LOS ANGELES -- A new version of Los Angeles' earthquake early warning app will now alert users of weaker shaking, Mayor Eric Garcetti announced Wednesday.
The change comes after many Angelenos were upset they didn't receive notice before shaking arrived in L.A. from two powerful quakes 125 miles north of Los Angeles on July 4 and 5.
The new threshold will trigger alerts on the city-owned ShakeAlertLA app when "weak" shaking is forecast to arrive in Los Angeles County -- a level of shaking, according to the Modified Mercalli Intensity Scale, that is felt quite noticeably by people inside, especially on upper floors of buildings, but may not be felt by other people, especially those outside.
When the Ridgecrest quakes occurred, the minimum level of shaking that would have triggered a public alert for app users was "light" shaking, a stronger level that can rattle dishes and windows and feel like a heavy truck striking a building.
But the city's app did not send an alert because the U.S. Geological Survey's ShakeAlert system forecast only "weak" shaking to arrive in L.A. The forecast was slightly off; in fact, "light" shaking was felt across much of Los Angeles.
Scientists familiar with the system thought the earthquake early warning system performed well; they pointed out that, generally speaking, the system was designed to sound an alarm only before shaking arrived with such an intensity that it could produce damage. The Los Angeles area had almost no reports of significant damage from the July 4-5 quakes.
But experts have concluded that the public would be better served by also receiving warnings to prepare for shaking that, while not damaging, can still be pretty scary.
"People want lower threshold alerts -- I think that is loud and clear in terms of what we're hearing from people," Berkeley Seismology Lab director Richard Allen told a webinar held by the National Academies of Sciences, Engineering and Medicine last month. "They don't just want warnings for damaging earthquakes. They want warnings for experienced earthquakes."
A lower threshold could have given Angelenos perhaps more than half a minute of warning that the shaking was on its way. While the city's ShakeAlertLA app remained silent, the USGS' backbone computer earthquake early warning network, ShakeAlert, did detect the July 5 earthquake about 49 seconds before the strongest shaking waves arrived at Los Angeles City Hall.
"People expect to get a warning if they're going to experience the earthquake, i.e. ... they're going to feel significant shaking," Allen added.