California’s earthquake alert app didn’t sound for 2 big ones. Why not?

Los Angeles residents were not pleased last week when a pair of earthquakes struck and the ShakeAlertLA app on their phones did just what it was supposed to do: nothing.

The app, released to the public on New Year’s Eve after more than a year of development, is intended to give at least a few precious seconds of notice before dangerous shaking from an earthquake. But users didn’t get the notifications they expected Thursday and Friday when two powerful earthquakes struck in the Mojave Desert, and many people assumed that the app had failed.

HOW IS THE APP SUPPOSED TO WORK?

Since 2006, the U.S. Geological Survey has been developing a network of sensors up and down the West Coast to detect potentially damaging seismic activity. The ShakeAlertLA app is meant to use that data to warn users when an earthquake strikes that could cause damage in Los Angeles County, and give them time to take action before the ground heaves beneath them.

“Earthquake! Earthquake! Expect strong shaking,” the alert warns users. “Drop, cover and hold on. Protect yourself now!”

How much warning users get depends on how far they happen to be from the epicenter of the quake. Those who are very close are likely to feel a major earthquake almost immediately, well before an alert could arrive. But a video produced by a group of seismology researchers says that people in more distant areas could get as much as 70 seconds’ warning before the strongest shaking reaches them.

WHY WAS THERE NO ALERT LAST WEEK?

The short answer is, the shaking wasn’t bad enough in Los Angeles.

The earthquakes Thursday and Friday were certainly very powerful, with magnitudes of 6.4 and 7.1 at their epicenters near the city of Ridgecrest, California. But Ridgecrest is about 100 miles north of Los Angeles, and the intensity of movement experienced during an earthquake diminishes greatly with distance.

So, though Dodger Stadium quivered and Los Angeles movie theaters were evacuated because of the tremors, the quakes’ effects did not exceed the system’s minimum threshold in Los Angeles County, the only place where the ShakeAlertLA app has been implemented.

The system is intended to notice an earthquake of 5.0 magnitude or greater, and then predict where in the county the quake is likely to be felt at an intensity of 4 or more. Alerts are sent to users in those areas.

At that intensity, which is measured on a different scale than magnitude, cars will rock, dishes will rattle and people will feel the earthquake indoors — and may even be roused from sleep.

The system did not issue alerts because, immediately after the quakes struck, it predicted that they would not exceed the intensity threshold anywhere in the 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.

WHAT IS CHANGING NOW?

In response to complaints, the city and the creators of the app announced that they were making changes to increase the app’s sensitivity.

“We hear you, and will lower the alert threshold with @USGS—ShakeAlert,” the City of Los Angeles wrote on Twitter after the first earthquake Thursday.

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, according to Jeff Gorell, the city’s deputy mayor for public safety.

Los Angeles officials expect the new lower threshold to be in place by the end of July. If it had been in effect last week, most of Los Angeles County would have received alerts.

Officials are also discussing lowering the app’s magnitude threshold.

Gorell said the move was in response to public concerns after last week’s earthquakes, when some users said they wanted to be alerted in advance of any significant shaking.

“We don’t want to have alert fatigue, but we also don’t want people to delete the app” or for the public to turn against the app, he said. “It really is a challenge to find where that fine line is.”

Robert de Groot, the national coordinator of outreach and education for ShakeAlert at the USGS, said the change was part of a move toward “more broad alerting.”

Los Angeles and USGS officials said they realized that users grew concerned when they felt the earth move without receiving the warnings they expected.

“Practicing ‘drop, cover and hold on’ is actually a good thing, but there also can be a level of annoyance, so we need to strike that balance,” de Groot said. “We want to do as much as we can to send messages to people when they are valuable, when they have meaning. And clearly, for these two earthquakes, meaning was, ‘I want to know this system works.’ “

WHAT ARE USERS SAYING?

Still, some users of the app were surprised and disappointed to hear that the system was working as intended when it issued no alert last week.

“So the app did nothing during today’s quake and therefore ‘performed as designed,’” a Twitter user named Kyle Krupinski wrote. “Seems like a pretty low bar to achieve.”

Some people suggested that the app be programmed to provide two kinds of alerts — one for relatively minor tremors and another for more serious earthquakes — or that users be allowed to set their own thresholds.

Another Twitter user, Sergio Treviño, tagged Mayor Eric Garcetti of Los Angeles and said he was “waiting for today’s excuse.”