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/sim/3306/

Suggested Citation: Watt, J.T., Sliter, R.W., Johnson, S.Y., and Ross, S.L., 2014, Seismic-reflection profiles, Offshore of San Gregorio 

map area, California, sheet 8 in Cochrane, G.R., Dartnell, P., Greene, H.G., Watt, J.T., Golden, N.E., Endris, C.A., Phillips, E.L., Hartwell, 

S.R., Johnson, S.Y., Kvitek, R.G., Erdey, M.D., Bretz, C.K., Manson, M.W., Sliter, R.W., Ross, S.L., Dieter, B.E., and Chin, J.L. (G.R. 

Cochrane and S.A. Cochran, eds.), California State Waters Map Series—Offshore of San Gregorio, California: U.S. Geological Survey 

Scientific Investigations Map 3306, pamphlet 38 p., 10 sheets, scale 1:24,000, http://dx.doi.org/10.3133/sim3306.

ISSN 2329-132X (online)

http://dx.doi.org/10.3133/sim3306

DISCUSSION

This map sheet shows seismic-reflection profiles from four different surveys of the Offshore of San Gregorio map area, providing 

imagery of the subsurface geology. This map area is largely characterized by a shallow (less than 65 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 San Gregorio 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 Pescadero Point (in this 

map area) and Pillar Point (14 km north of the map area, near Half Moon Bay) (see sheet 9; see also, Weber and Lajoie, 1980; Brabb and 

others, 1998). In the offshore, faults within this system are identified on seismic-reflection profiles by the abrupt truncation or warping of 

reflections and (or) the juxtaposition of reflection panels that have different seismic parameters such as reflection presence, amplitude, 

frequency, geometry, continuity, and vertical sequence.

In this map area, the San Gregorio Fault system forms a distributed, 2- to 5-km-wide shear zone that includes two main fault strands 

(see sheets 9, 10). The east strand (also known as the “Coastways Fault” or “Seal Cove Fault”) is mostly onshore in this map area; where it 

does go offshore (see sheet 10), it is located too close to the shoreline to image with marine seismic-reflection profiles. The west strand 

(also known as the “Frijoles Fault;” see figs. 1, 2, 3, 4, 5, 6, 7, 8, 10) extends offshore from Pescadero Point. Seismic-reflection data do 

show extensive faulting and folding of Neogene bedrock along the inner shelf between the east (Coastways Fault) and west (Frijoles Fault) 

strands of the San Gregorio Fault Zone (figs. 1 through 5). Cumulative lateral slip on the San Gregorio Fault Zone is thought to range from 

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

The west strand of the San Gregorio Fault Zone (Frijoles Fault) forms the east boundary of the Pigeon Point high (McCulloch, 1987) 

(fig. 12; see also, figs. 2, 4, 6, 7, 8, 9, 10, 11), a bedrock ridge that extends offshore northwest from Pescadero Point. The Pigeon Point 

high is characterized by reflection-free (that is, massive and highly deformed) rocks of the Upper Cretaceous Pigeon Point Formation that 

are unconformably overlain by mildly to highly reflective Neogene strata (figs. 2, 4, 6, 7, 8, 9, 10, 11). Deeper penetrating, industry 

seismic-reflection profiles (fig. 10) show the Pigeon Point high as an area that has very high amplitude reflection(s) near the seafloor, 

underlain by a relatively reflection-free zone that is bounded on the northeast and southwest by faults.

Several high-resolution seismic profiles show an upper unit (blue shading in profiles; see figs. 6, 7, 8, 9, 11) that is inferred to have 

been deposited in 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. In much of the map area, this upper unit is very thin (less than 2.5 m; see sheet 9) and, hence, is difficult to map on 

seismic-reflection profiles. In contrast, offshore of Pescadero Point, the thickness of this unit reaches 20 m. 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.

