U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet FS-2004-3017

February 2004 Printed on recycled paper

U n d e r s t a n d i n g E a r t h q u a k e H a z a r d s I n U r b a n A r e a s

High-Resolution Imaging within the Urban Environment—

“What’s beneath our streets”!

As part of the National Earthquake Hazards Reduction Program (NEHRP), detailed seismic-reflection studies 

(high-resolution imaging) are conducted in urban areas to gather information necessary to assess potential 

earthquake hazards within communities prone to damaging earthquakes.

Why Work in the Streets and 

How this Information is Used

National and regional earthquake hazards maps are critical to 

effective hazard-mitigation strategies for communities prone to earthquakes. For this reason, NEHRP 

conducts studies in urban areas to 

produce hazard assessments that 

reflect local variations in geology 

and seismic-wave propagation. The 

greatest challenge in producing 

credible and useful regional hazard 

assessments is reducing modeling 

uncertainties through more accurate 

and abundant geological and seismological observations. This pertains 

particularly to information on where 

earthquakes originate (source), their 

size (magnitude) and recurrence time, 

and site-specific ground-shaking 

effects. To acquire this information 

in urban areas, studies are conducted 

that measure near-surface attributes 

such as geologic setting, structure, 

seismic velocity, topography, ground 

water, and rock properties.

Northridge earthquake 1994 

aftermath. Over 60 fatalities and in excess of 

20 billion dollars in damage

Source truck sends sound-wave energy, described by one resident as “a garbage truck doing its pickup,” into the ground by 

vibrating a pad for 10–20 seconds at stations located typically 

15–33 feet apart. The generated signal produces a picture of 

the subsurface rock layers to a depth of 4,000–6,000 feet.

Geophone sensors, 

placed temporarily along 

the street curb, listen for 

the reflected “echo” generated when the source 

sound-wave is bounced 

back from the underlying 

rock layers. Urban studies 

often use hundreds of 

geophones in an array 

that can stretch more 

than a mile. 

Collecting Data in the Urban Community

 Cables connect 

the geophone 

array and carry 

the reflected echo 

information back 

to a recording 

computer. 
Taking the “Picture” —

Beneath the City Streets

Cities are filled with cultural noises that are caused by traffic, airplanes, and pedestrians within 

the community conducting activities 

associated with everyday life. Consequently, acquiring quality information to create a “picture” of what is 

beneath the streets under these less 

than ideal conditions (from a seismicimaging viewpoint) is challenging to 

research scientists. 

To meet this challenge, the U.S. 

Geological Survey has adapted and 

modified existing equipment and 

data- acquisition techniques. The 

equipment used to collect information 

must be powerful enough for survey 

needs without damaging streets and 

nearby structures or disrupting the 

flow of traffic or pedestrians. The signal-recording equipment (geophone 

sensors and cables) must be sensitive 

enough to record the seismic signal 

over nearly a mile in the noisy city 

environment without interfering with 

everyday community activities.

Hundreds of vibration points are 

integrated to produce a depth crosssection of the rocks beneath a city. 

Computer processing corrects for 

elevation changes, bends in line 

geometry and missing geophones 

(street crossings). Noise filtering and 

signal enhancement software is used 

to remove some of the degrading 

effects introduced by the everyday 

background noise. 

Using the “Picture”

The subsurface images or “pictures” that result from these studies in 

the streets of the urban community 

increase our knowledge of earthquake 

occurrence, potential earthquake 

ground shaking, and the geology that 

commonly controls water flow. From 

the subsurface “pictures” we are able 

to see faults and determine if and how 

they move. These images also help 

explain why different parts of a city 

shake more strongly than others and 

contribute to work that could lead to 

forecasting future earthquake ground 

shaking. Also, knowing the position 

and geometry of faults and the thickness of rock layers and unconsoli

dated materials will improve our ability to create ground-water models and, 

ultimately, lead to better management 

of the existing water supply.

Record produced when the energy 

source truck vibrates. Each vertical 

line (squiggle) represents information 

received from one geophone. The entire 

record contains information from an 

array of 240 geophones that are “listening” along nearly 1 mile in San Jose, 

Calif. The field record is a plot of time 

(vertical) versus distance (horizontal) 

for the sound wave to travel to a rock 

boundary below and return to the surface. The rock layer boundaries appear 

as heavy, coherent black bands (in the 

figure, some are highlighted in red). 

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This San Bernardino Basin profile created from hundreds of individual records (such as Record shown above) is an example of a high-resolution 

seismic-reflection “pict 

with thick sediments 

have broken, offset, and folded the underlying rocks. 

For more information, contact:

Earthquake Hazards Program

U.S. Geological Survey

Golden, CO 80401

Written by Jackson K. Odum, Robert A. Williams, 

William J. Stephenson, and David M. Worley

Graphics and layout design by Eleanor M. Omdahl

This Fact Sheet is available online at:

http://pubs.usgs.gov/fs/2004/3017/