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Did You Feel It?

Gauging earthquakes on the Web

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earthquake intensity

A quake's intensity, not its magnitude, is important for structures.

E. V. Leyendecker, US Geological Survey

Science isn't always about machinery and instruments. In the case of earthquakes, simple human observations have always been important, and today they are more important than ever.

Why Humans Still Matter

Earthquake researchers began in the 1800s by asking people what they felt and saw. The answers varied wildly. Some people panicked while others slept through it; some saw their homes crumble while others had no damage. Maybe one person noticed agitated animals as another dodged falling bookshelves. From these observations came scales of earthquake intensity. The United States uses the Modified Mercalli scale of quake intensity. Japan, China, Europe and many other places use similar intensity scales.

Seismic intensity is not the same as earthquake magnitude. Intensity, based on witnesses and damages, measures how hard an earthquake shakes a place while magnitude, based on seismograms, is a calculation of the total energy a quake releases. Charles Richter, who made the first scale of magnitude, often used the analogy of radio. Magnitude is like the broadcast power of the radio station, he would say, and intensity is like the strength of its signal at your receiver. If you ask me, magnitude is how big a quake is while intensity is how bad it is.

Earthquake researchers have always talked to people, but until recently they used to ignore you and me. After all, some people minimize or exaggerate things for reasons of their own. The U.S. Geological Survey's standard method was to send questionnaires to local postmasters after a significant quake, then interview expert observers like building engineers before carefully drawing a seismic intensity map for that quake.

But the Web makes you and me relevant again.

Building a Web-Based Intensity Scale

Now that scientists collect earthquake reports on the USGS Web site, they can quickly gather intensity reports from thousands of ordinary people. Visit the Survey's "Did You Feel It" page after an earthquake and you can watch these reports turn into an instant Community Internet Intensity Map. After one moderate California quake in late 2007, the system collected more than 63,000 reports without a hitch.

USGS researchers have been seeing what can be done with these maps. First, they've confirmed that Community Internet Intensity (CII) is a pretty good match to the offical Modified Mercalli Intensity (MMI). Compared to MMIs, the CIIs created by ordinary people seem to be slight overestimates, but they're close enough for government work.

Next they built some software tools:

  • One takes CII reports as they stream in and zeroes in on the quake's epicenter. This may be more accurate than an epicenter derived from a few distant seismometers.
  • With the help of some numbers characterizing the local rocks, a proper magnitude for the quake can be determined. Again, this serves as an independent check where seismometers are sparse.
  • Another tool translates CIIs into accelerations like those that a strong-motion detector would record. This allows a city with no strong-motion instruments to learn from earthquakes. Such places are very common, and not just in the third world—most American cities outside the mountain West are poorly instrumented.

The team tried out the tools on various quakes, then used it to model two events in the eastern United States. In the Evansville, Indiana quake of 8 June 2002 (M 5.0), for instance, there were few instrumental records but thousands of CII reports.

Current Capabilities

Today, the CII system is the Survey's default tool for determining intensities. It has impressive capabilities. For instance, CIIs can be used to map intensities in tenths of a unit. And the system can collect data for all U.S. earthquakes strong enough to be felt, even very small events below the detection limit of the widely spaced seismograph network in the eastern states. It also collects reports of large meteors (bolides) and sonic booms. And the extra comments submitted by citizens are a growing stockpile of sociological data about how people respond to earthquakes.

The new knowledge of people's mass response to earthquake shaking has spawned another new USGS service called PAGER. Given an earthquake anywhere in the world, PAGER provides automated estimates of the number of people exposed to different levels of shaking. This gives the world quick notice of major disasters with more useful detail than a bare magnitude.

Even more recently, a USGS project is monitoring real-time traffic on Twitter as a gauge of earthquakes. Called TED, for Twitter Earthquake Detector, it's easily followed as @USGSTed.

Going Beyond the Present

The information gathered by the Community Internet Intensity network helps in studying not just today's earthquakes, but also seismic events of the past and the future.

People of the distant past were untrained observers just like us. That means seismologists can use you and me to calibrate the evidence contained in old newspaper stories, diaries, letters and photographs. Because instrumental earthquake records have such a short history, document-based studies are valuable for extending the historical record. And the more we know about the past, the better we can plan for the future. Likewise, we can use this information to model how a city in the future—say, Tokyo in 2050—might respond to expected earthquakes.

Another indicator for the future is that thousands of ordinary people, just by using the CII system, are becoming better earthquake observers. As their skill grows, their awareness of earthquake issues also rises. That in turn promises to make each of them—each of you—more effective members of society. And that is one of the great benefits of this kind of citizen science.

PS: Take a look at this CII map for the Gilroy quake for another development: geocoded records. The first CII maps relied on you reporting your ZIP code, but if you send your street address instead, the system can pinpoint your location to within a few meters.

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