THE CHELAN SEISMIC ZONE, THE GREAT TERRACE AND THE DECEMBER 1872 WASHINGTON STATE EARTHQUAKE

The December 14, 1872 Washington State earthquake was felt over an area greater that 1 million square miles, and may be largest crustal earthquake on record in the Pacific Northwest. Recent research places the epicenter of this event near the Chelan Seismic Zone (CSZ), a region of persistent, modern earthquake activity. Earthquakes in the CSZ are typically shallow (less than 25 km), small (M less than 4), and frequent compared to the rest of the Pacific Northwest interior (more than 1/week in 2000 and 2001). Catalog locations of earthquakes from the Pacific Northwest Seismograph Network show the CSZ as a diffuse cloud of earthquakes between Lake Chelan and Wenatchee. Preliminary hypocenter relocations using HypoDD show that many of the recorded earthquakes cluster onto a blind surface striking approximately 130 and dipping 35SW. This attitude is similar to contractional structures in the region, including the Badger Mountain anticline. Thus, the CSZ may be generated by a blind structure related to the Yakima fold-and-thrust belt.

Published magnitude estimates for the 1872 event range 6.5-7.4. Using M~7 and a fault-plane area defined by the hypocenter relocation (~1.5*10⁸ m²), we determine average coseismic slip on the fault to be ~7m. Elastic dislocation models yield a coseismic surface displacement on the order of 2-3m (vertical) for a rupture surface with its upper tipline at 2-6 km depth, and assuming pure dip slip. Wavelength and amplitude of this signal vary with dip, fault-plane area, and depth beneath the surface.

The Great Terrace of the Columbia River runs through the 1872 epicentral region, providing a glacial-aged datum against which to measure surface deformation. Modeled surface deflection values for a single M~7 event are at about the limit of detectability in topographic deflection. If the recurrence interval for this event is 1000 years, 20-30 m of topographic deflection of the terraces might be expected. We report on field and GIS investigations to track down topographic evidence for the active structure and source of the 1872 earthquake.