The Borah Peak, Idaho Earthquake of October 28, 1983—Structural Control of Groundwater Eruptions and Sediment Boil Formation in the Chilly Buttes Area

Abstract

Groundwater eruptions attendant to the Borah Peak earthquake formed more than 40 sediment boils. The eruptions were strongly controlled by solution widened fractures in limestone underlying alluvium. The dominant fracture set oriented N15°-20°W is imprinted upon the thin alluvial cover which is a confining layer. The fractures and shear zones in the alluvium served as important conduits for the discharge of groundwater. Water and air expulsed from the underlying carbonate entered the alluvium under high pressure and entrained and fluidized the alluvium forming sediment eruptions. The pressure surge which caused the eruptions probably resulted from a porewater pressure increase caused by compression of the carbonate in the hanging wall as it dropped.