California’s QuakeAlertUSA, a new early-warning app for smartphones, has a “countdown� feature that shows the seconds before the shaking arrives. People can tailor the alert for a magnitude that triggers for weak, light, and moderate shaking. Other apps that have been around longer, are the U.S. Geological Survey’s (USGS) ShakeAlert system, powered by hundreds of sensors throughout California, LA City’s  ShakeAlertLA,  and UC Berkeley’s MyShake app that offers statewide coverage. Created by Santa Monica-based Early Warning Labs, the app uses the same data source as the other. Alerts via text message are also available through the Amber Alert-style Wireless Emergency System and don’t require downloading or a smartphone. According to the LA Times report, “The countdown feature should be used with caution. People shouldn’t wait around for the seconds to tick down before they Drop, Cover and Hold on; they should do it right away,� said USGS ShakeAlert scientist Robert de Groot. “Decent shaking can come even before the countdown hits zero. Don’t have a false sense of security; earthquakes are very complex,� he said. Based on the direction, speed, and magnitude of the first “P� wave, QuakeAlertUSA is able to quickly determine where ground shaking will occur, how intense the shaking will be, and when the shaking will start in different areas. The app then sends out alerts to app users based on their current location, providing early warning up to 60 seconds ahead of the damaging second “S� wave.  The notification you receive from Quake Alert USA tells you: • Expected arrival time of ground shaking • Expected intensity • Your distance from the epicenter • The magnitude of the earthquake at the epicenter • Suggested safety measures For every earthquake, there is a region near the epicenter where alerts will not arrive before shaking begins.