Recent California Earthquakes Had a Few Seconds’ Warning. In the Big One, That Could Save Lives

Unbeknownst to those walking around Pasadena at lunchtime on Monday, rapid, shallow waves rippling up to the Earth’s surface set off urgent warnings to nearby seismic sensors. The waves pinged a nearby earthquake processing center as they spread, and seconds later, phones began to buzz.

The magnitude 4.4 earthquake centered in northeast Los Angeles was the latest to rattle Southern California in recent days. Early warnings sent straight to people’s phones, once an impossibility, are partly thanks to the UC Berkeley Seismology Lab, which developed the MyShake app in partnership with the U.S. Geological Survey and state officials.

Between the shaking in L.A. and a magnitude 5.2 quake near Bakersfield last week, 800,000 devices got early detection notices through MyShake or other platforms.

Even that brief warning can be vital for keeping people safe, especially in a larger earthquake.

“As we’re talking right now, we can have a massive earthquake, and we have no way of knowing except for this tool, and what it does is it gives you valuable seconds that could potentially save your life,” said Jon Gudel, who works with earthquake early warnings at the California Governor’s Office of Emergency Services.

Sensors spanning the West Coast from Baja California to Canada, including 900 in California, detect the first pressure, or “p,” waves generated by an earthquake, said Angie Lux, a project scientist for the Berkeley Seismology Lab’s earthquake early warning program.

When these first p waves, which move fast but are usually weaker than the secondary “s” waves that follow, are picked up by four earthquake sensors, they send data to local processing centers where algorithms developed at Berkeley get to work. Within seconds, an estimated magnitude and level of shaking are generated, and a ShakeAlert message is sent to MyShake, Google and the federal Wireless Emergency Alerts system, which sends push alerts to registered devices in the area.

All of this happened within four seconds, though some devices might not have picked up the notification for a few more.

“It’s doing everything really, really fast. And the goal is simply to warn as many people who may feel shaking, particularly strong shaking, as much as we can,” she said.

Unlike main seismic networks that determine magnitude after a quake, the MyShake app uses a very small sample of data and works very quickly, she said. This means that sometimes, the initial detection might slightly miscalculate the magnitude, but it recalibrates quickly.

Getting a warning before an earthquake still feels unusual — Californians have learned from a young age that they’re unpredictable and come on quickly. The idea of an early notification feels impossible — and in some cases, it still is.

“If you’re right on top of the epicenter and you are closer than our four closest stations, you will probably not receive a warning before you feel the shifting,” Lux said. That area is called the late alert zone.

“Studies that show that this can still be useful to people because sometimes, especially if it’s not as big of an earthquake, sometimes that initial p wave, people kind of feel and they look around and be like, ‘Do you feel that?’ But if you have that alert come in, you’re like, ‘Oh yeah, it is an earthquake.’”

The farther from the epicenter you are, the longer the warning time you can usually get. Lux said the instructions for people who get the notification are the same: duck, cover and hold on.

“We’ve seen that for many earthquakes, particularly on the West Coast, in California, the people that are injured are injured by things falling on them because they’re trying to get to safety,” Lux said. “It’s really hard to move during an earthquake.”

While the recent string of earthquakes doesn’t mean much in terms of lessening tectonic pressures or subduing the Big One, installing early detection technology on your smartphone and taking other preparedness measures like making an emergency kit and coming up with a household plan might make you safer if one does hit.

KQED’s Billy Cruz contributed to this report.