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benelchi -> RE: Did you feel the Earth move this morning??? (4/21/2008 12:05:34 PM)
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ORIGINAL: joy2give2u
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"We don't have as many opportunities as in California," said Genda Chen, associate professor of engineering at the University of Missouri-Rolla, during an Associated Press interview
"We cannot even borrow on the knowledge they learn on the West Coast" she said, explaining that while quakes in California occur where tectonic plates beneath the Earth's surface collide, the Midwestern quakes happen far away from the edges of the nearest plates.
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Seismologists say Midwest earthquakes commonly radiate out for hundreds of miles due to the bedrock under much of the eastern United States.
"Our bedrock here is old, really rigid and sends those waves a long way," Bob Bauer, a geologist with the Illinois State Geological Survey, told the AP. Bauer equated the Midwest bedrock, which lies from a few feet to a few thousand feet below the earth’s surface, to a smooth bell that very efficiently transmits seismic waves like sound.
"California is young bedrock," he explained, "It's broken up ... like a cracked bell. You ring that, the waves don't go as far."
In California, the energy is dissipated on all the faults and the earthquakes are more local events
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Earthquakes in the central and eastern U.S., although less frequent than in the western U.S., are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi).
Who should go back to school ?????? [:)]
Those are actually apples and oranges.
The first is a question about how severe an earthquake is when different types of ground are compared; and USGS has done many studies about what types of soil result in the greatest amount of damage during an earthquake and which are the safest. They have earthquake maps that take the data from these studies and have acurately predicted the areas that would be most affected by an earthquake. Look at the maps and you will find that bedrock areas are always the safest areas.
The second question is about how far an earthquakes energy is dissipated from its epicenter, and although I might believe that an earthquake could be radiated over a larger area where the bedrock is one continuous piece; I don't buy the 10 times figure given in this article as the size and distances given for a midwest earthquakes are nearly identical to what is experienced out here on on the west coast; if there is any difference at all it is very minor. Even though this theory sounds a little more credible, I would have to see the data before I would accept it, and the examples given in this article actual go against the prediction he has made.
We actually have earthquakes here often enough that I have been on the phone with others when an earthquake has hit a number of times, so not only do I have a good idea about how far an earthquake is felt, but I also have a good idea about how fast they travel through the ground. When one person feels the earthquake, we will count the seconds until the other feels it as well, and then try and guess which direction it came from and where the epicenter is. I usually can guess the size of an earthquake within .5, and when I have been on the phone I have often been pretty close on the epicenter as well. It's one of those useless skills you learn when growing up in California.[:D]
Edited to add: Did you know that the epicenter for the 89' quake was almost 200KM away from the freeway collapse it caused i.e. the one that was in the news all over the entire country?
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