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doi:10.1130/2009.2454(4.4)Geological Society of America Special Papers 2009;454; 271-290 Michael A. Fisher, Christopher C. Sorlien and Ray W. Sliter Barbara Channel south to Dana PointPotential earthquake faults offshore Southern California, from the eastern Santa Geological Society of America Special Papers

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271

The Geological Society of AmericaSpecial Paper 454

2009

Potential earthquake faults offshore Southern California, from the eastern Santa Barbara Channel south to Dana Point

Michael A. Fisher*U.S. Geological Survey, MS 999, 345 Middlefi eld Road, Menlo Park, California 94025, USA

Christopher C. SorlienInstitute of Crustal Studies, University of California, Santa Barbara, California 93106, USA

Ray W. SliterU.S. Geological Survey, MS 999, 345 Middlefi eld Road, Menlo Park, California 94025, USA

ABSTRACT

Urban areas in Southern California are at risk from major earthquakes, not only quakes generated by long-recognized onshore faults but also ones that occur along poorly understood offshore faults. We summarize recent research fi ndings concern-ing these lesser known faults. Research by the U.S. Geological Survey during the past fi ve years indicates that these faults from the eastern Santa Barbara Channel south to Dana Point pose a potential earthquake threat. Historical seismicity in this area indicates that, in general, offshore faults can unleash earthquakes having at least moderate (M 5–6) magnitude.

Estimating the earthquake hazard in Southern California is complicated by strain partitioning and by inheritance of structures from early tectonic episodes. The three main episodes are Mesozoic through early Miocene subduction, early Miocene crustal extension coeval with rotation of the Western Transverse Ranges, and Pliocene and younger transpression related to plate-boundary motion along the San Andreas Fault. Additional complication in the analysis of earthquake hazards derives from the partitioning of tectonic strain into strike-slip and thrust components along separate but kinematically related faults.

The eastern Santa Barbara Basin is deformed by large active reverse and thrust faults, and this area appears to be underlain regionally by the north-dipping Channel Islands thrust fault. These faults could produce moderate to strong earthquakes and destructive tsunamis. On the Malibu coast, earthquakes along offshore faults could have left-lateral-oblique focal mechanisms, and the Santa Monica Mountains thrust

*mfi [email protected]

Fisher, M.A., Sorlien, C.C., and Sliter, R.W., 2009, Potential earthquake faults offshore Southern California, from the eastern Santa Barbara Channel south to Dana Point, in Lee, H.J., and Normark, W.R., eds., Earth Science in the Urban Ocean: The Southern California Continental Borderland: Geological Society of America Special Paper 454, p. 271–290, doi: 10.1130/2009.2454(4.4). For permission to copy, contact [email protected]. ©2009 The Geological Society of America. All rights reserved.

on June 12, 2014specialpapers.gsapubs.orgDownloaded from


272 Fisher et al.

INTRODUCTION

Urban Southern California faces an elevated risk for societal disruption by major earthquakes because the region combines high population density and economically critical infrastructure with a vigorously deforming Earth. Recent earthquakes, notably those at Whittier Narrows (1987, M

L 5.9; Hauksson and Jones,

1989; Lin and Stein, 1989) and at Northridge (1994, Mw 6.7; Hauksson et al., 1995), underscored the hazard and the eco-nomic threat posed by the numerous faults known to crosscut the cityscape in Southern California. For example, an M ~7 earth-quake along the Puente Hills blind thrust fault, which lies directly below the city of Los Angeles, could cause an economic loss of as much as $250 billion and a human toll numbering in the thou-sands (Field et al., 2005).

We summarize research fi ndings, by the U.S. Geological Survey (USGS) during the years 2000–2005, concerning offshore faults that pose a potential earthquake threat to urban areas of Southern California. The area encompassed by this report extends from the eastern Santa Barbara Channel south to Dana Point (Fig. 1). A similar summary for Dana Point to San Diego is given by Ryan et al. (this volume). Compilations of Southern California seismicity (Astiz and Shearer, 2000; Richards-Dinger and Shearer, 2000; Hauksson and Shearer, 2005; Shearer et al., 2005; Kagan et al., 2006) and Quaternary fault databases (e.g., U.S. Geologi-cal Survey, 2006) show the geographic distribution of potential earthquake sources (Fisher et al., this volume, Chapter 4.2, Fig. 2). For example, earthquakes in 1979 (M

L 5.2) and 1989 (M

L 5.0)

occurred ~12 km beneath Santa Monica Bay (Hauksson and Saldivar, 1989; Hauksson, 1990). The largest historic earthquake in coastal Southern California was the 1933 Long Beach event (M

L 6.4), which struck along the Newport-Inglewood Fault (Bar-

rows, 1974; Hauksson and Gross, 1991).Some offshore faults within ~50 km of the coast remain

poorly understood and are sometimes omitted from models of tectonic evolution and estimates of seismic hazard. However, some of these faults are more than 100 km long, and accord-ing to an empirical relationship between an earthquake’s rupture length and magnitude (Wells and Coppersmith, 1994), future earthquakes in Southern California larger than the moderate ones recorded are possible. Hence, coastal and offshore faults merit close scrutiny; they are the focus of this chapter. We describe them in geographic order from the eastern Santa Barbara Chan-nel to near Los Angeles.

