The Borah Peak, Idaho Earthquake of October 28, 1983—Hydrologic Effects

Abstract

Spectacular groundwater effects accompanied the October 28, 1983 Idaho earthquake (Ms = 7.3). Groundwater discharge increased from many springs and base flow of major rivers significantly increased over an 18,000 km2 region. In the epicentral area a 35-m surge in artesian pressure produced spectacular bursts of muddy water from carbonate bedrock hills at Chilly Buttes. This pressure surge was also manifested as a 1.4-km long zone of large sand boils erupted through the valley alluvium. At Smith's Fish Hatchery, sediment eroded by the surge of water clogged the fresh spring-water supply inlet pipes and led to suffocation of the entire crop of trout. North of the epicentral area a major warm springs dried up for 8 days and then recommenced flowing at an ever increasing rate until it stabilized at about 9 times the pre-earthquake flow. This flow of about 1400 l/s caused severe bank erosion of pastureland until diverted into an artificially excavated channel. At Clayton Silver Mine, 50 km north of the epicenter, increased seepage into the underground mine workings overwhelmed the mine-pump capacity and flooded the 330-meter level within 14 hrs. Thirty kms south of the epicenter a 200,000 m3 mudflow was apparently caused by increased spring flows. The valley fill failed two days after the earthquake and devastated a 3.5-km reach of Lupine Creek valley. Similar coseismic and post-seismic hydrologic effects were reported in studies of the 1952 Kern County, California earthquake, and the 1959 Hebgen Lake, Montana earthquake. Several mechanisms for these hydrologic phenomena are considered, but the most plausible cause is the sudden release of elastic strain in aquifer rocks. Sudden strain release of elastic rebound increases pore pressure in some confined aquifers thus leading to increase in discharge from springs that contribute to the base flow of major rivers.