A boulder

A boulder the size of a two-story house took out a section of Trail 101 west of Redfish Lake in the Sawtooth Mountains.

The impacts of a 6.5-magnitude earthquake that struck northwest of Stanley on March 31, and its aftershocks, are still being uncovered in the Sawtooth Mountains north of Ketchum. For geologists, though, the near-daily tremors offer a rare chance to measure seismic activity in the area—and, hopefully, to help predict long-term risks.

“Idaho has very little knowledge of its seismic hazard and the current state of tectonic stress that mountain ranges are experiencing,” said Claudio Berti, director and state geologist of the Idaho Geological Survey. “Nature doesn’t make big mountains without associated big geologic hazards, and there are very big mountains in central and southeast Idaho.”

The iconic Sawtooth Range saw some noticeable changes by April when backcountry guides noticed that blocks had toppled from the top of the Finger of Fate, a spire popular with rock climbers. The nearby Arrowhead rock formation had fallen completely.

As winter snows melted it became apparent that Stanley Lake’s inlet beach had collapsed as a result of the earthquake. Deputy Area Ranger Brian Anderson said numerous rockfalls around nearby Grandjean were also apparent, and that a chimney at the Grandjean lodge had collapsed.

Anderson said landslides on the north shore of Redfish Lake brought numerous old-growth trees into the water, many of which have drifted to shore and can be seen around Redfish Lake Lodge. Logjams and individual floating logs litter many locations around the lake’s perimeter.

Berti compared satellite imagery and concluded that the largest of the lakeshore slides occurred about a month after the March 31 quake.

“This late occurrence is not a direct effect of the March 31 main shock, yet it cannot be excluded,” he said. “In fact, there is a very good likelihood that the shaking from the main earthquake event and the long series of aftershocks may have triggered or facilitated the landslides.”

About 1.5 miles along Trail 101 above the inlet to Redfish Lake a house-size boulder fell from the north side of the valley wall and rolled about 1,000 yards to the valley floor, splintering its way through a swath of trees until coming to rest. The boulder took out a section of the trail, which has since been repaired.

“Whether this is from the earthquake, it’s hard to say,” Berti said. “Rock falls of this kind can happen at any time in steep mountain environments, yet the fact that it happened at this time makes it reasonable to

attribute to the ongoing shaking. Mountain environments are always in a delicate equilibrium between rock standing high and gravity wanting to pull them down, and anything helps.”

Berti said the combination of seismic shaking, snowmelt and spring precipitation created a “perfect storm” for landslides.

“Fortunately, we’re heading into a drier period and this will help in stabilizing the slopes,” he said. “The seismic sequence is ongoing and quite active, with many aftershocks, some of which are of respectable magnitude. I wouldn’t be surprised if more events will happen.”

As mountain snows retreat, Anderson said hikers will likely find more examples of changes to the landscape due to earthquakes farther into the high country, especially after the 4.1 quake that hit on June 9.

Berti said the Idaho Geological Survey, in collaboration with Boise State University, have deployed a number of seismometers and seismic stations to accurately record the aftershocks. This is giving important information on the geometry of the fault or faults that have been activated in this recent “seismic crisis.”

“The main shock and the early aftershocks occurred in an area where we had no previous evidence, yet we had suspicions, of active faults, with no superficial evidence of them, like fractures or fault traces,” Berti said. “More recently, the aftershocks are occurring in places that are consistent for location and movement with well-known faults, including the Sawtooth Fault.”

Berti said earthquake faults in the area have moved in the recent geologic past, with evidence of movement in the past 10,000 years, yet he does not have a good chronology of their activity.

“Hopefully,” he said, “these events will help generate support for more detailed and much-needed study.”

Email the writer: tevans@mtexpress.com

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