KALAMAZOO, MI -- Twenty years ago, a Michigan State University scientist wrote a paper suggesting there might be geological fault line in southeast Kalamazoo County.

A May 2 earthquake proved him right.

It appears the fault that caused Saturday's quake is part of the same fault involved in a 1947 quake in Coldwater, said Kazuya Fujita, a MSU geosciences professor.

"I always thought there was something there because of variations in the magnetic field," Fujita said about southeast Kalamazoo County. "When I heard the quake was in Kalamazoo, I figured it was probably that fault."

Based on data collected by the U.S. Geological Survey, the fault involved in the May 2 quake "matches almost perfectly" with Fujita's theory, said Chris Schmidt, a geoscientist for Western Michigan University.

The quake's epicenter was in Scotts, a rural farming community about 12 miles southeast of Kalamazoo.

"It looks like the fault goes between Coldwater and Kalamazoo," Schmidt said. "If it goes north of that, we don't know, and we're not sure if goes further south."

A big reason for the mystery is the fault line is about four miles underground.

"Don't bother looking for it" on the ground's surface, Schmidt said. "You won't find it."

However, based on the U.S. Geological Survey data, Schmidt said it appears the May 2 earthquake involved a strike-slip fault, a vertical fracture where the blocks move horizontally.

At 12:23 p.m., the northeast side of the fault suddenly shifted northwest while the southwest side slid southeast, releasing tension between the blocks that had been building for years, Schmidt said.

It was probably a movement of a few feet, "although that's a very off-the-wall guess," Schmidt said.

That movement triggered a 4.2 magnitude earthquake felt in five states and southern Ontario.

Like other scientists, Schmidt and Fujita said the quake appears to be the result of natural forces.

"This isn't a man-made quake," Schmidt said. "It's an old fault under strain. There's periodic build up and release. It's like stretching a rubber band, where it eventually snaps."

RELATED: Michigan quake not a result of fracking, scientists say

That strain is created, he said, by the fact that the North American continent -- or at least, this part of the continent -- shifts about 2 centimeters west a year. That's a continuation of the movement started hundreds of millions years ago when the continent broke away from Africa.

The May 2 quake provides a data point for predicting future seismic activity along the fault, Schmidt said.

There are historical records that make reference to an earthquake that occurred in nearby Climax in 1883. Sixty-four years later was Michigan's largest quake on record, the 1947 quake in Coldwater. Now, 68 years after the Coldwater event, is another quake apparently along the same fault line.

"So in 65 years or so, we might -- exclamation point, might -- get another," Schmidt said.

The quake also means future permit applications for drilling in southeast Kalamazoo County will get a closer look, said Hal Fitch, director of the Michigan Department of Environmental Quality's Office of Oil, Gas, and Minerals.

In the past few years, much has been made of the seeming connection between earthquakes and hydraulic fracturing -- also known as fracking -- to extract oil or gas.

The problem isn't the fracking itself, Fitch and other experts say. Rather, scientists have determined earthquakes can be triggered by the high-pressure injection wells used to dispose of wastewater that is the byproduct of fracking. Such human-induced quakes been seen in Colorado, Texas, Arkansas, Oklahoma and Ohio, according to the U.S. Geological Survey.

In the case of the May 2 quake, there currently are no high-pressure injection wells nearby the newfound fault.

In terms of the future, "we do consider proximity to the fault and to past seismic activity in evaluating permit applications in this area, and will continue to do so," Fitch said in an email to the Kalamazoo Gazette.

"The occurrence of the May 2 quake would not make us hesitant to issue a permit for a well that would be hydraulically fractured because hydraulic fracturing itself is not generally associated with induced seismicity," he said.

"If we receive an application for an injection well in this area in the future, the quake occurrence does give us more information that we would use in our evaluation."

* Complete coverage of the Michigan earthquake

Julie Mack is a reporter for Kalamazoo Gazette. Email her at jmack1@mlive.com, call her at 269-350-0277 or follow her on Twitter @kzjuliemack.