COMMUNITY INTERNET INTENSITY MAP: the science behind the maps
For more detailed information see the online CIIM paper in HTML
or as a PDF format file. (Warning: The PDF file is 2.7 Mb.)
- Introduction
- What is Intensity?
- Calculating Community Intensities
- NEW: What is geocoding?
- NEW: Bakun's Predicted Distance Attenuation
- CIIM and Trinet ShakeMaps
- CIIM and Modified Mercalli Intensities
- Importance of Your Contribution (and a Caveat)
- References
- Acknowledgments
This web site is intended to tap the abundant information available about
earthquakes from the people who actually experience them. By taking
advantage of the vast numbers of Internet users, we can get a more
complete description of what people experienced, the effects of the
earthquake, and the extent of damage, than traditional ways of gathering
felt information. And best of all, with your help we can do so almost
instantly.
By contributing your experience of the earthquake, either immediately
afterward, or whenever it is possible for you to do so, you will have made
a contribution to the scientific body of information about this
earthquake. You will also ensure that your area has been represented in
the compilation of the shaking map. This is a two-way street. Not only
will you add valuable information on the extent of ground shaking and
damage, but in the process we hope you will learn more about how other
communities fared and gain a greater understanding of the effects of
earthquakes.
There are two different ways to describe the size of an earthquake. One is
magnitude (often described by the Richter magnitude), which
is related to the amount of energy released by the earthquake source. It is
usually determined by measuring the amplitude (height) or by modeling
the character of the earthquake waves recorded by a seismograph.
The second way of describing earthquake size is to measure earthquake
effects. Intensity is a qualitative measure of the strength of
ground shaking at a particular site. Currently used in the U.S. is the Modified Mercalli Intensity Scale (see below). Each
earthquake which is large enough to be felt will have a range of
intensities. Usually (but not always) the highest intensities are measured
near the earthquake epicenter and lower intensities are measured farther
away. Roman numerals are used to describe intensities to distinguish them
from magnitudes. For example, the magnitude of the Northridge earthquake
was 6.7. The intensities ranged from IX (violent) close to the epicenter,
to V's (moderate) at distances of about 60 - 200 miles away, and finally
not felt at distances far away.
For more information consult the USGS National Earthquake Information
Center descriptions of
Magnitude and Intensity
The Community Internet Intensity Map (CIIM) summarizes the questionnaire
responses provided by you and other Internet users. An intensity number
has been assigned to each community from which we have received a
filled-out CIIM questionnaire; each intensity value reflects the effects of
earthquake shaking on the people and structures in the community. For
convenience, we define "communities" to be ZIP code regions. We consider
all the filled-out questionnaires from a given ZIP code and assign a single
intensity to the ZIP code. The form of the questionnaire and the method
for assignment of intensities are based on an algorithm developed by
Dengler and Dewey (1998) for determining a
"Community Decimal Intensity". The decimal intensity values computed by the
algorithm of Dengler and Dewey have been rounded off to integers for
the Community Internet Intensity Map and represented by Roman
Numerals.
A Community Internet Intensity Map is made and updated every few minutes
following a significant earthquake and then less frequently as additional
data is received. ZIP code areas for which data have been received are
color-coded according to the intensity scale below the map; ZIP codes in
gray are those for which we have not yet received data. At first only a
few ZIP codes will have intensities assigned, but over time others will
be assigned as data come in. Individual ZIP code zones may change color as
a new consensus is reached (that is, data from more respondents may change
the average intensity value for a ZIP code). Check back often and
remember to reload your browser to see the update! Note the date and time
on the lower left corner of the map to keep track of the updates.
NOTE: Since earthquake effects may vary significantly over small
distances, the average intensity shown for an entire ZIP code may differ
from the intensity that would be suggested by effects at a single location
within the ZIP code. Further, the input data is raw and unchecked, and may
contain errors. Finally, some details of the procedure for preparing the
CIIM may change as we gain experience with data collection from the
Internet. Hence, the nature of the CIIM may evolve with time. See the
Disclaimer for other considerations.
Recently, we've started adding geocoded maps for certain larger events with many
hundreds (or thousands) of responses. To do this, we take the addresses that
people provide when they fill out our questionnaire, and send them to
Etak, a company which turns regular street
addresses into very precise latitude and longitude coordinates (generally
6 digits of accuracy, enough to distinguish between different houses on a
street). (To test this geocoding on your own address, try Etak's
interactive script). We then
group nearby coordinates into regularly sized boxes, which are generally a few
km across, and calculate their intensities the same way we do for normal zip code
maps.
There are several advantages to using geocoding for maps, including greater
precision in locating intensities and cleaner-looking maps (since zip codes
tend to be very irregularly shaped). For rapid response to earthquakes, though,
zip codes are still preferable - almost everyone knows immediately which zip code
they are in, whereas few people know their latitude and longitude (and especially
not out to much accuracy).
