U.S. Department of the Interior

U.S. Geological Survey

US SG

Professional Paper 1687

Marine Geology and Earthquake Hazards of the 

San Pedro Shelf Region, Southern California

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Marine Geology and Earthquake Hazards of the

San Pedro Shelf Region, Southern California

Michael A. Fisher, William R. Normark, Victoria E. Langenheim, 

Andrew J. Calvert, and Ray Sliter

Professional Paper 1687

U.S. Department of the Interior

U.S. Geological Survey

Offshore geophysical and geological data are combined to determine the distribution of potential 

earthquake faults near the Los Angeles urban area.
ii 

U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

This report and any updates to it are available online at:

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Although this report is in the public domain, it contains copyrighted materials that are noted in the text. Permission 

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Cataloging-in-publication data are on file with the Library of Congress (URL http://www.loc.gov/).

Published in the Western Region, Menlo Park, California

Manuscript approved for publication, December 15, 2003

Text edited by Peter H. Stauffer

Production and design by Sara Boore

Front Cover Oblique image, looking north-northeast, of the Los Angeles region, California, and 

the adjacent Pacific Ocean, showing major geographic features and locations of main faults. 

Width of image is about 70 km, center of image located at about 34°N, 118.2°W. Image consists 

of an enhanced Landsat 5 satellite image (Bands 3, 2, and 1 as red, green, and blue, respectively) 

draped over topography from Space Shuttle radar data, with vertical exaggeration of 1.5 times. From 

National Aeronautical and Space Administration (NASA), available (without annotations) from: 

http://www.jpl.nasa.gov/srtm/california.html#PIA03348.
 iii

Contents

Abstract .......................................................................................................................................................... 1

Introduction ................................................................................................................................................... 1

Margin setting ............................................................................................................................................... 2

Tectonic evolution and regional structure ................................................................................................ 3

Regional geology .......................................................................................................................................... 5

Onshore rocks ...................................................................................................................................... 5

Age of offshore rocks .......................................................................................................................... 5

Depth to offshore metamorphic basement ...................................................................................... 7

Previous findings about the main offshore structures .................................................................. 8

Geophysical data ........................................................................................................................................ 10

Seismic-reflection data .................................................................................................................... 10

Rock-velocity data ............................................................................................................................. 10

Potential-field data ............................................................................................................................ 10

Offshore structure from seismic-reflection data ................................................................................... 10

Structural belt east of the Palos Verdes Fault Zone: the Wilmington Graben ......................... 11

Structural variation along the Palos Verdes Fault Zone .............................................................. 15

Structural belt west of the Palos Verdes Fault Zone ................................................................... 22

Discussion .................................................................................................................................................... 26

Geology of the San Pedro Shelf ...................................................................................................... 26

Faults and earthquake hazards ....................................................................................................... 28

Conclusions ................................................................................................................................................. 29

Acknowledgements .................................................................................................................................... 29

References ................................................................................................................................................... 30

Plates

[in pocket]

 1. Seismic-reflection sections showing structural variations along the Palos Verdes Fault 

Zone, Southern California

 2. Post-stack migrated and depth-converted multichannel seismic-reflection sections 

from the San Pedro Shelf, Southern California

Figures

 1. Location map of the study area near the San Pedro Shelf ................................................... 2

 2. Map showing tracklines of small-airgun seismic-reflection surveys in the 

study area during 1997-8 by the U.S. Geological Survey ...................................................... 3

 3. Map showing locations of regional cross sections in the study area, and diagram of 

Miocene and younger stratigraphy .......................................................................................... 4

 4. Cross sections comparing seismic-reflection data from this study with stratigraphic 

information from the Beta oil field ............................................................................................ 6

 5. Deep-crustal seismic-reflection data and simplified composite cross section across 

the San Pedro Shelf and the Palos Verdes Fault Zone .......................................................... 7
iv 

 6. Tomographic velocities across the San Pedro Basin and Shelf derived from data 

obtained during the LARSE experiment ................................................................................... 9

 7. Velocity-depth function derived from digitized and averaged sonic logs 

from five wells ............................................................................................................................ 11

 8. Aeromagnetic anomaly map and isostatic gravity map of the study area ....................... 12

 9. Index map of detailed seismic-reflection sections over selected features 

below the San Pedro Shelf and Lasuen Knoll ....................................................................... 13

 10. Seismic section over the Palos Verdes Fault Zone and Wilmington Graben 

under the San Pedro Shelf ....................................................................................................... 14

 11. Seismic sections showing the geologic structure near Dana Point. ................................ 14

 12. Detailed seismic section over the Palos Verdes Fault Zone .............................................. 15

 13. Detailed seismic sections over the Palos Verdes Fault Zone ............................................ 16

 14. Oblique shaded-relief diagrams of the San Pedro Shelf and Lasuen Knoll ..................... 18

 15. Detailed seismic section over the Palos Verdes Fault Zone under the 

bathymetric saddle between the San Pedro Shelf and Lasuen Knoll ............................... 19

 16. Detailed seismic section over the Palos Verdes Fault Zone and Lasuen Knoll .............. 19

 17. Detailed seismic section from near the crest of Lasuen Knoll .......................................... 20

 18. Detailed seismic section over the northwestern part of Lasuen Knoll ............................. 21

 19. Map summarizing the main known and inferred structures in the region of the San 

Pedro Shelf and Lasuen Knoll .................................................................................................. 22

 20. Detailed seismic section from the San Pedro Shelf ............................................................ 23

 21. Seismic sections over the slope west of the San Pedro Shelf, 

showing Miocene(?) rocks ....................................................................................................... 24

 22. Seismic section over the slope west of the San Pedro Shelf, showing the lower-slope 

compressional zone .................................................................................................................. 25

 23. Seismic sections over the slope northwest of the San Pedro Shelf and the San Pedro 

Basin ............................................................................................................................................ 25

 24. Magnetic and gravity models along seismic sections 115 and 200 .................................. 27
Marine Geology and Earthquake Hazards of the

San Pedro Shelf Region, Southern California

By Michael A. Fisher1, William R. Normark1, Victoria E. Langenheim1, Andrew J. Calvert2, and Ray Sliter1

Abstract

High-resolution seismic-reflection data have been combined with a variety of other geophysical and geological data 

to interpret the offshore structure and earthquake hazards of 

the San Pedro Shelf, near Los Angeles, California. Prominent 

structures investigated include the Wilmington Graben, the 

Palos Verdes Fault Zone, various faults below the western part 

of the shelf and slope, and the deep-water San Pedro Basin. 

The structure of the Palos Verdes Fault Zone changes markedly southeastward across the San Pedro Shelf and slope. 

Under the northern part of the shelf, this fault zone includes 

several strands, but the main strand dips west and is probably 

an oblique-slip fault. Under the slope, this fault zone consists of several fault strands having normal separation, most 

of which dip moderately east. To the southeast near Lasuen 

Knoll, the Palos Verdes Fault Zone locally is a low-angle fault 

that dips east, but elsewhere near this knoll the fault appears to 

dip steeply. Fresh sea-floor scarps near Lasuen Knoll indicate recent fault movement. The observed regional structural 

variation along the Palos Verdes Fault Zone is explained as the 

result of changes in strike and fault geometry along a master 

strike-slip fault at depth. The shallow summit and possible 

wavecut terraces on Lasuen knoll indicate subaerial exposure 

during the last sea-level lowstand. Modeling of aeromagnetic 

data indicates the presence of a large magnetic body under 

the western part of the San Pedro Shelf and upper slope. This 

is interpreted to be a thick body of basalt of Miocene(?) age. 

Reflective sedimentary rocks overlying the basalt are tightly 

folded, whereas folds in sedimentary rocks east of the basalt 

have longer wavelengths. This difference might mean that the 

basalt was more competent during folding than the encasing 

sedimentary rocks. West of the Palos Verdes Fault Zone, other 

northwest-striking faults deform the outer shelf and slope. 

Evidence for recent movement along these faults is equivocal, 

because age dates on deformed or offset sediment are lacking.

1

U.S. Geological Survey 

345 Middlefield Road, MS 999 Menlo Park, CA, 94025 

mfisher@usgs.gov, 650-329-5158 

2

Department of Earth Sciences 

Simon Fraser University 

Burnaby, B.C., V5A 1S6, Canada

Introduction

Numerous thrust and strike-slip faults cross cut the Los 

Angeles region (fig.1) and pose a significant earthquake 

hazard to this burgeoning population center (see, for example, 

Shaw and Suppe, 1996; Shaw and Shearer, 1999; Dolan and 

others, 2000; Weaver and Dolan, 2000; Tsutsumi and others, 

2001). Much effort has been expended to locate and determine 

the displacement history of these faults; even so, a category of 

fault that remains poorly understood includes active offshore 

faults within about 50 km of the coast. Largely because of a 

lack of information, such faults are often omitted from models 

of tectonic evolution and estimates of seismic hazard.

Recent earthquakes at Whittier Narrows (1987, ML

5.9; Hauksson and others, 1988; Lin and Stein, 1989) and 

at Northridge (1994, Mw 6.7; Hauksson and others, 1995) 

provide a measure of the regional seismic hazard and the 

economic dislocation that can result from earthquakes along 

onshore faults. In comparison, historical seismicity indicates 

that offshore faults can unleash earthquakes with at least 

moderate magnitude, as shown by compilations of southern 

California seismicity (for example, Ziony and Jones, 1989; 

Jennings, 1994; Petersen and Wesnousky, 1994; Astiz and 

Shearer, 2000; Richards-Dinger and Shearer, 2000). In 1933, a 

large (Mw 6.4) earthquake struck near Long Beach, probably 

along the offshore part of the Newport-Inglewood strike-slip 

fault (Barrows, 1974; Hauksson and Gross, 1991). Offshore 

epicenters in and around the area of this study are diffuse but 

tend to cluster over the western San Pedro Shelf and eastern 

San Pedro Basin (fig. 1). Two moderate, offshore earthquakes 

occurred near the coastal town of Malibu (M 5.2 in 1979 and 

M 5.0 in 1989; see inset to fig. 1). No large earthquakes have 

occurred in the recent past along the Palos Verdes Fault Zone, 

which strikes generally southeast across the San Pedro Shelf 

and nearshore areas (fig. 1). However, McNeilan and others 

(1996) estimate that this fault could produce an earthquake as 

large as about M 7.

The dearth of information about Holocene activity along 

offshore faults in this region provided the impetus to gain a 

more complete understanding of the offshore geologic structure. During 1998 through 2000, the U.S. Geological Survey 

collected high- and medium-resolution, marine seismic-reflection data (tracklines shown in fig. 2) with the goals of locating 
2 Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

active faults and estimating their displacement histories. A 

related research goal is understanding the susceptibility of the 

offshore region to earthquake-induced landslides and consequent local tsunamis. The Portuguese Bend landslide on the 

Palos Verdes Peninsula (see, for example, Woodring and others, 1946) and offshore landslides evident in seismic-reflection 

and multibeam-bathymetric data (for example, Locat and Lee, 

2002), indicate that sediment under the sea floor is locally susceptible to catastrophic downslope movement. The submarine 

Palos Verdes debris avalanche, which displaced more than a 

cubic kilometer of slope sediment, occurred 7,500 years ago 

(Normark and others, 2002).

This study focuses on the structure and stratigraphy 

southeastward from the Palos Verdes Peninsula to near Dana 

Point (fig. 1). Within the study area, the Palos Verdes Fault 

(Fischer and others, 1987; McNeilan and others, 1996) and the 

Newport-Inglewood Fault (Barrows, 1974; Fischer, 1992) play 

key roles in the regional earthquake hazard. 

Margin Setting

The study area encompasses the San Pedro continental 

shelf and the eastern part of the deep-water San Pedro Basin, 

lying west and southwest of the shelf (fig. 1). This area straddles the tectonic boundary between the Los Angeles Basin and 

the extensive California Continental Borderland (CCB). The 

Los Angeles Basin is filled with Miocene and younger rocks, 

at least 10 km thick, and the basin extends westward to near 

the Palos Verdes Fault Zone (Wright, 1991). West of the Los 

Angeles Basin, the CCB is 200 km wide and ends in the west 

at the Patton Escarpment.

Within the study area, rocks under the Palos Verdes 

Peninsula have long attracted the attention of geologists, 

because rapid uplift during the Quaternary caused an impressive flight of wavecut terraces to develop (Woodring and 

others, 1946; Bryan, 1987; Ward and Valensise, 1994). A 

concern of seismologists is that this intense vertical deformaFigure 1. The study area near the San Pedro Shelf, showing the main faults, earthquake epicenters (1978-1998, from Richards-Dingler 

and Shearer, 2000), and bathymetry. 

Palos Verdes Fault

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Study Area

Los Angeles

San

Diego

Santa Barbara

Long Beach 1933:

M 6.3

Malibu 1979, 1989: M 5.2 and 5.0

Santa Monica Mtns.

?

Thums-Huntington Beach Fault

Newport-Inglewood Fault

?

