MyShake is a free citizen science and public safety smartphone application that delivers the United States ShakeAlert program's Earthquake Early Warning to the public in the states of California, Oregon, andWashington. Although smartphone notifications have long been a component of the ShakeAlert warning delivery plan, very little data has been published on the efficiency and accuracy of such communication. MyShake records timestamps in its alert processing system, including the time it takes for a phone to receive and acknowledge an alert's delivery. We use data collected for five representative earthquakes-three in urban regions, two rural events-since October 2019 to assess MyShake alert delivery latencies and ground-motion prediction accuracy. For these events, MyShake was capable of efficiently processing and delivering warnings. For the smaller urban events that occur beneath the target population, about half of recipients received a warning before the estimated onset of the S-wave, and up to 90% received an alert prior to experiencing peak shaking. When earthquakes occur further from populated areas, the warning time naturally increases. Smartphone acceleration recordings can also be used to quantify the user experience during earthquakes because they are colocated with people. A review of waveforms collected by MyShake shows a systematic amplification of shaking recorded by smartphones relative to nearby traditional stations. The median amplification calculated using records from all five sample events is a factor of 3.1. Once this correction is applied, MyShake peak accelerations are generally consistent with the distribution of shaking intensities in the U.S. Geological Survey ShakeMap product, whereas also showing some individual sites with substantial amplification and de-amplification. In addition to delivering early warnings, therefore, MyShake provides a waveform observation dataset to densify shaking intensity observations and thereby improve our understanding of earthquake effects and evaluate the accuracy of ShakeAlert's alerting regions.