EASTERN SANTA BARBARA CHANNEL

The Ventura and Santa Barbara Basins lie within the east-west Transverse Ranges Province (Fig. 1), which cuts perpen-dicularly across the regional, north and northwest structural grain of major faults and mountain ranges found elsewhere in Southern California. The province ends to the south along the south fl ank of the Santa Monica Mountains and their westward extension along the northern Channel Islands. These mountains and islands are thought to be uplifted along deep-seated, north-dipping thrust faults (e.g., Bailey and Jahns, 1954; Davis and Namson, 1994a; Shaw and Suppe, 1994; Seeber and Sorlien, 2000). These faults extend east-west for more than 200 km and form the province boundary between the Western Transverse Ranges Province and the California Continental Borderland (Wright, 1991; Crouch and Suppe, 1993; Bohannon and Geist, 1998). Since the Miocene, the Ventura and Santa Barbara Basins have undergone strong north-south compression (Yeats et al., 1988; Yeats and Huftile, 1995; Sylvester and Brown, 1997; Sorlien et al., 2000). Global posi-tioning system (GPS) data revealed a high rate (~6 mm/yr) of north-south shortening across the onshore Ventura Basin (Don-nellan et al., 1993; Argus et al., 2005).

Despite decades of research into the geology of the Santa Barbara Basin by oil-company and academic geologists, the extent, age, and geometry of offshore faults are still controversial. Is crustal deformation thick or thin skinned (e.g., Yeats, 1998)? Do major thrust faults that appear to control the main aspects of basin structure dip north or south (e.g., Shaw and Suppe, 1994; Huftile and Yeats, 1995)?

Faults below the northern part of the Santa Barbara Chan-nel, including the North Channel, Pitas Point, and Red Mountain, illustrate this controversy. These faults are members of a system that in aggregate extends for ~100 km east-west through the Ven-tura and Santa Barbara Basins (Redin et al., 1998; Kamerling et al., 2001). To the east, this south-verging fault system merges with the San Cayetano and Santa Susana Faults. In one interpretation of the structure, a balanced section through the eastern Santa Bar-bara Channel indicates a north-dipping décollement at 5–10 km depth, and complicated folding of rocks above the décollement occurs along north-dipping thrust faults (Novoa, 1998). Another structural section through the same general area as the one just mentioned shows reverse faults below the northern part of the channel (Redin et al., 1998) (Figs. 2 and 3). Davis and Namson (1994a) provided two north-south geologic sections, 10 km apart,

fault, which underlies the oblique faults, could give rise to large (M ~7) earthquakes. Offshore faults near Santa Monica Bay and the San Pedro shelf are likely to produce both strike-slip and thrust earthquakes along northwest-striking faults. In all areas, transverse structures, such as lateral ramps and tear faults, which crosscut the main faults, could segment earthquake rupture zones.



Potential earthquake faults offshore Southern California 273

40 k

m

34 33

121

120

119

118

117

CIT

F

SPBF

NIF

PVFS

RIF

SC

IF

OR

F

PP

-RM

-NC

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F DF

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MT

F

Pro

vinc

e-bo

undi

ngfa

ult s

yste

mLo

ng B

each

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tura

SB

LA

New

port

Bea

ch

Oce

ansi

de San

Die

go

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a P

oint

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ico

Uni

ted

Sta

tes

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nt C

once

ptio

n

Pt.

Dum

e

Pal

os V

erde

s P

enin

sula

San

ta M

onic

a B

asin

San

Pedr

o Ba

sin

Santa Cruz B

asin

San

ta

B

arba

ra

B

asin

Santa Rosa --

Corte

s R

idge

S

anta C

ruz

- Cat

alin

a R

idge

Cat

alin

a Ba

sin

San N

ichol

as B

asin

Patton Ridge

Patton Escarpment

Corte

s B

ank

San Diego Trough

Coronado Bank

San

Cle

men

te I.

San

taC

atal

ina

I.S

an N

icho

las

I.

San

taB

arba

ra I.

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Mig

uel I

.S

anta

Cru

z I.

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taR

osa

I.

Ana

capa

I.

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A

na

Mou

ntain

s

San

ta M

onic

a M

ount

ains

SM

B

San

Cle

men

te B

asin

San

Ped

roS

helf

Tann

erB

ank

Nor

ther

n C

hann

el

Isla

nds

WEF

WT

RP

WT

RP

CC

BC

CB

Vent

ura

Bas

in

Fig

ure

Are

a

0-10

0

100-

500

500-

1000

1000

-150

0

1500

-200

0

2000

-250

0

2500

-300

0

3000

-350

0

3500

-400

0

Fig

. 2

Fig

. 6

Fig

. 9

Fig

. 11

Figu

re 1

. Reg

iona

l bat

hym

etry

of t

he S

outh

ern

Cal

ifor

nia

offs

hore

. Box

es o

utlin

e fo

ur a

reas

dis

cuss

ed in

the

text

. Red

line

s sh

ow fa

ults

(Jen

ning

s et

al.,

200

0; S

orlie

n et

al.,

200

0). A

bbre

via-

tions

: CC

B—

Cal

ifor

nia

Con

tinen

tal B

orde

rlan

d; L

A—

Los

Ang

eles

; DF—

Dum

e Fa

ult;

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—N

ewpo

rt-I

ngle

woo

d Fa

ult;

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F—M

alib

u C

oast

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lt; O

RF—

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ge F

ault;

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los

Ver

des

Faul

t; PP

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-NC

F—Pi

tas

Poin

t–R

ed M

ount

ain–

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th C

hann

el F

ault;

SA

F—Sa

n A

ndre

as F

ault;

SB

—Sa

nta

Bar

bara

; SC

IF—

Sant

a C

ruz

Isla

nd F

ault;

SPB

F—Sa

n Pe

dro

Bas

in

Faul

t; SR

IF—

Sant

a R

osa

Isla

nd F

ault;

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B—

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a M

onic

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ay; W

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ttier

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ult;

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tern

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nsve

rse

Ran

ges

Prov

ince

.



