COEUR d'ALENE - Two words are rarely mentioned together - Idaho earthquakes.

Now, though, researchers are scrambling to gather more data after a recent swarm of earthquakes struck the central Idaho town of Challis.

Central Idaho has had hundreds of small and medium earthquakes in the past few weeks.

One of the quakes was a magnitude 4.9, rattling items off walls and startling residents in the town of about 1,000. No one was hurt.

"It's pretty close to getting dangerous and may be widely felt" at that magnitude, said Bill Phillips, a research geologist for the Idaho Geological Survey at the University of Idaho.

The last time Idaho had an earthquake larger than 4.9 was April 21, 2001. That was a 5.3 near Grays Lake in southeast Idaho.

"You can see that 4.9 are pretty rare," Phillips said.

The Coeur d'Alene-Post Falls area has no known active faults and no recorded earthquakes.

Geologists haven't ruled out the Challis swarm as a foreshock sequence for a big one in central Idaho.

"Some large quakes have foreshocks - clusters of smaller quakes in the same area - before the larger quake," Phillips said. "But it's also important to recall that swarms like this - small earthquakes, generally shallow, tightly clustered - are relatively common in Idaho."

The Challis earthquakes began rumbling on March 24 northwest of Challis, about a 2-hour drive north from Sun Valley.

The swarm has been concentrated in a rectangular area measuring 12 to 14 miles north and south by 5 to 6 miles east and west.

"These swarms occur in multiple places in Idaho and we don't know what causes them," Phillips said.

There was a swarm around Christmas near the Teton County seat of Driggs, and another in December around McCall.

"We want to know if these things are caused by active faults," Phillips said.

If so, the "seismic hazard" increases, he said.

Beginning Tuesday, portable seismometers were placed in the area of the Challis swarm, as the state doesn't have a dense seismic-activity recording network.

The U.S. Geological Survey brought in two for deployment and the University of Utah is deploying four more.

The Wasatch (Mountains) Fault in Utah is capable of powerful earthquakes in the 7.5 magnitude range, putting people and infrastructure in places like Salt Lake City at risk, according to the USGS. So the University of Utah has to be a leader in the science.

One of the devices, a NetQuake box, is installed in a home with an Internet connection, enabling it to transmit data to researchers when an earthquake is detected.

Geologists don't know the source for the Challis swarm, which occurred in an area with no known active faults. Inactive faults crisscross the area.

"We hope it's a matter of weeks or months" before the source of the earthquakes is known, he said. "But it could be much longer."

In 1983, a 6.9 magnitude earthquake occurred about 10 to 15 miles from the current Challis swarm area.

The Trans-Challis fault zone hasn't been active in 45 million years. The Challis swarm is on the boundary of the zone.

"Swarms typically, we think, are occurring where there are variations in the usual structure along a fault," said Craig Weaver, a seismologist for the USGS at the University of Washington.

There are several hypotheses as to why the earth is moving in Challis.

A fault could be seeing some growth, as the tip of one end extends, Phillips said. Another is the ground is moving because of deep geothermal fluids moving, as there are plenty of known hot springs in the area.

"These earthquakes are near the surface," Phillips said.

They are believed to be occurring five or six kilometers deep, making them more easy for people to feel.

The 4.9 magnitude quake was felt in Salmon, Idaho, Sun Valley, and western Montana.

As for any connection to the volcano in Yellowstone National Park that's been getting attention lately: "I'm in no position to say they're linked, and I'm in no position to say they're not," Phillips said.

In 2001, there was a swarm of earthquakes under Spokane for about five months. That got people's attention.

The USGS released information about a new, shallow thrust fault under the city, which some researchers have named the Spokane Fault.

If a swarm is related to a fault, then that sends researchers down one set of explanations for why they're occurring.

"But if it's not on that fault, now we have another whole set of questions," Weaver said.

With the portable seismometers in place around the area of the Challis swarm and the regional stations in places like western Montana, researchers should be able to more precisely locate the quakes. They'll need some quakes big enough to register on both sets of machines.

"This is all part of being able to better characterize the hazard," Weaver said.