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: Sliter, R.W., Johnson, S.Y., Ross, S.L., Watt, J.T., and Chin, J.L., 2014, Seismic-reflection profiles, Offshore of Half 

Moon Bay map area, California, 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

DISCUSSION

This map sheet shows seismic-reflection profiles from four different surveys of the Offshore of Half Moon 

Bay map area, providing imagery of the subsurface geology. This map area is largely characterized by a shallow 

(less than 50 m) bedrock platform that is overlain locally by thin sediment cover. The seismic-reflection profiles 

provide the data for interpreting subbottom stratigraphy, sediment thickness, and geologic structure (see sheets 

9, 10 of this report).

The Offshore of Half Moon Bay map area straddles the right-lateral San Gregorio Fault, an important 

structure in the distributed transform boundary between the North American and Pacific plates (see, for 

example, Dickinson and others, 2005). This fault is part of a predominantly offshore, regional fault system that 

extends for about 400 km, from Point Conception in the south (where it is known as the Hosgri Fault) to Bolinas 

and Point Reyes in the north. The fault continues onland at coastal promontories such as Pillar Point (in this 

map area) and Pescadero Point (13 km south of the map area) (see sheet 9; see also, Weber and Lajoie, 1980; 

Brabb and others, 1998). Offshore parts of this fault system are identified on seismic-reflection profiles by the 

abrupt truncation or warping of reflections and (or) the juxtaposition of reflection panels that have differing 

seismic parameters, such as reflection presence, amplitude, frequency, geometry, continuity, and vertical 

sequence. 

In the Offshore of Half Moon Bay map area, the San Gregorio Fault forms a distributed, 2- to 5-km-wide 

shear zone that narrows northward and includes two main fault strands (see sheets 9, 10). The nearshore east 

strand (figs. 3, 5), also known as the “Seal Cove Fault” or “Coastways Fault,” forms a prominent bathymetric 

scarp (see sheets 1, 2). Weber (1990), Weber and others (1995), and Simpson and others (1997) suggested a 

dextral slip rate of 3.5 to 4.5 mm/yr for the onland part of the Seal Cove Fault on the basis of displacement of 

onshore marine-terrace shoreline angles and offset alluvial fan paleochannels. This estimated rate represents a 

minimum for the San Gregorio Fault system because the offshore west strand (also known as the “Frijoles 

Fault;” see figs. 1, 2, 3, 4, 5, 6, 7, 8, 10) also is active. Cumulative lateral slip on this fault zone is thought to 

range from 4 to 10 mm/yr in this area (U.S. Geological Survey and California Geological Survey, 2010). 

Several high-resolution seismic profiles show an upper unit (blue shading in profiles; figs. 1, 2, 3, 4, 6, 9, 

10) that is inferred to have been deposited in about the last about 21,000 years during the latest Pleistocene and 

Holocene post–Last Glacial Maximum sea-level rise. These deposits typically are 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); this seismic “facies” is attributable to the inferred uniform grain size caused by wave winnowing, which results in the general lack of 

acoustic-impedance contrasts needed to yield seismic reflections. This unit has its maximum thickness (about 12 

m) in the northern part of the map area (figs. 1, 2; see also, sheet 9). The contact with underlying units is a 

commonly planar, transgressive surface of erosion that is marked by a distinct downward change to a section 

characterized by moderate- to high-amplitude, variably continuous, parallel to subparallel, folded and faulted 

reflections.

Except for the profiles in figures 3, 5, and 8, all profiles displayed on this map sheet were collected in 2007 

on U.S. Geological Survey (USGS) cruise F–2–07–NC. Single-channel seismic-reflection data were acquired 

using two different sources, the SIG 2Mille minisparker (figs. 1, 2, 4, 6, 9, 10) and the EdgeTech 512 chirp (fig. 