Palos Verdes

Peninsula

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Catalina

Island

San

 Pedro Shelf

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Point

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Beach

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Beach

Laguna

Beach

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Pedro

Basin

Avalon

Knoll

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Knoll

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Monica

Bay

Wilmington

Graben

San Pedro

Sea Valley

San Gabriel Canyon

Cabrillo Fault

Newport

Beach

Axis of Wilmington Anticline

 15 km

Bathymetry in meters

Bathymetric contour interval

20 m for depth < 100 m

100 m for depth > 100 m

Offshore earthquake epicenter

118° 30' W 118° 15' 118° 00' 117° 45' 

33° 45' N

33° 30'

33° 15'
tion may extend south of the Palos Verdes Peninsula to involve 

offshore faults. This possibility has been difficult to evaluate, 

given the generally featureless morphology of the San Pedro 

Shelf, which is broad, flat, and shallow. Most of it is less than 

50 m deep (fig. 1), so it is unclear whether deformation rates 

measured onshore characterize offshore areas.

In contrast to the flat expanse of the San Pedro Shelf, the 

rest of the study area is characterized by varied topography. 

The San Pedro Shelf is bordered to the west and southeast by a 

steep slope (fig. 1). At the west end of the shelf, the San Pedro 

Sea Valley, a deep notch cut eastward into the shelf, is structurally controlled. South of the San Pedro Sea Valley, the San 

Gabriel Canyon incises the shelf break. This canyon extends 

south from the shelf edge, crosses the continental slope, and 

feeds sinuous turbidite channels on the floor of the San Pedro 

Basin. East of the San Gabriel canyon, the numerous tributaries of the Newport Canyon head near the coast at Newport 

Beach, where the shelf narrows greatly. A narrow shelf extends 

southeastward beyond the limit of the study area.

Tectonic Evolution and Regional Structure

Before the Miocene, the tectonic regime in the study 

area involved the eastward subduction of the Farallon 

oceanic plate beneath the area now occupied by the CCB 

(Atwater, 1970; Engebretson and others, 1985; Schwartz 

and Colburn, 1987; Wright, 1991). Subduction continued 

until the early Miocene, when the part of the Farallon Plate 

that lay west of southern California stopped subducting 

and began to move northward with the Pacific Plate (Nicholson and others, 1994). During this tectonic transition, 

regional oblique rifting and strike-slip deformation replaced 

100 m

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(fig. 5, 

top) 118

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?

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Peninsula

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Catalina

Island

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 Pedro

 Shelf

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Point

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Beach

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Beach

Huntington

Beach

Laguna

Beach

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Pedro

Basin

Avalon

Knoll

Lasuen

Knoll

Santa

Monica

Bay

Shaw (1999)

(fig. 5, bottom) 

Bathymetry in meters

Bathymetric contour interval 100 m

118° 30' W 118° 15' 118° 00' 117° 45' 

33° 45' N

33° 30' 

33° 15' 

 15 km

Major fault

Small-airgun seismic trackline

Large-airgun seismic trackline

Figure 2. Tracklines of small-airgun seismic-reflection data obtained during 1997-8 by the U.S. Geological Survey (USGS). These data 

were obtained with a 240-m-long streamer and with either a generator-injector airgun (each chamber was 35 in3

) or a 40 in3 Bolt airgun. 

Location of LARSE 1 deep crustal seismic-reflection line is also shown (Brocher and others, 1995) and seismic-reflection line of Shaw 

(1999). MCS 125 and MCS 129 indicate multichannel seismic-reflection data collected by the USGS during 1990.

Tectonic Evolution and Regional Structure 3
4 Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

margin-normal subduction. The resulting oblique extension 

of the margin of North America is thought to have occurred 

simultaneously with the clockwise rotation of the Transverse 

Ranges (Luyendyk and others, 1980, 1985; Kamerling and 

Luyendyk, 1985; Hornafius and others, 1986). The upper 

plate of the subduction zone underwent intense crustal extension (Vedder, 1987; Wright, 1991; Crouch and Suppe, 1993; 

Nicholson and others, 1994; Bohannon and Geist, 1998; ten 

Brink and others, 2000; Bjorklund and others, 2002), so that 

locally the continental margin may have doubled in width. 

Blueschist- and amphibolite-facies metamorphic rocks from 

the former subduction zone were uplifted from lower crustal 

depths and exposed at the surface. These rocks currently crop 

out as the Catalina Schist, and they make up the basement 

complex beneath much of the inner CCB, including this study 

area. Regional uplift of the Catalina Schist during the early 

Miocene accounts for the absence of sedimentary rocks of 

this and older age beneath the inner CCB. As the heat-flow 

anomaly associated with the crustal extension dissipated, the 

extended region subsided, and metamorphic basement rocks 

were buried beneath locally thick sediment.

Volcanism followed the crustal extension (Dickinson, 

1997; Bjorklund and others, 2002). Middle Miocene volcanic 

rocks, including pillow basalt and basalt sills, are exposed 

on the Palos Verdes Peninsula (fig. 3A; Woodring and others, 1946; Conrad and Ehlig, 1987; Stanley and others, 2000; 

McCulloh and others, 2002). As is shown below, some of these 

volcanic rocks produce offshore aeromagnetic anomalies.

Namson & Davis 11 Namson & Davis 11

Section 15 (Davis

and

Namson, 2002)

Section 12 (Davis and

Namson, 2002)

Section 13 (Davis and

Namson, 2002)

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Palos Verdes Fault

Newport-Inglewood Fault

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C

Fault

abrillo

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Peninsula

San

 Pedro

 Shelf

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Beach

Newport

Beach

Huntington

Beach

San

Pedro

Basin

Santa

Monica

Bay

Line 200

Line 84

Seismic reflection

and geologic data

in figure 5

Geologic

section of

Beta oil

field in

Figure 5

Section G (Wright, 1991)

Section F ([Wright, 1991)

Section E

(Wright, 1991)

Section D (Wright, 1991)

Mohnian

Delmontian

Repettian

Basalt

Catalina Schist

Volcanic tuff

Biogenic rocks

Sea-floor sedimentary rocks

Rocks on the Palos Verdes Peninsula

Miocene Monterey Formation

Metamorphic basement

Regional seismic reflection section (fig. 6)

Regional geologic cross section by Wright (1991)

Regional geologic cross section by

 Davis and Namson (2002)

Major fault

Well with digital sonic-log data

Explanation

Extent of magnetic

source from analysis

of maximum horizontal magnetic gradient

(Figure 9, top)

Hellman

Estates #1

Lakewood

Community #1

Pier F

Pier C

VT-600

 10 km

118° 30' W 118° 15' 118° 00'

33° 45' N

33° 30'

Bathymetric contour interval

20 m for depth < 100 m

100 m for depth > 100 m

A

Figure 3. Cross sections and stratigraphy in the study area. A, Locations of regional cross sections compiled by various authors 

and discussed in the text. Also shown are locations of wells from which sonic-log data were obtained. A cross section through the 

Beta Oil field (Wright, 1991) shows the approximate age of basin fill in figure 4. Biostratigraphic data from strata exposed at the sea 

floor are from Nardin and Henyey (1978). Onshore geology of the Palos Verdes Peninsula is from Woodring and others (1946). B, 

Stratigraphic nomenclature for Miocene and younger rocks in the study area (after Blake, 1991; Barron and Isaacs, 2001).
In late Pliocene and Quaternary time, mainly the past 

4 m.y., the extended rocks of the continental margin were 

deformed by regional transpression (Atwater, 1970; Wright, 

1991; Crouch and Suppe, 1993; Bohannon and Geist, 1998). 

The Pliocene transition in regional strain proceeded gradually and produced a structural grain that includes northwesttrending compressional and strike-slip faults (Hauksson, 

1990; Wright, 1991; Dolan and others, 1995; Shaw and 

Suppe, 1996; Schneider and others, 1996; Walls and others, 

1998; Bohannon and Geist, 1998; Shaw and Shearer, 1999; 

Argus and others, 1999). During this tectonic transition, 

some middle Miocene normal and oblique-normal faults 

were reactivated as reverse and strike-slip faults. Blind thrust 

faults and fault-propagation folds also may have begun to 

form offshore and beneath the densely populated Los Angeles region (see, for example, Namson and Davis, 1990; Shaw 

and Suppe, 1996).

Evidence for the Pliocene onset of this transpressive 

deformation comes from oil-industry wells that penetrate 

into the large Wilmington Anticline. This anticline (fig.1) 

deforms rocks northeast of the study area but did not exist 

before the deposition of upper Miocene and lower Pliocene 

rocks (Olson, 1974).Transpressive folding of this anticline 

apparently was completed by the end of the late Pliocene 

(Truex, 1973).

Regional Geology

Onshore Rocks

Metamorphic basement rocks exposed on the Palos 

Verdes Peninsula are overlain with marked unconformity by 

a sedimentary succession, middle Miocene through Pliocene in age, that is probably similar to some rock sequences 

revealed in seismic-reflection data collected offshore (for 

example, Fischer and others, 1987; Wright, 1991). Under the 

peninsula, middle and late Miocene deep-marine rocks form 

a thick, heterogeneous sequence assigned to the Monterey 

Formation (Woodring and others, 1946; Conrad and Ehlig, 

1987; Schwartz and Colburn, 1987; Blake, 1991; Isaacs, 2001; 

Barron and Isaacs, 2001) (figs. 3A, B). This formation was 

deposited extensively in western California and is exposed 

on the Palos Verdes Peninsula, where it includes three main 

lithologic units. The bottom unit of the Monterey Formation 

rests unconformably on metamorphic basement and includes 

not only cherty and phosphatic shale but also volcanic tuff 

and basalt. The volcanic rocks have been dated to between 

15 and 14 Ma (middle Miocene; Conrad and Ehlig, 1987; 

Stanley and others, 2000) (fig. 3B). The middle rock unit of 

the Monterey Formation is diatomite. This unit is important to 

the interpretation of offshore seismic-reflection data, because 

the low density and low acoustic velocity of diatomite, relative 

to such properties in over- and underlying rock units, likely 

cause strong seismic reflections. The top unit of the Monterey 

Formation is mudstone.

Lower Pliocene rocks on the Palos Verdes Peninsula 

unconformably overlie the Monterey Formation, and upper 

Pliocene rocks are absent. This stratal succession resulted 

because the area of the peninsula appears to have been a 

submarine high during the early Pliocene and received some 

sediment, but during the late Pliocene, this high apparently 

was exposed subaerially and eroded (Henry, 1987). Thick 

Pliocene strata in the Los Angeles Basin, east of the Palos 

Verdes Peninsula, were deposited predominantly in a deep 

marine basin. Pleistocene and Holocene sediment in the Los 

Angeles Basin was deposited atop a regional erosion surface. 

The upper part of the Pleistocene and Holocene rock sequence 

includes the transition from marine to generally nonmarine 

deposition.

Age of Offshore Rocks

The offshore stratigraphy is known primarily from 

oil industry coring and drilling. Numerous shallow cores, 

obtained south of the Palos Verdes Peninsula and west of the 

Palos Verdes Fault Zone, show that middle Miocene through 

Pliocene rocks are exposed at the sea-floor in a southeasttrending zone near the shelf break (Nardin and Henyey, 1978) 

(fig. 3A). These sea-floor rocks cause high backscatter in 

multibeam bathymetric data. We show below that seismic 

reflections from Miocene rocks are distinctive in that they 

Pleist.

Late Late Middle

E

Pliocene

arly

Miocene

0

2

4

6

8

10

12

14

Pico

Repettian

Mohnian

Delmontian

Age

(Ma)

B Epoch Stage

Figure 3—Continued

Regional Geology 5
6 Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

are parallel, uniform in appearance, and continuous over long 

distances (10-20 km).

Other information concerning the age of offshore rocks 

comes from geologic cross sections (locations shown on fig. 

3A) that are based on well data from the Beta oil field (Henry, 

1987; figure 14 in Wright, 1991). Comparison of one cross 

section from Wright (1991) to depth-converted, U.S. Geological Survey seismic-reflection data (fig.4) reveals not only the 

probable thickness and age of rocks on opposite sides of the 

Palos Verdes Fault Zone but also the depth to crystalline baseZone of distorted reflections

caused by sharp velocity gradients across the Palos Verdes fault

Line 200

Biostratigraphic data from wells in Beta oil field (Wright, 1991)

1

2

1

2

0

3

DEPTH, IN KIL

O

METERS DEPTH, IN KIL

O

METERS

Pico

Repettian

Delmontian

Mohnian

Catalina

Schist

Repettian rocks in sea-floor samples

(Nardin and Henyey, 1978)

multiple

Zone of distorted reflections

caused by sharp velocity gradients across the Palos Verdes fault

Biostratigraphic data from wells in Beta oil field (Wright, 1991)

1

2

1

2

0

3

Pico

Repettian

Delmontian

Mohnian

Catalina

Schist

Line 84

Sections

offset in depth

so the

sea floors

approximately

coincide

2 km

2 km

SW NE

Figure 4. Comparison of depthconverted seismic-reflection 

data along two lines from this 

study with stratigraphic information (colors) from the Beta oil 

field (Wright, 1991) and age of 

sea-floor samples (Nardin and 

Henyey, 1978). Rocks west of 

the Palos Verdes Fault Zone are 

mainly Miocene and Pliocene in 

age, whereas thick late Pliocene 

and Quaternary rocks and sediment lie on this fault zone’s east 

side. 
ment on the west side of the fault. The comparison indicates 

that near the Beta oil field and west of the Palos Verdes Fault 

Zone, Quaternary sediment is thin or absent and Miocene 

and Pliocene rocks are exposed at the sea floor. This exposure agrees with findings from the shallow coring mentioned 

above. East of the Palos Verdes Fault Zone, however, late 

Pliocene and Quaternary rocks and sediment aggregate in 

thickness to as much as 1.5 km.