Note that because roughly 5-10% of respondants do not leave their address there are
correspondingly fewer responses on a geocode map vs. a zip code map.
We have recently started making a variety of plots of your responses. The distance vs. intensity plot contains three different overlayed plots:
1. The calculated intensity for each zip code plotted against the distance from the center of the zipcode to the epicenter.
2. The average distance for each intensity (at 0.1 Intensity increments).
3. The predicted distance attenuation developed by Bakun and collaborators for the magnitude of the earthquake.
We are currently using these equations:
- Eastern USA: MMI = 1.41 + 1.68*Magnitude - 0.00345*Distance - 2.08*(log Distance)/(log 10)
- Western USA: MMI = 5.07 + 1.09*Magnitude -3.69*(log Distance)/(log 10);
These equations provide a predicted intensity vs. distance curve for various regions based on the known magnitude of the earthquke. Given enough intensity data, it is possible to find the magnitude of an unknown earthquake using these equations.
Notice that above the calculation is for MMI and not for CII.
The Community Internet Intensity Map (CIIM) is made to be compatible with
TriNet "ShakeMap"
Rapid Instrumental Intensity Maps (RIIM). Like the "ShakeMap", the CIIM's are
centered on the epicenter (star) of the earthquake and have similar
overall dimensions as the "ShakeMaps". However, the "ShakeMap" RIIM is
based on point location measurements of the ground motion as recorded by
seismometers, and the shaking intensity is inferred by empirically
relating the recorded ground motions to intensities and then interpolating the
ground motions between the recording sites. The "ShakeMap" RIIM does not
represent any averaging over ZIP code regions.
For United States earthquakes, intensities have for many years been
assigned on the basis of the
Modified Mercalli Intensity scale (Wood and Neumann,
1931; Richter, 1958). The Modified Mercalli Intensities are based in
part on postal questionnaires, in which respondents summarize the effects
of shaking in their communities. In addition, Modified Mercalli Intensities
are based on field study in areas of significant damage, on damage maps
produced by emergency response agencies, on reports produced by the
earthquake engineering community, and on press reports. For a destructive
earthquake, the process of collecting and interpreting damage data and
preparing a map of Modified Mercalli Intensities takes months.
The procedure used to calculate the Community Internet Intensity values was
calibrated so that the Community Internet Intensity values should, on
average, be similar to the Modified Mercalli Intensity values for the same
communities (Dengler and Dewey, 1998). We hope that the
CIIM will serve as
a useful first approximation to Modified Mercalli Intensity maps, in the
weeks to months following damaging earthquakes during which the final
Modified Mercalli Intensity maps are being prepared. We also envision that
the data collected from the CIIM questionnaires will be incorporated into
final assignment of USGS Modified Mercalli Intensities. Nevertheless,
because there are major differences in the data and procedures used to
assign the two types of intensities, the Community Internet Intensities
cannot be considered to be identical to the USGS Modified Mercalli
Intensities.
It is important that you fill out a questionnaire, even if your zip code is
already colored-in on the Community Internet Intensity Map for the
earthquake. The more questionnaires that are received for your ZIP code,
the more reliable will be the average intensity assigned to that ZIP code.
Even if you did not feel the earthquake, your questionnaire is
important: in areas of lighter shaking, the "not-felt" responses are
needed to prevent the average ZIP-code intensities from being too high.
Since, even with our encouragement to send in "not-felt" responses, it is
more likely that those that felt the earthquake will login and respond to
the questionnaire than those who did not feel it, there may be cases where
the lowest intensities are biased slightly upward.
Dengler, L. A., and J. W. Dewey (1998).
An Intensity Survey of Households Affected by the Northridge, California, Earthquake of 17 January, 1994,
Bulletin of the Seismological Society of America,
Vol. 88, p. 441-462.
Dewey, J. W., B. G. Reagor, L. Dengler, and K. Moley (1995).
Intensity distribution and isoseismal maps for the Northridge, California, earthquake of
January 17, 1994,
U. S. Geological Survey Open-File Report 95-92,
35 pp.
Richter, C. F. (1958).
Elementary Seismology.,
W. H. Freeman and Company, San Francisco, 135-149.
Wald, D. J. V. Quitoriano, L. A. Dengler, and J. W. Dewey (1999).
Utilization of the Internet for Rapid Community Intensity Maps
Seismological Research Letters, 70, No. 6, 680-697.
Wood, H. O., and F. Neumann (1931).
Modified Mercalli Intensity Scale of 1931,
Bulletin of the Seismological Society of America,
Vol. 21, p. 277-283.
We have benefited greatly from discussions and reviews by Jim Dewey of
the U.S. Geological Survey in Golden, Colorado,
who has the primary responsibility
for producing the Modified Mercalli Intensity Maps for the U.S.
Government. Lori Dengler at Humboldt State University also provided critical
input based on her experience with public responses to the Northridge
earthquake and to earthquakes in Northern California and Oregon.
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