The Borah Peak, Idaho Earthquake of October 28, 1983—Liquefaction

The most pervasive and damaging effects of liquefaction generated by the 1983 Borah Peak, Idaho earthquake occurred in the Big Lost River and Thousand Springs Valleys above Mackay Reservoir. Less severe effects occurred in the Big Lost River Valley south of Mackay Reservoir and in the Pahsimeroi Valley. Nearly all of the liquefaction effects developed in floodplain alluvium of late Holocene age. However, the sediment that liquefied beneath the alluvial fans on the east side of the Thousand Springs Valley was deposited in late Pleistocene time. The distance from the fault to the farthest effect of liquefaction was unusually short for an MS = 7.2 event. The distribution of liquefaction effects were consistent, however, with the distribution of MMI intensity and estimated peak ground motion parameters, both of which attenuated more rapidly than is generally expected for an earthquake of this type and magnitude.