Depth to Offshore Metamorphic Basement

In general, clear reflections from the top of metamorphic 

basement are not apparent in the small-airgun seismic-reflection data described here, but evidence from rock outcrop, oil 

industry operations, and seismic-refraction data all indicate 

that this basement lies at relatively shallow depth (< 3 km) 

beneath the shelf and slope. 

Regionally, the depth to basement decreases westward 

from within the Los Angeles Basin toward the offshore study 

area, and basement deepens offshore southeastward from its 

outcrop on the Palos Verdes Peninsula (see, for example, Olson, 

1974; Fischer and others, 1987; Wright, 1991). This configuration is demonstrated primarily by information compiled from oil 

industry sources. For example, regional cross sections (locations 

shown in fig. 3A) by several authors indicate that east of the 

Palos Verdes Fault Zone and Peninsula, the top of basement is 

mainly 1 to 1.5 km deep, except locally, where basement depth 

increases to about 3 km (section 12 of Davis and Namson, 

2002). A cross section drawn through the Huntington Beach oil 

field (section 15 of Davis and Namson, 2002) indicates basement rocks at about 3 km depth, except along the NewportInglewood Fault, where basement may be more than 5 km deep.

Another regional cross section, constructed in part from 

depth-converted seismic-reflection data (Shaw, 1999; summarized in fig. 5, bottom; section location shown in fig. 2), 

0

5

10

Top of

basement

(well control)

Top of

Repettian rocks (well control)

Palos Verdes Fault

(Shaw, 1999)

?

?

?

?

SW NE

SSW NNE

San Pedro Shelf

Coastline

Shelf break

DEPTH, IN KIL

OMETERS

Earthquake hypocenters. Those deeper than 8 km

show mainly thrust motion and may reveal the Palos

Verdes Fault Zone (Shaw, 1999)

Westward onlap of horizontal

basin fill onto top of basement

NNE-dipping strata

terminate against

the Palos Verdes Fault

and are also evident in

the cross section below and in seismic-reflection

sections 115 and 200 (Plate 2)

Basement top?

(Catalina Schist?)

Basement top

(Catalina Schist?)

Reflections poorly depicted Horizontal basin fill in Shaw [1999]

Modified from Shaw (1999)

LARSE 1 Stack

TWO-WAY TRAVELTIME, IN SECONDS

0

1

2

3

4

5

Avalon Knoll San Pedro Basin

San Pedro Basin

San Pedro Shelf

Palos Verdes

Fault Zone

East-dipping strata

See figure 2 for location

See figure 2 for location

0 5 10 15 20 25

0 5 10 15 20

DISTANCE, IN KILOMETERS

DISTANCE, IN KILOMETERS

Figure 5. Top, stack of LARSE line 1 (Brocher and others, 1995), deep-crustal, seismic-reflection data obtained across the 

San Pedro Shelf and the Palos Verdes Fault Zone (location shown in fig. 2). These data indicate that the fault zone dips west 

at depth. Bottom, a simplified view of the composite cross section by Shaw (1999; location shown in fig. 3A), which shows 

one interpretation of the deep-crustal structure of the Palos Verdes Fault Zone.

Regional Geology 7
8 Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

indicates that basement rocks below the coast southwest of 

the Newport-Inglewood Fault were encountered in a well 

at a depth of about 3 km. Furthermore, rocks that return 

strong seismic reflections extend southwestward from this 

well, toward the Palos Verdes Fault Zone. If these reflections 

represent the top of basement, then basement rocks vary 

irregularly in depth between 2 and 3.5 km, and adjacent to 

the Palos Verdes Fault Zone basement attains a depth of 3.5 

km. A cross section through the Beta oil field (figure 14 in 

Wright, 1991) provides the only direct information we have 

on basement depth southwest of the Palos Verdes Fault Zone, 

where a well encountered basement rocks at about 2.5 km 

depth (fig. 4).

Basement depth determined from drilling in the Beta oil 

field compares favorably to the depth estimated from tomographic analysis of wide-angle and refraction data collected 

during the LARSE experiment (ten Brink and others, 2000) 

(location of LARSE 1 seismic-reflection line shown in fig. 

2). Assuming that rock velocities exceeding 5 km/s indicate 

acoustic basement, either volcanic or metamorphic rock, then 

tomographic analysis of wide-angle seismic data indicates 

that basement northeast of the Palos Verdes Fault Zone is 

2.5 to 3 km deep (fig. 6, top). Southwest of the Palos Verdes 

Fault Zone, basement is about 2.5 km deep and deepens to 

the southwest. These depths agree, to within 0.2-0.4 km, with 

depths derived from oil industry operations, as described 

above.

Previous Findings About the Main Offshore 

Structures

The following discussion considers the main coastal 

structures in order from northeast to southwest, focusing 

closely on major faults that lie near the coast, especially the 

Newport-Inglewood, Thums-Huntington Beach, and Palos 

Verdes Fault Zones (fig. 1). The Newport-Inglewood rightlateral strike-slip fault has fundamental importance to the tectonic evolution of the CCB and Los Angeles Basin. Either this 

fault forms the contact between different basement types, or 

the fault closely follows this contact (Barrows, 1974; Wright, 

1991, Bohannon and Geist, 1998). North and east of the fault, 

basement rocks consist of Jurassic and Cretaceous crystalline 

continental crust, whereas south and west of it the basement 

is Catalina Schist, which is made up of highly extended and 

metamorphosed subduction-zone rocks. 

The Thums-Huntington Beach Fault splays southeastward 

away from the Palos Verdes Fault Zone (Wright, 1991) (fig. 

1). Interpretive cross sections differ on fundamental issues 

about this fault—one shows a normal fault that dips east and is 

downthrown on the east (Wright, 1991), another shows that it 

dips west and is downthrown on the west, merging downward 

with the Palos Verdes Fault Zone (section 12 of Davis and 

Namson, 2002).

The Palos Verdes Fault Zone has been postulated to 

extend southwestward from the northern part of Santa Monica 

Bay, to cross the bay’s southeastern shoreline at the Palos 

Verdes Peninsula, to strike under the San Pedro Shelf, and to 

continue southwest of Lasuen Knoll (for example, Greene 

and Kennedy, 1987; Wright, 1991; Petersen and Wesnousky, 

1994; Clarke and others, 1997; Marlow and others, 2000) (fig. 

1). Multibeam bathymetric data show recent scarps along this 

fault near Lasuen Knoll (Marlow and others, 2000).

The Palos Verdes Fault Zone passes northeast of the 

Palos Verdes Peninsula (Woodring and others, 1946; Yerkes 

and others, 1965; Ward and Valensise, 1994, McNeilan and 

others, 1996), where the total fault-slip rate appears to be 

around 3 mm/yr, based on analysis of wave-cut terraces and 

offset stream courses (Ward and Valensisie, 1994; Stephenson 

and others, 1995; McNeilan and others, 1996). McNeilan and 

others (1996) propose that, in the recent past, the main style of 

movement along the Palos Verdes Fault Zone has been strike 

slip. Stephenson and others (1995) interpret high-resolution 

seismic-reflection data collected onshore to show that five 

strands make up the shallow part of this fault zone and that 

these strands dip steeply southwest and are downthrown on the 

northeast.

Although the Palos Verdes Fault Zone has been postulated to end far to the northwest, near the Santa Monica 

Mountains, no indication of a recently active part of this fault 

has been observed in high-resolution seismic-reflection data 

collected in the southern part of Santa Monica Bay (Nardin 

and Henyey, 1978; Fisher and others, in press). If the active 

part of the fault does not extend northwest of the Palos Verdes 

Peninsula, then slip partitioning must occur among faults 

neighboring the peninsula to absorb the high strain rate measured along the fault.

The geometry of the Palos Verdes Fault Zone at depth is 

poorly characterized. For example, Davis and Namson (2002) 

depict this fault zone with a steep southwestern dip (about 

60°) in the upper 5 km of the crust. These authors indicate 

that, deeper than 7 km, the fault zone dips about 25° southwest. Similarly, Shaw (1999) used depth-converted seismicreflection sections to indicate that the Palos Verdes Fault Zone 

is steep to vertical in the upper 5 km of the crust, but below 

that depth, the fault is postulated to dip about 30° southwest. 

Shaw and Suppe (1996) tied movement along the Palos 

Verdes Fault Zone to deeper movement along the Compton 

thrust fault, which is blind and dips northeast. In opposition 

to these two interpretations, cross sections by Wright (1991) 

show the Palos Verdes Fault Zone as a vertical or steeply east 

dipping strike-slip fault, at least for depths shallower than 

about 9 km.

Nardin and Henyey (1978) propose that deformation of 

strata below the San Pedro Shelf is concentrated in zones that 

these authors referred to as anticlinoria. One anticlinorium 

deforms rocks under the Palos Verdes Peninsula, and another 

involves rocks that make up Lasuen Knoll, southeast of the 

San Pedro Shelf (fig. 1). A third anticlinorium underlies the 

western part of the San Pedro Shelf, along and west of the 

Palos Verdes Fault Zone. Formation of anticlinoria is attributed, in part, to convergent dextral shear along this fault zone.
> 5.0 km/s > 5.0 km/s

1

2

3

4

5

0 10 20 30 40

0

DE

PTH, IN KIL

O

METER

S DE

PTH, IN KIL

O

METER

S

Modified from ten Brink and others (2000)

Water

bottom

VERTICAL EXAGGERATION X 7

Probable Miocene volcanic and/or metamorphic rocks

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

V

6.0 ELOCITY, IN KIL

O

METER

S

PER 

SECO

N

D

V

ELOCITY, IN KIL

O

METER

S

PER 

SECO

N

D

LARSE 1 Wide-Angle Data

SW NE

Avalon Knoll

Palos Verdes

Fault Zone

San Pedro Basin

DISTANCE, IN KILOMETERS

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0.5

1.0

1.5

2.0

0.0

Water

bottom

LARSE 1 MCS Data

SW NE

0 20 30 40 10

Avalon

Knoll San Pedro Basin

VERTICAL EXAGGERATION X 10

DISTANCE, IN KILOMETERS

Figure 6. Tomographic velocities from wide-angle arrival times recorded during the LARSE 

experiment (top, after ten Brink and others, 2000) and derived from an analysis of arrivals 

recorded along the MCS streamer during the LARSE experiment (bottom).

Regional Geology 9
10 Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

Geophysical Data

Seismic-reflection Data

Multichannel seismic (MCS) reflection data used herein 

(fig. 2) were collected by the U.S. Geological Survey during three cruises (Normark and others, 1999a, b; Gutmacher 

and others, 2000). The seismic system included a 24-channel 

streamer with 10-m-long hydrophone sections. The resulting 

5-m common-depth-point (CDP) interval provides high spatial 

resolution. The sound source was a dual-chambered generator-injector airgun; each chamber had a volume of 573 cm3

(35 in3). The airgun was suspended from a float to maintain a 

constant source depth of 1 m, and it was fired every 12 s with 

an air pressure of 3,000 psi.

Seismic-reflection data were also collected using a highresolution Huntec system that included a boomer platform 

towed between 6 and 160 m below the sea surface. The frequency spectrum of the Huntec system spanned from 0.5 to 8 

kHz and peaked between 1.5 and 5 kHz.

 Deep-crustal seismic-reflection data were collected in 

1994 off Southern California for the Los Angeles Regional 

Seismic Experiment (LARSE; see fig. 2 for seismic line 

location) (Brocher and others, 1995). LARSE MCS data are 

40-fold and were collected aboard the R/V Maurice Ewing. 

The seismic source array consisted of 20 airguns having a total 

chamber volume of 137.7 L (8,470 in3

). The digital streamer 

was 4,200 m long and had 160 data channels. Results from the 

LARSE experiment are reported elsewhere (ten Brink and others, 2000; Fuis and others, 2001; Godfrey and others, 2002).

Rock-Velocity Data

Velocities used to convert seismic-reflection data to depth 

were obtained from sonic logs, a regional velocity model, 

and tomographic inversion of shallow arrivals in deep-crustal 

LARSE MCS data. Digitized sonic-log data (Suess and Shaw, 

1998) (fig. 7) are from wells, located in the southeastern 

part of the Los Angeles Basin, that penetrated Pliocene and 

Quaternary sedimentary rocks. Additional velocity data from 

wells drilled in the Los Angeles and San Pedro Harbors show 

the velocity variation in the upper 300 m of Pleistocene and 

Holocene sediment (D. Ponti, written commun., 2002). These 

wells are located east of the Newport-Inglewood Fault Zone, 

which is the boundary at depth between basement rock types. 

These wells, therefore, were drilled into rocks that may have 

accumulated and compacted at different rates from the rocks 

in our study area. Nonetheless, we use these sonic logs to infer 

velocity variation within Pliocene and younger rocks and sediment down to a maximum depth of 3 km. 

An average curve drawn through the sonic-log velocity data described above differs from a summary of regional 

rock-velocity data (Magistrale and others, 2000) (fig. 7). 

Deeper than 1 km, velocities from the regional compilation 

are substantially higher than the spread of values derived from 

sonic logs.

