EVIDENCE FAVORING AN EPICENTER NEAR ENTIAT FOR THE LARGE DECEMBER 1872 WASHINGTON STATE EARTHQUAKE

The December 1872 earthquake is the largest historic crustal earthquake in Washington State, and its location and magnitude are important both for the design of critical facilities and lifelines and for disaster planning. Five lines of evidence indicate that the earthquake occurred near Entiat, north-central Washington State, between Wenatchee and Chelan. First, consensus seismic intensities assigned for the earthquake suggest an Entiat area epicenter with a magnitude between 6.5 and 7. Second, paleoseismic studies of the WSW-trending and NW-dipping Spencer Canyon scarp, located several km southwest of Entiat, indicate that it likely formed during the earthquake. Third, aftershocks from the 1872 earthquake were reported after three months only in the Entiat area between Wenatchee and Chelan. Fourth, the numbers of modern, instrumentally recorded earthquakes occurring within a shallow ongoing earthquake cluster centered near Entiat are consistent with aftershock forecast models for an M6.5 1872 earthquake. The low slip rate (<0.01 mm/yr) inferred from the seismic moment rate of earthquakes in the cluster suggests that ongoing tectonic deformation in the region is unlikely to be the primary cause of the seismicity. Additionally, the Spencer Canyon scarp lies above the southern edge of the cluster and the 680 km² area of the modern seismicity cluster matches the rupture area expected for a M6.8 reverse earthquake. Fifth, although ground failures and water effects were widely reported in 1872, there was a concentration of such features along the Columbia River and its tributaries in the Entiat area. Based on these five lines of evidence I suggest that the earthquake be named the 1872 Entiat earthquake. The directions of shaking reported at 10 locations and the location of a precariously balanced rock, Omak Rock, are broadly consistent with but do not provide strong evidence for an Entiat epicenter.