OMAK ROCK AND THE 1872 PACIFIC-NORTHWEST EARTHQUAKE

Omak Rock is an about 40-ton granite boulder balanced on a narrow supp ort. The rock is located in northern Washington State, in the general vicinity of the 1872 Pacific Northwest M 7+ earthquake. The survival o f the balanced rock has in the past been used to place limits on peak acceleration and intensity at its location, and thus possibly on the e picenter of the earthquake. Using dynamical analysis, I find that the rock could have survived far greater peak ground motion than previousl y concluded, and might well have survived within a few kilometers of t he epicenter, thus invalidating the earlier conclusions.