Other velocity values are derived from tomographic 

inversion of seismic-reflection data collected along the MCS 

streamer during the LARSE experiment in 1994 (fig. 6, bottom). These velocities are important because they are the only 

detailed information we have on the velocity of sound in rocks 

southwest of the Palos Verdes Fault Zone. When calculating tomographic velocities, we edited LARSE seismic data 

to remove low-quality recording channels and bad shots, and 

the first arrivals were picked for every shot to within 8 ms. 

First-arrival traveltimes numbered nearly 180,000, and all 

were entered into an iterative, two-dimensional tomographic 

inversion algorithm that is based on a finite-difference solution 

to the eikonal equation (Aldridge and Oldenburg, 1993). The 

vertical and horizontal spacing of the grid of velocity values 

was 12.5 m. In general, velocities for given depths obtained 

from tomographic analysis of LARSE data are higher than 

either the average derived from sonic logs or the regional 

curve of Magistrale and others (2000) (fig. 7). The higher 

velocities result mainly from the shallow depth of Miocene 

rocks southwest of the Palos Verdes Fault Zone.

Potential-Field Data

Offshore aeromagnetic data were obtained during a 

regional survey (Langenheim and others, 1993) flown along 

flightlines spaced 1.6 km apart. A magnetic high is located 

over the western San Pedro Shelf, where seismic lines 115 and 

200 intersect (fig. 8, top). To help delineate the outline of the 

magnetic source body (shown as a dashed line in both parts of 

fig. 8), a computer algorithm was used to locate the maximum 

horizontal gradient of the magnetic field.

Offshore gravity values (fig. 8, bottom) are from a 3-km 

grid of gravity measurements derived from scattered field data 

collected by the National Oceanic and Atmospheric Administration and the U.S. Geological Survey and reduced to isostatic 

gravity values. These gravity data are accurate to a few mGal, 

so they are suitable for regional but not detailed studies. These 

data reveal mainly density variations within the middle and 

upper crust.

Offshore Structure from 

Seismic-Reflection Data

For the purposes of discussion, the study area is divided 

into two structural belts, separated by the Palos Verdes Fault 

Zone, that extend southeastward along the western fringe of 

the Los Angeles Basin. The eastern belt includes the Wilmington Graben (Fischer and others, 1987), an elongate, nearshore 

basin confined between the Newport-Inglewood and Palos 

Verdes Fault Zones (fig. 1). (The term “Wilmington Graben” 

is somewhat misleading because the feature is bounded by 
strike-slip, not normal, faults. Even so, the name “Wilmington 

Graben” is used here because of its precedence in the literature and common usage.) The western structural belt includes 

variably deformed rocks that lie west of the Palos Verdes Fault 

Zone, mainly rocks under the western San Pedro Shelf and 

slope, as well as rocks and sediment filling the deep-water San 

Pedro Basin. These structural belts are discussed in order from 

northeast to southwest.

0

500

1,000

1,500

2,000

2,500

3,000

1,500 2,000 2,500 3,000 3,500 4,000 4,500

DE

PTH, IN

METER

S

ROCK VELOCITY, IN METERS PER SECOND 

Magistrale and others (2000)

Average velocity-depth

relationship for area

east of the Palos Verdes

 Fault

Rock velocity from west San Pedro

shelf and upper slope, from 

tomographic analysis of LARSE 1 

MCS data (fig. 6, bottom)

Structural Belt East of the Palos Verdes Fault 

Zone: The Wilmington Graben

The Newport-Inglewood and Palos Verdes Fault Zones 

bound the Wilmington Graben on the northeast and southwest, 

respectively, and the Thums-Huntington Beach Fault deforms 

rocks within the graben (fig. 9). Unfortunately, owing to environmental restrictions placed on seismic surveys off CaliforFigure 7. Velocity-depth function derived from digitized sonic logs from five wells and averaged 

(Suess and Shaw, 1998; D. Ponti, written commun., 2002). This velocity-depth function was used 

to convert time to depth for seismic-reflection data collected east of the Palos Verdes Fault Zone, 

over the Wilmington Graben, and under deep-water areas. Rock velocities from depths >300 m are 

from sonic logs from the Tidewater Oil Hellman Estates #1 and the Amerada Hess Lakewood Community #1 wells (see fig. 3 for locations). The thick solid line shows the velocity distribution derived 

from tomographic analysis of LARSE MCS data from near the shelf break. These higher velocities 

from near the shelf break reflect the shallow depth of Miocene rocks.

Offshore Structure from Seismic-Reflection Data 11
12 Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

0

0

5

5

10 10

15

15

20

20 25 30

100

80

80

80

60

60

60

40

40

20

20

200

300

300 400

400

400

500

500

6

500

400

300

200

100 100

200

600

700

800

900

800

0

700

600

500

200

700

500

1

500

?

Palos Verdes

Fault

Newport-Inglewood Fault

Thums-Huntington Beach Fault

Palos Verdes

Peninsula

Santa

Catalina

Island

San

 Pedro

 Shelf

Long

Beach

Newport

Beach

Huntington

Beach

San

Pedro

Basin

Lasuen

Knoll

Santa

Monica

Bay

Avalon Knoll

-60

-56

-52

-48

-44

-40

-36

-32

-28

-24

-20

-16

-12

 -8

 -4

 0

 4

 8

 12

 16

ACCELERATION OF GRAVITY, IN MILLIGALS

115

200

 10 km

118° 30' W 118° 15' 118° 00' 33° 45' N 33° 30'

Isostatic Gravity Data

100

80

80

80

60

60

60

40

40

20

20

200

300

300 400

400

400

500

500

6

500

400

300

200

100 100

200

600

700

800

900

800

0

700

600

500

200

700

500

1

500

?

Palos Verdes

Fault

Newport-Inglewood Fault

Thums-Huntington Beach Fault

Palos Verdes Peninsula

Santa

Catalina

Island

San

 Pedro

 Shelf

Long

Beach

Newport

Beach

Huntington

Beach

San

Pedro

Basin

Lasuen

Knoll

Santa

Monica

Bay

Avalon Knoll

Anomaly caused by oilfield

Anomaly caused by

 oilfield

-145

-130

-115

-100

 -85

 -70

 -55

 -40

 -25

 -10

 5

 20

 35

 50

 65

 80

 95

 110

 125

 140

MAGNETIC FLUX DENSITY, IN NANOTESLAS

115

200

 10 km

118° 30' W 118° 15' 118° 00' 33° 45' N 33° 30'

Extent of magnetic

source from analysis

of maximum horizontal

magnetic gradient

Explanation

Mohnian

Delmontian

Repettian

Sea-floor rocks

Basalt

Catalina Schist

Volcanic tuff

Biogenic rocks

Rocks on the Palos

Verdes Peninsula

Miocene Monterey

Formation

Basement

Bathymetry in meters;

contour interval 20 m to

100-m depth, 100 m at

greater depths

Aeromagnetic Data

Figure 8. Aeromagnetic anomaly map (top) and isostatic gravity map (bottom) of the study area. An aeromagnetic 

anomaly near the northwest corner of the Palos Verdes Peninsula lies near an onshore basalt outcrop. Gravity and magnetic models along seismic-reflection lines 200 and 115 are shown in figure 24. Seismic-reflection sections are shown in 

plate 2. Ages of sea-floor rocks are from Nardin and Henyey (1978). Onshore geology is from Woodring and others (1946).
nia, none of the seismic-reflection lines shown here crosses the 

Thums-Huntington Beach Fault. However, geologic sections 

published elsewhere show this fault’s structure (see section 

locations in fig. 3A). For example, Wright (1991) shows that 

the northeast side of this fault dropped down, forming the 

large Wilmington Anticline. Basement rocks below the graben 

are at depths between 2 and 3 km (fig. 5).

Rocks within the Wilmington Graben are only mildly 

deformed, except in the far western part of the graben, directly 

adjacent to the Palos Verdes Fault Zone (fig. 10, location 

shown in fig. 9). Seismic-reflection data indicate that very 

shallow strata within the graben are subhorizontal and overlie 

a shallow unconformity at a traveltime of about 0.2 s, but this 

relationship is largely obscured by water-bottom multiples. 

This unconformity truncates the tops of underlying beds that 

have an apparent southwest dip. The unconformity and the 

uppermost part of the section in the Wilmington Graben cross 

the Palos Verdes Fault Zone and are deformed by it, indicating 

relatively recent fault movement.

Down to a traveltime of about 0.8 s (about 800-m depth), 

a deltaic sequence progrades across the graben in an apparent 

southwestward direction. A reverse fault within the graben, on 

figure 10, below shotpoint (SP) 1500, offsets this unconformity 

but does not extend far upward through the overlying rocks. A 

third unconformity is evident on the right half of figure 10, at a 

traveltime of 1.5 s.

In the southeastern part of the study area, east of Lasuen 

Knoll, the Newport-Inglewood Fault and the Palos Verdes 

Fault Zone diverge, and the widening deep-water area between 

these faults has received extensive turbidite fill from channels 

that make up the Newport Canyon. Rock or coarse-grained 

sediment that produces strong reflections are at shallow depth 

(fig. 11, top). For example, below the northeast end of seismicreflection line 76, little-deformed basin fill is only about 500 

m (0.4 s) thick over flat, strongly reflective rocks. These rocks 

extend southwestward to an inflection point, where the rocks 

abruptly dip more steeply southwest into the basin. The appearance of the reflections below the very flat and distinct reflection on seismic section 76 is consistent with the interpretation 

that the distinct reflection marks the contact between turbidite 

sediment, perhaps of latest Pleistocene age, and underlying, 

more consolidated rock. Discontinuous reflections extend to 

traveltimes at least as great as 2.5 s (nearly 2 km below sea 

level), suggesting that the basin deepens sharply southwest of 

the inflection. The upper part of the basin fill is subhorizontal.

Seismic section 74 (fig. 11, bottom; location shown in fig. 

9) shows a flat, strong reflection under the western area of the 

basin. At an inflection point below SP 600, this strong reflecPalos Verdes Fault

Thums-Huntington Beach Fault

Newport-Inglewood Fault

?

Palos Verdes

Peninsula

Santa

Catalina

Island

San

 Pedro Shelf

Dana

Point

Long

Beach

Newport

Beach

Huntington

Beach

Laguna

Beach San

Pedro

 Basin

Avalon

Knoll

Lasuen

Knoll

118° 15' W 118° 00' W 117° 45'

33° 45' N

33° 30'

 15 km

Major fault

Small-airgun seismic trackline

510P1,2

504P1

505P1

506P1 507P1

508P1

509P1

511P1

Sediment core stations

Bathymetry in meters;

contour interval 20 m to 100-m depth,

100 m at greater depth

Figure 10

Figure 15

Figure 11, top

Figure 11, bottom

1000

1

2000

5000

4000 5000

2000 3000

4000

2000

1000

1

4000

3000

Figure 12 1000

1

Figure 16

Figure 18 Figure 17

Figure 21, top 4000

2000 1000

3000

2000

1

3000

6000 6000

7000

5000

Figure 21, bottom

Figure 22

3000

2000

Figure 22

Figure 23, bottom

Figure 23, top

Figure 13 3000 4000

Figure 9. Index map of detailed seismic sections over selected features below the San Pedro Shelf and Lasuen Knoll. Small 

numbers indicate shot points along seismic lines. Dots with alphanumeric indicators show sediment core locations. 

Offshore Structure from Seismic-Reflection Data 13
14 Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

Seismic Section 118

0

0.5

1.0

1.5

TWO-WAY TRAVELTIME, IN SECONDS

Palos Verdes

Fault Zone

SW NE 2000 1500 1000 500

2000 m

200 m VERTICAL EXAGGERATION

IN WATER X 4

Apparent southwestward progradation

in fan-delta sediment

Wilmington Graben

Unconformities

Shallow rocks Unconformity

extend across fault

Main fault

strand

VERTICAL AND HORIZONTAL SCALES

(Shotpoints)

0.5

1.5

2.5

TWO-WAY TRAVELTIME, IN SECONDS

TWO-WAY TRAVELTIME, IN SECONDS

2000 m

200 m VERTICAL EXAGGERATION

IN WATER X 2

Seismic Section 76

3000 3500 4000 SW NE

Inflection

West dips point Turbidite

West dips lobe Strongly reflective flat

Strongly reflective flat

0.5

1.5

2.5

SW NE 2000 1500 1000 500

Seismic Section 74

Inflection point Filled channels

West-dipping rocks East-dipping rocks

Westward progradation onto flat East dipping rocks in graben

Multiple

Multiple

Multiple Multiple

(Shotpoints)

(Shotpoints)

VERTICAL AND HORIZONTAL SCALES

Figure 10. Seismic section over the Palos Verdes Fault Zone and Wilmington Graben under the San Pedro Shelf. Section location is shown in figure 9. 

Figure 11. Seismic sections showing the geologic structure near Dana Point. Section locations are shown in figure 9. 
tion begins an eastward descent into the deep part of the basin. 

Rocks below the strong reflection return coherent reflections. 

Below SP 1500 on seismic section 74, reflections from below 

the strong one dip primarily west, whereas below SP 800, 

other deep reflections dip east. Because of these deep reflections, the strong, flat reflection is unlikely to be from the top 

of metamorphic basement; instead the strong reflection is 

more likely to be from an unconformity cut across sedimentary rocks. The opposing dips of these rocks outline a truncated anticline. The strong reflection may be from a wave-cut 

terrace cut along the top of deformed Miocene sedimentary 

rocks. Deep-basin deposits below SP 500 on section 74 have 

moderate apparent dip to the east, and shallower parts of the 

basin fill prograde in the opposite direction, southwestward 

across the flat strong reflector. 

