You might say the continent was just letting off a little pressure, and the folks living along the southern edge of Lima, Ohio, were shook up over it.

About 2:30 yesterday morning, a 2.5 magnitude earthquake gave the Allen County community a little jiggle with the sound of a loud boom, sending about 75 worried residents to the phone to ask police what had blown up.

But Michael Hansen, coordinator of the Ohio seismic network, who was called by Lima police just a few minutes later, says the source of the rumble was quite a bit father away - about the middle of the Atlantic Ocean. In a way, anyway.

It turns out that endlessly flat, glacier-groomed terrain of western Ohio covers a multitude of faults - breaks in the Earth's crust where some movement has occurred through the ages. These faults make this part of the state one of the two most seismically active. On May 11, a small quake occurred in almost this same location in Lima.

The roots of all this rumbling go back about a billion years. Lima was a considerably different place then, when it lay below the equator with the rest of the world's land mass.

About that time, the continent tried to tear itself in two in what today is west central Ohio. Although the breakup halted as abruptly as it began, the result is a rift in the bedrock, an area of weakness some 3 miles below the ground.

Just as that calmed down, North America smashed into northern South America. The collision pushed a mountain range into existence in eastern Ohio that stretched from Canada to Alabama, Mr. Hansen said. The emerging mountains shoved rocks from eastern Ohio as far west as Toledo. The boundary of that rocky displacement is called the Grenville Front Tectonic Zone, which stretches far south of the Ohio River.

Geologist have traced a number of fault lines through western Ohio that stemmed from all this ancient sturm und drang.

So, how does this connect to the Atlantic Ocean? There's a long mountain range in the middle of the Atlantic, extending almost pole to pole. These mountains grow through the upwelling of magma from the Earth's mantle. The emerging mountains bump up against the North American continental plate, pushing it westward, said Mr. Hansen, also a professor at Ohio State University. Inevitably, that pressure has to be released, and sometimes the release is in the western Ohio fault zone.

The area has small earthquakes fairly frequently, compared with most of the rest of Ohio. Lima had its first recorded earthquake in 1884 - a 4.8-magnitude shiver, which makes it some 900 times more powerful than yesterday's rumble.

Anna, a town in Shelby County, two counties south of Lima, experienced earthquakes of greater than magnitude 4 in 1930, 1931, 1937, and 1986.

But the state record holder is Lake County in eastern Ohio, where a magnitude 5 earthquake broke windows, and cracked plaster just a few miles from the Perry nuclear power plant in 1986.

Lake County has been far more seismically active this year than the western part of the state, with 13 quakes above magnitude 2, compared with the two quakes in western Ohio.

There were no reports of property damage from the Lima quake this week, said Russ Decker, director of the Allen County Office of Homeland Security.

Still, Mr. Decker has invested in earthquake insurance for his home. It's inexpensive, he said, and who knows - one day it may pay off.

"We're like the little San Francisco of Ohio," he said.

Contact Jenni Laidman at:

jenni@theblade.com

or 419 724-6507.

First Published August 16, 2006, 11:21 a.m.