene and others (1973 #1323) and McCulloch and others (1990 #5406) suggested that the San Gregorio fault zone connects with onland strands of the Palo Colorado fault. Clark and Rosenberg (1999 #5394) postulated that an unknown component of slip is transferred from the San Gregorio (Sur region section) inland along oblique-slip, intra-Silurian block faults. Silver (1978 #5412), Graham and Dickinson (1978 #5398), and Weber (1994 #5420) suggested that the southern end of the San Gregorio (Sur region section) may connect with strike-slip traces of the Sur fault near Point Sur. For this compilation, the southern part of the San Gregorio (Sur region) section includes traces of the Palo Colorado and Sur fault zones. Fault ID: Refers to numbers 230 (Palo Colorado fault zone) and 237 (Sur fault zone) of Jennings (1994 #2878) and number A5 (San Gregorio, Sur Region) of Working Group on Northern California Earthquake Potential (1996 #1216). |
| County(s) and State(s) |
MONTEREY COUNTY, CALIFORNIA |
| Physiographic province(s) |
PACIFIC BORDER |
| Reliability of location |
Poor Compiled at 1:250,000 scale. Comments: Most of the traces for the Sur Region section are located offshore, hence their locations are poor. Fault locations are based on digital revisions to Jennings (1994 #2878), using original compilation mapping by McCulloch and Greene (1990 #5406) at 1:250,000 scale and Wagner and others (2002 #5418) at 1:100,000 scale. Onshore traces of the Sur fault zone are based on mapping by Gilbert (1971 #5400) and Clark and Rosenberg (1999 #5394) at a scale of 1:24,000, and mapping by Dibblee (1973 #5396) at a scale of 1:62,500.
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| Geologic setting |
Convergent dextral fault predominantly located offshore on the continental shelf of north-central California. The northern end of the fault has a complex interconnection with the San Andreas fault zone [1] over an approximately 15-km-long zone from the Golden Gate north to Bolinas Lagoon. The southern (offshore) extent of the fault zone is conjectural. Greene and others (1973 #1323) projected the southern part of thefault zone into the onshore Palo Colorado fault. Graham and Dickinson (1978 #5398) and Weber (1980 #5419; 1994 #5420) interpreted the San Gregorio fault zone as joining with the Sur fault. The general consensus is that the San Gregorio fault zone is a relatively continuous structure, linking to the south with the Hosgri [81] and San Simeon [80] fault zones (Graham and Dickinson, 1978 #5398; Silver, 1978 #5412; Weber, 1980 #5419; 1994 #5420; Dickinson, 1996 #5397). Alternatively, Clark and Rosenberg (1999 #5394) postulate that a component of slip from the San Gregorio fault zone is distributed onshore along intra-Salinian faults. Cumulative dextral strike-slip displacement along the San Gregorio fault zone since middle Miocene time has been reported to be between 115 km (Graham and Dickinson, 1978 #5398) and 156 km (Clark and others, 1984 #5395; Dickinson, 1996 #5397; 1998 #5393) based on stratigraphic and structural correlations.
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| Length (km) | This section is 121 km of a total fault length of 241 km. |
| Average strike | N30°W (for section) versus N26°W (for whole fault) |
| Sense of movement |
Right lateral
Comments: The Sur Region section is characterized by subparallel faults that exhibit both high-angle strike-slip and east-dipping reverse displacement. Onshore strands of the southern part of the section comprise a complex zone of both dextral strike-slip and west-vergent reverse displacement faults (Gilbert, 1971 #5400; Dibblee, 1973 #5396; Bryant, 1985 #5392; Clark and Rosenberg, 1999 #5394).
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| Dip |
50°E–90°
Comments: Moderately constrained seismicity data (focal plane solutions and focal depth distributions) indicate an eastward dip of 50? to 70? (McNally and Stakes, 1998 #5407). Gilbert (1971 #5400) and Dibblee (1973 #5396) mapped east-dipping reverse faults and near vertical strike-slip faults. Clark and Rosenberg (1999 #5394) described the Garrapata fault, a strand of the Palo Colorado fault zone originally mapped by Dibblee (1973 #5396), as a near vertical fault zone.