Structural Variation Along the Palos Verdes 

Fault Zone

The Palos Verdes Fault Zone is the most prominent 

and arguably the most complicated structure revealed by the 

seismic-reflection data. It bounds folded shelf rocks, primarily 

of Miocene and Pliocene age, to the west (Nardin and Henyey, 

1978), whereas a thick (typically 1 km) section of mainly flatlying upper Pliocene and Quaternary rocks and sediment abuts 

the fault on the east (fig. 4).

Cross sections (Wright, 1991; Davis and Namson, 2002) 

derived primarily from oil-industry seismic-reflection data 

(locations shown in fig. 3A) indicate that at shallow depth (< 

1 km) under the San Pedro Shelf the Palos Verdes Fault Zone 

is nearly vertical, but that it curves downward to dip steeply 

west. This geometry is also evident in a stack of multichannel 

seismic-reflection data obtained along LARSE line 1 (fig. 5, 

top). Furthermore, a summary diagram made from industry 

seismic-reflection data (Shaw, 1999; fig. 5, bottom) shows not 

only that this fault zone dips west but also that rocks below 

about 1 km depth along the west side of the fault dip east and 

terminate against the fault plane.

Some elements of the fault structure just described are 

also evident in migrated time sections of small-airgun seismicreflection data. These sections reveal that at shallow depth (< 

1 km) under the northern San Pedro Shelf, the Palos Verdes 

Fault Zone dips steeply west or is vertical (see, for example, 

pl. 1, seismic-reflection sections 115 and 118). A more detailed 

rendition of seismic-reflection section 115 (fig. 12) shows that 

this fault zone is composed of at least three fault strands that 

are nearly vertical to traveltimes less than about 1 s. This section also shows that shallow strata from within the Wilmington 

Graben extend westward through the Palos Verdes Fault Zone, 

are offset by the fault strands, and appear to unconformably 

overlie Miocene and Pliocene rocks on the fault’s west side. 

Structures along and within the Palos Verdes Fault 

Zone reveal that the intensity of compressional deformation 

Seismic Section 115

SW 1000 500 NE

0

1

TWO-WAY TRAV

ELTIME, IN

SECO

N

D

S

0.5

1.5

1000 m

100 m

in water

only

Anticlinally deformed

Miocene and Pliocene rocks

Flat-lying Quaternary

sediment in the

Wilmington graben

Palos Verdes

Fault zone

 

Shallow (Quaternary?)

rocks dip east

above the truncated

Miocene section

(Shotpoints)

VERTICAL EXAGGERATION

IN WATER X 2

Figure 12. Detailed seismic section over the Palos Verdes Fault Zone, showing that this zone includes several fault strands and offsets 

shallow rocks. Section location is shown in figure 9. 

Offshore Structure from Seismic-Reflection Data 15
16 Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

decreases southeastward across the San Pedro Shelf. In particular, 

the anticline along the west side of the fault zone (pl. 1, seismic 

section 115) decreases in structural relief and broadens southeastward (pl. 1, seismic section 118). Southeast of the shelf, below the 

upper part of the slope, this anticline appears to die out entirely, as 

little trace of it appears on seismic section 84 (pl. 1). In fact, this 

seismic section shows that the faults along the projected location 

of the Palos Verdes Fault Zone all exhibit normal separation. Such 

faults are even more readily evident on seismic-reflection section 

66 (pl. 1 and fig. 13A), where the main normal fault dips east.

The main strands of the Palos Verdes Fault Zone have 

moved recently, as shown by high-resolution Huntec seismicreflection data. These strands exhibit normal separation, down 

on the east, and they offset or warp the sea floor (seismic section 66 on pl. 1, figs. 13A, B). Huntec seismic-reflection data 

reveal an angular discordance within sediment west of the main 

fault strands (fig. 13B). Parallel-bedded rocks underlying this 

discordance can be tentatively correlated with parallel-bedded 

sediment across the fault. No stratigraphic ages exist for this 

sediment, but were they available, they would give the onset 

time for the most recent phase of fault movement. Another 

small normal fault, down on the west, within the Palos Verdes 

Fault Zone emerges within a sea-floor depression, indicating 

recent fault movement and possible fluid flow (fig. 13C).

A bathymetric saddle, where the sea floor attains a depth 

as great as 500 m, separates the southeastern edge of the San 

A

0.0

2500 3000 3500 4000 (Shotpoints)

0.5

TWO-W 1.0

AY TRAVELTIME, IN SECONDS

Fig. 13d Fig. 13e Fig. 13b

Fig. 13c

Main strand of the

Palos Verdes Fault Zone,

a normal-separation

fault

W E

Seismic Section 66

Sea-floor

Ridge A

Unfaulted sediment covers fault that bounds ridge A

on the east

Numerous

small normal faults

Subsidiary

strand of

Palos Verdes

Fault Zone

Channel of

San Gabriel

Canyon

2000 m

200 m

in water

only VERTICAL EXAGGERATION

IN WATER X 2

Figure 13. Detailed seismic sections over the Palos Verdes Fault Zone showing recent activity along some fault strands. Location of 

section 66 shown in figure 9. A, Small-airgun section over sea-floor ridge A and the Palos Verdes Fault Zone, where this comprises several 

normal-separation faults. B, Huntec seismic-reflection section over the main, normal-separation fault strands that make up the Palos 

Verdes Fault Zone. These faults offset the sea floor. C, Huntec seismic-reflection section showing a depression in the sea floor over a strand 

of the Palos Verdes Fault Zone. Fluids might escape along this fault strand. D, Huntec seismic-reflection section showing small normal faults 

that offset strata making up the west flank of ridge A. These faults offset shallow sediment but not the sea floor. E, Huntec seismic-reflection 

section shows that a sharp angular discordance separates sediment bodies that cover the fault bounding ridge A on the east. This discordance is overlain by shallow strata that are essentially flat, indicating that vertical movement on the east bounding fault has ceased.

Pedro Shelf from the high-standing Lasuen Knoll (bathymetry 

shown in fig.1 and the index map on pl. 1). Oblique projections of multibeam bathymetric data obtained over the San 

Pedro Shelf and slope (figs. 14A, B) show the incised head of 

the San Gabriel Canyon at the southeastern edge of the San 

Pedro Shelf. This channel head is located where underlying 

rocks are deformed by normal faults that make up the Palos 

Verdes Fault Zone, suggesting that the extensional deformation helped determine the course of the channel’s upper reach. 

The San Gabriel Canyon crosses the Palos Verdes Fault Zone 

at a highly oblique angle, and its channel is not obviously 

deflected, either horizontally or vertically. The channel, however, does bifurcate near this crossing. One channel branch 

extends southeastward along the west scarp of the Lasuen 

Knoll, which lies along the Palos Verdes Fault Zone (Marlow 

and others, 2000). The other channel branch veers southwest 

across the floor of the San Pedro Basin. Numerous arcuate 

sea-floor scars along the San Gabriel channel over the lower 

slope might be channel meanders or slump scars.

The bathymetric saddle marks an important structural 

transition along the Palos Verdes Fault Zone, because southeastward through the saddle both the dip and the vergence 

direction of the fault zone change. The sequence of seismic 

sections 67, 81, and 80 (pl. 1) shows that near Lasuen Knoll 

this fault zone exhibits reverse separation and dips east, which 

is opposite to the west dip of the fault under the northern San 
Pedro Shelf. This east dip is apparent on seismic section 67, 

so the dip change of the Palos Verdes Fault Zone occurs under 

the bathymetric saddle and possibly under the zone of normal 

faults described above.

Seismic section 66 (pl. 1) shows that rocks within the 

Wilmington Graben flex upward along a short-radius fold that 

clearly does not include shallow faulting. However, faulting 

becomes progressively more apparent southeastward along 

the east boundary of the dipping rocks. For example, kink 

bands clearly evident on seismic section 67 (pl. 1 and fig. 15, 

below SP’s 4000 and 4300) reveal topography on the plane of 

an underlying, east-dipping thrust fault, which we interpret to 

be the Palos Verdes Fault Zone, that lies below the depth of 

penetration of our seismic-reflection data. 

Clear faulting along the east flank of the dipping rock 

section is shown below SP 1500 on seismic section 80 (pl. 1), 

where the dip change occurs along a west-dipping reverse fault 

that offsets very shallow sediment and perhaps the sea floor. The 

increased intensity of faulting along this zone to the southeast 

mirrors the heightened bathymetric expression of Lasuen Knoll.

A shallow unconformity that truncates east-dipping rocks 

in the Wilmington Graben (fig. 15) crosses the Palos Verdes 

0.50

0.55

E

TWO-WAY TRAVELTIME, IN

SECO

N

D

S

Ridge A Sharp dip discordance

Shallow, flat-lying sediment

Reflections

continuous

across deep fault

0.50

0.55

D

TWO-WAY TRAVELTIME, IN

SECO

N

D

S

Small normal faults

offset all but shallowest

sediment and seafloor

Ridge A 

0.55

0.60

C

TWO-WAY TRAVELTIME, IN

SECO

N

D

S

Sea-floor

depression

Shallow offset along

fault strand 0.50

0.55

B

TWO-WAY TRAVELTIME, IN

SECO

N

D

S

Main strands of the

Palos Verdes Fault Zone, showing sea-floor offset

Unconformity with

parallel beds below

500 m

10 m

in water

only

VERTICAL EXAGGERATION

IN WATER X 10.9

Fault Zone. Strongly but irregularly reflective strata reveal 

coarse deposits, probably channel-fill and (or) lobe deposits, within the San Gabriel turbidite system that overlies this 

unconformity. Complex reflections from these deposits foiled 

our attempt to determine whether or not the unconformity 

extends unbroken across the Palos Verdes Fault Zone. However, between SP’s 4000 and 4200 on figure 15, rocks under 

the unconformity are parallel bedded and dip east, whereas 

strata above the unconformity are parallel bedded and flat 

lying. These relationships suggest that along the Palos Verdes 

Fault Zone near the north part of Lasuen Knoll, eastward 

tilting ended before the erosion that cut the unconformity. Significantly, tilting has not resumed since the erosion, because 

the turbidite channels of the San Gabriel Canyon have not 

been shifted away from what was once the locus of uplift and 

tilting (fig. 15). The vertical component of deformation ended 

before the cutting of the unconformity and the later establishment of the modern turbidite system.

A somewhat different picture of the deformation history is revealed by seismic section 68 (fig. 16), which crosses 

the Palos Verdes Fault Zone close to Lausen Knoll (location 

shown in fig. 9). This seismic section shows a sharp sea-floor 

Figure 13—Continued.

Offshore Structure from Seismic-Reflection Data 17
18 Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

Main strand of the

Palos Verdes Fault

66:3000

84:3000

84:1800 84:1000

115:500

81:3100

69:5300

San Gabriel

Channel

San Gabriel

Channel

Miocene(?) rocks at shallow depth or truncated

at the seafloor (fig. 21, top; shotpoints 5000-6000)

Sea-floor

ridge B

(pl. 1)

Sea-floor

ridge A

(pl. 1)

Underlying rocks

extensionally deformed

Sharp scarp (fig. 18)

Transverse scarp

Lasuen Knoll

Palos Verdes Peninsula

San Pedro

Shelf

A

N

San Gabriel Canyon

Newport Canyon

San Gabriel Channel

Sharp scarp

(fig. 18)

Lasuen

Knoll

Miocene? rocks at shallow

depth or truncated at the

sea floor (fig. 21,

top; shotpoints 5000-6000)

Sea-floor ridge A (pl. 1)

Sea-floor ridge B (pl. 1)

Transverse

scarp

San

 Pedro

 Shelf

B

N

Figure 14. Oblique shaded-relief diagrams derived from multibeam bathymetric data (Gardner and Dartnell, 2002). A, Looking northwest across Lasuen Knoll and the San Pedro Shelf. Numbers give locations; for example, 84:1800 refers to seismic section 84, shot point 

1800 on seismic sections in plate 1. Dashed lines show interpreted faults and sea-floor ridges A and B. B, Looking east, parallel to the 

continental slope that bounds the San Pedro Shelf. The transverse scarp and sharp scarp that demark Lasuen Knoll have sharp corners 

and edges, indicating recent movement along what are probably underlying faults.
Seismic Section 67

0

1

2

TWO-WAY TRAV

ELTIME, IN

SECO

N

D

S

Main strand of the

Palos Verdes Fault Zone

2000 m

200 m VERTICAL EXAGGERATION

IN WATER X 2

W E 5000 4500 4000

San Gabriel turbidite channels

Unconformity

Parallel bedding

Kink band

Kink band

Multiple

Multiple

(Shotpoints)

VERTICAL AND HORIZONTAL SCALES

Seismic Section 68

0

1

2

TWO-WAY TRAVELTIME, IN SECONDS

North Part of Lasuen Knoll

Unconformity at

base of westwardthinning section

2000 m

200 m

in water

VERTICAL EXAGGERATION

IN WATER X 2

W E

2000 2500 3000 3500

Parallel deep

reflections

Faulting dies

out upward

Sea floor truncation

Reflection appearance

differs across fault

Palos Verdes

Fault Zone

Sharp scarp evident in multibeam

bathymetric data

(figs. 13A, B)

Anticline dies out upward,

barely arches unconformity

Possible minor anticline

dies out upward

Multiple

Multiple

(Shotpoints)

VERTICAL AND HORIZONTAL SCALES

Figure 15. Detailed seismic section over the Palos Verdes Fault Zone under the bathymetric saddle between the San 

Pedro Shelf and Lasuen Knoll. Section location is shown in figure 9. Recent vertical movement along the main fault 

strand seems to have been small, because the San Gabriel Canyon cuts into the fault’s upper plate. 

