Abstract

Omak Rock is an about 40-ton granite boulder balanced on a narrow support. The rock is located in northern Washington State, in the general vicinity of the 1872 Pacific Northwest M 7+ earthquake. The survival of 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 epicenter of the earthquake. Using dynamical analysis, I find that the rock could have survived far greater peak ground motion than previously concluded, and might well have survived within a few kilometers of the epicenter, thus invalidating the earlier conclusions.

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