Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the 

U.S. Government

This map was printed on an electronic plotter directly from digital files. Dimensional calibration may vary between electronic plotters 

and between X and Y directions on the same plotter, and paper may change size due to atmospheric conditions; therefore, scale and 

proportions may not be true on plots of this map.

For sale by U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225, 1–888–ASK–USGS

Digital files available at http://pubs.usgs.gov/of/2014/1214/

Suggested Citation: Johnson, S.Y., Hartwell, S.R., Sliter, R.W., Watt, J.T., Phillips, E.L., Ross, S.L., and Chin, J.L., 2014, Local (Offshore 

of Half Moon Bay map area) and regional (offshore from Bolinas to Pescadero) shallow-subsurface geology and structure, 

California, sheet 9 in Cochrane, G.R., Dartnell, P., Greene, H.G., Johnson, S.Y., Golden, N.E., Hartwell, S.R., Dieter, B.E., Manson, M.W., 

Sliter, R.W., Ross, S.L., Watt, J.T., Endris, C.A., Kvitek, R.G., Phillips, E.L., Erdey, M.D., Chin, J.L., and Bretz, C.K. (G.R. Cochrane and S.A. 

Cochran, eds.), California State Waters Map Series—Offshore of Half Moon Bay, California: U.S. Geological Survey Open-File Report 

2014–1214, pamphlet 37 p., 10 sheets, scale 1:24,000, http://dx.doi.org/10.3133/ofr20141214.

ISSN 2331-1258 (online) 

http://dx.doi.org/10.3133/ofr20141214

Kilometers 0 5 10

0 5 10Nautical Miles

122°40' 122°20'

38°

37°40'

37°20'

San

Francisco

Bay

Area of

Map

Limit of California’s

State Waters

CALIF.

MAP LOCATION

Figure 2. Schematic map showing structural blocks within Offshore of Half Moon Bay 

map area. Pigeon Point block is separated from Santa Cruz Mountains block by 

Pomponio block, which is bounded by Frijoles Fault (to west) and Seal Cove Fault (to 

east), two strands of San Gregorio Fault Zone (SGFZ). Blue line shows coastline. 

Modified from Weber (1990). 

Frijoles Fault (west strand of SGFZ)

Seal Cove Fault (east strand of SGFZ)

SANTA CRUZ

MOUNTAINS

BLOCK

San Gregorio Fault Zone

0 1 2 3 KILOMETERS

PIGEON POINT

BLOCK

Half

Moon 

Bay

Pillar 

Point

Moss Beach

POMPONIO

BLOCK

Two-way travel time, in seconds

0

0.1

0.2

1 km VERTICAL EXAGGERATION ~ 17

160

0 

Approximate depth, in meters

80

WEST EAST

San Gregorio

Fault Zone,

west strand

(Frijoles Fault)

Figure 1. USGS high-resolution minisparker seismic-reflection profile HMS–006 (collected in 2007 on survey F–2–07–NC; see Map A for location). Dashed red line shows 

west strand (Frijoles Fault) of San Gregorio Fault Zone. Blue shading shows inferred uppermost Pleistocene and Holocene strata, deposited since last sea-level lowstand 

about 21,000 years ago. Dashed green lines highlight continuous reflections that reveal structure (not distinctive stratigraphic markers). Dashed yellow line is seafloor 

multiple (echo of seafloor reflector). Purple triangle shows location of California’s State Waters limit (yellow line on Maps A, B, C, D, E).