7). The SIG minisparker system used a 500-J high-voltage electrical discharge fired 1 to 4 times per second, 

which, at normal survey speed of 4 to 4.5 nautical miles per hour, gives a data trace every 0.5 to 2.0 meters. The 

data were digitally recorded in standard SEG-Y 32-bit floating-point format, using PC-based Triton Subbottom 

Logger (SBL) software that merges seismic-reflection data with differential GPS-navigation data. The 

EdgeTech 512 chirp subbottom-profiling system consists of a source transducer and an array of receiving 

hydrophones housed in a 500-lb fish towed at a depth of several meters below the sea surface. The sweptfrequency chirp source signal was 500 to 4,500 Hz and 50 ms in length, and it was recorded by hydrophones 

located on the bottom of the fish. After the survey, a short-window (20 ms) automatic gain control algorithm 

was applied to both the chirp and minisparker data, and a 160- to 1,200-Hz bandpass filter was applied to the 

minisparker data.

Figure 3 shows a seismic-reflection profile collected in 1997 on USGS cruise J–1–97–MB, using a 0.40- to 

5-kHz Geopulse system. Power was supplied by a double-plate, 910-J sound source fired at 0.5-second 

intervals. As with the minisparker and chirp systems, data were digitally recorded and merged with GPS 

navigation data. 

Figure 5 shows a depth-converted, multichannel seismic-reflection profile collected in 1995 on USGS 

cruise G–2–95–SF (Childs and others, 2000; Bruns and others, 2002). Two 0.65-L air guns fired at 12.5-m 

intervals provided the seismic source, and data were digitally recorded on a 24-channel, 150-m-long streamer 

merged with GPS navigation data. Data-processing steps included deconvolution, automatic gain control, 

filtering at 50 to 160 Hz, stacking, and migration.

Figure 8 shows a deep-penetration, depth-converted, multichannel seismic-reflection profile collected in 

1976 by WesternGeco on cruise W–14–76–SF. This profile and other similar data were collected in many areas 

offshore of California in the 1970s and 1980s when these areas were considered a frontier for oil and gas 

exploration. Much of these data have been publicly released and are now archived at the USGS National 

Archive of Marine Seismic Surveys (U.S. Geological Survey, 2009). These data were acquired with a largevolume air-gun source that has a frequency range of 3 to 40 Hz and recorded with a multichannel hydrophone 

streamer about 2 km long; shot spacing was about 30 m. These data can resolve geologic features that are 20 to 

30 m thick, down to subbottom depths of about 4 km.

REFERENCES CITED

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/.

Bruns, T.R., Cooper, A.K., Carlson, P.R., and McCulloch, D.S., 2002, Structure of the submerged San Andreas 

and San Gregorio Fault zones in the Gulf of the Farallones off San Francisco, California, from highresolution seismic-reflection data, in Parsons, T., ed., Crustal structure of the coastal and marine San 

Francisco Bay region, California: U.S. Geological Survey Professional Paper 1658, p. 77–117, available at 

http://pubs.usgs.gov/pp/1658/.

Childs, J.R., Hart, P., Bruns, T.R., Marlow, M.S., and Sliter, R., 2000, High-resolution marine seismic reflection 

data from the San Francisco Bay area: U.S. Geological Survey Open-File Report 00–494, available at 

http://pubs.usgs.gov/of/2000/0494/.

Dickinson, W.R., Ducea, M., Rosenberg, L.I., Greene, H.G., Graham, S.A., Clark, J.C., Weber, G.E., Kidder, S., 

Ernst, W.G., and Brabb, E.E., 2005, Net dextral slip, Neogene San Gregorio–Hosgri fault zone, coastal 

California—Geologic evidence and tectonic implications: Geological Society of America Special Paper 

391, 43 p. 

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.

Simpson, G.D., Thompson, S.C., Noller, J.S., and Lettis, W.R., 1997, The northern San Gregorio fault zone—

Evidence for the timing of late Holocene earthquakes near Seal Cove, California: Bulletin of the Seismological Society of America, v. 87, p. 1,158–1,170.

U.S. Geological Survey, 2009, National Archive of Marine Seismic Surveys: U.S. Geological Survey database, 

accessed April 5, 2011, at http://walrus.wr.usgs.gov/NAMSS/.

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.

