How Did an App Know an Earthquake Was Hitting California Before It Happened?

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Some people in Los Angeles knew about today’s earthquake before it even hit.

Those who had the beta Quake Alert application on their smartphones got about a 30-second warning before the shaking hit at 12:29 p.m. PDT (19:29 UTC). The magnitude-5.3 earthquake struck about 38 miles (61 kilometers) off the coast of California today (April 5), according to the U.S. Geological Survey (USGS).

“That was the first earthquake I’ve felt since I got access to the @EarlyWarningLab beta app,” tweeted Alissa Walker, an editor at the real estate blog Curbed. “I had 34 seconds warning — enough time to drop, cover, and hold on, which I would have done if I knew shaking was going to be strong.”

The app works thanks to a network of seismic sensors that detect P-waves — the first energy that radiates from an earthquake, according to Early Warning Labs, a Santa Monica-based company that works with the USGS on early warning systems. The seismic sensors also detect the location and the magnitude of the earthquake.

Once the longitudinal P-waves (short for pressure waves) are detected, the app sends a warning before the more powerful S-wave, or shear wave, arrives, which usually causes the most damage. Warnings are also sent to local and state public emergency response officials, infrastructure (such as to gas lines, subway systems and power plants) and to private businesses and the public, Early Warning Labs said.

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