U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 030-01

Revised July 2004

Did You Feel It?

Community-Made Earthquake Shaking Maps

REDUCING EARTHQUAKE LOSSES THROUGHOUT THE UNITED STATES

Since the early 1990’s, the 

magnitude and location of an 

Not long ago, the first thing that most 

people did after feeling an earthquake was to 

turn on their radio for information. Recently, 

however, after the Hector Mine earthquake 

in southern California and after a widely felt 

earthquake in upstate New York, many people 

logged onto the Internet instead, not only to 

get information, but also to share their own experience of the earthquake. After checking the 

U.S. Geological Survey (USGS) website for 

the location and magnitude of the earthquake, 

they went to a webpage called “Did You Feel 

It?” (http://earthquake.usgs.gov/shake/). 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 couple of hours, with more 

than several thousand responses for the southern California event, a Community Internet 

Intensity Map (CIIM) showed where and how 

strongly the earthquake had been felt.

Modifi ed Mercalli Intensities

The measure of shaking and damage from 

an earthquake is called “seismic intensity.” In 

general, the intensity decreases with increasing distance from the earthquake. However, 

a variety of factors, such as the type of earthquake, rupture direction, local geography, soil 

conditions, and type and age of buildings, reearthquake have been available 

within minutes on the Internet. 

Now, as a result of work by the U.S. 

Geological Survey (USGS) 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” (CIIM’s) contribute greatly 

in quickly assessing the scope of 

an earthquake emergency, even in 

areas lacking seismic instruments.

sult in an often complicated pattern of varying 

intensities from place to place. Since 1931, 

the USGS has been assigning intensities to 

earthquakes in the United States on the basis 

of the Modified Mercalli Intensity (MMI) 

scale. Typically, this is done by collecting 

responses to a postal questionnaire that is sent 

to each post office near the earthquake, and to 

a sparser sample of post offices with increasing distance from the earthquake. This way 

of preparing a seismic intensity map can take 

months to complete.

Community Internet Intensity Maps

In contrast to the old method, CIIM’s take 

advantage of the Internet to generate initial 

intensity maps almost instantly. Data are received through a questionnaire on the Internet 

answered by people who actually experienced 

the earthquake, reducing the process of preparing and distributing a shaking intensity 

map from months to minutes. 

A CIIM summarizes the responses, and an 

intensity number is assigned to each ZIP code 

for which a CIIM questionnaire is completed.

Community Internet Intensity Map (CIIM) for the February 28, 2001, magnitude 6.8 Nisqually, Washington, 

earthquake as of early April, 2001. Early estimates were that at least $2 billion in damage occurred in the 

Seattle region. Star shows epicenter, and colored areas on the map indicate the intensity for that ZIP-code 

area, as described in the key. White lines are major roads.

BREMERTON

BURLINGTON

EVERET/PAINE

FORT LEWIS

HOQUIAM

MCCHORD AFB

OAK HARBOR

OLYMPIA

PORT ANGELES

LAYUTE

RENTON

SEATTLE

SHELTON TACOMA STAMPEDE PAS

TOLEDO

WHIDBEY IS

12092 responses in 694 ZIP areas. Max intensity: VIII

0 7 KILOMETERS 0

INTENSITY

SHAKING

DAMAGE

I

Not felt

none

II-III

Weak

none

IV

Light

none

V

Moderate

Very light

VI

Strong

Light

VII

Very strong

Moderate

VIII

Severe

Moderate/Heavy

IX

Violent

Heavy

X+

Extreme

Very Heavy

0 MILES 50
For More Information

Earthquake Hazards Program

U.S. Geological Survey

DFC P.O. Box 25046, MS 966

Denver, CO 80225

http://earthquake.usgs.gov/

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 colorcoded 

according to the intensity scale below the map; 

other areas are gray.

A CIIM is automatically made after each 

widely felt earthquake in the United States. 

The system can start receiving responses within 

about 3 minutes after the earthquake. Internet 

users can also enter data for U.S. earthquakes 

they have experienced in the past.

CIIM values have been calibrated to be, on 

average, similar to MMI values. Comparisons 

between traditional MMI values obtained by 

using postal questionnaires and CIIM values 

show that they agree well except in areas of 

low shaking, where the CIIM values tend to be 

more reliable.

Why “Did You Feel It?”

In regions with few earthquakes and, hence, 

few seismic instruments, which includes most 

of the United States and most of the world, 

large numbers of intensity observations for a 

small to moderate event can indicate which 

areas will be more prone to shaking in the less 

frequent larger earthquakes.

