Hundreds of earthquakes strike central Idaho, rattling nerves

[ Challis, Idaho ( KMVT-TV / KSVT-TV ) For the last two weeks, a cluster of nearly 20 earthquakes of magnitude 2.5 or greater has shook Custer County]

By Laura Zuckerman

SALMON, Idaho (Reuters) - Hundreds of low-level and medium-sized earthquakes have struck central Idaho since last month, puzzling geologists who wonder whether the ruptures portend a much larger temblor to come or are merely the rumblings of a seismic fault previously thought to be dormant.

The recent earthquake swarm, beginning on March 24 and climaxed by a 4.9 magnitude tremor on Saturday, has produced no reports of injuries or severe damage but has rattled nerves in a region where Idaho's most powerful known quake, measured at 6.9, killed two children in 1983.

Saturday's earthquake was the strongest recorded in the state since 2005 and was followed on Monday by a magnitude 4.4 event that struck 10 miles north of the small ranching community of Challis, Idaho, according to the U.S. Geological Survey. 

The Challis tremor knocked pictures and animal mounts from walls, rattled dishes off tables and was felt by residents in neighboring Montana more than 100 miles from the quake's epicenter, officials said.

The latest seismic surge, including 100 small to moderate quakes on Monday alone, has galvanized government scientists, who planned to install special seismometers in the area as early as Tuesday to more closely track the activity.

The likelihood of a severe earthquake coming on the heels of the recent swarm is low, but much is perplexing about the series of tremors, said Bill Phillips, a geologist with the Idaho Geological Survey at the University of Idaho.

Such earthquake swarms typically are associated with the movement of molten rock below ground, which geologists credited for the recent quake cluster at Yellowstone National Park, or they are linked to an active fault, he said on Tuesday.

"What has many of us scratching our heads is the present-day swarm doesn't appear to be on the big, active fault in the area that ruptured in 1983 and caused the largest earthquake in Idaho," Phillips said. 

He was referring to the magnitude 6.9 temblor that struck Mount Borah, Idaho's tallest peak, killing two children in Challis and damaging hundreds of homes and businesses.

Idaho sits at the center of a seismic belt in the intermountain West that runs from northwestern Montana to southern Nevada and contains thousands of faults in the Earth's crust, said Michael Stickney, director of earthquake studies at the Montana Bureau of Mines and Geology.

Carl Alexander, disaster coordinator in Challis, said schools have stepped up earthquake drills, and he has requested that emergency responders in Idaho and Utah be available if disaster strikes.

Alexander is advising local residents to keep bottled water and canned goods on hand just in case "a big shaker" should strike.

"It does make your heart race a little bit to see your windows vibrating," he said of the recent tremors.

(Reporting by Laura Zuckerman; Editing by Steve Gorman and Ken Wills)