DISCUSSION

This sheet includes maps that show the interpreted thickness and the depth to base of uppermost Pleistocene and Holocene deposits in California’s State Waters for the Offshore of Half Moon 

Bay map area (Maps A, B), as well as for a larger area that extends about 91 km along the coast 

from Bolinas to Pescadero (Maps C, D) to establish a regional context. This uppermost stratigraphic 

unit (blue shading in seismic-reflection profile of fig. 1; see also, figs. 1, 2, 3, 4, 6, 7, 9, 10 on sheet 

8) is inferred to have been deposited during the post–Last Glacial Maximum (LGM) sea-level rise in 

the last about 21,000 years (see, for example, Peltier and Fairbanks, 2006; Stanford and others, 

2011). The unit commonly is characterized either by “acoustic transparency” or by parallel, 

low-amplitude, low- to high-frequency, continuous to moderately continuous, diffuse reflections 

(terminology from Mitchum and others, 1977). The acoustic transparency can be caused by extensive wave winnowing, which results in a uniform sediment grain size and the consequent lack of 

acoustic-impedance contrasts needed to produce seismic reflections. On the continental shelf, the 

contact with underlying units is a transgressive surface of erosion commonly marked by angularity, 

channeling, or a distinct upward change to lower amplitude, more diffuse reflections.

To make these maps, water bottom and depth to base of the post-LGM horizons were mapped 

from seismic-reflection profiles (fig. 1; see also, sheet 8). The difference in the two horizons was 

exported for every shot point as XY coordinates (UTM zone 10) and two-way travel time (TWT). 

The thickness of the post-LGM unit (Maps B, D) was determined by applying a sound velocity of 

1,600 m/sec to the TWT. The thickness points were interpolated to a preliminary continuous surface, 

overlaid with zero-thickness bedrock outcrops (see sheet 10), and contoured, following the methodology of Wong and others (2012). 

The thickness data points are dense along tracklines (about 1 m apart) and sparse between 

tracklines (1 km apart), resulting in minor contouring artifacts. To incorporate the effect of a few 

rapid thickness changes along faults, to remove irregularities from interpolation, and to reflect other 

geologic information and complexity, minor manual editing of the preliminary thickness contours 

was undertaken. Contour modifications and regridding were repeated several times to produce the 

final sediment-thickness maps. Information for the depth to base of the post-LGM unit (Maps A, C) 

was generated by adding the sediment-thickness data to water depths determined by multibeam 

bathymetry (see sheet 1). 

The thickness of the post-LGM unit in the Offshore of Half Moon Bay map area ranges from 0 

to 12 m (Map B), and the depth to base of the unit ranges from less than 5 to 56 m (Map A). This 

relatively thin sediment cover suggests a lack of sediment “accommodation space” (Catuneanu, 

2006), consistent with regional uplift expressed by the young, high topography of the Santa Cruz 

Mountains and Coast Ranges, as well as nearby uplifted marine terraces (Weber, 1990; Lajoie and 

others, 1991; Brabb and others, 1998). This uplift has been attributed to the northwesttranspressional bend in the San Andreas Fault (see, for example, Zoback and others, 1999), which 

lies about 12 km northeast of the map area. The San Gregorio Fault Zone, which cuts through the 

Offshore of Half Moon Bay map area, consists of an east strand (“Seal Cove Fault” or “Coastways 

Fault”), a west strand (“Frijoles Fault”), and related splay faults (fig. 2). The thickest sediment 

(about 12 m) in the map area is west of of the Frijoles Fault, and it results from offset and (or) gentle 

downwarping associated with that structure (for example, fig. 1).

Five different “domains” of sediment thickness are recognized on the regional sedimentthickness map (Map D): (1) the Bolinas shelf, located west of the east strand of the San Gregorio 

Fault Zone, in the northwestern part of the regional map (Map D); (2) the San Andreas graben, 

located between the San Gregorio Fault Zone and the Golden Gate Fault, east-southeast of the 

Bolinas shelf and both southwest and southeast of the Marin shelf; (3) the Marin shelf, located both 

northeast and northwest of the San Andreas graben and north of the San Francisco ebb-tidal delta 

paleovalley; (4) the northeast-trending San Francisco ebb-tidal delta paleovalley, located outside the 

Golden Gate at the mouth of San Francisco Bay, between the Marin shelf and San Andreas graben 

on the north and the Pacifica-Pescadero shelf on the south; and (5) the Pacifica-Pescadero shelf, 

which is located south of the San Francisco ebb-tidal delta paleovalley and which extends south all 

the way to Pescadero Point (including all of the Offshore of Half Moon Bay map area). 

