Seismic swarm near sawtooth fault raises concerns among experts
Central Idaho experienced a series of more than 30 earthquakes between Monday afternoon and early Tuesday, according to the U.S. Geological Survey (USGS). The strongest, a magnitude 3.9 quake, struck near the remote town of Stanley at 9:06 a.m. ET on Tuesday, shaking the sparsely populated region and renewing fears about the Sawtooth Fault, a dormant geological structure capable of powerful seismic activity.
Quakes strike Idaho in rapid succession
The swarm began at 2:13 p.m. Monday, with USGS recording 33 quakes by early Tuesday morning, many of which clustered tightly around the town of Stanley, home to fewer than 150 residents. While several of the tremors surpassed magnitude 3.5, no injuries or structural damage were reported.
By 3:34 a.m. Tuesday, a magnitude 4.0 earthquake, the strongest in the past three weeks, added to a total of 133 recorded earthquakes in the area during that period.
Sawtooth Fault: A ‘sleeping giant’
The Sawtooth Fault, a 40-mile-long fault line identified in 2010, drew national attention in 2020 when a magnitude 6.5 earthquake, one of Idaho’s strongest in decades, shook the region. Though the fault had remained largely quiet until recently, geologists fear that its dormant but volatile nature could produce a devastating magnitude 7.0 or higher earthquake in the future.
“This recent seismic activity is unusual and worth monitoring closely,” said Idaho State Geologist Claudio Berti in an interview with the Daily Mail. “It’s impossible to predict what will happen next, but clusters like these can occasionally precede larger quakes.”
Seismic swarms: A sign or a fluke?
Seismic swarms — clusters of small-to-moderate quakes without a single dominant mainshock — are not uncommon in tectonically active areas. While they can sometimes precede larger quakes, scientists stress there is no certainty a major event will follow.
Quakes in the magnitude 2.5 to 5.4 range are typically felt by residents and may cause minor structural damage, but none has been reported in Stanley so far.
Mapping the Earth’s cracks
According to Berti, the swarm is offering valuable data about the location and behavior of the Sawtooth Fault. “Each quake helps us understand how the Earth’s crust is moving,” he noted, explaining that the shifting tectonic plates, massive rock slabs covering the Earth, generate earthquakes when built-up stress overcomes friction along fault lines.
The Sawtooth Fault is part of the broader Basin and Range Province, a seismically active region shaped over millions of years of tectonic stretching and faulting.
Residents remain alert
Though Stanley residents are used to occasional tremors, the volume and frequency of recent quakes have some on edge. Officials are urging locals to stay informed, review emergency preparedness plans, and report any damage or unusual ground movement.
While no major quake has followed this week’s activity, geologists will be watching closely, and so will Stanley.
