Here are the details of this morning's earthquake that shook southern Illinois and Indiana.

WFIE-TV weatherman Byron Douglas was on the air in Evansville, Ind., when the quake hit. Watch the video of the tremor and the cool, calm way he handled it. WAVE-TV in Louisville, Ky., also was broadcasting live.

Earthquake Info from the U.S. Geological Survey

• Constantly updated, worldwide earthquake activity from the Earthquake Hazards Program.
• Info, including history, for each state.
• List of the largest earthquakes by state.
• Seismic hazard maps showing where earthquakes are most likely.
• "Did You Feel It" page, where you can see maps of recent events with color-coded areas indicating how strong people reported the tremors. You can submit your own report there, too.
• "Top Ten" earthquake facts for the U.S. and the world, including deadliest, largest, most recent and most significant.

As you look at the USGS maps, you recognize that the earth's crust is constantly shifting. By some estimates there is a quake somewhere on the globe every 11 seconds. The U.S. Geological Survey says that there are 500,000 detectable earthquakes in the world each year; of those, 100,000 can be felt, and 100 of them cause damage.

To put today's quake in perspective, I am filing this report from Alaska, where I am teaching this weekend. Earlier this week there was a 6.6-magnitude quake out in the Aleutians that you never heard about. You also have not heard much about what scientists are calling an "earthquake swarm" occurring off the coast of Oregon. This swarm involves up to 60 quakes per day.

How are quakes measured?

The Richter scale, which goes from 1 to 10, measures the movement -- the shaking -- from earthquakes. Each whole-number increase represents a tenfold increase in movement. So the movement (scientists call it wave amplitude) in a level 5 earthquake such as today's is 10 times greater than in a level 4 earthquake.

I have heard some journalists get this wrong today. They said each whole-number increase was a tenfold increase in energy, not movement. Actually, when movement increases by 10 times, the amount of energy multiplies by 30.

We also now know that quakes of the same "scale" might create more damage depending on where they strike. In part that is because of regional differences in geology.

The old measure was the Richter scale, but now scientists use the Modified Mercalli Scale, which measures from 1 to 12. The Mercalli scale is more interested in the effects of a quake on people and structures than the shaking itself. Japanese scientists use the Omori Scale.

What are faults, where are they and how do they work?

Click here for a primer on the three kinds of faults found on Earth.

Can we predict earthquakes?

In a word, no, but scientists know more then they used to. Scientists are especially interested in gas discharges and electrical changes in the earth's surface prior to quakes. The USGS and the California Earthquake Authority recently published this 30-year outlook for California quake activity.

Should you get earthquake insurance?

The answer depends on where you live. Earthquake insurance makes as much sense in some parts of the country as flood insurance makes for those of us living near water. A basic homeowner's policy does not cover quake damage, just like basic policies do not cover flood damage. Read more from:

• Insure.com
• San Francisco Chronicle
• Business First of Louisville

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