Two large earthquakes rattle LA—but where was the early warning?

One of the largest earthquakes to strike Southern California in decades rocked Los Angeles residents on Friday evening, just a day after the region experienced another major quake. The magnitude 7.1 quake on Friday night came less than 36 hours after a 6.4 earthquake rippled through the LA area on Fourth of July morning.

A new early warning app, which the City of LA made available at the end of 2018, did not send alerts before either of the earthquakes.

The 6.4 and 7.1 quakes (Friday’s magnitude 7.1 was downgraded to 6.9, then revised back to 7.1)—were both epicentered near Ridgecrest, California, a city located in Kern County’s high desert, about 100 miles from LA. Some fires, rockslides, and damage to buildings, roads, and infrastructure were reported near the epicenter.

There was no major damage reported in LA, but millions of people throughout the LA area—and all the way to Las Vegas—felt both quakes, which were strong enough to disrupt live newscasts, shake cameras filming the Dodger game, splash water out of pools, and sway chandeliers.

At a January press conference for the early warning app, which is named ShakeAlertLA, city representatives said the app was intended to send an alert to local residents for earthquakes of magnitude 5.0 and above when shaking is felt in Los Angeles County.

The app’s description as installed on users’ phones reads: “ShakeAlertLA sends you notifications when a 5.0 or greater earthquake happens in Los Angeles County.”

On Thursday, Angelenos who had downloaded the alert system posted screengrabs of their apps, wondering why those parameters wouldn’t have included a 6.4 earthquake that was felt across the county.

They also questioned why the app did not list the 6.4 event under the “recent earthquakes” page, which showed a blank screen on Thursday.

The confusion was compounded when the Los Angeles Times tweeted about an hour after the quake that the alert did not go out because the ShakeAlertLA system is “designed to cover quakes that occur within LA County.”

Los Angeles deputy mayor Jeff Gorell said in a tweet on Thursday that the system only sends alerts for events when the LA-area shaking intensity is 5.0 on the Modified Mercalli Intensity (MMI) scale or higher, regardless of the earthquake’s magnitude. “You will not get a warning every time there is shaking,” he said in a separate tweet. “Only if it’s dangerous.”

AP Photo/John Antczak

The shaking intensity in LA County for the Fourth of July event was below 4.5, USGS confirmed Thursday. According to seismologist Lucy Jones, who gave multiple press conferences at Caltech after the quake, LA’s shaking intensity was closer to 3.

On Saturday morning, USGS reported that Friday evening’s shaking intensity was also below the 4.5 threshold.

The City of Los Angeles confirmed Thursday that the city will lower the alert threshold with USGS. On Friday, before the 7.1 quake struck, LA’s chief information officer Jeanne Holm told the Los Angeles Times that the city was already planning to lower the magnitude threshold to 4.5, and that the app will be updated by the end of the month.

On Monday, a New York Times story provided different information about the current thresholds, with deputy mayor Gorell saying the current shaking intensity threshold is 4. “Once it is updated, the app will send alerts to areas where an earthquake is expected to be felt at an intensity of 3 or more, down from 4 or more now,” Gorell said. Officials are still discussing lowering the magnitude threshold.

The New York Times also reports that shaking intensity of 4.5 or more was recorded in LA County: “As it turned out, parts of western Los Angeles felt the second quake more strongly than predicted—an intensity reading of 4.5 was recorded in one area of western Los Angeles—and downtown just missed the threshold at 3.9. But that was not known until after the fact.”

Robert-Michael de Groot, the national coordinator for the ShakeAlert program at USGS, told the The Verge the reactions were “excellent feedback” that helped to determine how much the threshold will be reduced.

“Yes, thresholds are going to come down. Absolutely,” he said. “But there’s also our need to make sure that the system is stable and it’s working as it should. Our threshold at the moment is to maximize public safety and minimize over-alerting.”

In the same interview, de Groot also said that the LA app developers were “on the verge of doing a speed test” to practice sending a message all the app’s users at once.

USGS posted a tweet late Friday night confirming that the agency had been working with the city to improve alert delivery when the second earthquake hit. “We are working harder than ever on these issues and will have news shortly.”

On Friday, the ShakeAlertLA app pushed an update for iPhone users that added all earthquakes of magnitude 3.0 and above for the past 30 days to the “recent earthquakes” list. An update was pushed Monday for Android users.

Although the city’s app did not send alerts, a statewide app that also uses USGS data named QuakeAlert, which is in use by some commercial organizations and will be launched to the public this summer, gave LA beta testers an average of 45 seconds of warning, according to their post-event report.

Josh Bashioum of Early Warning Labs, the Santa Monica-based company that’s developing the QuakeAlert app, says the city’s warning system should have sent an alert to Angelenos who have the app on their phone.

“It should have worked,” Bashioum tells Curbed. “The app says they will only alert people in Los Angeles County, but they need to alert them for earthquakes that happen anywhere outside of LA County, too. The San Andreas Fault isn’t in LA County.”

One major difference between ShakeAlertLA and QuakeAlert is that Early Warning Labs uses raw USGS data to calculate customized countdown alerts based on the user’s location, says Bashioum. “This is expensive, to create ‘intensity’-based alerts but we believe it’s the best method when lives are on the line,” he says. “We hope our consumer app will be ready soon; our commercial platform already protects over 100,000 California residents.”

QuakeAlert users can also set their own thresholds for what alerts to get for any quakes—as long as they’re above magnitude 3.5. But the app will only give an estimate of shaking intensity if the quake is expected to be felt by the user.

The first few hours after the initial earthquake would have been a good time for the city of Los Angeles to issue clarification on what the city’s app does and how to use it, says Deanna Sellnow, chair of the communication department at the University of Central Florida’s Nicholson School of Communication and Media, who has worked on crisis messaging for earthquake early warning systems. This might have included new onboarding screens like the ones that are displayed when users install the app, showing what the experience of receiving an alert will be like, and the factors that determine if an alert is sent.

“The debrief has to happen right after the fact, because that’s when people are most alert,” she tells Curbed. “Now people are saying they don’t want the app because they say it doesn’t work.”

LA’s app was made publicly available over six months ago, but the city has still not released video or sound showing what users will see and hear in the moments before a quake. At a Caltech press conference, Jones was asked what the ShakeAlertLA alert sounded like and she said she didn’t know.

Viewers watching live local news coverage of Caltech’s early warning system on Friday night as aftershocks rolled in could see how LA’s system might have performed.

Jones said to expect more aftershocks, some of which could be felt in the LA area. After Friday’s 7.1 earthquake, Thursday’s 6.4 earthquake is now considered to be a foreshock. As of Saturday night, the chance of another earthquake occurring as part of this same sequence that’s magnitude 6 or larger is 12 percent. The chance that another earthquake will be magnitude 7 or larger is now 1 percent.

This story was originally published at 11:19 a.m. on July 4, 2019 and has been updated with new information.