274 Fisher et al.

which suggest strong along-strike structural complexity. Both sections show a fl at or shallowly north-dipping décollement at ~9 km depth. In the eastern section, however, rocks above the décollement are deformed along south-dipping thrust faults, whereas the western section shows the opposite vergence along north-dipping thrust faults that are blind above ~4 km depth.

Seismic-refl ection data available to us indicate that the Pitas Point and Red Mountain Faults dip moderately (40°–60°) north (Figs. 4A and 4B). The relationship between the Pitas Point thrust fault and faults in the Red Mountain fault zone to the north is not clearly evident in these seismic-refl ection data, but the faults probably merge downward to the north. Other researchers (Sor-lien and Kamerling, 2000; Kamerling et al., 2001, 2003) have proposed that although the fault system includes many splays,

the overall system dips 40°–50° north. Near the town of Car-pinteria, the Red Mountain Fault splays into two main branches (Fig. 3), and displacement is transferred between these branches by northeast-striking cross faults (Gurrola and Kamerling, 1996; Kamerling et al., 2001, 2003). The cross faults are thought to be important to the analysis of earthquake hazards because they could delimit earthquake-rupture segments (Gurrola and Kamer-ling, 1996; Kamerling and Gurrola, 1997).

The Oak Ridge Fault forms the southern boundary of the Ventura Basin (e.g., Yeats, 1988; Yeats et al., 1988; Sorlien et al., 2000). This fault is thought to pose a substantial earthquake haz-ard, because not only does it appear to be the westward continu-ation of the fault system responsible for the 1994 M 6.7 North-ridge earthquake (Yeats and Huftile, 1995), but also the fault

-500

0

500

600

500

400

300200

200

200100

100

100

10 km

-120˚ 00’

34˚ 30’

34˚ 00’

-119˚ 30’

PPF

RMF

ORF

Surface traceOak Ridge fault

Redin98

Santa Cruz I.

SantaRosa I.

Santa Barbara Channel

Santa Barbara

Ventura

Santa Cruz-Catalina Ridge

SRIFSCIF

Fig. 4AFig. 4B

Fig. 5

A B C

Fig. 3

Figure 2. Location of a cross section and seismic lines through eastern Santa Barbara channel. High-resolution seismic lines in Figure 5 are shown as short lines across the Oak Ridge Fault west of Ventura. The dashed black line shows the surface trace of the Oak Ridge Fault. Bathymetry is in meters. Faults in Santa Barbara Channel are from Heck (1998); the locations are where the faults cut the top of the Monterey Formation. Faults south of the Santa Cruz Island are from Sorlien et al. (2000); locations show surface fault traces. Abbreviations: ORF—Oak Ridge Fault; PPF—Pitas Point Fault; RMF—Red Mountain Fault; SCIF—Santa Cruz Island Fault; SRIF—Santa Rosa Island Fault.




Depth (km)0 2 4 6 8

San

ta C

ruz

Isla

ndO

RF

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FP

PF

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F

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ta B

arba

ra C

hann

elN

S

Ada

pted

from

Red

in e

t al.

(199

8)

Geo

logi

c ex

plan

atio

n

Plio

-Ple

isto

cene

and

Rec

ent

Late

Plio

cene

(P

ico

For

mat

ion)

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Mio

cene

to e

arly

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cene

(R

epet

to F

orm

atio

n)

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cene

(S

isqu

oc F

orm

atio

n)

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dle

to la

te M

ioce

ne (

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tere

y F

orm

atio

n)

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ly M

ioce

ne (

San

Ono

fre

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ccia

(es

p. s

outh

), T

opan

gaF

orm

atio

n an

d R

inco

n S

hale

(es

p. n

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))

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e (S

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ion

and

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uero

s S

ands

tone

)

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to la

te E

ocen

e (C

ozy

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l, S

acat

e an

d C

lear

wat

er F

orm

atio

ns)

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dle

Mio

cene

(S

anta

Cru

z Is

land

Vol

cani

cs (

sou

th))

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ly a

nd m

iddl

e E

ocen

e (M

antil

ija a

nd A

nita

For

mat

ions

)

Late

Cre

tace

ous

(Jal

ama

For

mat

ion)

Mes

ozoi

c (m

etam

orph

ic a

nd g

rani

tic b

asem

ent)

10 k

m

VE

2

Figu

re 3

. Geo

logi

c cr

oss

sect

ion

thro

ugh

east

ern

Sant

a B

arba

ra C

hann

el s

how

ing

the

Pita

s Po

int R

ed M

ount

ain

Faul

ts, w

hich

mer

ge d

ownw

ard

to th

e no

rth,

as

wel

l as

the

Oak

Rid

ge F

ault,

w

hich

dip

s so

uth.

Sec

tion

loca

tion

show

n in

Fig

ure

2. A

bbre

viat

ions

: OR

F—O

ak R

idge

Fau

lt; P

PF—

Pita

s Po

int F

ault;

RM

F—R

ed M

ount

ain

Faul

t; V

.E.—

Ver

tical

exa

gger

atio

n; W

EF—

Wor

ld’s

End

Fau

lt.