Figure 16. Detailed seismic section over the Palos Verdes Fault Zone and Lasuen Knoll, showing the stratigraphy and structure 

of the sharp scarp (see also fig. 14). Section location is shown in figure 9.

Offshore Structure from Seismic-Reflection Data 19
20 Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

scarp that is perched above and west of an anticline and an 

east-dipping fault below SP 2500. These structures lie along 

the Palos Verdes Fault Zone, and following Marlow and others 

(2000) we propose that they are the southeastward extension 

of this fault zone along the west flank of Lasuen Knoll. The 

sea-floor scarp, probably caused by vertical uplift along the 

Palos Verdes Fault Zone (fig. 16), is clearly evident in the 

oblique views of multibeam bathymetric data (figs. 14A, B, 

labeled “sharp scarp”). This scarp has sharp edges that contrast 

strongly with the rounded contours of the sea floor elsewhere. 

On this basis, the scarp appears to be young and indicates 

recent movement along the Palos Verdes Fault Zone. Taken 

together, seismic-reflection and multibeam bathymetric data 

show that this fault zone near Lasuen Knoll exhibits reverse 

separation and dips east.

An unconformity (indicated by an arrow below SP 3200 

on fig. 16) separates rocks below, which have nearly constant 

thickness, from sediments above, which thin markedly westward. Sediments just above the unconformity thin gradually 

westward, whereas beds higher up in the section thin more 

rapidly. The geometry of these strata indicates that broad eastward tilting of the upper plate of the Palos Verdes Fault Zone 

began shortly after the unconformity formed and was buried.

An anticline underlies the sea-floor scarp along the east 

side of the Palos Verdes Fault Zone (fig. 16, below SP 2500). 

This anticline dies out upward through the rock section, so 

that shallow rocks are barely arched. Likewise, a minor fault 

east of the anticline, below SP 2700, offsets deep basin rocks 

and extends upward through the unconformity but dies out just 

above the unconformity. An apparent minor anticline below SP 

3000 also dies out upward. All of these structures indicate a 

decreasing intensity of local folding and faulting through time 

along the Palos Verdes Fault Zone near the Lasuen Knoll.

East-dipping rocks under the crest of Lasuen Knoll are 

truncated at the sea floor. This truncation may have occurred 

subaerially, as suggested by a modified version of seismic section 69 (fig. 17). To make this figure, the seismic section was 

rotated 3° counter-clockwise so that discontinuous, flat parts 

of the knoll’s crest, as well as notches in the sea floor, could be 

connected by dashed horizontal lines. We propose that these 

lines connect wave-cut platforms and terraces. This interpretation is bolstered by the recovery of shallow benthic fauna 

from deep water along the knoll flank. A piston core sample 

obtained from 373-m water depth on Lasuen Knoll about 5 km 

north of the seismic section in figure 17 contains a shelf-edge 

benthic fauna (Mary McGann, written commun., 2003). This 

finding is consistent with the proposed terraces having been 

cut at sea level. The youth of the tilting derived here agrees 

with the point made above about the youth of the tilting of 

rocks along the west side of the Wilmington Graben.

A point worth noting is that the sharpness of the sea-floor 

scarp (figs. 14 and 16) indicates that the Palos Verdes Fault 

Zone was reactivated recently and is most likely responsible 

for the eastward tilting of rocks in the Wilmington Graben and 

Proposed ancient terraces

Normal

faults

Lasuen Knoll

Parallel to present sea level

Present sea level

1000 m

100 m

in water

only

VERTICAL EXAGGERATION

IN WATER X 5

3

o

Parallel to terraces

Seismic Section 69

TWO-WAY TRAVELTIM

E, 

IN

SECON

DS

0.0

0.5

Multiple

Multiple

3 actual rotation without

vertical exaggeration

o

Figure 17. Detailed seismic section from near the crest of Lasuen Knoll, showing inferred wave-cut terraces. The seismic section is 

vertically exaggerated and has been rotated counter-clockwise by 10°, which corresponds to a 3° rotation in natural coordinates, as 

annotated on the section. Section location is shown in figure 9.
under Lasuen Knoll. The style of deformation that involves 

broad eastward tilting differs from the earlier, local folding 

and faulting that preceded cutting of the unconformity.

Another, transverse scarp (labeled in figs. 14A, B) is 

nearly perpendicular to the Palos Verdes Fault Zone. This 

scarp also has sharp edges and merges with the beginning of 

the sharp scarp that for nearly 30 km extends along the foot of 

the Lasuen Knoll and marks the location of the Palos Verdes 

Fault Zone. We propose that a fault underlies the transverse 

scarp and that it played a key role in the recent uplift of 

Lasuen Knoll. 

A detailed view of the Palos Verdes Fault Zone near the 

north part of Lasuen Knoll (fig. 18) shows that this zone here 

includes at least four fault strands. The fault zone is about 1 

km wide, and deep rocks (1.5 s traveltime) on the fault’s west 

side are deformed anticlinally. The main, western fault strand 

is shown as vertical (pl. 1 and fig. 18), but complicated rock 

structure on both sides of this fault makes the interpretation 

ambiguous. This fault, however, might curve downward to 

dip west below the anticline. The right-hand part of seismic 

section 80 (pl. 1) shows rocks in the knoll truncated at the 

sea floor, as noted above for other parts of the knoll. Poorly 

reflective rocks make up the west part of the fault’s upper plate 

(fig. 18). This loss of reflectivity could be due to high strain 

along closely spaced faults. Another possibility is that the poor 

reflectivity indicates an ancient landslide. 

Southeast of the crest of Lasuen Knoll (seismic sections 

69 and 70 on pl. 1), deep rocks on the west side of the Palos 

Verdes Fault Zone are faulted and folded. Younger rocks onlap 

these deep, deformed rocks with sharp angularity. The dip of 

the Palos Verdes Fault Zone in the area of seismic sections 69 

and 70 is ambiguous but appears to be steep.

The Palos Verdes Fault Zone is difficult to trace southeast of Lasuen Knoll, partly because seismic lines are widely 

spaced, but also because the geologic structure is less distinct 

than farther northwest. Seismic section 213 (pl. 1), for 

example, shows only subdued sea-floor and structural relief, 

and reflections indicate several candidate features that could 

be the fault zone’s southeastward extension. On the basis 

of the eastward curve of bathymetric contours that outline 

Lasuen Knoll between seismic lines 70 and 213 (see index 

map on pl. 1), the Palos Verdes Fault Zone might intersect 

seismic-reflection line 213 between SP 5000 and SP 6000. 

Another possibility is that motion on the Palos Verdes Fault 

Zone is transferred onto several fault splays southeastward 

from the knoll.

Palos Verdes Fault Zone

0.5

1.0

1.5

2.0

TWO-WAY TRAVELTIME, IN

SECO

N

D

S

Sharp scarp

(fig. 14)

W E

1 km

200 m

in water

only

Seismic Section 80

San Gabriel

turbidite

channel

500

Base of poorly reflective rocks

Lasuen Knoll

Multiple

Multiple

VERTICAL EXAGGERATION

IN WATER X 2

Landslide

deposit?

Erosional truncation Erosional truncation

Anticline

(Shotpoints)

?

?

Figure 18. Detailed seismic section over the northwestern part of Lasuen Knoll, showing a scarp along the upper plate of the Palos 

Verdes Fault Zone and poorly reflective rocks that may be disrupted by closely spaced faulting or by a landslide. Section location is 

shown in figure 9. 

Offshore Structure from Seismic-Reflection Data 21
22 Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

Structural Belt West of the Palos Verdes 

Fault Zone

The western structural belt includes rocks west of the 

Palos Verdes Fault Zone and under the western San Pedro 

Shelf, as well as those below the slope and within the deepwater San Pedro Basin (fig. 1). The western belt contains 

much evidence for strong deformation and active tectonics. For 

example, the floor of the San Pedro Sea Valley, a deep notch 

cut southwestward into the shelf edge, is covered by debris 

from recent submarine landslides. Furthermore, the main geologic structures include a large anticline and syncline (figs. 19 

and 20) that border the Palos Verdes Fault Zone on the west.

The subshelf anticline grows in structural relief northwestward beneath the San Pedro Shelf. Projecting this 

anticline’s axis (fig. 19) northwestward indicates that the axis 

would traverse the area where Miocene and Pliocene rocks 

crop out at the sea floor (Nardin and Henyey, 1978) (fig. 19) 

In addition, the projected axis would merge with the trend of 

the high topographic elevation of the Palos Verdes Peninsula. 

The axis of the syncline under the west part of the San 

Pedro Shelf (fig. 19) can be extended northwestward along 

the axis of the San Pedro Sea Valley. This connection strongly 

suggests that the sea valley’s location was controlled by the 

synclinal structure of underlying rocks. The walls of this valley, especially the north wall, are known to have spawned one 

or more submarine landslides (Locat and Lee, 2002). A debris 

avalanche, 14 km long and partly filling the San Pedro Sea 

Valley, occurred about 7,500 years ago and is much larger than 

more recent failures (Normark and others, 2002). The dips 

of rocks along both sea valley walls are toward the colinear 

axes of the syncline and sea valley. This inward dip probably 

explains why landslides were more commonly shed here than 

elsewhere along the San Pedro margin. 

Seismic-reflection line 85 (fig. 20; location shown on 

figs. 2 and 9) provides a nearly perpendicular section through 

the anticline and syncline just described. Both structures are 

cut transversely by a swarm of normal faults that have offsets 

of less than about 100 m. These faults extend to within a short 

distance of the sea floor, but do not appear to offset it.

?

100

80

80

60

0

60

40

40

20

200

300

300

400

400

400

500

500

600

500

400

200 300

100

100

200

600

700

800

900

800

700 600

500

200

700

50

500

Ridge A (pl. 1)

Ridge B (pl. 1)

Palos Verdes Fault zone

Thums-Huntington Beach Fault

Cabrillo Fault

 10 km

118¡ 15’ W 118¡ 00’ 

33¡ 45’ N

33¡ 30’

Anticline

Syncline

Thrust or reverse fault

Normal fault

West limit of Miocene

basin (Henry, 1987)

Bathymetry in meters; 

 contour interval 20 m down to 100-m depth,

 100 m for greater depths

Palos Verdes Explanation

Peninsula

San

 Pedro

 Shelf

San

Pedro

Basin

Lasuen

Knoll

Avalon

Santa Knoll

Catalina

Island

Newport Canyon

San Pedro

Sea Valley

Sea-floor outcrop

Miocene and Pliocene

 rocks

Extent of magnetic source from analysis of maximum horizontal magnetic gradient

Lower-slope compressional zone (see, for example, fig. 22)

Area with closely spaced,

small-offset normal faults

Figure 19. Map summarizing the main known and inferred structures in the region of the San Pedro Shelf and Lasuen Knoll.
The origin of the swarm of normal faults is not obvious. Although the orientation of individual faults cannot be 

determined because of the coarse grid of seismic-reflection 

lines, as a group these faults affect an east-west trending zone 

below the shelf (fig. 19). This normal-fault zone has a peculiar orientation with respect to neighboring structures, in that 

it crosscuts the axes of the large anticline and syncline. Also, 

this zone in not obviously related to whatever rocks cause the 

western-shelf magnetic anomaly (fig. 8, top; causative body 

outlined in fig. 19). Olson (1974) describes potentially analogous structures—the Wilmington Anticline is cut transverse to 

strike by numerous small normal faults.

South of the San Pedro Shelf, an important category of 

fault lies west of and subparallel to the Palos Verdes Fault Zone. 

For example, the northeastward view of multibeam bathymetric data (fig. 14A) shows a sea-floor ridge, labeled “A,” on the 

slope west of the Palos Verdes Fault Zone. Seismic-reflection 

sections 84 and 66 (pl. 1) show that this ridge is underlain by 

an anticline and by a west-dipping fault and that the anticline 

decreases in amplitude and radius of folding northwestward. 

Bathymetric data (fig. 14A) show that sea floor ridge A, and 

presumably its underlying fold and fault, is convex to the east. 

This convexity is located directly west of where normal-separation faults make up the Palos Verdes Fault Zone (fig. 19). An 

important point is that the sea-floor ridge has clearest expression and the underlying anticline has maximum relief directly 

west of where the Palos Verdes Fault Zone comprises a series of 

faults with normal separation. This structural arrangement, best 

depicted in seismic section 66 (pl. 1), suggests that these faults 

and folds are connected within a displacement-transfer zone, but 

our data are insufficient to unravel this connection.

Seismic-reflection data indicate little or no vertical 

movement during the recent past along faults associated with 

sea-floor ridge A. Many small-offset normal faults cut rocks 

forming the west flank of the ridge (fig. 13A). A Huntec 

seismic-reflection section collected over these faults (fig. 

