The value of real‐time GNSS to earthquake early warning

Global Navigation Satellite Systems (GNSS)‐based earthquake early warning (EEW) algorithms estimate fault finiteness and unsaturated moment magnitude for the largest, most damaging earthquakes. Because large events are infrequent, algorithms are not regularly exercised and insufficiently tested on few available data sets. We use 1300 realistic, time‐dependent, synthetic earthquakes on the Cascadia megathrust to rigorously test the Geodetic Alarm System. Solutions are reliable once six GNSS stations report static offsets, which we require for a “first alert.” Median magnitude and length errors are −0.15 ± 0.24 units and −31 ± 40% for the first alert, and −0.04 ± 0.11 units and +7 ± 31% for the final solution. We perform a coupled test of a seismic‐geodetic EEW system using synthetic waveforms for aMw8.7 scenario. Seismic point‐source solutions result in severely underestimated peak ground acceleration. Geodetic finite‐fault solutions provide more accurate predictions at larger distances, thus increasing warning times. Hence, GNSS observations are essential in EEW to accurately characterize large (out‐of‐network) events and correctly predict ground motion.