Earthquake Early Warning: ShakeAlert in the Pacific Northwest

We evaluate the performance of earthquake early warning algorithm ElarmS-2 (earthquake alarm system v. 2) in the Pacific Northwest. Real-time and prerecorded seismic data from Oregon, California, and Washington in the United States and British Columbia in Canada are used. The earthquakes tested range up to moment magnitude 7.2, the limit for which the ElarmS-2 magnitude method is accurate. ElarmS-2 reliably detects catalog magnitude 3 and larger earthquakes within the network, but the cut-off magnitude for accurate event recognition is higher for events offshore and on the edges of the network. We have made several adjustments that make the ElarmS-2 algorithm less likely to falsely report multiple alerts for earthquakes. Replaying of past earthquakes shows that the new settings improve the algorithm's behavior for edge cases while not degrading previously well-constrained solutions within the dense part of the network. We expect few false alerts, none that would predict significant shaking anywhere in our region. Even though ElarmS-2 assumes a fixed earthquake depth, the epicenter and magnitude estimates are accurate enough to provide good predictions of shaking intensities.