A 4.2-magnitude Michigan earthquake: How did that happen?

The 4.2-magnitude earthquake that hit near Kalamazoo, Mich., Saturday afternoon speaks to how much scientists are still learning about the hidden faults that crisscross the United States.

Saturday's quake shook walls and knocked some items off shelves in southwestern Michigan but resulted in no reported damage.

Scientists have long known that faults lie under the state. The strongest recorded quake ever to originate in Michigan measured at 4.6 on the Richter scale and hit roughly the same area in 1947.

But the lack of seismic activity, combined with the fact that the faults lie so deep beneath the surface, has meant that that such faults have been difficult to find or study. The 30 recorded Michigan quakes that have been related to fault movement had their energy source more than 60 miles deep and left no evidence of the fault on the surface, wrote D. Michael Bricker, a Lansing, Mich., geologist in 1977.

The "bedrock is covered by a mantle of glacial deposits which obscures any surface evidence of faulting," he wrote in a report called "Seismic Disturbances in Michigan." "In these areas fault zones are considered to be stable, most movement having occurred in the distant geologic past."

But sometimes even such "stable" faults move. The United States Geological Survey's seismic hazard map for Michigan shows a 6 to 10 percent chance of seismic activity in the next 50 years for the southernmost part of Michigan, running from the Kalamazoo area east to the shore of Lake Erie. By comparison, the USGS seismic hazard map for California puts the probability of seismic activity for many parts of the coast at greater than 80 percent.

Even in exhaustively-studied southern California, though, scientists are still finding and learning about hidden faults.

The third Uniform California Earthquake Rupture Forecast, released earlier this year, included data on 350 faults in the state. The second UCERF, released in 2007, studied only 200. And the original 1988 study monitored only 16.

"As we’ve added more and more faults based on new science in California, we've come to discover it’s really an interconnected fault system," Ned Field, a USGS seismologist and lead author of the report, told the Monitor in March.

Reports have suggested that the Michigan quake was not the result of fracking and put its depth at only 3.7 miles. The activity will give scientists a new peek at the seismic mechanics at work beneath Michigan.