U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet-110-99

The “LARSE” Project—Working Toward a Safer Future

for Los Angeles

UNDERSTANDING EARTHQUAKE HAZARDS IN SOUTHERN CALIFORNIA

T

US SG

he Los Angeles region is

underlain by a network of

active faults, including many that

are deep and do not break the

Earth’s surface. These hidden

faults include the previously

unknown one responsible for the

devastating January 1994

Northridge earthquake, the

costliest quake in U.S. history. So

that structures can be built or

strengthened to withstand the

quakes that are certain in the

future, the Los Angeles Region

Seismic Experiment (LARSE) is

locating hidden earthquake

hazards beneath the region to

help scientists determine where

the strongest shaking will occur.

On January 17, 1994, the costliest earthquake in the history of the United States struck

the Los Angeles region, killing 57 people,

leaving 20,000 homeless, and causing more

than $20 billion in damage to homes, public

buildings, freeways, and bridges. This magnitude 6.7 quake occurred 10 miles beneath the

town of Northridge on a previously unknown

ramp-like (“thrust”) fault not visible at the

Earth’s surface. Many faults, including such

hidden or “blind” faults, in the Los Angeles

region are capable of producing even stronger

earthquakes.

In the aftermath of the Northridge

earthquake, residents of the Los Angeles

region are asking whether a strong quake

can strike near their home and, if so, how

hard the ground will shake. Scientists are

working to answer these questions so that

damage can be reduced in the next big

earthquake and future quakes. A crucial step

is getting an accurate picture of the network

of active faults that underlie the Los

Angeles region.

In 1993, scientists from the Southern

California Earthquake Center (SCEC), the

U.S. Geological Survey (USGS), and other

organizations began the Los Angeles Region

Seismic Experiment (LARSE) to collect

seismic images, or pictures, of the Earth’s

crust beneath the region. The goals of

LARSE are to obtain images of faults at

depth, especially of blind thrust faults, and

also to obtain data on the subsurface shapes

of sedimentary basins (large valleys filled

with sedimentary deposits). Knowing the

configuration of buried faults is crucial to

understanding how the earthquake-producing “machinery” works in the Los Angeles

region, and information on the thickness and

shape of the region’s sedimentary basins is

essential for predicting how hard the ground

will shake in future quakes.

LARSE uses sound waves traveling beneath the Earth’s surface to produce seismic

images. These sound waves are generated

by underwater bursts of compressed air in

the offshore region and by small underground explosions on land. The sound

waves from airgun bursts and buried explosions are received by hundreds of portable recording instruments (seismographs). The recorded data are then analyzed by powerful

computers to produce images of the subsurface. LARSE also makes use of seismic waves

from natural earthquakes. Because the

project’s explosive charges are small and are

set off at the bottoms of holes drilled to more

than 60 feet below the ground surface, they do

not cause property damage, and there is no

chance of their triggering an earthquake.

By 1998, the LARSE project had made

several important discoveries. Images of the

Home to millions of people, the Los Angeles region is underlain by

numerous active faults, many of which cannot be seen at the Earth’s

surface. One of these hidden faults produced the January 17, 1994,

Northridge earthquake, which caused 57 deaths and widespread

damage—photo at right shows vehicles stranded by the collapse of

sections of Interstate 5. (Top photo courtesy Arnesen Photography/

LACVB, PictureLA.com; by and copyright Erik Arnesen. Photo at right

by Brant Ward; copyright San Francisco Chronicle.)

The U.S. Geological Survey (USGS) scientists shown

here are using one of the seismographs that record

underground sound waves in the Los Angeles Region

Seismic Experiment (LARSE). These recordings are

analyzed by powerful computers to produce images of

the subsurface of the L.A. region. To generate sound

waves, LARSE in part uses small charges set off at the

bottoms of holes drilled to more

than 60 feet below the ground

surface (see inset photo of

mobile drill rig). (Photograph by

Michael Diggles, USGS.)
SAN ANDREAS 

FAULT ZONE

WHITTIER

SIERRA MADRE THRUST

FAULT

SAN GABRIEL

 MOUNTAINS

MOJAVE DESERT

WHITTIER

NARROWS

SIERRA

MADRE

FORT

TEJON

CRUST

MANTLE

BRIT TLE

PACIFIC 

PLATE NORTH 

AMERICAN 

PLATE

1991

5.8

1857

7.8

1987

5.9

LOS

ANGELES

DU C

LE

I T

Sedimentary

rocks

 

?

