21-09-22 earthquake courtesy.jpg

A 4.0 aftershock near Trap Creek Campground was felt “lightly” in Boise, Ketchum, Challis and Idaho City on Saturday night, according to the USGS.

A year and a half since a 6.5-magnitude earthquake—Idaho’s second-largest—struck the Stanley region, the U.S. Geological Survey continues to log daily aftershocks in the Sawtooth and the Salmon-Challis national forests.

The agency has recorded 1,400 aftershocks in the area so far in 2021, putting the daily average at about five, according to Michael Stickney, director of the Earthquake Studies Office at the Montana Bureau of Mines and Geology.

The strongest aftershock this year measured a magnitude 4.0 and occurred Saturday at 10:18 p.m., with an epicenter near Trap Creek Campground northwest of Stanley. The tremor was felt “lightly” in Boise, Ketchum and Challis, according to the USGS Community Intensity Map. The second-strongest tremor this year measured a magnitude 3.8 and occurred Sunday around 10:40 p.m. just north of East Basin Lake, about 3 miles west of the Yankee Fork River.

Aftershocks above magnitude 4 are more likely to damage rock formations and buildings, according to the USGS. In August 2020, for example, a 4.2-magnitude earthquake in the Sawtooths triggered a large rockslide that resulted in the destruction of Baron Spire, a 120-foot granite rock formation also known among climbers as “Old Smoothie.”

Stickney said he did not expect “any significant damage” from the 4.0 aftershock on Saturday.

This month alone, the USGS has recorded 12 earthquakes magnitude 3.0 or higher in the Sawtooths area. On Sept. 10, four tremors that measured between 3.1 and 3.3. occurred near Anderson Creek, north of Stanley Lake, near Elk Mountain and north of Copper Mountain. The following Saturday, Sept. 11, a 3.1-magnitude aftershock was recorded about 6 miles beneath Stanley Lake. The under-lake aftershock was “too small to generate any sort of wave or currents, and thus should not have presented any dangers,” Stickney noted.

Seven tremors between magnitude 3 and 4 were logged this past Saturday, primarily in the Copper Mountain, Swamp Lake and Newman Creek areas.

According to Paul Earle, seismologist and director of operations at the U.S. Geological Survey’s National Earthquake Information Center, earthquakes in the Stanley region will be considered aftershocks until the earthquake frequency around the original 6.5-magnitude epicenter returns to its “background level.”

“This time depends on the size of the earthquake, and varies between earthquakes sequences,” he said.

A similar series of aftershocks played out in the Stanley region in the mid-1940s following two earthquakes in 1944 and 1955 that measured 6.1 and 6.0, respectively, according to Stickney.

“Both of these earthquakes generated aftershock sequences, but seismic monitoring in this region was in its infancy back then,” he said. “None of the aftershocks had instrumentally determined epicenters.”

Email the writer: ejones@mtexpress.com

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