276 Fisher et al.

6

SectionOffsetA Santa Barbara Basin

0

1

2

3

4

5

Two-

way

trav

el ti

me

(s)

Oak Ridge FaultPitas Point Fault

5 km

VE = 5.7 in water

0

1

2

3

4

5

Two-

way

trav

el ti

me

(s)

BOak Ridge

Fault Pitas PointFaultSanta Barbara Basin

5 km

VE = 5.7 in water

Figure 4. (A) Migrated multichannel seismic section across the Pitas Point and related faults and the Oak Ridge Fault in eastern Santa Barbara channel. (B) Migrated seismic section showing the same faults south of Santa Barbara. Line locations shown in Figure 2.




extends along strike for ~130 km and deforms Holocene rocks. The fault extends east-west and crosses the coastline near Ventura (Fig. 2). Shaw and Suppe (1994), using offshore seismic-refl ection data, proposed that this “fault” is actually an active kink band developed above a ramp in the north-dipping Channel Islands thrust fault (but see also Shaw et al., 1996; Stone, 1996). Others, however, maintain that the Oak Ridge is a major south-dipping reverse fault that has been inactive for the past 500 ka (Huft-ile and Yeats, 1995), or that this fault or splays of it have been recently active (Kamerling and Nicholson, 1996; Sorlien et al., 2000; Fisher et al., 2005a). Cumulative offset and slip-rate vary signifi cantly along the Oak Ridge Fault. In the east, fault dis-placement since 500 ka ago amounts to 2.5 km and occurred at high rates (5 mm/yr; Huftile and Yeats, 1995). In the west, how-ever, fault displacement occurred during the middle Pleistocene, but since 500 ka ago, this displacement has been only 1–2 mm/yr (Yeats et al., 1988; Huftile and Yeats, 1995; Azor et al., 2002).

High-resolution, seismic-refl ection data show that offshore near Ventura, the main strand of the Oak Ridge reverse fault extends upward to within ~80 m of the seafl oor (Figs. 5A and 5B). Consequently, movement along the nearshore part of this fault continued until sometime during the late Pleistocene or early Holocene (Fisher et al., 2005a). Fisher et al. (2005a) posit two generations of the Oak Ridge Fault—the older is the main basin-bounding reverse fault, which is beveled by an unconformity possibly at the base of late Pleistocene and Holocene sediment (Fig. 5B). This fault shows no evidence for Holocene movement. The younger fault generation includes possible left-slip along a vertical fault that is indicated in seismic-refl ection data by verti-cally aligned infl ections (Figs. 5B and 5C). This possible fault may be related to ones involved in the formation of the Montalvo mounds, which are pressure ridges, located over the Oak Ridge Fault, that indicate recent left or left-oblique slip on one or more shallow faults (Hall, 1982).

THE NORTHERN CHANNEL ISLANDS AND MALIBU COAST

The Dume, Santa Monica, Malibu Coast, and related faults are part of the regional fault system that forms the tectonic boundary between the Western Transverse Ranges Province, on the north, and the California Continental Borderland, on the south (e.g., Wright, 1991; Crouch and Suppe, 1993; Dolan et al., 1995) (Fig. 1). This fault system extends for ~200 km, from near the city of Los Angeles in the east to the far western end of the northern Channel Islands. The province-bounding fault system poses a signifi cant earthquake threat because of its length, recent activity, and the proximity of the fault’s eastern end to the city of Los Angeles. For example, Dolan et al. (2000) estimated that the Malibu Coast and the Santa Monica Faults, members of the regional fault system, could each unleash earthquakes as large as Mw ~7. To date, the Point Mugu earthquake (Mw = 5.3; Ells-worth et al., 1973; Stierman and Ellsworth, 1976) is the largest one to have struck near the fault system west of Point Dume

(Fig. 1). The main shock was caused by left-lateral-reverse slip along a north-dipping fault plane.

The main uncertainties in understanding fault deformation along the regional fault system concern the Channel Islands thrust fault (Shaw and Suppe, 1994; Seeber and Sorlien, 2000), which is proposed to dip north below Anacapa Island and the other islands farther west as well as below the Santa Barbara Basin. This thrust fault is similar in structural position to the Santa Monica Moun-tains thrust fault, east of Point Dume, which dips north beneath the Santa Monica Mountains (Bailey and Jahns, 1954; Davis et al., 1989; Davis and Namson, 1994b). Whether the Channel Islands and Santa Monica Mountains thrust faults link along strike is uncertain. If they do, the throughgoing thrust fault could unleash a larger earthquake than would the two separate faults. However, Shaw and Suppe (1994) propose that the faults do not connect, that east of Anacapa Island the Channel Islands thrust fault strikes northeastward across the Ventura Basin.

Rocks exposed on the northern Channel Islands are deformed by the left-lateral, strike-slip Santa Cruz Island and Santa Rosa Island Faults (Pinter et al., 1998a, 1998b, 2001). These faults were active during the late Quaternary, as shown by defl ected stream channels and offset terraces (Pinter and Sorlien, 1991). One structural interpretation is that the strike-slip faults are confi ned to the hanging wall of the underlying Channel Islands thrust fault and that they terminate downward into this thrust fault (Pinter et al., 2003). Together the thrust and strike-slip faults accommodate partitioned crustal strain (e.g., Pinter et al., 1998a), so both thrust and strike-slip earthquake are pos-sible. A major thrust earthquake could generate a tsunami that threatens the populated coast along the north side of the Santa Barbara Channel (Borrero et al., 2001).