The five sediment-thickness domains have distinct geologic controls. The Bolinas and 

Pacifica-Pescadero shelves are uplifting and are relatively sediment poor (mean sediment thicknesses of 0.8 and 3.6 m, respectively). Thicker sediment accumulations (as much as 20 m) on the 

western margins of the Pacifica-Pescadero shelf (within California’s State Waters) are associated 

with west-side-down slip on the west strand of the San Gregorio Fault Zone and with deposition on 

the outboard, west-dipping Pigeon Point block (McCulloch, 1987; see also, fig. 2) farther south, 

offshore of Pescadero Point. The San Andreas graben is a rapidly subsiding, fault-controlled 

sedimentary basin (Cooper, 1973; Ryan and others, 2008) that has sediment thicknesses of as much 

as 57 m; the Marin shelf forms the uplifted northeastern and northwestern margins of this basin. The 

San Francisco ebb-tidal delta is filling a paleovalley that formed during the last sea-level lowstand, 

with sediment thicknesses of as much as 32 m along the trough axis. Although the southern part of 

the San Andreas graben may extend into the paleovalley, the north flank of the paleovalley is used 

here as the boundary when calculating sediment volumes for the five sediment-thickness domains 

(see table 7–1 in accompanying pamphlet). Subsidence in the San Francisco ebb-tidal delta paleovalley and the San Andreas graben can be partly attributed to the northward change in strike of both 

the San Andreas and San Gregorio Fault Zones offshore of San Francisco, which has resulted in the 

local change from contractional deformation to extensional deformation (Zoback and others, 1999).

Map E shows the regional pattern of major faults and of earthquakes occurring between 1967 

and April 2014 that have inferred or measured magnitudes of 2.0 and greater. Fault locations, which 

have been simplified, are compiled from our mapping within California’s State Waters (see sheet 10) 

and from the U.S. Geological Survey’s Quaternary fault and fold database (U.S. Geological Survey 

and California Geological Survey, 2010). Earthquake epicenters are from the Northern California 

Earthquake Data Center (2014), which is maintained by the U.S. Geological Survey and the University of California, Berkeley, Seismological Laboratory. Map E also shows the inferred location of 

the devastating great 1906 California earthquake (M7.8, 4/18/1906), thought to have nucleated on 

the San Andreas Fault offshore of San Francisco (see, for example, Bolt, 1968; Lomax, 2005). Map 

E clearly shows that the largest number of earthquakes in the region occur within the broad San 

Andreas Fault Zone between Pacifica and Bolinas; events west of the east strand of the San Gregorio Fault Zone and east of the Golden Gate Fault are much less common. The Offshore of Half 

Moon Bay map area is notably devoid of seismicity during this time period.

REFERENCES CITED

Bolt, B.A., 1968, The focus of the 1906 California earthquake: Bulletin of the Seismological Society 

of America, v. 58, p. 457–471.

Brabb, E.E., Graymer, R.W., and Jones, D.L., 1998, Geology of the onshore part of San Mateo 

County, California—A digital database: U.S. Geological Survey Open-File Report 98–137, 

scale 1:62,500, available at http://pubs.usgs.gov/of/1998/of98-137/.

Catuneanu, O., 2006, Principles of sequence stratigraphy: Amsterdam, Elsevier, 375 p.

Cooper, A.K., 1973, Structure of the continental shelf west of San Francisco, California: U.S. 

Geological Survey Open-File Report 73–48, 65 p.

Lajoie, K.R., Ponti, D.J., Powell, C.L., II, Mathieson, S.A., and Sarna-Wojcicki, A.M., 1991, 

Emergent marine strandlines and associated sediments, coastal California—A record of 

Quaternary sea-level fluctuations, vertical tectonic movements, climatic changes, and coastal 

processes, in Morrison, R.B., ed., The Geology of North America, Quaternary nonglacial 

geology—Conterminous U.S.: Geological Society of America, Decade of North American 

Geology, v. K–2, p. 190–213. 