All profiles displayed on this map sheet, except for line WSF–050 in figure 10, are single-channel seismic-reflection data acquired 

using the SIG 2Mille minisparker: figures 1, 2, 4, 5, 6, 7, 8, 9, and 11 show minisparker profiles collected in 2010 on USGS cruise 

S–15–10–NC, and figure 3 shows a minisparker profile collected in 2007 on USGS cruise F–02–07–NC. For these two USGS surveys, the 

SIG minisparker system used a 500-J high-voltage electrical discharge fired 1 to 4 times per second, which, at normal survey speeds 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 Triton Subbottom Logger (SBL) software that merges seismic-reflection data with differential GPS-navigation 

data. After the survey, a short-window (20 ms) automatic gain control algorithm was applied to the data, along with a 160- to 1,200-Hz 

bandpass filter and a heave correction that uses an automatic seafloor-detection window (averaged over 30 m of lateral distance covered). 

Figure 10 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 large-volume 

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/.

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. 

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.

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.

Kilometers 0 5 10

0 5 10Nautical Miles

122°40' 122°20'

38°

37°40'

37°20'

San

Francisco

Bay

San

Francisco

Area of

Map

Limit of California’s

State Waters

CALIF.

MAP LOCATION

Seismic-Reflection Profiles, Offshore of San Gregorio Map Area, California

By

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

2014

Figure 12. Schematic map showing structural blocks within Offshore of San Gregorio 

map area. Pigeon Point block and Pigeon Point high (McCulloch, 1987) are separated 

from Santa Cruz Mountains block by Pomponio block, which is bounded by Frijoles 

Fault (to west) and Coastways Fault (to east), two strands of San Gregorio Fault Zone 

(SGFZ). Blue line shows coastline. Modified from Weber (1990).

0 1 2

Frijoles Fault (west strand of SGFZ)

SANTA CRUZ

MOUNTAINS

BLOCK

San Gregorio

State Beach

Pescadero

State Beach

axis of Pigeon Point high

San Gregorio Fault Zone

3 KILOMETERS

PIGEON POINT

BLOCK

POMPONIO

BLOCK

Coastways Fault (east strand of SGFZ)

0

0.1

0.2

0.3

0

80

160

240

Approximate depth, in meters

SOUTHWEST NORTHEAST

Two-way travel time, in seconds

Figure 9. USGS high-resolution minisparker seismic-reflection profile P10–15 (collected in 2010 on survey S–15–10–NC), which crosses shelf west of Pescadero Point, roughly perpendicular to 

shoreline; see trackline map for location. Pigeon Point high (McCulloch, 1987; see also, fig. 12) is depicted here as shallow, reflection-free basement rock (inferred to be rocks of the Upper 

Cretaceous Pigeon Point Formation) exposed on seafloor near northeast end of profile; these basement rocks are unconformably overlain by west-dipping strata inferred to be of Neogene and 

Pleistocene age that are, in turn, unconformably overlain by 2- to 20-m-thick layer of shelf strata (blue shading), inferred to be of latest Pleistocene and Holocene age (that is, deposited since last 

sea-level lowstand about 21,000 years ago). This uppermost unit thins to northeast and to southwest, suggesting deposition within shallow basin (see Maps B, D on sheet 9). Dashed green lines 

highlight continuous, southwest-dipping reflections on west flank of Pigeon Point high. Dashed blue line shows erosional unconformity. 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).

1 km

VERTICAL EXAGGERATION ~ 8

Pigeon Point high

0

0.1

0.2

0.3

0

80

160

240

Approximate depth, in meters

SOUTHWEST NORTHEAST

Two-way travel time, in seconds

 Fig. 7

1 km

VERTICAL EXAGGERATION ~ 8

Figure 11. USGS high-resolution minisparker seismic-reflection profile P10–12 (collected in 2010 on survey S–15–10–NC), which crosses shelf west of Pescadero State Beach, roughly perpendicular to shoreline; see trackline map for location. Dashed red line shows interpreted fault. Pigeon Point high (McCulloch, 1987; see also, fig. 12) is depicted here as shallow, reflection-free 

basement rock, inferred to be rocks of the Upper Cretaceous Pigeon Point Formation. Southwest flank of Pigeon Point high is unconformably overlain by west-dipping strata (inferred to be of 

Neogene and Pleistocene age) that include internal unconformities; these strata are unconformably overlain by 2- to 10-m-thick layer of shelf strata (blue shading), inferred to be of latest 

Pleistocene and Holocene age (that is, deposited since last sea-level lowstand about 21,000 years ago). This uppermost unit thins to northeast and to southwest, suggesting deposition within 

shallow basin (see sheet 9). Dashed green lines highlight continuous, west-dipping reflections on west flank of Pigeon Point high (see fig. 12). Dashed blue lines show erosional unconformities. 

