Central Idaho quakes likely stemmed from undocumented fault
According to the Associated Press, the U.S. Geological Survey had recorded 47 aftershocks, with one measuring magnitude 4.6, by mid-morning on Wednesday.
People may feel aftershocks for the next couple of weeks, but the USGS predicts the odds of an aftershock above magnitude 6.5 occurring during the next week is less than 1 percent, and the odds of an aftershock reaching at least magnitude 5 are about 20 percent.
"The reason for large aftershocks, sometimes when one fault moves it stresses another part of the fault and there's another earthquake," said Glenn Thackray, an Idaho State University geosciences professor.
Thackray said Boise State University seismologists were at the scene Wednesday seeking to record the aftershocks to better document the faults in the area.
"That will allow the fault to be mapped in the subsurface," Thackray said. "The aftershocks will tell us a lot about the fault itself."
Thackray said the major active fault in the vicinity extends along the front of the Sawtooth Range near Stanley, sloping underneath the valley and Red Fish Lake.
Thackray said the earthquakes don't appear to have originated from that large fault, which he's personally studied. Rather, they seem to have been caused by a strike-slip fault, where one block slides horizontally past an adjacent block. He said the fault was likely previously undocumented, or the earthquakes may have stemmed from some older faults in the area that have only been suspected to still be active.
"This is the sort of stuff that gets people excited about science and geology," Thackray said. "It's like bread and butter to us."
Thackray said there's no geological connection linking the recent earthquakes to a magnitude 5.9 earthquake that caused damage in Salt Lake City about two weeks earlier. Thackray described the occurrence of the two strong earthquakes in the same region within less than a month, on separate faults, as "a very strong coincidence but only a coincidence."
Central Idaho quakes likely stemmed from undocumented fault