?

?

?

0 5 10 15 20 MILES 

MASTER

BLIND THRUST

FAULT

Strongly reflective (red)

0

5

DEPTH BELOW SEA LEVEL, IN MILES

0

5

10

15

20

25

15

25

30

10

20

30

SAN ANDREAS

 FAULT

SOUTHWEST LOS ANGELES 

BASIN

SAN GABRIEL MOUNTAINS NORTHEAST SAN GABRIEL

Seal Beach VALLEY Undergroud charges 

NEWPORT- (small triangles)

INGLEWOOD

FAULT

WHITTIER

 FAULT

WHITTIER

NARROWS FAULT

SIERRA MADRE

 THRUST 

MOJAVE

DESERT

Arrow pairs show

relative movement

on thrust faults

MASTER

BLIND THRUST

FAULT

Strongly reflective (red)

Base of Earth's Crust Upper Mantle

The LARSE project is completing the onshore part of the

second of two major lines of data collection across the

Los Angeles region. LARSE uses hundreds of portable

seismographs to record underground sound waves,

which are analyzed to create subsurface images of the

Earth’s crust. In 1994, these sound waves were generated

by underwater bursts of compressed air in the offshore

region on both Lines 1 and 2 and by small, buried

explosive charges on land on Line 1. To complete Line 2,

drilling of holes for underground charges starts in the

summer of 1999, and actual data collection begins in

October. Data collected in 1994 along Line 1 were used to

produce the image and diagram at right. Red stars with

numbers (magnitudes) are earthquakes of magnitude 5.8

and greater since 1932.

sedimentary basin beneath the San Gabriel

Valley show that its depth reaches 3 miles,

50% more than earlier estimates. Because

deeper sedimentary basins have greater

shaking potential, earthquake hazards in the

San Gabriel Valley need to be reevaluated.

Another major finding is a strongly reflective zone located deep beneath the San Gabriel

Mountains. This zone begins at about 12 miles

depth near the vertical San Andreas Fault and

rises in a ramp-like fashion southward toward

the Los Angeles Basin. It appears to connect to

the fault system responsible for the 1987 magnitude 5.9 Whittier Narrows earthquake,

which occurred on a blind thrust fault. This reflective zone is interpreted as a “master” blind

thrust fault that transfers stress and strain upward and southward to a network of faults in

the San Gabriel Valley and Los Angeles Basin.

The LARSE data obtained so far have provided answers that could not have been obtained in any other way.

The LARSE project is completing the onshore part of the second of two major lines of

data collection across the Los Angeles region.

This 60-mile line (Line 2) extends from the

coast to the Mojave Desert. Drilling of holes

for the placing of underground charges starts

in the summer of 1999, and actual data collection begins in October.

The cooperative efforts of SCEC, USGS,

and other scientists in the LARSE project are

helping to find hidden faults and areas of potential strong earthquake shaking in the Los

Angeles region. This knowledge is essential to

making new and existing structures in the region better able to withstand earthquakes. The

work of SCEC and USGS scientists in

LARSE is only part of the National Earthquake Hazards Reduction Program’s ongoing

efforts to protect people’s lives and property

from the earthquakes that are inevitable in

southern California and elsewhere in the

United States.