Lomax, A., 2005, A reanalysis of the hypocentral location and related observations for the Great 

1906 California earthquake: Bulletin of the Seismological Society of America, v. 95, p. 

861–877, doi:10.1785/0120040141.

McCulloch, D.S., 1987, Regional geology and hydrocarbon potential of offshore central California, 

in Scholl, D.W., Grantz, A., and Vedder, J.G., eds., Geology and resource potential of the 

continental margin of western North America and adjacent ocean basins—Beaufort Sea to Baja 

California: Circum-Pacific Council for Energy and Mineral Resources, Earth Science Series, v. 

6, p. 353–401.

Mitchum, R.M., Jr., Vail, P.R., and Sangree, J.B., 1977, Seismic stratigraphy and global changes of 

sea level, part 6—Stratigraphic interpretation of seismic reflection patterns in depositional 

sequences, in Payton, C.E., ed., Seismic stratigraphy—Applications to hydrocarbon 

exploration: Tulsa, Okla., American Association of Petroleum Geologists, p. 117–133.

Northern California Earthquake Data Center, 2014, Northern California earthquake catalog: 

Northern California Earthquake Data Center database, accessed April 5, 2014, at 

http://www.ncedc.org/ncsn/.

Peltier, W.R., and Fairbanks, R.G., 2006, Global glacial ice volume and Last Glacial Maximum 

duration from an extended Barbados sea level record: Quaternary Science Reviews, v. 25, p. 

3,322–3,337, doi:10.1016/j.quascirev.2006.04.010.

Ryan, H.F., Parsons, T., and Sliter, R.W., 2008, Vertical tectonic deformation associated with the San 

Andreas Fault Zone offshore of San Francisco, California: Tectonophysics, v. 429, p. 209–224, 

doi:10.1016/j.tecto.2008.06.011.

Stanford, J.D., Hemingway, R., Rohling, E.J., Challenor, P.G., Medina-Elizalde, M., and Lester, 

A.J., 2011, Sea-level probability for the last deglaciation—A statistical analysis of far-field 

records: Global and Planetary Change, v. 79, p. 193–203, doi:10.1016/j.gloplacha.2010.11.002.

U.S. Geological Survey and California Geological Survey, 2010, Quaternary fault and fold database 

of the United States: U.S. Geological Survey database, accessed April 5, 2014, at 

http://earthquake.usgs.gov/hazards/qfaults/.

Weber, G.E., 1990, Late Pleistocene slip rates on the San Gregorio fault zone at Point Año Nuevo, 

San Mateo County, California, in Greene, H.G., Weber, G.E., Wright, T.L., and Garrison, R.E., 

eds., Geology and tectonics of the central California coast region—San Francisco to Monterey: 

American Association of Petroleum Geologists, Pacific Section, volume and guidebook, v. 67, 

p. 193–204.

Wong, F.L., Phillips, E.L., Johnson, S.Y., and Sliter, R.W., 2012, Modeling of depth to base of Last 

Glacial Maximum and seafloor sediment thickness for the California State Waters Map Series, 

eastern Santa Barbara Channel, California: U.S. Geological Survey Open-File Report 

2012–1161, 16 p., available at http://pubs.usgs.gov/of/2012/1161/.

Zoback, M.L., Jachens, R.C., and Olson, J.A., 1999, Abrupt along-strike change in tectonic style—

San Andreas Fault zone, San Francisco Peninsula: Journal of Geophysical Research, v. 104 

(B5), p. 10,719–10,742.

