U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2005-3016

March 2005 Printed on recycled paper

Then and Now

Not so long ago, the first thing that 

most people did after feeling an earthquake was to turn on their radio for 

information. That practice is changing, 

however. After the 2003 San Simeon

earthquake in central California, for 

example, many people logged onto the

Internet, not only to get information, 

but also to share their own experience 

of the earthquake. After checking the 

U.S. Geological Survey (USGS) Web 

site for the location and magnitude of 

the earthquake, they went to a Web page 

called “Did You Feel It?” (at http://

earthquake.usgs.gov/). They entered 

their ZIP Code and answered a list of questions such as 

“Did the earthquake wake you up?” and “Did objects fall off 

shelves?” In minutes a map began taking shape on the Internet, 

and in a few hours, with more than 14,000 responses for the 

central California event, a Community Internet Intensity Map 

(CIIM) showed where and how strongly the earthquake had 

been felt and where damage occurred.

Did You Feel It?

Citizens Contribute to Earthquake Science

Since the early 1990s, the magnitude and location of an earthquake have been

available within minutes on the Internet. Now, as a result of work by the U.S.

Geological Survey and with the cooperation of various regional seismic networks,

people who experience an earthquake can go online and share information

about its effects to help create a map of shaking intensities and damage. Such

“Community Internet Intensity Maps” (CIIMs) contribute greatly toward the quick

assessment of the scope of an earthquake emergency and provide valuable

data for earthquake research.

Photographs represent effects of the 2003 San Simeon, Calif., earthquake. Following this event, over 18,000 

responses to “Did You Feel It?” were shared with the USGS. The earthquake was Intensity VIII in Paso 

Robles (left) and Intensity VII near Atascadero (right). 

VII

VIII

Macroseismic Intensity

Macroseismic intensity describes the strength of shaking 

from an earthquake at a particular location, as determined from 

effects that people can readily observe without special instruments or special training. Such macroseismic effects include 

damage caused by the earthquake and the strength of shaking as 

perceived by people.

In general, the macroseismic intensity is highest near the 

earthquake source and decreases with distance from the source. 

However, a variety of factors—such as the direction in which 

the earthquake fault ruptures and variations in the soil conditions underlying different sites—may lead to complicated patterns of intensities that vary strongly from place to place.

Community Internet Intensity Maps

In contrast to the intensity maps prepared from paper-copy 

questionnaires, CIIMs take advantage of the Internet to generate intensity maps almost instantly. Data are received through 

questionnaires on the Internet answered by people who experienced the earthquake. The Internet approach reduces the time for 

preparing and distributing a shaking-intensity map from months 

to minutes.

Since 1931, the USGS has assigned macroscopic intensities to 

United States earthquakes on the basis of the Modified Mercalli 

Intensity (MMI) scale. Until recently, most of the macroseismic 

observations used to assign intensities were collected with questionnaires that were mailed to post offices in the earthquake 

region. The process of sending the questionnaires by standard 

mail, waiting for written responses, manually interpreting the 

responses, and preparing intensity maps could take months. In the 

late 1990s, the USGS began collecting data and publishing CIIMs 

on the “Did You Feel It?” Web page.
Did You Feel It? Community Internet Intensity Map data for the 

years 1999 to 2004 are plotted on the United States map below. These data 

indicate shaking levels reported nationwide by over 500,000 individuals. Earthquake shaking has been reported from many regions of long-term high seismic 

hazard (see hazard map on next page). Most areas with long-term high hazard, 

however, have experienced only low-intensity shaking during the relatively short 

period during which the CIIM Web site has been in operation.

Did You Feel It?

Fill out the questionnaire 

by selecting appropriate 

answers and filling in 

the blanks. You may also 

contribute longer descriptions of your experience. 

Review the CIIM Web 

site to see your response 

contribute to building the 

detailed map!

Go to URL http://

earthquakes.usgs.

gov/ and select 

“DID YOU FEEL IT?” 

Select the region 

where you felt the 

earthquake.

From the list of recent 

earthquakes in your 

region, either select 

the earthquake that 

corresponds to your 

location and the time 

you felt the event 

or select “New or 

Unknown Earthquake.” 

Fill out the questionnaire.

Your input does make a difference!
The USGS National Seismic-Hazard Map: Earthquake hazards exist in many parts of the United States. The probability that an 

earthquake hazard will occur in any given area is shown by this USGS National 

Seismic-Hazard Map. USGS national and regional seismic-hazard maps forecast 

the amount of shaking expected over specified time periods. Many parts of the 

central and Eastern United States have long-term moderate to high hazards, even 

though those areas have not experienced recent large quakes. USGS seismichazard maps such as this one are used as a basis for building codes and are also 

widely used by structural engineers and government agencies.

Probability of an

Earthquake Hazard

A Unique Tool for Understanding Earthquakes

In areas such as California where there are networks of seismic

instruments, CIIMs provide a very rapid means of displaying the

pattern of shaking independent of strong-motion seismographs.

CIIMs provide descriptions of actual damage, rather than inferred

damage indicated by instrumental shaking records. Also, the

potential number of Internet responses far exceeds the number of

seismic instruments, so very dense sampling of earthquake effects

is possible, providing details that would not be possible with the

instruments alone.

In regions with few seismic instruments, which includes most 

of the United States and most of the World, intensity observations for a small to moderate event can indicate which areas will 

be more prone to shaking in larger, but less frequent earthquakes. 

After a damaging earthquake in those sparsely instrumented 

areas, CIIMs can provide information about which areas experienced the most shaking and, therefore, the most potential damage. 

