he 1872 Washington State earthquake is the largest known crustal earthquake in the Pacific Northwest, with an estimated magnitude exceeding most recent damaging U.S. earthquakes. The fault that produced the 1872 earthquake is not known, nor has a record of earlier events been produced. Recent work on the precise relocation of modern earthquakes traces a blind thrust in the presumed 1872 epicentral region around Lake Chelan, Washington. Preliminary mapping suggests observable deformation of glacial terraces along the Columbia River that may be associated with the 1872. This project is exploring two hypotheses. First, The 1872 earthquake was produced by slip on a northwest-striking, blind thrust (delineated by hypocenter relocation) that produced a broad fold visible in the topographic deflection of glacial deposits along the Columbia River valley between the Okanogan confluence and the city of Wenatchee. Mapping and detailed GPS surveying of the terrace surfaces coupled with surface exposure dating methods are being used to test this hypothesis. Second, ground shaking caused deformation of sediments in Lake Chelan and other near-by lakes. Earlier events are recorded by similar deformation at lower stratigraphic horizons. Cores are being collected from Lake Chelan to determine if deformation related to the 1872-event is present and to seek evidence for earlier large earthquakes, lower in the stratigraphic section.
