DaytonDailyNews.com

DAYTON — Allen McGrew, chairman of the geology department at the University of Dayton, said he didn't feel tremors from the earthquake Friday morning, "but my son said he woke up to what sounded like a garage-door opening, and my sister-in-law said she thought a train was passing by, but we don't have any trains near our house."

McGrew lives in Beavercreek, and his son and sister-in-law were sleeping upstairs in the home, he said.

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At 5.2 magnitude, "it was a big quake, around the level where you could have chimneys topple or poorly built houses receive damage within maybe a 20-to-25 mile radius of the epicenter, though sturdier homes and one-story ranches wouldn't be affected," McGrew said. "Southwest Indiana and Southeast Illinois, where the quake hit, probably got a pretty stiff jolt," he said.

The quake "was probably on the Wabash Valley fault system, which is peripherally related to the New Madrid system in southeast Missouri," McGrew said.

The Wabash Valley system has quakes "much less frequently and probably less strong" than the New Madrid fault, he said. The New Madrid system had a series of three great quakes in 1811 and the beginning of 1812 that rang church bells in Boston, he said.

"The thing about earthquakes in the Midwest is that they have an extremely large felt area," McGrew said.

A 1968 quake with an epicenter in southern Illinois was felt in upper floors of a building in Boston, Mass., he said. The reason tremors are felt so far away is that in the Midwest the strata are flat line and they're mostly continuous, so the energy transmission is much more efficient, he said.

In California, you have so many fault areas, that when they pass through the fault, they attenuate in the fault area and do not spread out as far, he said.

McGrew said the Indiana quake appears to have been caused by a side slip motion, which wasn't predicted by any foreshocks.

"There have already been three aftershocks so far" from Friday's quake, McGrew said. " They were recorded at a magnitude 2," he said, which wouldn't be felt.

"Typically after a large earthquake like this, there can be a hundred small aftershocks for days," he said. "The rule of thumb on earthquakes is that an aftershock of about one magnitude less than the original earthquake occurs within 24 hours. We may still have a magnitude 4 in store," he said, "but that's not certain."

Studies about 10 years ago of the ground disruption features showing where river sediment is disrupted by earthquakes in the Wabash fault area showed that "in the last 25,000 years there were seven or eight major earthquakes with a magnitude of up to 7.5, which could create very major damage, but there's a very low probability of that happening in our life time," he said.

California's big quakes have been around 6.7 magnitude, he said.

The tectonic plates of the Midwest are "probably the least understood," McGrew said. The quakes come probably from plates in Illinois with stresses from old ancient plains of weakness. There's a long scar where 10 million years ago the continent almost split apart, he said.

"When you have one earthquake, there's always a risk of having more, but it's not at all probable there would be a bigger one," McGrew said, "though it would not be impossible."

No one has successfully predicted earthquakes, he said. "They don't happen frequently enough in our area to estimate recurrence intervals."

McGrew said Charles Richter, inventor of the Richter magnitude scale, said only fools, charlatans and liars predict earthquakes. "Basically it's snake water," McGrew said.

Contact this reporter at (937) 225-2341 or Kullmer@DaytonDailyNews.com.

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