Weber, G.E., and Lajoie, K.R., 1980, Map of Quaternary faulting along the San Gregorio fault zone, San Mateo 

and Santa Cruz Counties, California: U.S. Geological Survey Open-File Report 80–907, scale 1:24,000.

Weber, G.E., Nolan, J.M., and Zinn, E.N., 1995, Determination of late Pleistocene-Holocene slip rates along the 

San Gregorio fault zone, San Mateo and Santa Cruz Counties, California—Final Technical Report: 

National Earthquake Hazard Reduction Program, Final Technical Report, Contract No. 1434–93–G–2336, 

70 p., 4 sheets.

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

Seismic-Reflection Profiles, Offshore of Half Moon Bay Map Area, California

By

Ray W. Sliter, Samuel Y. Johnson, Stephanie L. Ross, Janet T. Watt, and John L. Chin

2014 

Two-way travel time, in seconds

0

0.1

0.2

1 km VERTICAL EXAGGERATION ~ 17 WEST

Fig. 9

EAST

160

0 

Approximate depth, in meters

80

Figure 10. USGS high-resolution minisparker seismic-reflection profile HMS–034 (collected in 2007 on survey F–2–07–NC), which crosses shelf south-southwest of city of Half Moon Bay; 

see trackline map for location. Dashed red lines show faults, including west strand (Frijoles Fault) of San Gregorio Fault Zone. Magenta symbols show fold axes (diverging arrows, 

anticlines; converging arrows, synclines). Blue shading (west end of profile) shows local, very thin sediment cover, inferred to represent 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 trackline map).

San Gregorio

Fault Zone, 

west strand 

(Frijoles Fault)

NORTH

Two-way travel time, in seconds

0

0.1

0.2 160

0 

Approximate depth, in meters

80

1 km VERTICAL EXAGGERATION ~ 17 SOUTH Fig. 10

Figure 9. USGS high-resolution minisparker seismic-reflection profile HMS–036E (collected in 2007 on survey F–2–07–NC), which crosses shelf 

west and south of city of Half Moon Bay; see trackline map for location. Profile is subparallel to, and lies between, west (Frijoles Fault) and east 

(Seal Cove Fault) strands of San Gregorio Fault Zone. Dashed red line shows inferred fault. Magenta symbols show fold axes (diverging arrows, 

anticline; converging arrows, syncline). Blue shading shows local, very thin sediment cover, inferred to represent 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).

Approximate depth, in meters

0

500

1,500

EAST

2,000

1,000

WEST VERTICAL EXAGGERATION ~ 1.6

1 km

Figure 8. Deep-penetration, industry, 2-D, multichannel air-gun seismic-reflection profile WSF–054 (collected in 1976 on survey W–14–76–SF; from USGS National 

Archive of Marine Seismic Surveys [U.S. Geological Survey, 2009]), which crosses shelf west of city of Half Moon Bay; see trackline map for location. Dashed 

yellow lines show west strand (Frijoles Fault) of San Gregorio Fault Zone, imaged as complex positive flower structure. Magenta symbols show fold axes (diverging 

arrows, anticline; converging arrows, syncline). Local Tertiary sedimentary basin that contains at least 2 km of sediment is imaged east of fault zone. Purple triangle 

shows location of California’s State Waters limit (yellow line on trackline map). Fig. 7

Fig. 4

San Gregorio Fault Zone,

west strands (Frijoles Fault)

Two-way travel time, in seconds

0

0.1

0.2 160

0 

Approximate depth, in meters

80

1 km

WEST VERTICAL EXAGGERATION ~ 17 EAST Fig. 8 Fig. 4

Figure 7. High-resolution chirp profile HMS–001 collected by USGS in 2007 (survey F–2–07–NC), which crosses shelf west of city of Half Moon Bay, showing tightly folded strata of 

Pliocene and upper Miocene Purisima Formation; see trackline map for location. Dashed red lines show faults, including west (Frijoles Fault) and east (Seal Cove Fault) strands of San 

Gregorio Fault Zone; faults are mapped primarily on basis of multibeam bathymetric data (see sheets 1, 2) but also by comparison with adjacent seismic-reflection profiles (see trackline 

map) on which more sediment is present. Magenta symbols show fold axes (diverging arrows, anticlines; converging arrows, synclines). Blue shading shows local, typically very thin 

sediment cover, inferred to represent uppermost Pleistocene and Holocene strata deposited since last sea-level lowstand about 21,000 years ago. Dashed yellow line is seafloor multiple 

(echo of seafloor reflector). Purple triangle shows location of California’s State Waters limit (yellow line on trackline map).

