Site for 3-km-deep

San Andreas Fault Observatory

at Depth (SAFOD)

Site for 3-km-deep

San Andreas Fault Observatory

at Depth (SAFOD)

N

0 10 KILOMETERS

Paso

Robles

San Miquel Cholame

Coalinga

Parkfield

101

46

46

1966 M 6

rupture zone

San Andreas Fault San Andreas Fault

Initiation point

for 1934

and 1966

earthquakes

North end

of 1857 M 7.9

rupture zone

Initiation point

for 1934

and 1966

earthquakes

CALI FORN I A

Area of

map

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 049-02

Revised July 2004

The Parkfield Experiment—

Capturing What Happens in an Earthquake

REDUCING EARTHQUAKE LOSSES THROUGHOUT THE UNITED STATES

To better understand what happens on and near a fault before, 

Moderate earthquakes (magnitude about 

6) have occurred on the Parkfield section 

of the San Andreas Fault at fairly regular 

intervals—in 1857, 1881, 1901, 1922, 

1934, and 1966. All these Parkfield earthquakes have struck in the same area—historical seismograms show that at least the 

1934 and 1966 shocks initiated at the same 

point on the fault.

This fairly regular repetition of similar-magnitude earthquakes prompted the 

U.S. Geological Survey (USGS) in 1985 

to forecast the likelihood of another moderate earthquake in the Parkfield region 

within the next several years. Scientists 

also recognized that Parkfield was an ideal 

site to capture the physical processes leading up to and immediately following an 

earthquake. As a result, the USGS and the 

California Geological Survey (then the 

Division of Mines and Geology) placed 

during, and after an earthquake, the 

U.S. Geological Survey (USGS) and the 

California Geological Survey began the 

Parkfield Earthquake Experiment in the 

1980’s. Researchers from the USGS and 

collaborating institutions have created 

a dense network of instruments on the 

San Andreas Fault at Parkfield, California, where moderate earthquakes have 

occurred at fairly regular intervals. Data 

from these instruments are revealing the 

earthquake process in unprecedented 

detail and will aid in predicting the time 

and severity of future shocks. The National Science Foundation (NSF) is expanding the Parkfield Experiment by drilling a 

deep borehole and installing instruments 

at the actual depths where earthquakes 

initiate, creating a San Andreas Fault 

Observatory at Depth (SAFOD).

instruments in the ground at Parkfield to 

seek methods for making short-term predictions of earthquakes based on precursory events in the preceding hours or days. 

This initial joint effort developed into the 

Parkfield Earthquake Experiment.

The goals of the experiment (see box) 

have both scientific and public safety implications that reach far beyond the rural 

Parkfield region of central California. To 

achieve those goals, scientists develop, 

install, and operate real-time monitoring 

systems for seismic activity and for various physical and chemical parameters in 

the rocks, soil, and ground water. Sensitive 

instruments have been deployed to measure the buildup and release of strain during the earthquake cycle and to precisely 

record strong ground shaking during the 

next magnitude 6 earthquake.

Although the anticipated earthquake has 

not yet occurred, strain continues to accumulate on this section of the fault. Indeed, 

more energy has now accumulated there 

since the last magnitude 6 earthquake in 

1966 than was released in that quake. Thus, 

the hazard remains high and the occurrence 

of another earthquake seems certain.

• Observe the buildup and release of 

stresses on the San Andreas Fault 

through multiple earthquake cycles.

• Test the feasibility of short-term earthquake prediction.

• Measure near-fault shaking during 

earthquake rupture and learn better how 

to predict the amplifi cation of shaking 

caused by different soil types for improving building codes and designs.

 MAJOR GOALS OF THE 

PARKFIELD EARTHQUAKE EXPERIMENT

Study area for the Parkfi eld Earthquake 

Experiment at Parkfi eld, California. The 

Parkfi eld area is considered an ideal 

place to study the earthquake process. 

The San Andreas Fault (red line) last 

ruptured here in 1966 (yellow zone). A 

similar rupture in 1857 was a foreshock to the last great earthquake in 

southern California, the 1857 Fort Tejon 

magnitude 7.9 event (pink zone). Brown 

squares represent sites of geophysical 

monitoring instruments, including seismometers, strainmeters, creepmeters, 

and global positioning system (GPS) 

receivers. Drilling for SAFOD began in 

June 2004 (photo).
Surface

trace of

San Andreas

Fault

0 3 KILOMETERS

Pilot hole

Fault Zone

SAFOD

For more information contact:

Earthquake Information Offi ce (650) 329-4668

U.S. Geological Survey, Mail Stop 977

345 Middlefi eld Road

Menlo Park, CA 94025

http://quake.usgs.gov/research/parkfi eld

http://www.earthscope.org

COOPERATING AGENCIES

California Geological Survey

California Offi ce of Emergency Services

EarthScope

International Continental Scientifi c Drilling Program

National Science Foundation

Stanford University

Steve Hickman and John Langbein

Edited by

Peter H. Stauffer

Graphic design by

Susan Mayfi eld and Sara Boore

Creation of a San Andreas Fault 

Observatory at Depth (SAFOD)

Building on more than 15 years of experience from the Parkfield Earthquake Experiment, the National Science Foundation 

(NSF) is drilling a deep hole in order to install instruments directly within the San Andreas Fault Zone near the initiation point of 

previous magnitude 6 Parkfield earthquakes. 

