Millions of people across Southern California learned about a 5.2-magnitude earthquake that hit San Diego County on Monday not from their windows rattling or their picture frames clattering, but from a noisy alert on their mobile phones seconds before the shaking started.
The urgent messages — advising people to “drop, cover, hold on. Protect yourself” — were created with a tool from the United States Geological Survey known as ShakeAlert. The system detects earthquakes as soon as they start happening and quickly gathers information to determine which areas are likely to experience strong shaking. ShakeAlert operates on the notion that each and every fraction of a second is precious time that people can use to react and avoid injury before shaking from an earthquake occurs.
Here’s how the system works:
Picture a small car six feet underground.
The ShakeAlert early warning system monitors for earthquakes across the West Coast with the help of tens of thousands of seismic stations, each the size of a compact car and buried about six feet underground. The stations are equipped with sensors that detect the full spectrum of seismic waves, the vibrations that travel through the Earth and toward the surface after an earthquake, similar to the way ripples radiate across water when a pebble is dropped into a lake.
The waves travel at different speeds, and the success of the system is built on this. The fast-moving primary waves, or P-waves, travel at about four miles per second; they are gentler and less likely to cause destruction. The stronger secondary waves, or S-waves, move much more slowly, about two and a half miles per second. They are more likely to cause damage in an earthquake.
The goal of ShakeAlert is to capture the P-wave data, process it on the U.S.G.S.’s high-speed computers and make it available to government agencies and private companies like apps that send out alerts before the S-waves hit the surface.
“It’s all automated, and it’s all happening really fast,” said Robert de Groot, a physical scientist at the agency and a coordinator for ShakeAlert.
The closer you are, the less notice you’ll get.
California gets a lot of earthquakes, many of which cause little or no damage. The alerts go out for only the strongest: when a quake’s strength is estimated to be magnitude-4.5 and above. Separately, shaking generated from an earthquake is measured on a scale from 1 to 10, and if the shaking in an area is estimated to be level 3 or higher, an alert is sent.
The closer you are to the epicenter, the less likely you are to get a warning before the strongest shaking arrives — because people living near the quake are going to feel seismic waves as soon as the system starts processing the data.
On Monday, people in downtown San Diego received alerts about one or two seconds before the shaking, Mr. de Groot said. People in Los Angeles got messages up to about 10 seconds early.
“Distance matters,” Mr. de Groot said. “For people farther away from the earthquake, they had a lot of time before the earthquake.”
Here’s how to sign up for the alerts.
You can sign up for alerts online, and you are likely to receive them automatically if you’re in the area because they’re enabled by default on most mobile phones.
The early warning system, the only one in the nation, was launched in California in 2019 and was expanded to Oregon and Washington in 2021, serving some 50 million residents and visitors. (Mr. de Groot said that the program could expand to Alaska when funding allows.)
The MyShake app, which you can download onto your phone, uses data from ShakeAlert to send out early earthquake warnings. Wireless Emergency Alerts (the same technology used to send Amber alerts), which most people receive thanks to a default setting on phones, also delivers early earthquake warning messages. Google has its own system that’s integrated into its Android phones.
(These alert systems should not be confused with the U.S.G.S. Earthquake Notification Service, which sends out emails and texts after an earthquake hits.)
‘I thought it was an Amber alert.’
Monday’s quake hit at 10:08 a.m. Pacific time about two and a half miles south of Julian, a small mountain town about 60 miles northeast of downtown San Diego, and was felt as far away as Los Angeles.
Allison Gill, a liberal political podcaster, was at home in San Diego on Monday morning working on a script when her phone started vibrating and making a loud sound.
“I thought it was an Amber alert,” Ms. Gill said.
By the time she figured out it was an earthquake warning, she was feeling what she described as a “sizable jolt and then mellow rolling.”
The talk show host Jimmy Kimmel recounted his own experience during Monday night’s show. He said that he and wife were in their bedroom on their computers when they both received the alert telling them to drop and cover. When he finished reading the message, he said, the earthquake was there.
“I’m trying to figure out if this is a good thing or a bad thing,” Mr. Kimmel joked.
Amy Graff is a Times reporter covering weather, wildfires and earthquakes.
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