About Community Internet Intensity Maps

Introduction

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.

What is Intensity?

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. We currently use 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.

More information on Magnitude and Intensity.

Calculating Community Internet Intensities

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 postal code regions. We consider all the filled-out questionnaires from a given postal code and assign a single intensity to the postal 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. Postal code areas for which data have been received are color-coded according to the intensity scale below the map; postal codes in gray are those for which we have not yet received data. At first only a few postal codes will have intensities assigned, but over time others will be assigned as data come in. Individual postal 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 postal 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 postal code may differ from the intensity that would be suggested by effects at a single location within the postal 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.

Examples of intensity maps

Examples of scientific evaluation of intensity for some significant earthquakes, and the associated intensity maps, can be seen by following links marked "intensity map" or "intensity report" from the page Significant Earthquakes of the 20th Century.

Bakun's Predicted Distance Attenuation

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 postal code plotted against the distance from the center of the postalcode 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 + 1.68Magnitude - 0.00345Distance - 2.08(log Distance)/(log 10) * Western USA: MMI = 5.07 + 1.09Magnitude -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.

CIIM and TriNet "ShakeMaps"

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 postal code regions.

CIIM and Modified Mercalli Intensities

In Canada and the United States, intensities have for many years been assigned on the basis of the Modified Mercalli Intensity scale (MMI) (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 theweeks 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 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 GSC Modified Mercalli Intensities.

Importance of Your Contribution, and a Caveat

It is important that you fill out a questionnaire, even if your postal code is already colored-in on the Community Internet Intensity Map for the earthquake. The more questionnaires that are received for your postal code, the more reliable will be the average intensity assigned to that postal 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 postal-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.

References

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.