Latency and Geofence Testing of Wireless Emergency Alerts intended for the ShakeAlert earthquake early warning system (West Coast, United States of America)

ShakeAlert, the United States Geological Survey-managed earthquake early warning system for the West Coast of the United States, provides crucial warnings before strong shaking occurs. However, because the alerts are triggered only when an earthquake is already in progress, and the alert latencies and delivery times are platform dependent, the time between these warnings and the arrival of shaking is variable. The Federal Emergency Management Agency (FEMA) uses the Integrated Public Alert & Warning System (IPAWS) gateway as an alerting platform with Wireless Emergency Alerts (WEAs) informing people via their smartphones and other mobile devices about various events, such as natural hazards or child abductions, in their geographic location. However, little is known about the IPAWS latency (the time between when a ShakeAlert Message is sent to the IPAWS gateway and when it appears on a users mobile phone). Given that people may have only a few seconds of notice when they receive a ShakeAlert Message, quantifying WEA latencies is critical to understanding whether the IPAWS system is useful for EEW. In this study, we tested the WEA distribution system's performance, both with devices in a controlled environment, and with a 2019 community-based survey in the City of Oakland and San Diego County, California. The controlled environment test used smartphones and associated devices to determine alert receipt times; the community survey had participants self-report their receipt times. By triangulating the data between the controlled test environment and the community surveys, we determined the latency and whether the geofence (the geographic area where the alert was intended to be sent) held broadly. We found that the latencies were similar between the two tests despite the large difference in population sizes. WEA messages were received within a median time frame of 6-12 s, and the geofence held with only a few exceptions. We use this latency to assess how the system would have performed in two large earthquakes, the 1989 M6.9 Loma Prieta and 2019 M7.1 Ridgecrest earthquakes, which both occurred in California near our WEA test locations. Our analysis revealed that had IPAWS been available during those earthquakes, it would have provided crucial seconds of notice that earthquake shaking was imminent.