Abstract

Estimates of the epicentral location and maximum intensity of the earthquake of 14 December 1872, the largest and oldest historic earthquake documented in the Pacific Northwest, are controversial largely because the estimates are based on ground effects. The Ribbon Cliff landslide is one of the more critical ground effects used to argue that the epicenter was in the vicinity of Lake Chelan in central Washington. Sketchy historical accounts link the Ribbon Cliff landslide to the 1872 earthquake, but a subsequent study disputed the historical accounts and, on the basis of dendrochronology, concluded that the landslide occurred more than 100 yr prior to the earthquake. However, Quaternary stratigraphic relations and the results of multiple dating techniques reported here indicate that the main Ribbon Cliff landslide probably occurred within a 14-yr period that includes the time of the 1872 earthquake. Although our study supports the historical accounts that link the landslide to the December 1872 earthquake, it does not prove that seismic shaking triggered the landslide.

Geomorphic and stratigraphic relations show that Ribbon Cliff has a long and complex history and that the landslide was not primarily a rockfall-rockslide that originated from the cliff itself, as some believe, but rather that the main failure occurred in colluvium that had accumulated beneath the cliff. Four colluvial units and two volcanic ash deposits, the Mazama ash bed (erupted about 6.8 ka), and the Mount St. Helens set W (erupted about 1480 A.D.), underlie the slopes below Ribbon Cliff. Most of the colluvium involved in the landslide accumulated after deposition of the Mazama ash bed. Loss of support at the foot of the colluvial wedge because of undercutting by the Columbia River probably was an important contributing factor to slope failure.

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