RIBBON CLIFF LANDSLIDE, WASHINGTON, AND THE EARTHQUAKE OF 14 DECEMBER1872

Estimates of the epicentral location and maximum intensity of the eart hquake of 14 December 1872, the largest and oldest historic earthquake documented in the Pacific Northwest, are controversial largely becaus e the estimates are based on ground effects. The Ribbon Cliff landslid e is one of the more critical ground effects used to argue that the ep icenter was in the vicinity of Lake Chelan in central Washington. Sket chy historical accounts link the Ribbon Cliff landslide to the 1872 ea rthquake, but a subsequent study disputed the historical accounts and, on the basis of dendrochronology, concluded that the landslide occurr ed more than 100 yr prior to the earthquake. However, Quaternary strat igraphic relations and the results of multiple dating techniques repor ted here indicate that the main Ribbon Cliff landslide probably occurr ed within a 14-yr period that includes the time of the 1872 earthquake . Although our study supports the historical accounts that link the la ndslide to the December 1872 earthquake, it does not prove that seismi c shaking triggered the landslide. Geomorphic and stratigraphic relati ons 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 o ccurred in colluvium that had accumulated beneath the cliff. Four coll uvial 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 invo lved in the landslide accumulated after deposition of the Mazama ash b ed. Loss of support at the foot of the colluvial wedge because of unde rcutting by the Columbia River probably was an important contributing factor to slope failure.