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).

Pigeon Point high 

0

2

1

Frijoles Fault

Approximate depth, in kilometers

1 km

VERTICAL EXAGGERATION ~ 2.5

SOUTHWEST NORTHEAST

Figure 10. Deep-penetration, industry, 2-D, multichannel air-gun profile WSF–050 (collected in 1976 by WesternGeco on survey W–14–76–SF; from USGS National Archive of Marine Seismic 

Surveys [U.S. Geological Survey, 2009]), which extends southwest across shelf offshore of San Gregorio State Beach; see trackline map for location. Dashed yellow lines show faults. Magenta 

symbol shows antiform axis. Along this profile, Frijoles Fault (west strand of San Gregorio Fault Zone) is imaged as vertical structure. Pigeon Point high (McCulloch, 1987; see also, fig. 12) is 

depicted here with two unnamed faults (fault A, fault B) imaged along its southwest (left) edge: fault A appears to be northeast-dipping fault that has northeast-side-up motion; fault B is also 

northeast dipping, but sense of motion across fault is difficult to determine in this profile. Tertiary-age strata form relatively continuous and gently folded reflections southwest of Pigeon Point 

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

Fault B Fault A Pigeon Point high

SOUTHWEST NORTHEAST

0

0.1

0.2

0.3

0

80

160

240

Approximate depth, in meters

Frijoles Fault

1 km

VERTICAL EXAGGERATION ~ 8

Figure 8. USGS high-resolution minisparker seismic-reflection profile P10–10 (collected in 2010 on survey S–15–10–NC), which crosses shelf west of Pescadero State Beach, roughly 

perpendicular to shoreline; see trackline map for location. Dashed red lines show inferred faults. West strand of San Gregorio Fault Zone (Frijoles Fault) is imaged in shallow (about 20 m) 

water near northeast end of profile, near where it continues onshore at Pescadero State Beach. Pigeon Point high (McCulloch, 1987; see also, fig. 12), which is depicted here as 

reflection-free basement rock southwest of Frijoles Fault, is inferred to consist of rocks of the Upper Cretaceous Pigeon Point Formation; these rocks are unconformably overlain by 

undeformed to gently folded strata of inferred Tertiary and Pleistocene age. Erosional unconformity separates these folded strata from westward-thickening (from 2 to 10 m) layer of shelf 

strata (blue shading), inferred to be of latest Pleistocene and Holocene age (that is, deposited since last sea-level lowstand about 21,000 years ago). Dashed green lines highlight 

continuous reflections on west flank of Pigeon Point high (McCulloch, 1987; see also, fig. 12). Dashed blue lines show erosional unconformities. 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. 7

Two-way travel time, in seconds

Pigeon Point high

0

0.1

0.2

0.3

0

80

160

240

Approximate depth, in meters

SOUTH NORTH

Frijoles Fault

 Fig. 11

 Fig. 8

 Fig. 6

 Fig. 4

 Fig. 2

 Fig. 5

Two-way travel time, in seconds

0.4

1 km

VERTICAL EXAGGERATION ~ 7

Figure 7. USGS minisparker seismic-reflection profile P10–41 (collected in 2010 on survey S–15–10–NC), which crosses shelf west of San Gregorio and Pescadero State Beaches, parallel to shoreline; see trackline 

map for location. Dashed red lines show inferred faults. West strand of San Gregorio Fault Zone (Frijoles Fault) is imaged in shallow (about 20 m) water near north end of profile. Magenta symbols show fold axes 

(diverging arrows, antiforms; converging arrows, synform). Pigeon Point high (McCulloch, 1987; see also, fig. 12), depicted here as reflection-free basement rock south of Frijoles Fault (as imaged within 100 m of 

seafloor), is inferred to consist of rocks of the Upper Cretaceous Pigeon Point Formation; these rocks are unconformably overlain by undeformed to mildly folded strata of presumed Tertiary and Pleistocene age. 

Along southern part of profile, erosional unconformity separates these folded strata from westward-thickening (to as much as 20 m) layer of shelf strata (blue shading), inferred to be of latest Pleistocene and 

Holocene age (that is, deposited since last sea-level lowstand about 21,000 years ago). Dashed green lines highlight continuous reflections in fault-bounded folds at north end of profile. Dashed blue lines show 

erosional unconformities. Dashed yellow line is seafloor multiple (echo of seafloor reflector).

