ShakeAlert app alerts Los Angeles when an earthquake hits

Los Angeles is the first US city to publicly release an early earthquake warning app.

Known as ShakeAlertLA, the app alerts residents in the event of a magnitude 5.0 earthquake or greater.

The high alert threshold explains why LA residents weren't warned of the 7.1-magnitude quake that struck Ridgecrest, a city 125 miles away, over Fourth of July weekend.

In the aftermath of the earthquake, the city promised to lower its threshold.

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Southern California witnessed a painful reminder of its vulnerability to earthquakes over Fourth of July weekend. On July 4, a 6.4-magnitude quake produced gas leaks and power outages in Ridgecrest, a small city about 125 miles northeast of Los Angeles. The following day, that same community witnessed an even more destructive 7.1-magnitude quake.

Though some LA residents felt the ground shaking, many were surprised that they didn't receive an alert from the city's earthquake warning app.

In January, LA became the first US city to release an app that alerts the public of an impending earthquake via their Android and Apple smartphones — a rollout made possible by a contract with AT&T. The app, known as ShakeAlertLA, relies on a system developed by the United States Geological Survey (USGS), which has been refining the technology for years.

Part of their motivation has been to prepare citizens for “The Big One,” a magnitude 8.0 earthquake that could ripple through California at any moment, toppling its infrastructure, cutting off power, collapsing buildings, and taking lives. Scientists say the earthquake is inevitable, though no one is quite sure when or where it will happen.

The quake could come from the San Andreas Fault, which stretches almost the full length of California. It's possible that the Big One could occur near San Francisco, though scientists have said the fault near Los Angeles is “riper, more ready to go.”

Read more: Earthquake maps reveal never-before-seen detail about threats to buildings and people — here's who's most at risk around the world

A few seconds of advanced notice would give citizens enough time to duck and cover, and reduce the likelihood of injury. Tens of seconds could allow time for life-saving measures, like a surgeon finishing an operation or an industrial worker shutting off a gas pipeline.

Though other US cities have yet to reveal the same technology to citizens, early warning apps have already found some success in nations like Mexico and Japan.

The timing of an alert depends on both a quake's magnitude and a person's location. Warnings could range from two seconds to 90 seconds of advanced notice, said Kate Hutton, a coordinator at LA's Emergency Management Department.

But according to Richard Allen, the director of the Berkeley Seismological Laboratory, which developed a key algorithm for the ShakeAlert system, earthquake researchers aren't entirely ensure whether the warning will work as planned.

“We don't know how quickly and effectively apps like ShakeAlertLA, or any app for that matter, could deliver the alert to a very large number of people,” he told Business Insider in January. “What the city [of LA] is doing is trying to push the envelope by putting the app out there, and we'll see how it performs.”

Hutton said the technology will have to be refined with time, but she also said the USGS made sure the app was working well before unveiling it to the public.

“This is relatively new system for us,” she said. “What you don't want to have happen are false alerts that … people don't trust anymore or a situation where it doesn't alert when a major earthquake is happening.”

To account for this, the app only issues an alert when an earthquake's magnitude is at or above 5.0 — the point at which minor damage is possible. That explains why LA residents didn't receive a warning about the quakes over Fourth of July weekend. By the time they reached Los Angeles, the shaking was below the alert threshold.

Allen estimated that there are 100 times as many 5.0 earthquakes as there are 7.0 earthquakes.

As a Bay Area resident, Allen said he “fully expects” there to be major earthquake in his lifetime, though he doesn't believe in inciting chaos.

“It's not about panicking,” he said. “It's about being cautious.”

But the Ridgecrest quakes have LA reconsidering whether residents should be alerted more often. Late last week, the city announced that it would lower its threshold for the ShakeAlert app to at least a 4.5 magnitude. The change, which should be effective by the end of the month, could prove timely as scientists anticipate another quake in Southern California.