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| Paleoseismology studies |
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| Geomorphic expression |
Most traces of the Sur Region section are offshore. Eittreim and others (1998 #5399) reported that the Sur Region section southwest of Carmel is characterized by two subparallel traces that are clearly expressed on the seafloor, as seen on multibeam bathymetric data. A 10-km-long eastern strand is delineated by geomorphic features indicative of dextral strike-slip displacement, including scarps, linear ridges, shutter ridges, and prominent drag folds in pre-Quaternary bedrock. A western trace, located about 0.5-km west of the eastern trace, is characterized by low linear scarps developed on the modern seafloor and probably represents the most recently active strand of the Sur Region section. Onshore strands of the southern part of the Sur Region section are moderately to poorly defined by geomorphic features indicative of strike-slip offset, such as linear and deflected drainages, linear alignment of saddles and benches and scarps on late Quaternary marine terrace surfaces (Bryant, 1985 #5392; Clark and Rosenberg, 1999 #5394).
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| Age of faulted surficial deposits | McCulloch and Greene (1990 #5406) reported that the faults offset Holocene sediment. Clark and Rosenberg (1999 #5394) reported that a prominent strand of the Palo Colorado fault zone, the Garrapata fault, juxtaposes two colluvial units that have conventional 14C ages of 1,200?60 years B.P. (organic sediment) and 9,750?60 years B.P. (detrital charcoal). |
| Historic earthquake | |
| Most recent prehistoric deformation |
latest Quaternary (<15 ka)
Comments: Linear scarps on the seafloor reported by Eittreim and others (1998 #5399) suggest late Holocene displacement.
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| Recurrence interval | |
| Slip-rate category |
Between 1.0 and 5.0 mm/yr
Comments: The slip rate for the San Gregorio (Sur region) section is unknown. Clark and Rosenberg (1999 #5394) estimated late Quaternary dextral slip rates of as much as about 3 mm/yr for the Palo Colorado fault. They estimated a Holocene dextral slip rate of 0.4-3.5 mm/yr on the basis of 4 m offset of a colluvial unit along the Garrapata fault. Pleistocene dextral slip-rate estimates based on stream channel offsets range between 0.9 to 3 mm/yr (Hamilton, 1984 #5402; Tuttle, 1985 #5417; Clark and Rosenberg, 1999 #5394). Late Quaternary estimates of dextral slip rates range from 0.42 to 1.35 mm/yr, based on dextral offset of shoreline angles (Tuttle, 1985 #5417; J. Weber, personal commun. in Clark and Rosenberg, 1999 #5394). |
| Date and Compiler(s) |
1999
William A. Bryant, California Geological Survey Sereyna E. Cluett, California Geological Survey |
| References |
#5381 Branner, J.C., Newsom, J.F., and Arnold, R., 1909, Description of the Santa Cruz quadrangle, California: U.S. Geological Survey, Geologic Atlas, Folio 163, 11 p. #5392 Bryant, W.A., 1985, Palo Colorado and Sur fault zones, Monterey County: California Division of Mines and Geology Fault Evaluation Report FER-169, microfiche copy in Division of Mines and Geology Open-File Report 90-11, 14 p. #5393 Clark, J.C., 1998, Neotectonics of the San Gregorio fault zone: Age dating controls on offset history and slip rates [abs.]: American Association of Petroleum Geologists Bulletin, v. 82, p. 844-845. #5394 Clark, J.C., and Rosenberg, L.I., 1999, Southern San Gregorio fault displacement—Stepover segmentation vs. through-going tectonics: U.S. Geological Survey NEHRP Final Technical Report, Grant Number 1434-HQ-98-GR-00007, 50 p., scale 1:24,000. #5395 Clark, J.C., Brabb, E.E., Greene, H.G., and Ross, D.C., 1984, Geology of the Point Reyes Peninsula