East of the Channel Islands, in the Santa Monica Moun-tains, the Malibu Coast and Santa Monica Faults are members of the province-bounding fault system and were active dur-ing the Holocene as left-lateral-reverse faults (Dibblee, 1982; Drumm, 1992; Dolan et al., 2000). The probable offshore extension of the Malibu Coast Fault dips steeply in its upper part, but at depth, it fl attens and dips shallowly north (Fisher et al., 2005b; Sorlien et al., 2006).

The Dume Fault dips north and is the main fault along this part of the coast, in the sense that other faults merge downward into it. Sorlien et al. (2006) propose that the Santa Monica basin, located south of these faults, subsided by 4 km since 5 Ma ago and that this rate of subsidence might continue today. Combining this subsidence with the rate of uplift of the Santa Monica Moun-tains indicates that substantial slip occurs along a blind thrust fault under the mountains.

West of Point Dume, the fl at-topped Sycamore knoll marks an abrupt along-strike discontinuity in the structure of the regional fault zone that encompasses the Dume and Malibu Coast Faults (Fig. 6). Just 2 km west of the knoll, the structural relief near the tip of the Dume Fault decreases abruptly. Compare seismic sections WG85-394 and WG85-390 (Figs. 7A and 7B). Sorlien et al. (2006) explain the anomalous fault structure near the knoll



278 Fisher et al.

No

vert

ical

sep

arat

ion

of u

ncon

form

ityab

ove

faul

t tip

Infle

ctio

n

0.1

0.0

0.2

0.3

Two-way traveltime (s)

Hun

tec

min

ispa

rker

sec

tion

1854

SS

EN

NW

Oak

Rid

ge F

ault

500

mS

ea-s

urfa

ce e

cho

VE

~6.

1

Oak

Rid

ge fa

ult

Diff

ract

ions

Infle

ctio

n

0.50

0.250

0.75

Two-way travel time (s)

17 in

3 ai

rgun

sec

tion

811

No

vert

ical

sep

arat

ion

of u

ncon

form

ityab

ove

faul

t tip

SS

EN

NW

500

mV

E ~

4.8

0.050.0

Chi

rp s

ectio

n 81

1

Wat

er-c

olum

n ga

s


Infle

ctio

n

NN

WS

SE

500

m

VE

~ 1

2

Oak

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ge F

ault

AB

C

Figu

re

5.

Stac

ked

high

-res

olut

ion

seis

mic

se

ctio

ns

acro

ss th

e O

ak R

idge

Fau

lt w

est o

f Ven

tura

. Sec

tion

lo-

catio

ns s

how

n in

Fig

ure

2. V

.E.—

Ver

tical

exa

gger

atio

n.




11

2

3

4

56

7

8

WG

85 3

90

WG

85 3

94

WG

85 4

08

600 600

500500

400

300

400

300

400

300

200 200 DF

MCF

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Seismic stratigraphy in Santa Monica basin(Sorlien et al., 2006)

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Explanation for this figure and seismic sections in Figures 7A and 7B

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Figure 6. Bathymetry and seismic section locations for the area of the Malibu coast. Area located in Figure 1. Faults are from Sorlien et al. (2000), and the earthquake focal mechanism is from Ellsworth et al. (1973).



280 Fisher et al.

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Figure 7. (A) Migrated multichannel seismic section shows high structural relief in the hanging wall of the Dume Fault. (B) shows much lower relief. The loss of structural relief along the Dume Fault over a short along-strike distance may signify that a transverse structure extends west of the knoll. This structure may form a boundary to earthquake rupture. Section locations shown in Figure 6. V.E.—Vertical exaggeration.




as resulting from deformation within a restraining double bend formed within a broad left-oblique fault zone.

Another possibility to explain the structural differences evi-dent in these seismic sections (Figs. 7A and 7B) is that near Syca-more knoll, a transverse structural zone strikes north across the province-bounding fault system (Fisher et al., 2005b). Transverse structures are fundamental to the development of many foreland fold-and-thrust belts (e.g., Wheeler et al., 1979; Thomas, 1990). Typically, such structures include lateral ramps, transverse faults, and displacement-transfer zones. A transverse structure would be important to the analysis of earthquake hazards because the structure could bound earthquake-rupture zones along a fault, thereby limiting the maximum earthquake magnitude because of the constrained rupture-zone length (e.g., Wells and Cop-persmith, 1994). A local example of this effect is the two lateral ramps in the Santa Susana Fault that constrained the rupture zone of the 1994 Northridge earthquake (Hauksson et al., 1995). Simi-lar fault segmentation has been described for faults in the eastern Santa Barbara Channel (Gurrola and Kamerling, 1996; Kamer-ling and Gurrola, 1997).

South of Point Dume, the Dume Fault offsets the seafl oor, and fault-related folding extends upward to arch the seafl oor (Fig. 8). This shows very recent fault movement, especially in view of the probable high sedimentation rates associated with the Dume submarine fan.

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SANTA MONICA BAY

The area of concern in this section includes the continen-tal shelf under Santa Monica Bay as well as the eastern part of the deep-water Santa Monica and San Pedro Basins to the southwest (Figs. 1 and 9). This area is bounded on the north by

Figure 8. Migrated multichannel seismic section showing a seafl oor offset along the Dume Fault and other evidence for recent fault activ-ity. Section location shown in Figure 6.

Figure 9. Location of seismic sections shown in Figure 10 that were collected in Santa Monica Bay across the loca-tion of the San Pedro Basin Fault and the reported location of the Palos Verdes Fault. Map area shown in Figure 1.