These instruments, set 2 to 3 km beneath 

the Earth’s surface, will form a San Andreas 

Fault Observatory at Depth (SAFOD). This 

project, under the joint leadership of Stanford University and the USGS, will directly 

reveal, for the first time, the physical and 

chemical processes controlling earthquake 

generation on a seismically active fault.

In June 2004, drilling the hole for 

SAFOD began west of the San Andreas 

Fault and is using advanced directionaldrilling technology developed by the petroleum industry to angle the hole through 

the entire fault zone until relatively 

undisturbed rock is reached on the east 

side. Fault-zone rocks and fluids will be 

retrieved for laboratory analyses, and geophysical measurements will be made within the active fault zone. SAFOD’s longterm monitoring activities will include 

detailed observations of small to moderate 

earthquakes and continuous measurement 

of rock deformation and other parameters 

during the earthquake cycle.

SAFOD will provide direct information 

on the composition and mechanical properties of rocks in the fault zone, the nature 

of stresses responsible for earthquakes, the 

role of fluids in controlling faulting and 

earthquake recurrence, and the physics 

of earthquake initiation and rupture. By 

observing quakes “up close,” SAFOD will 

mark a major advance in the pursuit of a 

rigorous scientific basis for assessing earthquake hazards and predicting earthquakes.

EarthScope and the SAFOD Pilot Hole

Funding for SAFOD was obtained by 

NSF as part of an ambitious scientific 

initiative called EarthScope. The other 

elements of EarthScope are USArray and 

the Plate Boundary Observatory (PBO). 

USArray consists of a large transportable broadband seismic array, augmented 

by smaller seismic arrays and coordinated with the USGS Advanced National 

Seismic System. The PBO is a network 

of deformation sensors for the western 

United States similar to that deployed at 

Parkfield. PBO will enhance the USGS's 

existing network of GPS receivers and 

strain meters at Parkfield. Support for scientific studies using SAFOD and the other 

EarthScope facilities will come through 

programs within NSF, USGS, and other 

agencies, both in the U.S. and abroad.

In preparation for SAFOD, a 2.2-kmdeep vertical pilot hole was drilled adjacent 

to the San Andreas Fault at Parkfield in the 

summer of 2002. This pilot project was developed as a collaborative effort among the 

International Continental Scientific Drilling 

Program, NSF, and USGS and is guiding 

This Fact Sheet and any updates to it are available 

online at:

http://pubs.usgs.gov/fact-sheet/fs049-02/

• Creation of the most complete active 

fault observatory in the world.

• Development and continuous operation for more than 15 years of real-time 

warning system and expansion of such 

rapid earthquake-reporting capability 

to cover all of California.

• Open and unrestricted access to 

monitoring data through the Internet, 

permitting the entire scientifi c community to create and test models of the 

earthquake cycle.

• Direct measurement of stress buildup 

on the San Andreas Fault and recognition that stress buildup is not uniform 

with time.

• Discovery that many small earthquakes 

at Parkfi eld are virtually identical and 

repeatedly rupture the same area on 

the fault.

Schematic cross section 

of the San Andreas 

Fault Zone at Parkfi eld, 

showing the drill hole 

for the San Andreas 

Fault Observatory at 

Depth (SAFOD) and the 

pilot hole drilled in 2002. 

Red dots in drill holes 

show sites of monitoring 

instruments. White dots 

represent area of persistent minor seismicity 

at depths of 2.5 to more 

than 10 km. The colors 

in the subsurface show 

electrical resistivity of the 

rocks as determined from 

surface surveys; the lowest-resistivity rocks (red) 

above the area of minor 

earthquakes may represent a fl uid-rich zone.

subsequent SAFOD scientific investigations 

within the active fault zone.

The cooperative efforts of USGS and 

other scientists in the Parkfield Earthquake 

Experiment and SAFOD will help achieve 

a better understanding of what happens on 

and near a fault during the earthquake cycle and will aid in predicting the time and 

severity of future quakes. This work is part 

of the National Earthquake Hazard Reduction Program’s ongoing efforts to protect 

people’s lives and property from the earthquakes that are inevitable in California and 

elsewhere in the United States.

Printed on recycled paper

SOME ACHIEVEMENTS OF THE 

PARKFIELD EARTHQUAKE EXPERIMENT