A TOPOGRAPHIC AND STRUCTURAL LINEAMENT NEAR CHELAN, WASHINGTON

Topography and structure infer a 10 km long, N75E trending lineament located in the middle Columbia River valley between Entiat and Chelan, Washington. This lineament lies within the epicentral region, identified by Bakun et al. (2002), of the December 1872 Washington State earthquake. Evidence for the lineament includes a one to two hundred meter wide comminuted zone of pre-Tertiary crystalline bedrock that outcrops on the left bank of the Columbia River across from Stayman. The comminuted zone is bound on the west by similar rocks exhibiting a very persistent fracture set with a primarily E-W trend and near vertical inclination. Defining the lineament to the ENE of Stayman, the Columbia River flows through a notably straight reach for about 6 km, much of which is bedrock confined. Extending about three kilometers to the WSW from the outcrop of comminuted rocks, are two linear depressions. The depressions are roughly 400 m and 800 m long, 100 to 150 m wide, and 10 to 30 m deep. Curiously, the Ribbon Cliffs landslide falls on the lineament about 11 km WSW of Stayman. Madole et al. (1995) constrained the landslide occurrence within a 14 year period around the 1872 earthquake. Earthquake records between 1976 and 2001 are summarized in Bakun et al. (2002) and show active seismicity in the immediate area. If the lineament is of tectonic origin, it could be a source of recent seismicity and possibly the 1872 earthquake.