Commentary: The Role of Geodetic Algorithms for Earthquake Early Warning in Cascadia

The ShakeAlert earthquake early warning (EEW) system issues public alerts in California and will soon extend to Oregon and Washington. The Cascadia subduction zone presents significant new challenges and opportunities for EEW. Initial publications suggested that EEW algorithms based on Global Navigation Satellite System (GNSS) data could provide improved warning for intraslab events and dramatically improved warning for offshore megathrust events, both of which contribute significantly to hazard in Cascadia. We find that some expectations in these publications were unrealistic, and we demonstrate that in general geodetic algorithms would not produce timely warnings for intraslab events nor warning times of two minutes or more for severe shaking from megathrust earthquakes. Nonetheless, lessons from recent earthquakes in Japan and California, for which alerts from seismic algorithms suffered from magnitude saturation and high data latencies, demonstrate the urgent need for rigorous testing of geodetic EEW as a potential complement to seismic EEW. Plain Language Summary ShakeAlert was initially implemented and optimized in California where most earthquakes occur on shallow faults in the Earth's crust. These earthquakes typically start just a few miles underground and can begin directly underneath population centers. Thus, earthquake early warning (EEW) needs to rely on the very first detections of seismic waves to be able to issue warnings before stronger shaking arrives. In a subduction zone, many damaging earthquakes are located offshore on the plate boundary megathrust, and many inland large earthquakes occur in the subducted plate at depths of 30+ miles. Because some seismic EEW systems have underestimated large earthquakes in the past, approaches are being tested that use GNSS data as the primary input for quantifying the magnitude and length of a large, growing rupture. We discuss realistic expectations from this approach and identify key topics for future research.

Automated summary

ShakeAlert earthquake early warning system issues alerts in California, and will soon expand to Oregon and Washington. Challenges and opportunities for EEW in the Cascadia subduction zone are significant. Initial expectations for geodetic algorithms providing improved warnings for specific types of earthquakes in Cascadia were found to be unrealistic, highlighting the need for rigorous testing.