Did You Feel It? The Science Behind the Maps

  1. Introduction
  2. What is Intensity?
  3. Calculating Community Intensities
  4. What is geocoding?
  5. Global DYFI
  6. Predicted Distance Attenuation
  7. CIIM and ShakeMap
  8. CIIM and Modified Mercalli Intensities
  9. Importance of Your Contribution and a Caveat
  10. Acknowledgments
  11. Publications
Did You Feel It? Citizens Contribute to Earthquake Science

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. Currently used in the U.S. is the Modified Mercalli Intensity Scale (see below). Each earthquake that 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.

Calculating Community Internet Intensities

The Community Internet Intensity Map (CIIM) summarizes the questionnaire responses provided by you and other Internet users. An intensity value 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 five minutes following a significant earthquake and then less frequently as additional data are 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.

What is geocoding?

We have the capability of 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 TomTom, a company which turns regular street addresses into precise latitude and longitude coordinates (generally 6 digits of accuracy, enough to distinguish the nearest 1/2 block on a street). 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. To test this geocoding on your own address, try this interactive script.

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 to the needed precision).

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.

Global DYFI

We have recently expanded CIIM to a global scale. Users from all over the world can now contribute their earthquake experiences using the Did You Feel It? system for the "Outside US" region. It works very similarly to the US-based system. Since ZIP codes do not exist or are not available in most countries, we have simplified the system to base location on the approximate latitude and longitude of the city which the respondent was closest to. A city with a response is displayed as a filled circle. The size of the circle is dependent on the approximate population of the city, and the color represents the average intensity for the city. Since we need to know the city's location to map it, only cities in our database can be selected by observers.

Although we receive fewer responses on average per event, the global system is a very useful tool for rapidly assessing earthquake effects worldwide.

Predicted Distance Attenuation

We have also been making a variety of plots of your responses. The distance vs. intensity plot contains three different overlaid plots:

  1. The average intensity for each ZIP code plotted against the distance from the ZIP code centroid to the epicenter. Each is represented by an open green circle.
  2. The average intensity for each distance "bin" in log space. Effectively, we combine ZIP codes with similar distances away from the epicenter into bins, similar to concentric rings. These bins are evenly spaced in log space, but grow in size in linear space. For each of these bins, we take the average intensity from the ZIP code intensities. See this example image, taken from eventID: us2003teak, for a visual reference. These are represented by filled, blue boxes with error bars of one standard deviation. The purple line is a sixth order polynomial that best fits the blue boxes.
  3. The predicted distance attenuation developed by Hotovec and Wald for the magnitude of the earthquake, labeled in orange. The form of the equation closely resembles Bakun's equation for the change of MMI with distance, but is formulated using CDI from CIIM. As you may see on several graphs, the equation doesn't quite fit the data, especially close in, and we are working to find a better functional form.

We are currently using these equations:

  1. Eastern USA: CDI = 1.60 + 1.29*Magnitude - 0.00051*Distance - 2.16*(log Distance)/(log 10)
  2. Western USA: CDI = 1.15 + 1.01*Magnitude - 0.00054*Distance - 1.72*(log Distance)/(log 10)

These equations provide a predicted distance vs. intensity curve for various regions based on the known magnitude of the earthquake. Given enough intensity data, it is possible to find the magnitude of an unknown earthquake using these equations.

CIIM and ShakeMap

The Community Internet Intensity Map (CIIM) is made to be compatible with ShakeMap rapid instrumental intensity maps. Like the ShakeMap, the CIIM's are centered on the epicenter (star) of the earthquake and have similar overall dimensions as the ShakeMaps. However, ShakeMap 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. ShakeMap does not represent any averaging over ZIP code regions.

CIIM and Modified Mercalli Intensities

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.

Importance of Your Contribution and a Caveat

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.

Acknowledgments

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.

Publications

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