282 Fisher et al.

the Santa Monica Mountains and the active faults that make up the province-bounding fault system, namely the Santa Monica, Dume, and Malibu Coast Faults (see the previous section). The Los Angeles Basin borders the east side of the shelf, and along the southeast side, the Palos Verdes Hills jut above the other-wise low-lying coastal plain. The main faults below and border-ing Santa Monica Bay include the Santa Monica and San Pedro Basin Faults as well as the closely related Palos Verdes Fault and Compton thrust ramp.

The northwest-striking San Pedro Basin and Palos Verdes Faults abut to the northwest against the west-striking Santa Monica and related faults, which are associated with the Western Transverse Ranges (Fig. 1). In 1979 and 1989, the two largest (M

L

~5) historic earthquakes under Santa Monica Bay nucleated at a depth of ~12 km and show west-vergent, thrust-fault focal mecha-nisms (Hauksson and Saldivar, 1986, 1989; Hauksson, 1990).

The Palos Verdes Fault is not evident in high-resolution, seismic-refl ection data collected in Santa Monica Bay (Nardin and Henyey, 1978; Fisher et al., 2003); however this fault appar-ently is evident in proprietary multichannel seismic data acquired there (Sorlien et al., 2004). In small-airgun, seismic-refl ection data, a northeast-dipping refl ection, likely from the top of the Catalina Schist, ends in the northeast near but not at the reported location of this fault (Fisher et al., 2003) (Fig. 10A). This refl ec-tion end might show the fault’s location, but taken alone, the evi-dence is equivocal. The Palos Verdes Fault passes northeast of the Palos Verdes Peninsula, sharing the regional northwest strike of most major structures south of the Transverse Ranges (Woodring et al., 1946; Yerkes et al., 1965; Wright, 1991; Legg, 1992; Shaw and Suppe, 1996). This fault apparently extends for more than 200 km southeastward from below Santa Monica Bay to near Lasuen Knoll (Clarke et al., 1983, 1985; Greene and Kennedy, 1987; McNeilan et al., 1996; Marlow et al., 2000; Fisher et al., 2004a) (Fig. 1).

The main question concerning potential earthquake faults under Santa Monica Bay is: Do large faults and folds there result from thrust or strike-slip faults? Unfortunately, the structural geometry of the Palos Verdes Fault below Santa Monica Bay remains unclear. In one interpretation, this fault splays upward to the east from the underlying blind Compton thrust ramp, on the basis of an analysis of fault-bend folding of rocks associated with the fault (Shaw and Suppe, 1996). Below the east shore of Santa Monica Bay, the thrust ramp is proposed to be at a depth of ~5 km, and its hanging wall verges westward from and is rooted within the Los Angeles Basin (Shaw and Suppe, 1996). This vergence direction agrees with the results of modeling of GPS data, which suggests west-directed thrusting from within the basin (Argus et al., 2005). Whether or not this thrust ramp exists is an important issue for earthquake-hazards analysis because the ramp would partly underlie coastal urban areas and could cause an earthquake estimated to be as large as M 6.6 (Shaw and Suppe, 1996).

An alternative interpretation to explain the development of the offshore structure involves strike-slip faulting. Three large anticlinoria underlie Santa Monica Bay, the Palos Verdes

Peninsula, and the San Pedro shelf (Nardin and Henyey, 1978; Ward and Valensise, 1994; Fisher et al., 2003, 2004a; Legg et al., 2004). These structures could have developed within restrain-ing bends along a strike-slip fault. Explanations for the disconti-nuities between anticlinoria include: (1) these structures may be separated by lateral ramps along a deep thrust fault (Shaw and Suppe, 1996); (2) they could have developed during convergent dextral shear along the Palos Verdes Fault (Nardin and Henyey, 1978); or (3) they may have formed along restraining bends in this fault (Ward and Valensise, 1994; Legg and Borrero, 2001; Fisher et al., 2003; Legg et al., 2004).

The San Pedro Escarpment marks an important geologic boundary between the Santa Monica shelf and the deep-water (~800 m) Santa Monica and San Pedro Basins (Figs. 9, 10B, and 11). This escarpment is the steep section of seafl oor that extends nearly 60 km southeastward from Santa Monica Canyon to southeast of the Palos Verdes Peninsula (Fig. 9). Along much of the escarpment’s length, active folds and faults cut the seafl oor and form the escarpment’s foot. Southwest-dipping rocks under the escarpment indicate the presence of a fault at depth, possibly one that dips southwest (e.g., Davis and Namson, 1994a), but the dip and mode of offset along this fault are controversial.

The San Pedro Basin fault zone dips steeply and is probably strike slip, as indicated by the fl ower structures developed along it (Fisher et al., 2003). Where this fault and the San Pedro Escarp-ment converge near the mouth of Santa Monica Canyon (Fig. 9), the escarpment ends abruptly and a low-relief continental slope extends farther north and northwest. Anticlines that deform rock below this low-relief slope show that the fault continues north-west nearly to intersect with the Dume Fault.

The Redondo Canyon Fault has been proposed (e.g., Nardin and Henyey, 1978; Wright, 1991; Bohannon et al., 2004) to fol-low Redondo Canyon southwestward across the continental shelf (Fig. 9). However, high-resolution, seismic-refl ection sections across this proposed fault do not show fault-plane refl ections or other clear evidence for faulting (Fisher et al., 2003).