13D) shows them cutting shallow rocks and sediment but not 

the sea floor. In addition, the east side of the sea-floor ridge 

is bounded at depth by a reverse fault that appears to dip west 

(seismic section 66 on pl. 1; fig. 13A). Huntec seismic-reflection data over the buried fault reveal a sharp angular discordance within shallow strata that blanket the fault (fig. 13D). 

Shallow strata above the discordance and fault are horizontal 

and have not been tilted by vertical fault motion, and some 

dipping strata (for example at 0.54 s traveltime on fig. 13D) 

extend unbroken across the fault.

The discontinuous sea-floor ridge labeled “B,” revealed 

by bathymetric data (figs. 14A, B), is subparallel to and west 

of ridge A, and ridge B is also underlain by an anticline and 

west-dipping fault. Seismic section 84 (pl. 1) shows that rocks 

under ridge B are mainly deformed by an anticline. Section 65 

(fig. 21; location shown on fig. 9), however, crosses the ridge 

farther northwest, where faulting is more readily evident. A 

fault (below SP 5100 on fig. 21) is clearly expressed by the 

different appearance of reflections from the rock masses the 

fault separates. This seismic section shows that the fault dips 

steeply west, and this fault’s tip is covered by only a thin sediment drape, indicating relatively recent activity.

Rocks under the slope and west of ridge B dip west and are 

truncated at the sea floor along the narrow crest of the ridge (fig. 

21, seismic section 65). These rocks have the “Miocene” reflective appearance that was described above, in which the reflections are strong, parallel, uniform in appearance, and continuous 

over long distances. Multibeam bathymetric data (fig. 14A) show 

approximately where these rocks are truncated along the lower 

slope of the San Pedro margin. We propose that the sea-floor 

truncation of indurated, Miocene(?) rocks evident on seismic 

section 65 and on other nearby sections occurred subaerially. 

Figure 20. Detailed seismic section from the San Pedro Shelf, showing the large anticline and syncline that border the Palos Verdes 

Fault Zone (fig. 19). These structures are cut by numerous, small normal faults. Section location is shown in figure 9.

Seismic Section 85

0

1

2

TWO-WAY TRAVELTIME, IN SECONDS

Main strand

Palos Verdes

Fault Zone

2000 m

200 m

in water

only

VERTICAL EXAGGERATION

IN WATER X 2

W E

3000 2500 2000 1500

Syncline Anticline

Normal faulting ends at

the Palos Verdes Fault zone

Shallow sediments cross

Palos Verdes Fault Zone

(Shotpoints)

HORIZONTAL AND VERTICAL SCALES

Offshore Structure from Seismic-Reflection Data 23
24 Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

Rocks under the slope, between the shelf break and the 

flat sea floor over the San Pedro Basin, produce reflections 

that are continuous for about 2 km (fig. 21, at traveltimes 

between 1.0 and 1.5 s below SP 5700 in seismic section 65), 

but these rocks end abruptly to the east against a rock mass 

that returns only few reflections. One interpretation of this 

reflection geometry is that the abrupt end of reflections reveals 

buried relief of deep rocks, possibly metamorphic basement or 

Miocene volcanic rocks.

Some rocks under the slope have the “Miocene” reflective appearance and pinch out westward in an obvious wedge 

(fig. 21, seismic sections 65 and 37). The apex of the wedge 

probably marks the western limit of the basin that contained 

the Miocene rocks. This interpretation accords with Henry 

(1987), who shows the regional extent of the Miocene basin 

edge near the Palos Verdes Peninsula and San Pedro Shelf 

(reproduced in fig. 19).

The lower-slope compressional zone (fig. 19) includes 

complicated deformation of rocks that underlie the slope and a 

small part of the San Pedro Basin (fig. 22; location of seismic 

section is shown in fig. 9). This compressional zone is about 

8 km wide, and structural growth therein has produced a complicated stratigraphy with numerous unconformities and other 

bed truncations. Folds and thrust faults within the zone appear 

to belong to two generations. Some deep-seated faults, like 

the one below SP 3800 (fig. 22), appear to be inactive because 

they do not extend upward to deform rocks at or at near the 

sea floor. Other faults (for example, below SP’s 3000 and 3500 

in fig. 22) are part of a younger generation that arches the 

sea floor and deforms shallow sediment. Seismic-reflection 

data shown here lack the penetration to determine how these 

structures are related at depth, but we propose that they merge 

into a decollement.

A regionally flat sea floor at a depth of about 800 m 

characterizes the eastern part of the San Pedro Basin (fig. 19), 

which contains turbidite sediment and rocks that are little 

deformed except near the lower-slope compressional zone. 

Away from that zone, seismic section 86 (fig. 23, bottom) 

2000 m

200 m

in water

only

VERTICAL EXAGGERATION

IN WATER X 2

Seismic Section 65

Seismic Section 37

7500 7000 6500 6000 5500

6000 5500 5000 4500

Onlap Basement?

Onlap

Onlap

Wedging in deep rocks

(Pinch-out at margin of

Miocene(?)-Pliocene(?) basin)

Wedging in deep rocks.

(Pinch-out at margin of Miocene(?)-Pliocene(?)

basin)

Eastward onlap of

Pliocene and younger rocks

Eastward onlap of Pliocene(?)

and younger rocks

Sea-floor truncation

Miocene(?) rocks

Miocene(?) rocks

at shallow depth

or truncated at seafloor

0

1

2

0

1

2

TWO-WAY TRAVELTIME, IN SECONDS TWO-WAY TRAVELTIME, IN SECONDS

Sea-floor ridge B

(fig. 14a, b)

Sea-floor ridge B

(fig. 14A, B)

Abrupt end to reflections

Fault

Fault

Multiple

Multiple

Multiple

Multiple

Fault

(Shotpoints)

(Shotpoints)

W E

SW NE

Figure 21. Seismic sections over the slope west of the San Pedro Shelf, showing what may be Miocene(?) rocks at shallow 

subsurface depth or truncated at the sea floor, and the wedging of Miocene(?) rocks at the margin of the basin in which they 

were deposited. Locations of sections are shown in figure 9.
Seismic Section 115

SW NE

0

1

2

4000 3500 3000 2500

TWO-WAY TRAVELTIME, IN SECONDS

2000 m

200 m

in water

only

San Pedro Basin

Edge of San Pedro Shelf

Active

thrust fault Inactive

thrust fault

Multiple

Multiple

Stacked

unconformities

Onlap

Onlap

Active fault

Low-angle dip discordance;

unconformity or thrust fault?

Lower-slope compressional zone

VERTICAL EXAGGERATION

IN WATER X 2

(Shotpoints)

Active normal faults

Seismic Section 35 Palos Verdes

Peninsula

San Pedro Basin

San Pedro Basin

SW NE

Mound of poorly

reflective rocks

Seismic Section 86

Onlap interfaces

2000 1500 500

2500 3000 3500

2000 m

200 m

in water

only

Eastward onlap onto top

of Miocene(?)

W E

0

1

2

TWO-WAY TRAVELTIME, IN SECONDS TWO-WAY TRAVELTIME, IN SECONDS

0

1

2

Multiple

Multiple

Multiple

Channeled-lobe area of Redondo fan

Palos Verdes

debris avalanche

VERTICAL EXAGGERATION

IN WATER X 2

(Shotpoints)

(Shotpoints)

Figure 23. Seismic sections over the slope northwest of the San Pedro Shelf and the San Pedro Basin. 

The shallow part of the fill in the San Pedro Basin that is shown in seismic section 86 includes the Palos 

Verdes debris avalanche. Locations of sections are shown in figure 9.

Figure 22. Seismic section over the slope west of the San Pedro Shelf, showing the lower-slope compressional zone. Section location is shown in figure 9.

Offshore Structure from Seismic-Reflection Data 25
26 Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

shows normal faults that cut shallow sediment or the sea floor, 

particularly below SP 1500. These faults extend downward 

and offset the irregular top of a mound of poorly reflective 

rocks. Basin fill and rocks under the slope onlap this mound’s 

east and west flanks.

Seismic-reflection data collected along line 35 (fig. 23, 

top; location shown in fig. 9) suggested the possibility that the 

geology known from the Palos Verdes Peninsula (Woodring 

and others, 1946) extends into the offshore area. In general, 

a high level of low-frequency noise in these data prevents 

insightful interpretation of the deeper geology. Seismic section 

35, however, does show an unconformity that is at a traveltime of about 1.3 s below SP 3500. This unconformity dips 

southwest within or below the San Pedro Basin. Sedimentary 

fill overlying the unconformity is nearly horizontal, and dip 

discordance across the unconformity can be traced westward 

to about the center of this seismic section (SP 3000), where the 

unconformity’s reflection has a traveltime of about 1.8 s. We 

propose that Miocene rocks, like those exposed on the Palos 

Verdes Peninsula, underlie this unconformity. 

Discussion

Geology of the San Pedro Shelf

The regional shelf structure described above has been 

combined with other geological and geophysical information 

to describe the geology of the San Pedro Shelf, to a maximum 

depth of about 2 km. Two post-stack-migrated seismic sections 

converted from time to depth (pl. 2) formed the basis for modeling of aeromagnetic and gravity anomalies (figs. 8 and 24) 

to probe parts of the subsurface that are nonreflective. These 

results, when combined with stratigraphic information derived 

from sea-floor rock samples (Nardin and Henyey, 1978) and 

from drilling at the Beta oil field (Wright, 1991), yielded cross 

sections in depth, shown with only moderate (2:1) vertical 

exaggeration (pl. 2).

Both seismic sections in plate 2 cross the source of a 

large aeromagnetic anomaly (fig. 8; index map on pl. 2). In the 

area of this anomaly on both seismic sections, rocks are poorly 

reflective below about 0.5 km (pl. 2). Because of the absence 

of information in the seismic-reflection data about the deeper 

structure and rock types, we modeled aeromagnetic and gravity anomalies along these two sections.

Seismic-reflection data were used to constrain the models 

by placing the top of the magnetic source rock at the base of 

the shallow (< 0.5 km) reflective strata. Sedimentary rocks 

are usually weakly magnetic, and therefore the source of the 

magnetic anomaly is unlikely to be shallower than 0.5 km. 

Assuming reasonable magnetizations, which are based on 

measurements of a wide variety of rock samples, the resulting 

magnetic models (fig. 24) indicate that a large mass, as thick 

as 5.5 km, of moderately to strongly magnetic rock underlies 

the outer part of the San Pedro Shelf (pl. 2).

The most likely source for the magnetic anomaly is 

middle Miocene basalt. Susceptibility measurements as high 

as 0.0019 x 10-3 cgs units made on basalt samples indicate 

that these rocks are moderately to strongly magnetic. Catalina 

Schist, which forms the crystalline basement in this region, 

does not produce significant aeromagnetic anomalies where 

exposed on Catalina Island (fig. 8, top). Supporting the idea 

that the magnetic source is basalt, the other high-amplitude 

aeromagnetic anomaly in the study area, near the northern 

part of the Palos Verdes Peninsula, is adjacent to exposures 

of Middle Miocene basalt on the Palos Verdes Peninsula (see, 

for example, Woodring and others, 1946). The exposed basalts 

themselves do not produce large-amplitude anomalies, but a 

nearby well drilled within the high-amplitude, nearshore aeromagnetic anomaly encountered a minimum thickness of 1 km 

of basalt (Dibblee, 1999). This implies that a significant thickness of basalt is needed to produce an aeromagnetic anomaly 

of equivalent amplitude and areal extent as the anomaly 

traversed by the seismic sections on the outer San Pedro Shelf.

The gravity map and models indicate that the magnetic 

source rocks do not produce an associated gravity high. This 

lack of correspondence may be an artifact of the grid size of 

the gravity data (3 km), which may be too coarse to resolve 

such a localized mass. However, density measurements of 

Tertiary basalt near the Los Angeles Basin indicate an average 

density of only 2.58 g/cm3

 (29 samples), which is substantially less dense than the Catalina Schist but greater than our 

modeled density. Thus, the lack of an associated gravity high 

is most likely a result of the basalt’s low density. The modeled 

geometry of the basalt suggests that the source of the outershelf anomaly includes intrusive rocks, because the magnetic 

rock extends deeper into the subsurface than the depth extent 

of the adjacent sedimentary deposits.

The compiled seismic-reflection, potential-field, and 

stratigraphic data sets give considerable insight into the geology of the San Pedro Shelf (pl. 2). First, magnetic models 

indicate that the subshelf basalt is as thick as 5 km (fig. 24; in 

pl. 2, we chose to end the magnetic bodies downward at 3 km), 

although in modeling magnetic data, a tradeoff can be made 

between body magnetizaton and thickness. Even so, basalt 

probably makes up a substantial part of the thickness of the 

upper crust.

The second main result is the difference in rock deformation around the magnetic body. Stratified rocks of probable Miocene age that directly overlie the magnetic body are 

deformed by short-wavelength folds and are offset along small 

faults of undetermined dip and throw (see, for example, pl. 

2, section 115, SP 2200). In contrast, stratified Miocene and 

Pliocene sedimentary rocks east of the magnetic mass are 

deformed into the large anticline and syncline that underlie the 

northern part of the San Pedro Shelf (pl. 2, section 115; fig. 

19). The east flank of the magnetic body marks the juncture 

between the differently folded rocks. Seismic section 200 (pl. 