San Gregorio

Fault Zone,

west strand 

(Frijoles Fault)

San Gregorio Fault Zone,

east strand (Seal Cove Fault)

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 Fig. 4 Fig. 3

Figure 6. USGS high-resolution minisparker seismic-reflection profile HMS–006 (collected in 2007 on survey F–2–07–NC), which crosses shelf west-southwest of Pillar Point; 

see trackline map for location. Dashed red lines show faults, including 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 trackline map).

Figure 5. USGS migrated and depth-converted, multichannel seismic-reflection profile 128 (collected in 1995 on survey G–2–95–SF), which crosses shelf west of Point Montara, showing San 

Gregorio Fault Zone offshore of Point Montara (modified from Bruns and others, 2002); see trackline map for location. Dashed red lines show faults, including west (Frijoles Fault) and east (Seal 

Cove Fault) strands of San Gregorio Fault Zone. Both west (Frijoles Fault) and east (Seal Cove Fault) strands of fault system form positive flower structures with multiple splays. East strand (Seal 

Cove Fault) is also imaged as prominent lineation in high-resolution bathymetric data (see sheets 1, 2). Magenta symbols show fold axes (diverging arrows, anticline; converging arrows, syncline). 

Purple triangle shows location of California’s State Waters limit (yellow line on trackline map).

Approximate depth, in meters

0

500

1,000

1,500

WEST EAST

VERTICAL EXAGGERATION ~ 2

1 km

Fig. 1

San Gregorio Fault Zone,

west strands (Frijoles Fault)

San Gregorio Fault Zone,

east strands (Seal Cove Fault)

San Gregorio

Fault Zone,

west strand 

(Frijoles Fault)

Two-way travel time, in seconds

0

0.1

0.2

NORTHWEST

1 km VERTICAL EXAGGERATION ~ 17 SOUTHEAST

160

0 

Approximate depth, in meters

80

Fig. 4

Fig. 8

Fig. 7

Figure 4. USGS high-resolution minisparker seismic-reflection profile HMS–036D (collected in 2007 on survey F–2–07–NC), which crosses shelf west and south of Pillar Point; see trackline map for 

location. Dashed red lines show faults, including west strand (Frijoles Fault) of San Gregorio Fault Zone. Magenta symbols show fold axes (diverging arrows, anticlines; converging arrows, synclines). 

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 trackline 

map).

San Gregorio 

Fault Zone,

west strands 

(Frijoles Fault)

Two-way travel time, in seconds

0

0.1

0.05

0 

Approximate depth, in meters

80

1 km

NORTHWEST VERTICAL EXAGGERATION ~ 17 SOUTHEAST

 San Gregorio Fault Zone,

east strand (Seal Cove Fault)

Figure 3. USGS high-resolution geopulse seismic-reflection profile 714 (collected in 1997 on survey J–1–97–MB), which crosses shelf west and south of Pillar Point; see trackline map for location. 