SAN PEDRO SHELF

The San Pedro shelf extends southeastward from the Palos Verdes Peninsula to where the continental shelf narrows abruptly about halfway between Huntington Beach and Dana Point (Fig. 11). The Los Angeles Basin borders this shelf on the east, and the deep-water San Pedro Basin delimits the shelf’s west-ern fl ank. The major faults under this shelf are the Newport-Inglewood, Thums–Huntington Beach, and Palos Verdes. Near the San Pedro shelf, the Newport-Inglewood right-lateral, strike-slip fault is known primarily where the fault cuts onshore (Yeats, 1973; Barrows, 1974; Hauksson, 1987; Wright, 1991, Fischer, 1992; Freeman et al., 1992; Grant et al., 1997; Grant and Shearer, 2004). The Newport-Inglewood Fault either forms the contact between different basement types, or the Fault closely follows this contact. North and east of the fault, basement rocks consist of Jurassic and Cretaceous crystalline continental crust, whereas




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284 Fisher et al.

south and west of the fault the basement is composed of highly extended and metamorphosed subduction-zone rocks.

Below the Long Beach area, the Thums–Huntington Beach Fault diverges southeastward away from the Palos Verdes Fault (Wright, 1991). Unpublished oil-company, seismic-refl ection data indicate that this fault has been inactive since during the Pliocene (T.L. Wright, 2005, written commun.) because an anticline in the fault’s hanging wall is overlain by undeformed Pliocene and Qua-ternary strata. Interpretive cross sections by different authors dis-agree on fundamental issues about this fault—one section shows the fault to be a normal fault that dips east and is downthrown on the east (Wright, 1991); another interpretation shows that it dips west and is downthrown on the west and merges downward with the Palos Verdes Fault (Davis and Namson, 1994a).

The Palos Verdes Fault strikes southwestward from the Palos Verdes Peninsula to underlie the San Pedro shelf and the area near Lasuen Knoll (Greene and Kennedy, 1987; Wright, 1991;

Petersen and Wesnousky, 1994; Clarke et al., 1997; Marlow et al., 2000; Bohannon et al., 2004; Fisher et al., 2004a, 2004b; Baher et al., 2005) (Fig. 11). The slip rate along the Palos Verdes Fault is ~3 mm/yr, based on analysis of wave-cut terraces and offset stream courses (Ward and Valensise, 1994). McNeilan et al. (1996) pro-posed that recently the main style of movement along the Palos Verdes Fault was strike slip. Stephenson et al. (1995) interpreted high-resolution, seismic-refl ection data collected onshore and proposed that fi ve strands make up the shallow part of this fault zone. These strands dip steeply southwest and are downthrown on the northeast. Multibeam bathymetric data show recent scarps along this fault near Lasuen Knoll (Marlow et al., 2000).

The earthquake and tsunami hazards from faults below the San Pedro shelf appear to be high (Legg, 1992; Legg and Borrero, 2001; Fisher et al., 2004b), as has been deduced, in part, from the rapid Quaternary uplift of rocks on the Palos Verdes Peninsula (Woodring et al., 1946; Bryan, 1987; Ward and Valensise, 1994).

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Figure 11. Location of seismic sections over San Pedro shelf. Area located in Figure 1. Faults are from Wright (1991).




However, the geometry of the Palos Verdes Fault at depth and whether it splays upward from a thrust fault that extends west-ward from within the Los Angeles Basin are unknown. One pos-sibility is that this uplift resulted from right-oblique reverse slip along faults making up a restraining bend in the Palos Verdes Fault (Ward and Valensise, 1994). This style of deformation may extend south from the peninsula to involve faults under the San Pedro shelf, thereby heightening the regional earthquake hazard.

No large historical earthquakes have occurred along the Palos Verdes Fault; even so, McNeilan et al. (1996) estimate that this fault could produce an earthquake as large as M 7. Earth-quakes in Hauksson (1990) revealed shallow strike-slip focal mechanisms along this fault. Deep focal mechanisms in the same area show thrust-fault fi rst motions. These results are in appar-ent agreement with proposals by Brankman and Shaw (2004) that displacement along the Palos Verdes Fault is partitioned into right-lateral, strike-slip motion on shallow, nearly vertical faults and thrust motion along deep, gently to moderately dipping, blind thrust faults.

Using high-resolution, seismic-refl ection data obtained south of the Palos Verdes Peninsula, the offshore part of the Palos Verdes Fault can be divided into three segments, on the basis of the fault’s structure in the upper 2 km of the crust (Fisher et al., 2004a, 2004b). The northwestern segment underlies the San Pedro shelf; the middle fault segment underlies the bathymetric saddle that separates the San Pedro shelf from Lasuen Knoll; and the south-eastern segment offsets the seafl oor near Lasuen Knoll (Fig. 11).

Close to the Palos Verdes Peninsula, the northwest segment of the Palos Verdes Fault zone deforms young rocks and the sea-fl oor (McNeilan et al., 1996; Clarke et al., 1998). Along this part of the fault zone, high-resolution, seismic-refl ection data (Francis et al., 1996) reveal a restraining bend and attendant fl ower struc-ture that coincides with a low seafl oor swale ~1–4 m high. This deformation apparently accumulated during the past 10 ka, as sea level rose after the last glacial lowstand. The middle segment of the Palos Verdes Fault underlies the part of the seafl oor that deepens southeastward from the San Pedro shelf. This fault seg-ment includes numerous normal-separation faults that probably formed within a releasing bend along the right-lateral, strike-slip fault (Fisher et al., 2004a, 2004b). Along the southeast segment of the Palos Verdes fault zone, sharp seafl oor scarps bordering Lasuen Knoll, possible tilted wave-cut terraces, and disrupted shallow sediment all attest to recent fault movement (Marlow et al., 2000; Fisher et al., 2004a, 2004b). Lasuen Knoll appears to be underlain by a popup structure, like those that develop along a restraining bend or stepover in a strike-slip fault system.

