Starting this month, earthquakes of magnitude 4.5 or higher will draw alerts on ShakeAlertLA; the previous threshold was 5.0.
California has been testing the Shake Alert System, which is run by the US Geological Survey and other state and university partners; it's designed to detect significant earthquakes and alert as many residents as possible seconds before any shaking begins. The system will eventually cover the entire West Coast, according to its website.
The US Geological Survey, the Annenberg Foundation and AT&T, CNN's parent company, worked together to combine the Geological Survey's sensor network with mobile app technology.
In July, Los Angeles area residents didn't get a warning from the app about the 6.4- and 7.1-magnitude earthquakes in Ridgecrest, about 150 miles from the city. Those quakes were strong enough draw alerts near their epicenters, but farther away, the magnitude was considered too low to cause damage.
The earthquakes caused some structure and road damage, and fallen power lines meant electrical outages. Although there were no deaths or major injuries in either quake, the jolts were a wakeup call, California Gov. Gavin Newsom said.
CalTech seismologist Lucy Jones described the effect like throwing a rock into a pond, with strong waves where the rock breaks the water and longer, slower waves farther out.
"Many people were disappointed and surprised that the app didn't go off for an earthquake that they felt," US Geological Survey Early Warning System Coordinator Doug Given told CNN affiliate KCBS. "But the threshold for alerting was set to a level of shaking that might start to cause some damage. But in the L.A. County area, the ground motion didn't reach that level. It was felt, but it didn't do any damage."
Garcetti said in a statement, "Every day we are communicating the importance of preparedness, so that every Angeleno has the tools and resources they need to build a better life — and then protect that life when disaster strikes. Updates to ShakeAlertLA will result in an even more responsive application making our city stronger and our families safer."
The mayor's office said that tools like ShakeAlertLA are meant to help warn residents but not to replace the need to plan and prepare for an earthquake.
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