2) shows the same relationship among folded rocks, except 

for the complication that the style of folding changes abruptly 

along a fault east of the magnetic body (pl. 2, seismic section 
Metamorphic basement

(Catalina Schist)

Quaternary Basalt

sedimentary rocks

Pre-Quaternary

sedimentary rocks

Explanation

0 5 10 15 20

DISTANCE, IN KILOMETERS

6

5

4

3

2

1

0

D=2.07, S=0

D=2.42, S=0.0011

D=2.52, S=0.0012

D=2.37, S=0 D=2.32, S=0

PVF

D=2.42, S=0.0011

D=2.52, S=0.0012

D=2.70, S=0

Water: D = 1.0, S=0

NO VERTICAL

EXAGGERATION

DEPTH,

IN KILOMETERS

GRAVITY ANOMALY,

IN MILLIGALS

MAGNETIC ANOMALY,

IN NANOTESLAS

SEISMIC-REFLECTION LINE 115

Intersection with seismic line 200 -35

-30

-25

-20

-15

-10

-5

0

-50

0

50

100

Observed Calculated

SW NE

0 5 10 15 20 25 30

DISTANCE, IN KILOMETERS NO VERTICAL

EXAGGERATION

6

5

4

3

2

1

0

D=2.32, S=0

D=2.02, S=0

D=2.37, S=0

Intersection with seismic line 115

D=2.47

S=0.0012

D=2.37

S=0.001

PVF

D=2.70, S=0

Water: D = 1.0, S=0

DEPTH,

IN KILOMETERS

GRAVITY ANOMALY,

IN MILLIGALS

MAGNETIC ANOMALY,

IN NANOTESLAS

-50

-40

-30

-20

-10

0

-80

0

80

W SEISMIC-REFLECTION LINE 200 E

Could indicate

another shallow

basalt body Observed

Calculated

Figure 24. Magnetic and gravity models (2.5-dimensional) along seismic sections 115 (top) and 200 (bottom). D refers to the density of the body in g/cm3

; S is the susceptibility assigned to the body in 10-3 cgs 

units. PVF, Palos Verdes Fault Zone. Differences in physical properties assigned to the magnetic body (here 

labeled basalt) can be attributed to inadequate modeling of the three-dimensional shape of the source.

Discussion 27
28 Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

200, SP 3800). This fault is the northwestward continuation 

of the fault that forms sea-floor ridge A (for example, pl. 1, 

seismic section 66, SP 3300). The zones of different folding 

may be unrelated, in the sense that all folds may be relatively 

old and are now fortuitously juxtaposed across this fault (pl. 

2, seismic section 200, SP 3800). On the other hand, the zones 

may mean that the basalt(?) was more competent mechanically than the encasing sedimentary rocks and thereby exerted 

considerable influence on the style of deformation.

Following this vein of thought, there may be a genetic link 

among the mass of magnetic rocks, the lower-slope compressional zone, which extends along the southwest margin of the 

magnetic rocks (figs. 19, 22), and the puzzling east-west zone 

of numerous small normal faults, which lies along the east margin of the basalt(?) (figs. 19, 20). For example, the distribution 

of compressive and extensional faulting (fig. 19) could have 

resulted from clockwise rotation and perhaps westward translation of the basalt(?) relative to its encasing sedimentary rock.

Faults and Earthquake Hazards

Achieving a full understanding of the earthquake threat 

posed by offshore faults near Los Angeles requires that substantial interpretive differences be resolved that now persist 

among researchers. For example, Namson and Davis (1990) 

and Shaw and Suppe (1996), among others, assume that only 

limited strike-slip movement occurs along major faults; they 

postulate instead that mainly thrust or reverse movement 

occurs at depth along blind faults. Such faults are commonly 

expressed at Earth’s surface as broad folds and as discontinuous fault strands that may have styles of deformation and offset that are dissimilar to what occurs along the deeper, master 

faults. Using this faulting style, hypocenters 8 to 12 km deep 

are interpreted to lie along a downdip, southwestward projection of the Palos Verdes Fault Zone (Shaw, 1999); the aligned 

hypocenters show mainly reverse fault movement in this depth 

range. 

Other researchers, however, emphasize the importance of 

strike- or oblique-slip offset (for example Nardin and Henyey, 

1978; Wright, 1991; McNeilan and others, 1996; Francis and 

others, 1996; Bohannon and Geist, 1998). According to this 

view, movement along the Palos Verdes Fault Zone is mainly 

strike slip, and large positive structures have developed along 

it, possibly owing to restraining bends in the fault zone. 

The controversy about predominant slip mode along 

faults remains unresolved because data presented here do not 

reveal sufficiently deep structural levels to unravel the relationships among offshore faults. In our view, however, the 

diversity in the near-surface structural expression of the Palos 

Verdes Fault Zone is best explained by movement along a deep 

strike-slip fault.

Using seismic-reflection data presented here, the offshore 

stretch of the Palos Verdes Fault Zone can be divided into 

three segments on the basis of the fault zone’s structure in the 

upper 2 km of the crust. The northwestern segment underlies 

the San Pedro Shelf (fig. 19), where the shallow fault zone is 

vertical or dips steeply west. The middle fault-zone segment 

underlies the bathymetric saddle that separates the San Pedro 

Shelf from Lasuen Knoll. The fault zone in this segment is 

made up of east-dipping faults with normal separation. Near 

Lasuen Knoll, the southeastern segment of the Palos Verdes 

Fault Zone shows reverse separation and locally dips east at 

shallow angles, but along much of the knoll’s west scarp, the 

fault zone appears to dip steeply.

The northwest segment of the Palos Verdes Fault Zone 

evidently deforms young rocks and the sea floor (McNeilan 

and others, 1996; Clarke and others, 1998). Along this part of 

the fault zone, some high-resolution seismic-reflection data 

(Francis and others, 1996) reveal a restraining bend and attendant flower structure that coincides with a low sea-floor swale 

about 1-4 m high. This deformation apparently postdates submergence and erosion of the shelf during the past 10 k.y.

The middle segment of the Palos Verdes Fault Zone 

includes normal-separation faults under the slope that deepens southeastward from the San Pedro Shelf. These normal 

faults may have developed analogously to such faults in the 

Inglewood oil field, which is located along the strike-slip 

Newport-Inglewood fault. There, a central graben developed in 

the upper 1 km of the crust, at the crest of an anticline, where 

the main and underlying, strike-slip fault undergoes a minor 

(about 2°-5°) change in strike (figures 19 and 20 in Wright, 

1991). The normal-separation faults in the middle segment of 

the Palos Verdes Fault Zone probably formed within a releasing bend along a right-lateral strike-slip fault. 

The middle segment of the Palos Verdes Fault Zone ends 

abruptly between the San Pedro Shelf and Lasuen Knoll, and 

near this end the modern San Gabriel Canyon is incised into 

the upper plate of the Palos Verdes Fault Zone, close to the 

fault zone’s trace (fig. 15). Seismic-reflection data indicate 

that the channel has been in this position while the upper part 

of the sediment along the fault’s east side was being deposited 

over a prominent unconformity. These observations require 

that as the channel meandered laterally, movement along the 

Palos Verdes Fault Zone involved little or no vertical separation, otherwise the channel would have been diverted elsewhere. This limitation does not preclude pure strike-slip movement along the fault zone, but the reverse fault evident under 

the channel (fig. 15) has had little or no vertical movement.

Along the southeast segment of the Palos Verdes Fault 

Zone, sharp sea-floor scarps bordering Lasuen Knoll, possible 

tilted wave-cut terraces, and disrupted shallow sediment all 

attest to recent fault movement. Lasuen Knoll appears to be 

underlain by a pop-up structure, like those that develop along 

a restraining bend or stepover in a strike-slip fault system.

Probable wave-cut terraces and fresh fault scarps evident 

near Lasuen Knoll indicate a complex history of vertical motion 

that will hinder efforts to decipher recent fault offset. The shallow unconformity along the west side of the Palos Verdes Fault 

Zone at Lasuen Knoll (fig. 16) suggests a sequence of events 

that includes sediment deposition, uplift, erosion, and then subsidence. The sharp fault scarps bordering Lasuen Knoll indicate 

renewed uplift that postdates the final subsidence.
The Cabrillo Fault (fig. 19) extends at least as far offshore, southeast from the Palos Verdes Peninsula, as seismicreflection line 115, as indicated by Marlow and others (2000). 

In our seismic-reflection data, this fault lies along an obvious 

and abrupt change in the appearance of reflections from Miocene rocks (pl. 2, section 115, SP 800). The fault cannot be 

followed with certainty southeast of line 115.

A maze of reverse faults that form the lower-slope 

compressional zone (figs. 18 and 22) appears to be restricted 

to the area of the margin that lies west of the San Pedro Shelf. 

Whether or not these discontinuous faults connect with deeper, 

more extensive faults that would constitute an earthquake 

hazard cannot be determined using our data.

Offshore faults that parallel the Palos Verdes Fault Zone 

include those that developed along with sea-floor ridges A 

and B (pl. 1). Ridge A has its greatest structural development 

due west of where normal-separation faults make up the Palos 

Verdes Fault Zone. These faults are probably linked in the 

subsurface and must be considered as a whole when evaluating 

earthquake hazards.

Conclusions

1. Geological and geophysical data reveal the structural geology and earthquake hazards of the area just offshore 

from the Los Angeles urban region. Prominent structures 

include the Wilmington Graben, the Palos Verdes Fault 

Zone, and faults below the western part of the shelf and 

slope. 

2. In the shallow (<2 km) crust, the Palos Verdes Fault Zone is 

segmented into three parts. Under the San Pedro Shelf, 

the main fault strand in this zone dips west and is probably an oblique-slip fault. Southeast of the shelf, the fault 

zone comprises several normal-separation faults, most 

of which dip east. Near Lasuen Knoll, the Palos Verdes 

Fault Zone exhibits reverse separation and dips east at a 

low angle, and what appear to be fresh sea-floor scarps 

indicate recent fault movement. Such segmentation probably results from changes in geometry along a master 

strike-slip fault zone at depth. This segmentation could 

complicate efforts to apportion seismic hazard along 

this fault zone, because an earthquake rupture might not 

propagate across segment boundaries or the inferred, 

underlying fault bends.

3. Rocks under Lasuen Knoll and those under the Palos Verdes 

Peninsula are involved in concentrated deformation and 

uplift that may have occurred similarly. Both the relief of 

Lasuen Knoll and some secondary faults associated with 

this knoll terminate abruptly northward along a bathymetric scarp that is transverse to the Palos Verdes Fault Zone. 

In like fashion, the high relief of the Palos Verdes Peninsula ends abruptly along the south shore of Santa Monica 

Bay. Perhaps structures under the peninsula end along a 

transverse fault like the transverse one near Lasuen Knoll.

4. Seismic-reflection and multibeam bathymetric data reveal 

the extent of several other northwest-striking faults under 

the outer shelf and slope. These faults deform rocks 

into anticlines that have clear geomorphic expression as 

sea-floor ridges. Evidence for recent movement along 

these faults is equivocal, because at many locations, 

undated sediment just under the sea-floor appears to be 

undeformed, indicating the absence of very recent fault 

movement.

5. Significant tsunamis can result from major submarine 

landslides, and the debris from one or more landslides 

is evident along the axis of the San Pedro Sea Valley. 

This sea valley follows the axis of a large syncline under 

the San Pedro Shelf, so that synclinal rocks, of probable 

Miocene age, dip toward the axis of the sea valley. This 

indicates that more landslides, and hence tsunamis, could 

occur because the synclinal rocks dip favorably for the 

development of coherent slide masses.

6. The stratigraphy under the San Pedro Shelf was determined 

by combining depth-converted seismic-reflection data 

with rock information from oil wells and sea-floor 

samples. Results show that east of the Palos Verdes Fault 

Zone the Wilmington Graben contains thick (as much as 

1.5 km) Quaternary deposits, whereas west of the fault 

zone Miocene and Pliocene rocks are truncated at the 

sea floor. Rocks under the slope are assigned provisional 

Miocene and Pliocene ages on the basis of reflection 

appearance, because these slope rocks have a distinctive 

reflectivity in which reflections are parallel, uniform in 

amplitude, and continuous over long distances. Provisionally dated Miocene and Pliocene rocks under the 

slope terminate in the west in a wedge that probably 

marks the margin of the basin in which these rocks were 

deposited.

7. Modeling of aeromagnetic data reveals a large magnetic 

body under the western part of the San Pedro Shelf and 

the adjacent upper slope. This body is probably Miocene basalt. The basalt mass may have influenced the 

deformation of sedimentary rocks that encase the mass. 

Sedimentary rocks that overlie the basalt are deformed 

into short-wavelength, small-amplitude folds, whereas 

sedimentary rocks east of the basalt mass are deformed 

into folds that have much longer wavelengths and greater 

amplitude. This difference in style of folding may have 

resulted because the basalt was more competent during 

deformation than the sedimentary rocks.

Acknowledgments

We thank Gary Fuis, Daniel Ponti, Craig Nicholson, and 

Thomas Brocher for critical comments on this report. Christina Gutmacher, Florence Wong, and Carolyn Degnan assisted 

our effort greatly in data collection, processing, and display.

Conclusions 29
30 Marine Geology and Earthquake Hazards of the San Pedro Shelf Region, Southern California

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