U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 086-02

August 2002

Probing the Los Angeles Basin—Insights Into 

Ground-Water Resources and Earthquake Hazards

Millions of southern Californians 

depend on ground water from 

the Los Angeles Basin. The U.S. Geological Survey (USGS) is working with 

local water-management agencies to 

gain a better understanding of groundwater flow within the basin and to create an improved model of the sediment 

layers below its surface. These efforts 

are providing crucial information for 

managing and ensuring the sustainability of the region’s ground-water 

supply. The data obtained are also being used to help locate areas especially 

susceptible to earthquake shaking.

��� �������

���� �����

����� ������ ���������

��� �������� ������

����� ������ ���������

��� ������� ���������

������ ���� ����� ������

��� ����� ���

������� �����

�����

������ ���

��������� �����

������

��� �������

������� �����

���� ����� �����

������ �����

��������� �����

������ �����

������� ����� ������� �����

�������� ������� �������

��������� ���

������� ������� ���� �����

������� ���� ����

����

�����

������� �������

��� ������� �������

���������� �������

���

���

���

���

���

��

��

��� �

��

��

��

����� ��� ��������� ������ �� ���� ����

�����

����

��������� ����� ����� ������

�����

� � � � � � � � � �

� � � � � �

� � � � � �

� � � � � �

��������� ���� ����

������� ��������������� ����

���� ���������� ����

More than 10 million people live in the 

Greater Los Angeles metropolitan area. 

Much of this population depends on water 

pumped from beneath the surface of the Los 

Angeles Basin. For example, one-third of 

the water consumed by the 4 million residents in the central and west coast basins is 

ground water. Successful management of 

this ground-water resource requires knowing 

where and how fast the water moves in the 

various layers of sediment that fill the basin.

Since 1995, the U.S. Geological Survey 

(USGS) has been working with the Water 

Replenishment District of Southern California 

in a cooperative ground-water and geologic 

study of the Los Angeles Basin. Now, with additional support from the Los Angeles County 

Department of Public Works, the USGS is expanding this program to study the distribution 

and geologic history of sediments as deep as 

1,500 feet (450 meters) below city streets. This 

information is critical not only to ground-water 

management but also to earthquake hazard assessment in the densely populated region. During a strong earthquake, ground shaking may 

cause water-saturated sediment to “liquefy,” 

resulting in ground failure. Therefore, knowing the composition of near-surface sediment 

is important for assessing the vulnerability of 

buildings to liquefaction and other damaging 

effects of earthquakes.

The USGS has already drilled more than 

30 monitoring wells throughout the Los 

Angeles Basin and has collected chemical, 

geologic, hydrologic, and geophysical data 

from these and other wells in the region. One 

critical issue being examined is seawater 

intrusion into freshwater aquifers along the 

coastal parts of the basin. Another such issue 

is the transport and fate of water diverted 

into spreading ponds in the Montebello Forebay, an area of the basin where the underlying sediment is relatively coarse and clay 

poor and so allows surface water to infiltrate 

into and recharge the ground-water system.

Ideally, when the monitoring wells are drilled 

in the Los Angeles Basin, attempts are made 

to recover intact sediment cores from the full 

length of a well. Because drilling is costly, large 

parts of the basin must be explored by other 

means. Geophysical techniques, such as seismic imaging and gravity surveys, are used to 

interpolate the character of sediments between 

wells. Geologic mapping of surface deposits in 

the basin provides clues to what lies beneath. In 

harbors and offshore areas, geologic structure 

below the water surface is revealed by using a 

ship-towed system that probes the sediments 

with acoustic pulses. The data obtained using 

this range of techniques help to characterize 

the three-dimensional geologic structure and 

ground-water flow system of the basin.

The present distribution of gravel, sand, 

and clay in the Los Angeles Basin reflects the 

interplay between the seashore processes and 

streams carrying sediment from the upland 

areas surrounding the basin. Similar processes 

operated in the past; therefore, knowing the 

geologic history of changes in sea-level, 

climate, and rivers allows prediction of the 

distribution of subsurface sediments in the 

basin and their possible effects on the regional 

ground-water flow and seismic hazards.

Greater Los Angeles depends on ground water for much of its water supply. As part of a cooperative project 

with local water-management agencies to better understand the ground-water system and geology of the Los 

Angeles Basin, the U.S. Geological Survey has drilled more than 30 monitoring wells. Critical issues being examined include the transport and fate of recharge water infiltrated into the ground-water system from spreading ponds in the Montebello Forebay (an area where surface water easily infiltrates the ground) and saltwater 

intrusion into freshwater aquifers along the coast. In an effort to halt saltwater intrusion, barriers (yellow lines) 

consisting of arrays of freshwater injection wells have been installed by local water-management agencies.
For more information contact:

Ground Water – Eric Reichard (858) 637-6834

U.S. Geological Survey

5735 Kearny Villa Road

San Diego, CA 92123

e-mail: egreich@usgs.gov

Earthquake Hazards – Daniel Ponti (650) 329-5679

U.S. Geological Survey MS 977

345 Middlefield Rd.

Menlo Park, CA 94025

e-mail: dponti@usgs.gov

See also Saltwater Intrusion in Los Angeles Area 

Coastal Aquifers—The Marine Connection (USGS Fact 

Sheet 030-02).

