ShakeAlertLA designers and officials said the app actually worked pretty much as it was supposed to. They agreed, however, to an upgrade after the system failed to alert residents to a 6.4 magnitude quake on Thursday and a 7.1 quake Friday near the town of Ridgecrest that rattled LA from nearly 150 miles away. The upgrade will increase the sensitivity of the system, they said.
The app relies on a system of sensors on the West Coast used by the U.S. Geological Survey. It’s intended to give residents precious seconds to prepare for peak shaking — to, say, rush out of their house or not get on the elevator they’re waiting for.
“Earthquake! Earthquake! Expect strong shaking,” the alert is supposed to warn app users. “Drop, cover and hold on. Protect yourself now!”
But the app — currently available only for LA area residents — is focused on Los Angeles County, according to officials. It didn’t alert residents there last week because the two quakes were too far away and the shaking in the city wasn’t strong enough.
That was the ShakeAlertLA assessment, even though the tremors caused cranes and the seating decks of Dodger Stadium to sway, windows to rattle and water to slosh out of pools in Los Angeles.
The system is designed to track an earthquake of at least 5.0 magnitude — but will only issue an alert when it predicts the impact will measure a level 4 on the Modified Mercalli Intensity Scale. The system didn’t forecast that level of intensity, though it actually did occur in some areas.
“It didn’t meet the threshold for the LA area,” Doug Given, the U.S. Geological Survey’s earthquake early warning coordinator, told the Los Angeles Times. “The level of shaking in the city of Los Angeles was not damaging.”
City officials said the app’s alert threshold will be adjusted, but it was not immediately clear how quickly that could be done. “We hear you and will lower the alert threshold,” the city’s Twitter account said.
Designers have hesitated to make the system too sensitive because they fear it may alert people unnecessarily, and people may ignore repeated warnings.
Scientists believe the quake fracture at Ridgecrest remains active.
“The fault is growing,” retired USGS seismologist Lucy Jones told the Los Angeles Times on Friday. “This is an earthquake sequence. It will be ongoing. It is clearly a very energetic sequence, so there’s no reason to think we can’t have more large earthquakes.”