Earthquake Fault Found In Snake River Plain Discovery Implies Fault Zone Under Boise May Be Active

A graduate student at Boise State University has documented the first active earthquake fault found anywhere in Idaho’s Snake River plain.

Boise State geoscientists said Monday that the fault indicates earthquakes could occur again someday in the Treasure Valley, and it implies a fault zone beneath Boise may be active.

Gregg Beukelman and Boise State geoscience professors Charles Waag and James Zollweg identified what they named the Water Tank fault about 57 miles southeast of Boise and 13 miles from Grand View.

The area near the fault is sparsely populated, but Zollweg said a magnitude 7 earthquake there likely would cause damage in Boise and Mountain Home. A magnitude 7 earthquake is considered a major vent capable of widespread and heavy damage.

“There’s no way to forecast exactly when the next earthquake will occur,” Zollweg said. “It could be tomorrow or it could be thousands of years from now. But what we found does indicate that Boise needs to start taking earthquake hazards seriously.”

Beukelman found evidence of six prehistoric earthquakes on the Water Tank fault at estimated intervals ranging from 2,000 to 9,000 years. The most recent earthquake occurred about 3,000 years ago, and all the prehistoric quakes probably had magnitudes between 6.7 and 7.3, he said.

The largest earthquake in Idaho’s recorded history, the October 1983 Borah Peak temblor, had a magnitude of about 7.3 and killed two children in Challis. A January 1995 earthquake in Kobe, Japan, had a magnitude of about 7.2 and killed 5,300 people.

Faults are classified as active in the United States if there is historic or geologic evidence of a major earthquake in the last 10,000 years or repeated major earthquakes in the last 150,000 years. Zollweg and Waag said the Water Tank fault probably is only a minor fault in a larger system known as the Halfway Gulch fault zone.

They said a detailed study of the zone is essential to understanding the earthquake hazard in the Treasure Valley. However, the most important part of the fault zone cannot be investigated in as much detail as they would like because it is in a protected wilderness study area.

Zollweg said the discovery also implies the Boise Front fault zone beneath Boise may be active because of its geologic position across the western Snake River Plain from the Halfway Gulch fault zone. The two fault zones form opposite sides of a basin that has dropped down more than a mile to form part of the western Snake River Plain.

The U.S. Geological Survey recently awarded Zollweg and Waag a $16,184 grant to fund 80 percent of a one-year study of the Halfway Gulch fault zone. Boise State is contributing the other 20 percent.