Dashed red lines show faults, including west (Frijoles Fault) and east (Seal Cove Fault) strands of San Gregorio Fault Zone. Location of east strand is based primarily on prominent lineament in 

multibeam bathymetry (see sheets 1, 2). Magenta symbols show fold axes (diverging arrows, anticline; converging arrows, synclines). Blue shading shows inferred uppermost Pleistocene and Holocene 

strata, deposited since last sea-level lowstand about 21,000 years ago. Dashed orange lines highlight zone of reflections that show offshore-dipping clinoforms. Dashed green lines highlight continuous 

reflections that reveal structure (not distinctive stratigraphic markers). Dashed yellow line is seafloor multiple (echo of seafloor reflector). Fig. 6

San Gregorio 

Fault Zone,

west strands 

(Frijoles Fault)

Two-way travel time, in seconds

0

0.1

0.2

WEST

1 km VERTICAL EXAGGERATION ~ 17

Figure 2. USGS high-resolution minisparker seismic-reflection profile HMS–008 (collected in 2007 on survey F–2–07–NC), which crosses shelf west of Pillar Point, perpendicular to 

shoreline; see trackline map for location. Dashed red lines show faults, including west strands (Frijoles Fault) of San Gregorio Fault Zone. Magenta symbol shows syncline axis. Blue 

shading shows inferred uppermost Pleistocene and Holocene strata, deposited since last sea-level lowstand about 21,000 years ago. In underlying unit, dashed orange lines 

highlight reflections that show offshore-dipping clinoform. 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 trackline map).

San Gregorio 

Fault Zone,

west strands 

(Frijoles Fault)

Fig. 1

160

0 

Approximate depth, in meters

EAST

80

SOUTHWEST Two-way travel time, in seconds

0

0.1

0.2 160

0 

Approximate depth, in meters

NORTHEAST

80

1 km VERTICAL EXAGGERATION ~ 17

Figure 1. USGS high-resolution minisparker seismic-reflection profile HMS–036C (collected in 2007 on survey F–2–07–NC), which crosses shelf 

west-southwest of Point Montara; see trackline map for location. Dashed red lines show faults, including west strands (Frijoles Fault) of San Gregorio Fault 

Zone. Magenta symbol shows syncline axis. Blue shading shows inferred uppermost Pleistocene and Holocene strata, deposited since last sea-level 

lowstand about 21,000 years ago. Dashed orange lines highlight zone of reflections that show offshore-dipping clinoforms. 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 trackline map). Fig. 2

Fig. 5

San Gregorio 

Fault Zone,

west strands 

(Frijoles Fault)

ONE MILE = 0.869 NAUTICAL MILES

1 0.5 0 1 KILOMETER

7000 3500 0 7000 FEET

2 1 0 2 MILES

14°

SCALE 1:50 000

APPROXIMATE MEAN

DECLINATION, 2014

MAGNETIC 

 NORTH

TRUE NORTH

GIS database and digital cartography by Eleyne L. 

Phillips, Florence L. Wong, and Stephen R. Hartwell

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. Offshore shaded-relief bathymetry from map on 

sheet 2, this report. California’s State Waters limit from 

NOAA Office of Coast Survey

Universal Transverse Mercator projection, Zone 10N

NOT INTENDED FOR NAVIGATIONAL USE

10

20

30

40

50

Pillar

Point

Point

Montara

Half

Moon

Bay

715A

HMC-05T

HMS-08T

HMS-10T

122°35' 122°30' 122°25'

122°25' 122°35' 122°30'

37°30'

37°25'

37°30'

37°25'

Trackline Map

EXPLANATION

Tracklines of select seismic-reflection 

surveys—Thick lines show locations of 

profile lines depicted in figures; thin lines 

show full extent of surveys

Survey F–2–07–NC (USGS)

Survey J–1–97–MB (USGS)

Survey G–2–95–SF (USGS)

Survey W–14–76–SF (WesternGeco)

3-nautical-mile limit of California’s State 

Waters

Area of “no data”—Areas near shoreline not 

mapped owing to insufficient high-

 resolution seafloor mapping data; areas 

beyond 3-nautical-mile limit of 

California’s State Waters were not mapped 

as part of California Seafloor Mapping 

Program

Figure 2

Figure 5

Figure 3

Figure 8

Fig. 5

Fig. 2

Fig. 6

Fig. 7

Fig. 10

Fig. 8

Fig. 3

Fig. 1

Fig. 4

Fig. 9

Open-File Report 2014–1214

Sheet 8 of 10

Pamphlet accompanies map

U.S. Department of the Interior

U.S. Geological Survey