Multichannel seismic-refl ection data show the structural variability along the Palos Verdes Fault (Figs. 12A, 12B, and 12C). Near the Palos Verdes Peninsula, this fault dips at a low angle southwest, and the basement complex, made up of Cata-lina Schist, rises to shallow depth and is arched in a fault-related fold. Southeast from the peninsula, the Palos Verdes Fault dips at increasingly steeper angles, at least in the upper refl ective part of the crust, and deformation of the top of basement becomes corre-

spondingly reduced (Figs. 12B and 12C). Figure 12C shows that refl ections from the basement top overlap across the Palos Verdes Fault, suggesting that the deep part of the fault dips southwest at a low angle. Folding and structural relief associated with this fault is greatly reduced, compared to deformation in Figure 12A, where the seismic line (Fig. 12C) crosses the San Pedro shelf.

A south-facing continental slope delimits the San Pedro shelf on the south and separates the broad part of the shelf from the narrow part that extends southeast toward Dana Point (Fig. 11). Conceivably, this abrupt narrowing might result from a deep-seated structure that trends transversely across the strike of major offshore faults. Multichannel seismic section 191 (Fig. 13), however, shows that none of the relief evident in the top of basement, possibly including a Miocene normal fault, extends upward through the sedimentary section to infl uence the sea-fl oor morphology.

The Cabrillo Fault extends offshore, southeast from the Palos Verdes Peninsula (Marlow et al., 2000; Fisher et al., 2004a, 2004b) (Fig. 11). On high-resolution, seismic-refl ection sections, this fault lies along an obvious and abrupt change in the appear-ance of refl ections from Miocene rocks.

CONCLUSIONS

Coastal and nearshore faults described above show evidence for Recent movement, and some have large cumulative displace-ment; thus these faults pose a signifi cant earthquake hazard to neighboring urban areas. The foregoing discussion clarifi es that a large degree of uncertainty still clouds our understanding of many of these faults. Two main sources for uncertainty are: fi rst, the absence of data on offshore-fault slip rates and on deep-fault confi gurations, and second, the complicated sequence of struc-tural inheritance of offshore structures, a product of the rap-idly changing tectonic environment during the late Cenozoic in Southern California.

Among the most critical yet elusive data needed to analyze earthquake hazards are fault-slip rates. Such rates along some onshore faults in Southern California are being estimated from GPS-campaign data (e.g., Southern California Integrated GPS Network, 2006) and from the results of trenches dug across faults (e.g., Dolan et al., 2000). These techniques, however, are inap-plicable or diffi cult to apply to offshore faults, most of which will need to be examined using high-resolution, seismic-refl ection data coupled with careful dating of sediment samples obtained by coring (e.g., Normark et al., 2004; this volume, Chapter 2.6).

Uncertainty about the confi guration of the deep parts of faults, specifi cally those parts within the seismogenic zone, leads to ambiguity in estimates of how seismic energy is radiated dur-ing an earthquake. Consequently, ground-motion modeling might not be as accurate as civic planners require. A potential means to reduce uncertainty related to the seismogenic zone involves the newly released multichannel seismic-refl ection data (National Archive of Marine Seismic Surveys, 2006) that allow geologists to investigate deeper levels along faults than previously possible.



286 Fisher et al.

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A main source for uncertainty in analyzing offshore faults is the complicated geologic history of the offshore area. This history developed during three tectonic episodes: Mesozoic through early Miocene subduction, early Miocene extension, and Pliocene and later transpression (Kamerling and Luyen-dyk, 1979; Luyendyk et al., 1980; Kamerling and Luyendyk, 1985; Hornafi us et al., 1986; Atwater, 1989; Wright, 1991; Crouch and Suppe, 1993; Nicholson et al., 1994; Bohannon and Geist, 1998). Each episode left its mark on offshore struc-tures, and early-formed structures controlled or at least infl u-enced the location and development of later ones. Unraveling the sequence of structural inheritance will shed more light on how strain currently is distributed throughout the crust and on the geometry of faults at seismogenic depths. For exam-ple, since the Miocene, rocks once deformed under extension have undergone transpression. In general as rifts form, faults commonly develop that are transverse to and bridge across the opening rift (e.g., Acocella et al., 2005), and under sub-sequently reoriented contraction, these transverse structures could form structural discontinuities that segment earthquake rupture zones. Furthermore, the reoriented contraction since the Miocene could have transformed major rift-bounding extensional faults into thrust, reverse, strike- and oblique-slip faults. Clearly, continued research is necessary to understand the hazard to urban areas from offshore and coastal faults hav-ing such a complicated development history.

ACKNOWLEDGMENTS

We thank Eric Geist and Shirley Baher for critical comments that greatly improved this report.

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st

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nor

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t?

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sver

se s

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ture

evid

ent i

n bx

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ow s

outh

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ing

bath

ymet

ric s

lope

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re 1

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orth

wes

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uthe

ast m

igra

ted,

mul

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nnel

sei

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sec

tion

acro

ss th

e Sa

n Pe

dro

shel

f an

d sl

ope

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uctu

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ol o

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enta

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pe th

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