COOPERATING ORGANIZATIONS

Water Replenishment District of Southern California

Los Angeles County Department of Public Works

John W. Hillhouse, Eric G. Reichard, and Daniel J. Ponti

Edited by

James W. Hendley II and Peter H. Stauffer

Graphic design by

Susan Mayfield, Rudolph Contreras, and Sara Boore

This fact sheet and any updates to it are available 

online at:

http://geopubs.wr.usgs.gov/fact-sheet/fs086-02/

������� �� �������� �����

In the cores recovered in the Los Angeles 

Basin so far, several past cycles of advance 

and retreat of the sea have been identified. 

Each cycle leaves a signature in the sediments by producing distinctive layering or 

bedding, evidence of erosion, and telltale 

assemblages of fossils. Preliminary age 

determinations of core samples indicate a 

correlation of these cycles with the sea-level 

changes that occurred in the past as a result 

of the storing and release of vast quantities 

of water during alternating glacial periods 

(“ice ages”) and intervening warm periods.

An example of how sediments can be interpreted to reveal past conditions was provided 

during recent drilling of a USGS water-monitoring well in the City of Long Beach. Shells 

and thick pieces of wood were discovered 

in coarse sand at a depth of 400 feet (120 

meters). This sediment layer, although found 

hundreds of feet below the surface and several 

miles inland from the present shoreline, is an 

ancient beach deposit and has been dated at 

about 300,000 years old. This date correlates 

to a high stand of sea level during a warm 

period between ice ages. The ancient beach 

is now well below the current ground surface 

because the Los Angeles Basin has subsided 

with time as it filled with sediment.

Much information has already been collected in the ongoing study of the Los Angeles 

Basin. To evaluate the potential effects of 

alternative strategies for water management, 

USGS scientists are using these data to develop 

a computer simulation of ground-water flow 

in the basin. In addition, to further evaluate the 

ground-water resource, as well as earthquake 

hazards, the scientists are using the data to 

develop a model of the three-dimensional geology of the basin. This model includes ages, 

thicknesses, grain sizes, and environments of 

deposition of sedimentary beds; locations and 

quality of water-bearing zones; and locations 

of major fractures and faults. This geologic 

model will incorporate the concept of “sequence stratigraphy,” which recognizes the 

influence of climate-driven depositional cycles 

on the distribution of sediment types.

During the next several years, the USGS 

will be drilling and coring more wells in the 

Los Angeles Basin and applying geologic 

and ground-water models to address a host of 

practical questions for the region: What are 

the main pathways for seawater intrusion? 

What are the best strategies for recharging the 

ground-water supply, such as pumping, injecting, and spreading water? Where are faults 

located underground and what earthquake 

hazards do they pose? Which areas face greater 

risks to life and property during earthquakes 

because they are more likely to experience intensified shaking or soil liquefaction?

The cooperative efforts of the USGS, the 

Water Replenishment District of Southern 

California, and the Los Angeles County 

Department of Public Works are providing 

a better understanding of the ground-water 

system and geology of the Los Angeles Basin. This knowledge is essential to ensuring 

future adequate ground-water supplies for 

this populous urban region and to mitigating 

the threat posed by earthquakes.

Sediment cores collected during the drilling 

of monitoring wells by the U.S. Geological 

Survey (USGS) in the Los Angeles Basin 

(see photo) provide information that is vital 

to understanding ground-water flow and 

seismic hazards in the region. Scientists 

study and sample the cores in a USGS mobile 

laboratory. This diagram shows various types 

of information obtained from study of cores 

from a single well. Shown are the thickness 

and grain size of sediment layers down the 

length of the well; environments of deposition, such as river channels and tidal flats, 

interpreted from fossils and other features; 

changing trends of the height of the depositional surface relative to sea level (blue 

dashed arrows), and location of well screens 

that enable sampling of water-bearing layers.

����������

���������

��� ����

��������

��� �����

������������

�� ����������

���������� ��� ��� �������� �������������� ��

���� ������������� ���� ����

� � � � �

����

������������

���� ��

������ �� ���� ����� ���� �������

�

���

���

���

�����

���

�����

�����

����

���

����

������

�����

����

����� ����������

����� �������

����� ������������

��������� ����������

����� ������������ ��������� ����������

����� ������������

��������� ����������

������� ������

�������� ������ ���������� ������

���� ����� ������������ �����

����� �������

����� �������������

������� ������

�������� ������ ������ ������������

������� ������

����� ����

������

����

����

����

�������� ��������������

�������� ����

During earthquakes, 

ground shaking may cause 

water-saturated sediment 

to “liquefy,” resulting in 

ground failure. In the 1933 

Long Beach earthquake, 

liquefaction of the underlying 

soil damaged this section of 

the Pacific coast highway 

south of Los Angeles (photo 

courtesy City of Long Beach 

Public Library History Collection). In the 1994 Northridge 

earthquake, liquefaction also 

caused considerable damage 

in the Los Angeles Basin. 

Data being obtained by the U.S. Geological Survey on the distribution and character of near-surface 

sediments in the basin are providing information that is crucial to evaluating local susceptibility to earthquake damage caused by liquefaction and other forms of ground failure.