and implications for San Gregorio fault history, in Crouch, J.K., and Bachman, S.B., eds., Tectonics and sedimentation along the California margin: Society of Economic Paleontologists and Mineralogists, Pacific Section, Book 38, p. 67-86. #5396 Dibblee, T.W., Jr., 1973, Geologic map of the Point Sur 15-minute quadrangle, Monterey County, California: U.S. Geological Survey Open-File Report 74-1021, scale 1:62,500. #5397 Dickinson, W.R., 1996, Kinematics of transrotational tectonism in the California Transverse Ranges and its contribution to cumulative slip along the San Andreas transform fault system: Geological Society of America Special Paper 305, p. 46. #5399 Eittreim, S.L., Stevenson, A.J., Maher, N., and Greene, H.G., 1998, Southern San Gregorio fault and Sur Platform structures revealed [abs.]: EOS, Transactions, American Geophysical Union, v. 79, no. 45, p. F825. #5400 Gilbert, W.G., 1971, Sur fault zone, Monterey County, California: Palo Alto, Stanford University, unpublished Ph.D. dissertation, 80 p., 3 pls. #5398 Graham, S.A., and Dickinson, W.R., 1978, Apparent offsets of on-land geologic features across the San Gregorio-Hosgri fault zone, in Silver, E.A., and Normark, W.R., eds., San Gregorio-Hosgri fault zone, California: California Division of Mines and Geology Special Report 137, p. 13-23. #1323 Greene, H.G., Lee, W.H.K., McCulloch, D.S., and Brabb, E.E., 1973, Faults and earthquakes in the Monterey Bay region, California: U.S. Geological Survey Miscellaneous Field Studies Map MF-518 (U.S. Department of the Interior and U.S. Department of Housing and Urban Development Basic Data Contribution 58), 14 p. pamphlet, 4 sheets. #5401 Hall, N.T., Hunt, T.D., and Vaughan, P.R., 1994, Holocene behavior of the San Simeon fault zone, south-central California, in Alterman, I.B., McMullen, R.B., Cluff, L.S., and Slemmons, D.B., eds., Seismotectonics of the Central California Coast Ranges: Geological Society of America Special Paper 292, p. 167–189. #5402 Hamilton, D.H., 1984, The tectonic boundary of coastal central California: Palo Alto, Stanford University, unpublished Ph.D. dissertation, 290 p. #5403 Hanson, K.L., and Lettis, W.R., 1994, Estimated Pleistocene slip rate for the San Simeon fault zone, south-central coastal California, in Alterman, I.B., McMullen, R.B., Cluff, L.S., and Slemmons, D.B., eds., Seismotectonics of the Central California Coast Ranges: Geological Society of America Special Paper 292, p. 133–150. #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. #4969 Lawson, A.C., chairman, 1908, The California earthquake of April 18, 1906—Report of the State Earthquake Investigation Commission: Washington, D.C., Carnegie Institution of Washington Publication 87. #5405 Lewis, S.D., 1994, Seismic reflection profiles of folds and thrust faults, offshore San Francisco region [abs.]: EOS, Transactions, American Geophysical Union, vol.75, no.44, supplement, p.681. #5406 McCulloch, D.S., and Greene, H.G., 1990, Geologic map of the central California continental margin, Map No. 5A (Geology), in Green, H.G., and Kennedy, M.P., eds., Geology of the central California continental margin: California Division of Mines and Geology California Continental Margin Geologic Map Series, Area 5 of 7, scale 1:250,000. #5407 McNally, K.C., and Stakes, D.S., 1998, Implications of the San Gregorio fault zone seismicity for Monterey Bay Coastal hazard assessment [abs.]: EOS, Transactions, American Geophysical Union, v. 79, no. 45, p. F589. #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. #5411 Sedlock, R.L., 1999, San Gregorio fault zone: late Cenozoic and modern dextral slip and slip rate, and southward linkage with the Sur San Simeon, and Hosgri faults [abs.]: Geological Society of America Abstracts with Programs, Cordilleran Section, v. 31, no. 6, p. A94. |