COOPERATING ORGANIZATIONS

California Department of Parks and Recreation

California Department of Transportation

California Division of Mines and Geology

California Office of Emergency Services

California State University, Northridge

Canadian Geological Survey

City of Los Angeles

County of Los Angeles

Copenhagen University, Denmark

GeoForschungsZentrum, Potsdam, Germany

Incorporated Research Institutes for Seismology

Karlsruhe University, Germany

Lamont-Doherty Earth Observatory

Los Angeles Unified School District

Metropolitan Water District of Southern California

Santa Monica Mountains Conservancy

Southern California Earthquake Center

University of Texas at El Paso

U.S. Army Corps of Engineers

U.S. Bureau of Land Management

U.S. Department of Agriculture, U.S. Forest Service

U.S. Naval Weapons Station, Seal Beach

Many private and government landowners

Thomas L. Henyey, Gary S. Fuis, Mark L. Benthien, Thomas R.

Burdette, Shari A. Christofferson, Robert W. Clayton, Edward E.

Criley, Paul M. Davis, James W. Hendley II, Monica D. Kohler,

William J. Lutter, John K. McRaney, Janice M. Murphy, David A.

Okaya, Trond Ryberg, Gerald W. Simila, and Peter H. Stauffer

Graphic design by

Sara Boore, Susan Mayfield, and Stephen Scott

For more information contact:

Southern California Earthquake Center

University of Southern California, Mail Code 0742

Los Angeles, CA 90089-0742, Tel (213) 740-0323

http://www.scec.org/ or http://quake.wr.usgs.gov/

See also Southern Californians Cope with Earthquakes

(USGS Fact Sheet 225-95).

SAN

ANDREAS

FAULT

DESERT

Lines where data are collected

1994 underground charges

MOJAVE

LINE 1

LINE 2

WHITTIER FAULT

WHITTIER FAULT

FAULT

SEAL

 BEACH 6.3

5.9

6.7

6.7

SIERRA MADRE FAULT ZONE

LOS ANGELES BASIN

SAN GABRIEL

 VALLEY

1994 airgun

bursts

Dry Lakes

5.8

Recently

active

faults (red)

SAN GABRIEL

MOUNTAINS

ZONE ZONE

NEWPORT-INGLEWOOD

PACIFIC

OCEAN

LOS

ANGELES

AREA

OF MAP

CALIFOR NI A

0 10 20 30 MILES

ultrasound images (reflected sound waves) are created.

Transmitted sound waves pass through the area being imaged,

revealing the geologic structure by the ways they are bent or

slowed. Reflected sound waves bounce off faults and rock

layers, showing the shapes and depths of those features. Data

collected along Line 1 of LARSE were used to create this

composite image of the geologic structure beneath the Los

Angeles region. Color bands in the upper part of the image,

derived from computer analysis of transmitted sound waves,

show the speed at which waves pass through different rock

This diagram, based on the

subsurface image made along Line 1

of the LARSE project (see above),

shows an interpretation of the

geologic structure beneath part of

the Los Angeles region. Many faults

in the region are capable of

producing powerful earthquakes.

Vertical faults, such as the San

Andreas Fault, commonly are clearly

visible at the Earth’s surface. However,

ramp-like faults, or “thrust” faults,

such as the one responsible for the

1987 Whitter Narrows earthquake,

often do not break the surface; these

hidden faults are targeted by the

LARSE project. In this diagram,

relative movement on faults is

shown by pairs of small arrows.

Large white arrows show the

oblique direction in which the Pacific

Plate (left) and the North American

Plate (right) are converging. Red

stars with dates and numbers

(magnitudes) are earthquakes.

Disponible también en Español: USGS Fact Sheet 111-99

The LARSE project uses

powerful computers to analyze

transmitted and reflected

underground sound waves to

produce images of the Earth’s

crust, similar to the way in

which medical CAT-scan images

(transmitted X-rays) and

types—slower speeds (cooler colors) are in less dense rock. The deeper rectangular part of the image shows results from

reflected sound waves—strongly reflective areas are in bright red. The intensely reflective zone deep beneath the San

Gabriel Mountains is interpreted by LARSE scientists as a “master” blind thrust fault—a thrust fault that transfers stress and

strain from near the San Andreas Fault to the network of faults in the San Gabriel Valley and Los Angeles Basin.