Afteshocks felt following Midwest quake

April 18, 2008 10:52:31 AM PDT
Reports of aftershocks from the Midwest earthquake have been received. ABC7 Chicago meteorologist Phil Schwarz said there had been at least six aftershocks as of approximately 11:20 a.m., one of which was reportedly in Chicago. According to the U.S. Geological Survey, the strongest aftershock was a 4.6 in magnitude. The rest of them have registered at a magnitude of 2. ABC7 Chicago received calls from people who said they felt the aftershocks.

For answers to questions about aftershocks, ABC7 Chicago turned to Dr. Timothy Larson of the Illinois State Geological Survey.

ABC7: Are we really feeling these aftershocks of magnitude 2? Wouldn't that be hard to feel?

Larson: Magnitude 2 earthquakes you wouldn't feel, normally. Although, we did have a 4.5 aftershock at approximately 10:14 a.m.

ABC7: We expect aftershocks after earthquakes. At this point, what more can we expect?

Larson: Aftershock sequences, usually for smaller earthquakes, they will diminish with time. It's unusual to have a felt aftershock at 4.5 [magnitude]. I felt it in Champaign. Usually in the Champaign area, we have smaller ones [aftershocks] that are only instrumented and are not felt.

ABC7: Considering that people could be nervous about aftershocks, how long do they usually last? Will it most likely be the rest of the day that they will occur or will it go on for a longer period of time?

Larson: Aftershocks that you can feel might continue through the day or the rest of the weekend. Some of my colleagues at other universities were sending out instruments to measure the earthquakes. Those very sensitive instruments will be able to measure aftershocks for weeks, possibly up to a month or so. With that, we'll be able to have much greater resolution on the three-dimensional location of where this is occurring.

ABC7: Is it possible for an aftershock to be stronger than the original quake?

Larson: Then it wouldn't be an aftershock. But, yes, it is possible, but it would be unusual for that to occur in this area. However, very large earthquakes are known to be clustered, but this is not a very large earthquake. Typically, in this area, we have moderate earthquakes followed by smaller aftershocks.

ABC7: We've known for years that there was this fault line. Should we be expecting anything else or could it be years before we experience anything like this again in our area?

Larson: They've been happening like clockwork every 20 years. There was one in 1987 and one in 1968. That's the pattern. But, you never know. We don't have enough data to say for sure and these are natural events that happen whenever they happen, but the pattern is every 20 years.


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