Ohio is in the middle of a crustal plate. I thought earthquakes only occur at the plate edges, where they are crashing together.
Earthquakes in Ohio and adjacent areas are known as Intraplate Earthquakes; that is, they occur in the middle of a large crustal plate (North American plate). We know, of course, that the vast majority of earthquakes occur at plate boundaries, where two plates are crashing together or sliding past one another, as in the case of the San Andreas fault in California. However, intraplate earthquakes might be viewed as remnants of ancient plate boundaries where events of long ago have left persistent zones of weakness.
About a billion years ago, Ohio was a tectonically active area. An episode of crustal extension (pulling apart) created an area of down-dropped crust, known as a rift zone, in western Ohio. Crustal extension stopped before the crust was completely split, creating a failed rift. Numerous faults marked the boundary of the failed rift, now known as the Anna or Fort Wayne rift, and these faults form the zone of weakness where many earthquakes occur today.
After the rifting event, the eastern margin of the North American continent collided with a continental mass to the east, creating a chain of mountains that ran north-south from Canada to Tennessee and beyond. These mountains, known as the Grenville Mountains for exposures in Canada, were eventually worn down by erosion and buried beneath sediments deposited by continental seas during the Paleozoic Era. Many deeply buried faults were created during this mountain-building event.
The faults associated with the Grenville Mountains and the Anna rift are now ancient zones of weakness. As the North American continent is now pushed slowly westward as the Atlantic Ocean widens, stresses are created in these crustal rocks. The ancient fractures are zones of weakness along which stress is relieved by occasional earthquakes.
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