This information can serve not only as a tool for postearthquake 

response, but also for estimating losses from future earthquakes.

The interactive nature of the Internet questionnaire and mapping

provides an unprecedented opportunity for community involvement.

The CIIM interactive Web site provides an avenue for feedback

among the communities affected by earthquakes, scientists studying

earthquake effects, and agencies responding to the events. By allowing people in an area struck by an earthquake to share their experiences, the CIIM Web site may help them cope with the emotional

impact of the earthquake.

A CIIM summarizes the responses. An intensity number is

assigned to each ZIP Code for which one or more CIIM questionnaires are completed. The intensity values in each ZIP-Code

area are averaged, and the map is updated as additional data are

received. ZIP-Code areas for which data have been received are

color-coded according to the intensity scale shown below the

map on the CIIM Web page; other areas are gray.

A CIIM is automatically made after each widely felt earthquake in the United States. The system can start receiving

responses immediately after the earthquake. Internet users can

also enter data for significant U.S. earthquakes they have experienced in the past.

The CIIM procedure was constructed so that CIIM intensities would agree on average with MMI values produced by the 

traditional procedure that is based on postal questionnaires. For 

some specific levels of shaking, differences are likely between 

intensities produced by the two procedures, just because the 

procedures are different. Five years experience with the CIIM 

procedure, however, has shown that the CIIM values usually 

agree well with MMI values that would be assigned by the traditional procedure. In cases where the new CIIM methodology 

produces values that differ from those produced by the traditional procedure, the CIIM values are usually more self-consistent, because they are usually based on many more observations 

in a given ZIP Code.
For More Information:

Earthquake Hazards Program

U.S. Geological Survey

Golden, CO 80401

http://earthquake.usgs.gov/

Written by David J. Wald and 

James W. Dewey

Graphics and Layout Design by 

Eleanor M. Omdahl 

This Fact Sheet is available online at 

http://pubs.usgs.gov/fs/2005/3016/

0 10 20

km LONG BEACH

LOS ANGELES

NORTHRIDGE

PASADENA

SANTA MONICA

Comparison of intensity distribution for magnitude 7.1 (left) and magnitude 4.4 (right) earthquakes in southern California. 

Note that the small event produced intensity IV to V near the epicenter (star), whereas the large event produced intensity 

VIII to IX near the causative fault (black line) and intensities IV to V (comparable to the smaller event) at great distances 

from the epicenter and fault.

Typical Effects of Different Intensity Levels.

[Most of the effects are sometimes observed in special circumstances at lower intensities than suggested by the table. Some of the effects are used to help define intensity levels in the CIIM procedure; other effects do not influence calculation of intensity values, but are nonetheless commonly observed]

PAHRUMP

LANCASTER

ANZA

AVALON

KERSFIELD BAKER

BARSTOW

BLYTHE

BRAWLEY

CASTAIC

DESERT CENTER

EDWARDS AFB

EL CENTRO

FORT IRWIN

FURNACE CREEK RANCH

GORMAN

IRVINE

JULIAN

L.ISABELLA

LAS VEGAS

LITTLE LAKE

LONG BEACH

LOS ANGELES

MECCA

MEXICALI

MOJAVE

SAN CLEMENTE IS.

NEEDLES

NORTHRIDGE

OCEANSIDE

OLANCHA

PALM SPRINGS

PALMDALE

PALOMAR

PARKER PASADENA

RTERVILLE

RIVERSIDE

SALTON CITY

SAN CLEMENTE

SAN DIEGO

SHOSHONE

TIJUANA

TRONA

TWENTYNINE PALMS

VICTORVILLE

WRIGHTWOOD

YUMA

0 50 100

km

CIIM

Intensity

People’s

Reaction

Furnishings Built Environment Natural Environment

I Not felt Changes in level and clarity of well 

water are occasionally associated 

with great earthquakes at distances 

beyond which the earthquake is felt 

by people.

II Felt by a few. Delicately suspended objects 

may swing.

III Felt by several; 

vibration like passing of truck.

Hanging objects may swing 

appreciably.

IV Felt by many; sensation like heavy 

body striking 

building.

Dishes rattle. Walls creak; windows rattle.

V Felt by nearly all; 

frightens a few.

Pictures swing out of place; 

small objects move; a few 

objects fall from shelves 

within the community.

A few instances of cracked plaster and cracked windows 

within the community.

Trees and bushes shaken 

noticeably.

VI Frightens many; 

people move 

unsteadily.

Many objects fall from 

shelves within the community.

A few instances of fallen plaster, broken windows, and 

damaged chimneys within the community.

Some fall of tree limbs and tops, 

isolated rockfalls and landslides, 

and isolated liquefaction.

VII Frightens most; 

some lose balance.

Heavy furniture overturned. Damage negligible in buildings of good design and construction, but considerable in some poorly built or badly 

designed structures; weak chimneys broken at roof line, 

fall of unbraced parapets.

Tree damage, rockfalls, landslides, 

and liquefaction are more severe 

and widespread with increasing 

intensity.

VIII Many find it difficult to stand.

Very heavy furniture moves 

conspicuously.

Damage slight in buildings designed to be earthquake 

resistant, but severe in some poorly built structures. Widespread fall of chimneys and monuments.

IX Some forcibly 

thrown to the 

ground.

Damage considerable in some buildings designed to be 

earthquake resistant; buildings shift off foundations if not 

bolted to them.

X Most ordinary masonry structures collapse; damage 

moderate to severe in many buildings designed to be 

earthquake resistant.

N

N