This Fact Sheet and any updates to it are available 

online at:

http://pubs.usgs.gov/fs/fs030-01/

After a damaging earthquake in those 

sparsely instrumented areas, CIIM’s can provide information about which areas experienced the most shaking and therefore the most 

potential damage. This information can serve 

as a post-earthquake response tool and for estimating losses from future earthquakes.

In areas such as California where there are 

networks of seismic instruments, CIIM’s provide 

a very rapid means of displaying the pattern of 

shaking independently of strong-motion seismographs. CIIM’s provide descriptions of actual 

damage, rather than inferred potential 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 can 

be collected, providing details that would not be 

possible with the instruments alone. Eventually, 

the automated use of voluntary Internet contributions for the collection of intensity data following an earthquake will significantly reduce 

the manpower required to collect and interpret 

intensity observations.

Finally, the interactive nature of the Internet-based questionnaire and mapping provides 

an unprecedented opportunity for community 

involvement. The CIIM interactive website 

provides an avenue for feedback among the communities affected by earthquakes, the scientists 

studying their effects, and agencies responding 

to the events. The CIIM website allows people in 

an area struck by an earthquake to share their experiences, and this type of communication after 

a stressful event is an important tool for dealing 

with the emotional impact of such an experience.

Most areas of the United States, even many 

of those prone to earthquakes, do not have 

dense seismic networks. For such areas, CIIM’s 

provide the only rapid way to assess the distribution of shaking intensities and levels of damage. This quick information can aid in making 

the most effective use of emergency response 

resources and assist in preparing for future 

earthquakes. The work of the USGS and its cooperators in developing CIIM’s are only part of 

the ongoing USGS efforts to protect lives and 

property from future earthquakes throughout 

the United States.

Printed on recycled paper

Community Internet Intensity Maps are at:

http://earthquake.usgs.gov/shake/

Written by David Wald, Lisa Wald, James Dewey, 

Vince Quitoriano, and Elisabeth Adams

Graphic design by Lisa Wald 

Community Internet Intensity Map for the January 17, 1994, magnitude 6.7 Northridge, California, earthquake as of 

early April, 2001. This earthquake killed 57 people and caused more than $20 billion in damage in the Los Angeles 

region. Star shows epicenter, and colored areas on the map indicate the intensity for that ZIP-code area, as described in the key. White lines are major roads. CIIM data for the Northridge earthquake were collected beginning 

3 years after the event. CIIM’s are now being created immediately after earthquakes throughout the United States.

CASTAIC

LOS ANGELES

MALIBU

NORTHRIDGE

PALMDALE

PASADENA

2208 responses in 482 ZIP areas. Max intensity: IX

0 10 20 30KILOMETERS

INTENSITY

SHAKING

DAMAGE

I

Not felt

none

II-III

Weak

none

IV

Light

none

V

Moderate

Very light

VI

Strong

Light

VII

Very strong

Moderate

VIII

Severe

Moderate/Heavy

IX

Violent

Heavy

X+

Extreme

Very Heavy

0 2 10 0 MILES

How is an earthquake like a lightbulb?

The magnitude, or size, of an earthquake is like the wattage of a lightbulb. Just as the wattage represents

the amount of power of the lightbulb, the magnitude is related to the total amount of energy released by

the earthquake source. It is based on the amplitude of the earthquake waves recorded on instruments

that have a common calibration. The magnitude of an earthquake is thus represented by a single,

instrumentally determined value.

The intensity, or shaking level, is like the amount of light from a lightbulb that is received at any spot in a

room. A lightbulb in one corner of a room will make that corner bright (a high intensity value), but it will

leave the opposite corner of the room dim (a low intensity value). Intensity is based on the observed

effects of ground shaking on people, buildings, and natural features. Like the brightness of light in a room,

the intensity generally decreases as one moves away from the fault, but other factors such as rupture

direction and soil type also influence the amount of shaking. It also varies from place to place within the

disturbed region, depending on the location of the observer with respect to the earthquake epicenter.

Roman numerals are often used to describe intensities to distinguish them from magnitudes. The Modified

Mercalli Intensity scale is currently used in the United States.

For example, the magnitude of the October 16, 1999, Hector Mine, California, earthquake was 7.1, and the

shaking intensities ranged from VII (very strong) in towns nearest the epicenter to II-III (weak) at

distances as much as 300 miles (500 km) away.