Pigeon Point high

0

0.1

0.2

0.3

SOUTHWEST NORTHEAST

0

80

160

240

Approximate depth, in meters

 Fig. 7

Frijoles Fault

1 km

VERTICAL EXAGGERATION ~ 8

Two-way travel time, in seconds

Figure 6. USGS high-resolution minisparker seismic-reflection profile P10–08 (collected in 2010 on survey S–15–10–NC), which crosses shelf southwest of San 

Gregorio State Beach, roughly perpendicular to shoreline; see trackline map for location. Dashed red lines show interpreted faults. West strand of San Gregorio Fault 

Zone (Frijoles Fault) is imaged in shallow (about 20 m) water near northeast end of profile. Magenta symbol shows antiform axis. Pigeon Point high (McCulloch, 1987; 

see also, fig. 12) is depicted here as reflection-free basement rock (inferred to be rocks of the Upper Cretaceous Pigeon Point Formation) southwest of Frijoles Fault; 

these basement rocks are unconformably overlain by undeformed to gently folded strata of presumed Tertiary and Quaternary age. At southwest end of profile, 

erosional unconformity separates these folded strata from overlying, westward-thickening (from 2 to 10 m) layer of shelf strata (blue shading), inferred to be of latest 

Pleistocene and Holocene age (that is, deposited since last sea-level lowstand about 21,000 years ago). Dashed green lines highlight continuous reflections (not 

distinctive stratigraphic markers). Dashed blue lines show erosional unconformities. 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).

Pigeon Point high

0

0.1

0.2

0.3

SOUTHWEST NORTHEAST

Two-way travel time, in seconds

0

80

160

240

Approximate depth, in meters

 Fig. 7

Figure 5. USGS high-resolution minisparker seismic-reflection profile P10–01 (collected in 2010 on survey S–15–10–NC), which crosses shelf northwest of San 

Gregorio State Beach, roughly perpendicular to shoreline; see trackline map for location. Dashed red lines show inferred faults. San Gregorio Fault Zone is imaged 

as zone of faults in center of profile: of these, Frijoles Fault is southwesternmost strand. Magenta symbols show fold axes (diverging arrows, antiforms; converging 

arrows, synforms). Dashed green lines highlight folded strata: northeast of Frijoles Fault, folded strata is inferred to be rocks of the upper Miocene and Pliocene 

Purisima Formation; southwest of Frijoles Fault, gently folded Tertiary and Quaternary strata unconformably overlie reflection-free bedrock that probably consists of 

rocks of the Upper Cretaceous Pigeon Point Formation. Dashed blue lines highlight erosional unconformities. 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).

1 km

VERTICAL EXAGGERATION ~ 8

Frijoles Fault

0

0.1

0.2

0.3

SOUTHWEST NORTHEAST

0

80

160

240

Approximate depth, in meters

 Fig. 7

Frijoles Fault

Figure 4. USGS high-resolution minisparker seismic-reflection profile P10–07 (collected in 2010 on survey S–15–10–NC), which crosses shelf west of San Gregorio State Beach, 

roughly perpendicular to shoreline; see trackline map for location. Dashed red lines show interpreted faults. Western part of San Gregorio Fault Zone is imaged as two strands, 

one of which is Frijoles Fault. Magenta symbol shows synform axis. Dashed green lines highlight dipping and mildly folded reflections from within rocks inferred to be of the 

upper Miocene and Pliocene Purisima Formation. Dashed blue line highlights erosional unconformity on east flank of Pigeon Point high (McCulloch, 1987; see also, fig. 12), which 

is depicted here as reflection-free basement rock (probably consisting of rocks of the Upper Cretaceous Pigeon Point Formation) southwest of Frijoles Fault. 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).