Local (Offshore of Half Moon Bay Map Area) and Regional (Offshore from Bolinas to Pescadero) Shallow-Subsurface Geology and Structure, California

By

Samuel Y. Johnson, Stephen R. Hartwell, Ray W. Sliter, Janet T. Watt, Eleyne L. Phillips, Stephanie L. Ross, and John L. Chin

2014

ONE MILE = 0.869 NAUTICAL MILES

1 0.5 0 1 KILOMETER

7000 3500 0 7000 FEET

2 1 0 2 MILES

SCALE 1:50 000

ONE MILE = 0.869 NAUTICAL MILES

10 5 0 10 KILOMETERS

40000 30000 20000 10000 0 40000 FEET

10 5 0 10 MILES

14°

SCALE 1:200 000

APPROXIMATE MEAN

DECLINATION, 2014

MAGNETIC 

 NORTH

TRUE NORTH

14°

APPROXIMATE MEAN

DECLINATION, 2014

MAGNETIC 

 NORTH

TRUE NORTH

Depth and thickness mapped by Samuel Y. Johnson and 

Stephen R. Hartwell, 2011-2012

GIS database and digital cartography by Stephen R. 

Hartwell, Eleyne L. Phillips, and Florence L. Wong

Manuscript approved for publication October 2, 2014

Onshore elevation data collected by Photoscience in 

2005 for U.S. Geological Survey and County of San 

Mateo. California’s State Waters limit from NOAA Office 

of Coast Survey 

Universal Transverse Mercator projection, Zone 10N

Onshore elevation data from U.S. Geological Survey 

(2012) National Elevation dataset. California’s State 

Waters limit from NOAA Office of Coast Survey 

Universal Transverse Mercator projection, Zone 10N

Depth and thickness mapped by Samuel Y. 

Johnson and Stephen R. Hartwell, 2010-2011

GIS database and digital cartography by 

Stephen R. Hartwell, Eleyne L. Phillips, and 

Florence L. Wong

122°20'

122°20'

122°40'

122°40'

37°40' 37°40'

37°20' 37°20'

SAF

SAF

GGF

PF

SPF

SGE

SGE

SGW

PPF

SGW

San Francisco Bay

Pacific Ocean

 Marin 

Headlands

Bolinas

Pescadero

 Point

San Gregorio

Half 

Moon 

Bay

Pacifica

San Francisco

Golden Gate

SANTA CRUZ MOUNTAINS

Offshore of Half

Moon Bay map area

and area of Maps A, B

3-nautical-mile limit of 

California’s State Waters

Earthquake magnitude

7.8

Great 1906 California 

earthquake

Regional faults and 

earthquake epicenters, 

1967 - 2014

EXPLANATION

2.0 - 2.5

2.5 - 3.0

3.0 - 3.5

3.5 - 4.0

4.0 - 5.5

Fault—Dashed where

locations inferred. 

Abbreviations: GGF, 

Golden Gate Fault; PF, 

Pilarcitos Fault; PPF, 

Potato Patch Fault; 

SAF, San Andreas Fault 

Zone; SGE, San 

Gregorio Fault Zone, 

east strand; SGW, San 

Gregorio Fault Zone, 

west strand

Pillar

Point

Map E

122°20'

122°20'

122°40'

122°40'

37°40' 37°40'

37°20' 37°20'

Pacific Ocean

San Francisco Bay

Half 

Moon 

Bay

Pescadero

San Gregorio

Pacifica

San Francisco

Golden Gate

 Marin 

Headlands

Bolinas

Map D

Pigeon Point 

structural block

Offshore of Half

Moon Bay map area

and area of Maps A, B

Map D

Map D

PACFICA-PESCADERO

SHELF DOMAIN

SAN FRANCISCO

PALEOVALLEY

DOMAIN

SAN ANDREAS

GRABEN DOMAIN

BOLINAS

SHELF

DOMAIN

MARIN SHELF

 DOMAIN

0 – 0.1

0.1 – 2.5

2.5 – 5

5 – 7.5

7.5 – 10

10 – 12.5

12.5 – 15

15 – 17.5

17.5 – 20

20 – 22.5

22.5 – 25

25 – 27.5

27.5 – 30

30 – 32.5

32.5 – 35

35 – 37.5

37.5 – 40

40 – 42.5

42.5 – 45

45 – 47.5

47.5 – 50

50 – 52.5

52.5 – 55

55 – 57.5

EXPLANATION

Thickness of uppermost Pleistocene and 

Holocene sediment, in meters

Thickness contours

Index (10-m intervals)

