One of the largest earthquakes to strike Southern California in decades rocked Los Angeles residents on the evening of July 5, just a day after the region experienced another major quake. The magnitude 7.1 quake on that 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 were both epicentered near Ridgecrest, 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 July 4, 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 July 4.
Why L.A. Residents Weren’t Alerted
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 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.”
The shaking intensity in LA County for the Fourth of July event was below 4.5, USGS confirmed. 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 the July 5 shaking intensity was also below the 4.5 threshold.
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 July 8, a New York Timesreported that shaking intensity of 4.5 or more was recorded in LA County: “…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.”
Set to Avoid ‘Over-alerting’
Robert Michael de Groot, the national coordinator for the ShakeAlert program at USGS, told the The Vergethe reactions were “excellent feedback” that helped to determine how much the threshold will be reduced, and that they “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 to all the app’s users at once.
USGS posted a tweet confirming that the agency had been working with the city to improve alert delivery when the second earthquake hit.
On July 5, 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 for Android users on July 8.
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.
“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.”
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.
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 as aftershocks rolled in could see how LA’s system might have performed.