1 km

VERTICAL EXAGGERATION ~ 8

Two-way travel time, in seconds

Pigeon Point high

0

0.1

0.2

1 km

VERTICAL EXAGGERATION ~ 8

SOUTHWEST NORTHEAST

Two-way travel time, in seconds

Approximate depth, in meters

0

80

160

Frijoles Fault

 Fig. 9

Figure 3. USGS high-resolution minisparker seismic-reflection profile HMS–033 (collected in 2007 on survey F–02–07–NC), which crosses shelf northwest of San Gregorio State Beach, roughly perpendicular to shoreline; see trackline map for location. Dashed red lines show interpreted faults. San Gregorio Fault Zone is imaged as multiple strands of faults in center of profile; of these, Frijoles Fault is 

southwesternmost strand. Magenta symbols show fold axes (diverging arrows, antiforms; converging arrows, synforms). Dashed green lines highlight dipping and folded strata: northeast of Frijoles 

Fault, strata probably represent rocks of the upper Miocene and Pliocene Purisima Formation. Dashed blue line highlights erosional unconformity southwest of Frijoles Fault; unconformity separates 

rocks inferred to be of the Upper Cretaceous Pigeon Point Formation from overlying reflective unit that is probably of Tertiary and (or) Quaternary age. 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). 

0

0.1

0.2

0.3

Two-way travel time, in seconds

SOUTHWEST NORTHEAST

0

80

160

240

Approximate depth, in meters

Frijoles Fault

 Fig. 7

Figure 2. USGS high-resolution minisparker seismic-reflection profile P10–05 (collected in 2010 on survey S–15–10–NC), which crosses shelf west of San Gregorio State Beach, 

roughly perpendicular to shoreline; see trackline map for location. Dashed red lines show interpreted faults. San Gregorio Fault Zone is imaged as multiple strands, southwesternmost of which is Frijoles Fault. Magenta symbols show fold axes (diverging arrows, antiform; converging arrows, synforms). Pigeon Point high (McCulloch, 1987; see also, fig. 

12) is depicted here as relatively reflection-free basement rock southwest of Frijoles Fault that probably consists of rocks of the Upper Cretaceous Pigeon Point Formation; 

dashed blue line shows erosional unconformity on east flank of this basement high. Dashed green lines highlight continuous reflections: northeast of Frijoles Fault, reflections 

probably are within rocks of the upper Miocene and Pliocene Purisima Formation. 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).

1 km

VERTICAL EXAGGERATION ~ 8

Pigeon Point high

0

0.1

0.2

0.3

Two-way travel time, in seconds

SOUTHWEST NORTHEAST

0

80

160

240

Approximate depth, in meters

Figure 1. USGS high-resolution minisparker seismic-reflection profile P10–43 (collected in 2010 on survey S–15–10–NC), which crosses shelf northwest of San Gregorio State Beach, perpendicular to shoreline; see trackline map for location. Dashed red lines show faults. San Gregorio Fault Zone 

is imaged as multiple strands: of two faults in western part of fault zone (southwestern [left] side of profile), Frijoles Fault (on right) is more prominent 

strand; second steeply northeast-dipping reverse fault southwest (to left) of Frijoles Fault may link with Frijoles Fault at depth. Magenta symbols show 

fold axes (diverging arrows, antiforms; converging arrows, synforms). Dashed green lines highlight dipping and folded reflections, probably within 

rocks of the upper Miocene and Pliocene Purisima Formation. 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). 

Frijoles Fault

1 km

VERTICAL EXAGGERATION ~ 8

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

APPROXIMATE MEAN

DECLINATION, 2014

MAGNETIC NORTH

13 / ° 1 2

TRUE NORTH

GIS database and digital cartography by Janet T. Watt, 

Eleyne L. Phillips, and Florence L. Wong

Edited by Taryn A. Lindquist

Manuscript approved for publication June 30, 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

600

400

122°25'

122°25'

122°30'

122°30'

37°20'

37°20'

37°15'

37°15'

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 S–15–10–NC (USGS)

Survey F–2–07–NC (USGS)

Survey W–14–76–SF (WesternGeco)

Survey W–18–75–NP (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 1

Figure 10

Figure 3

Fig. 1

Fig. 3

Fig. 5

Fig. 2

Fig. 4

Fig. 6

Fig. 8

Fig. 11

Fig. 9

Fig. 7

Fig. 10

Pescadero

 Point

Pescadero

State Beach

San Gregorio

State Beach

Scientific Investigations Map 3306

Sheet 8 of 10

Pamphlet accompanies map

U.S. Department of the Interior

U.S. Geological Survey