Intermediate

3-nautical-mile limit of California’s 

State Waters

Boundary of sediment-thickness domain

Area of “no data”—Areas where data are 

insufficient for contouring

37°40' 37°40'

37°20' 37°20'

122°20'

122°20'

122°40'

122°40'

San Francisco Bay

Pacific Ocean

Pescadero

San Gregorio

Half 

Moon 

Bay

Pacifica

San Francisco

Golden Gate

 Marin 

Headlands

Bolinas

Map C

Offshore of Half

Moon Bay map area

and area of Maps A, B

-10 – -5

-5 – 0

-85 – -80

-80 – -75

-75 – -70

-70 – -65

-65 – -60

-60 – -55

-55 – -50

-50 – -45

-45 – -40

-40 – -35

-35 – -30

-30 – -25

-25 – -20

-20 – -15

-15 – -10

EXPLANATION

Depth to base of uppermost Pleistocene and 

Holocene sediment, in meters

Depth-to-base contours

Index (10-m intervals)

Intermediate

3-nautical-mile limit of California’s 

State Waters

Area of “no data”—Areas where data are 

insufficient for contouring

122°25'

122°25'

122°30'

122°30'

122°35'

122°35'

37°30' 37°30'

37°25' 37°25'

Half

Moon

Bay

Map B

 PACIFICAPESCADERO

SHELF Seal Cove Fault (east strand of San Gregorio Fault Zone)

Frijoles Fault (west strand of San Gregorio Fautt Zone)

Pillar

Point

0 – 0.1

0.1 – 2.5

2.5 – 5

5 – 7.5

7.5 – 10

10 – 12.5

12.5 – 15

15 – 17.5

17.5 – 20

20 – 22.5

22.5 – 25

25 – 27.5

27.5 – 30

30 – 32.5

32.5 – 35

35 – 37.5

37.5 – 40

40 – 42.5

42.5 – 45

45 – 47.5

47.5 – 50

50 – 52.5

52.5 – 55

55 – 57.5

EXPLANATION

Thickness of uppermost Pleistocene and 

Holocene sediment, in meters

Thickness contours

Index (10-m intervals)

Intermediate

Fault—Solid where location is certain, 

dashed where location is inferred or 

concealed

Folds—Solid where location is certain, 

dashed where location is inferred or 

concealed, arrow on axial trace 

indicates direction of plunge

 Anticline

 Syncline

3-nautical-mile limit of California’s 

State Waters

Area of “no data”—Areas where data are 

insufficient for contouring

122°25'

122°25'

122°30'

122°30'

122°35'

122°35'

37°30' 37°30'

37°25' 37°25'

Half

Moon

Bay

PACIFICAPESCADERO

SHELF Pillar

Point

Seal Cove Fault (east strand of San Gregorio Fault Zone)

Frijoles Fault (west strand of San Gregorio Fautt Zone)

Figure 2

EXPLANATION

Depth to base of uppermost Pleistocene and 

Holocene sediment, in meters

Depth-to-base contours

Index (10-m intervals)

Intermediate

Fault—Solid where location is certain, 

dashed where location is inferred or 

concealed

Folds—Solid where location is certain, 

dashed where location is inferred or 

concealed, arrow on axial trace 

indicates direction of plunge 

 Anticline

 Syncline

Trackline of seismic-reflection profile 

shown in figure 2

3-nautical-mile limit of California’s 

State Waters

Area of “no data”—Areas where data are 

insufficient for contouring

-10 – -5

-5 – 0

-85 – -80

-80 – -75

-75 – -70

-70 – -65

-65 – -60

-60 – -55

-55 – -50

-50 – -45

-45 – -40

-40 – -35

-35 – -30

-30 – -25

-25 – -20

-20 – -15

-15 – -10

Map A

Open-File Report 2014–1214

Sheet 9 of 10

Pamphlet accompanies map

U.S. Department of the Interior

U.S. Geological Survey