How low should we go when warning for earthquakes?

A key goal of earthquake early warning (EEW) systems is to alert populations who may be affected by a particular level of ground shaking so that they can take action to reduce impacts of that shaking, such as injuries, damages to physical infrastructure, or emotional distress. Most EEW systems work by rapidly determining the location and size of an earthquake, estimating shaking levels, and then distributing an alert to potentially affected populations. But EEW systems are limited by how rapidly the size of an earthquake can be determined as well as the details of the earthquake rupture process, the path of the seismic waves, and the alert distribution mechanism. And we are just beginning to understand how people respond to earthquake alerts, often relying on anecdotes. Determining the appropriate shaking intensities for public warnings requires understanding the range of individual and societal responses to earthquake alerts. The decision on when to issue earthquake alerts must balance the technical capabilities and potential outcomes, both desired and undesired, when choosing a ground-motion alerting threshold. Only when benefits outweigh the risks and users are prepared for alerts should they be used to warn the public about the possibility of earthquake shaking.