How often can Earthquake Early Warning systems alert sites with high intensity ground motion

Although numerous Earthquake Early Warning (EEW) algorithms have been developed to date we lack a detailed understanding of how often and under what circumstances useful ground motion alerts can be provided to end‐users. In particular, it is unclear how often EEW systems can successfully alert sites with high ground motion intensities. These are the sites that arguably need EEW alerts the most, but they are also the most challenging ones to alert because they tend to be located close to the epicenter where the seismic waves arrive first. Here we analyze the alerting performance of the PLUM, EPIC and FinDer algorithms by running them retrospectively on the seismic strong motion data of the 219 earthquakes in Japan since 1996 that exceeded Modified Mercalli Intensity (MMI) of 4.5 on at least 10 sites (Mw 4.5‐9.1). Our analysis suggests that, irrespective of the algorithm, EEW end‐users should expect that EEW can often but not always provide useful alerts. Using a conservative warning time (t_w) definition, we find that 40% ‐ 60% of sites with strong to extreme shaking levels receive alerts with t_w >5 s. If high intensity shaking is caused by shallow crustal events, around 50% of sites with strong (MM~6) and =8) shaking receive alerts with t_w >5 s. Our results provide detailed quantitative insight into the expected alerting performance for EEW algorithms under realistic conditions. We also discuss how operational systems can achieve longer warning times with more precautionary alerting strategies.