Why L.A.’s early warning system didn’t send an alert before the magnitude 6.4 quake

Did the ShakeAlertLA system fail to provide an earthquake early warning?

Los Angeles residents were asking that question after Thursday’s earthquake that was felt throughout Southern California, when they didn’t get an early warning from the much-anticipated ShakeAlertLA app, released by the city of Los Angeles earlier this year.

Did it fail? Not quite. The ShakeAlertLA smartphone app was only designed to alert users of cellphones physically located in Los Angeles County if there was a decent chance of destruction, with the warning system forecasting at least “light shaking,� or level 4 on the Modified Mercalli Intensity Scale.

What was actually felt Thursday in Los Angeles County, while seemingly scary, was actually not that bad. It was forecast by the earthquake early warning system as bringing shaking too weak to cause significant damage in Los Angeles County, said U.S. Geological Survey research geophysicist Rob Graves.

“It didn’t meet the threshold for the L.A. area,� said Doug Given, the U.S. Geological Survey’s earthquake early warning coordinator.

�The level of shaking in the city of Los Angeles was not damaging,� Graves said.

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ShakeAlertLA is a mobile phone app developed by the city that transmits earthquake early warnings based off a separate, but similarly named, system called ShakeAlert and run by the U.S. Geological Survey.

The USGS’ system relies on hundreds of earthquake sensors scattered throughout the West Coast. There is no public smartphone app yet available that sends earthquake early warnings throughout all of California.

However, scientists are continuing to test, refine and perfect the USGS’ ShakeAlert system that aims to provide earthquake early warnings throughout California, and eventually Oregon and Washington.

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That ShakeAlert system worked — it’s just that the public does not yet have access to that information as scientists continue to refine its public delivery system. The USGS’ ShakeAlert system issued an alert about 6.9 seconds after the shaking began, Given said.

Had there been a public warning system in place for Kern and San Bernardino counties, the USGS ShakeAlert system would not have been fast enough to issue an early warning for Ridgecrest — at 10 miles away from the epicenter too close to get a warning, but enough to give some warning to California City, about 50 miles southwest of the epicenter.

Caltech seismologist Egill Hauksson said he received a warning that shaking was coming toward his location in the Pasadena area before the shaking arrived from an earthquake that began some 200 miles away.

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The intensity of shaking was obviously worse closer to the epicenter, maxing out at intensity level 8, or severe shaking, but that occurred in a much more remote area.

The city of Ridgecrest, population 29,000, endured intensity level 6 or “very strong� shaking, in which damage that might occur could result in broken chimneys, considerable damage in poorly built or badly designed buildings, but negligible damage in buildings of good design and construction.

The ShakeAlert forecast of the shaking intensity in L.A. County was slightly off. Although the ShakeAlert forecast conducted almost instantaneously did not forecast Intensity 4 or “light,� shaking in Los Angeles County, such shaking actually did occur in the northern part of the Los Angeles County, including East L.A., Palmdale, Pasadena, Santa Clarita, Santa Monica, Hollywood and Pomona. Other parts recorded Intensity 2 or 3, or “weak,� shaking, such as USC, Los Angeles International Airport, Long Beach, Malibu and Watts.

But generally speaking, there were no reports of significant damage in L.A. County.

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Officials have debated how sensitive to make earthquake early warnings.

“The question is what should the threshold be,� Graves said. “This is a subjective judgment. You don’t necessarily want too many alerts to go out when nothing of consequence happens. On the other side, you don’t want to set it too high when you don’t send out an alert when you should have.

“In this case, we’re right at that fuzzy boundary,� Graves said.

Officials with the city of Los Angeles, who manage the ShakeAlertLA app, suggested they would consider lowering the threshold for alerts. “We hear you and will lower the alert threshold,� the city’s Twitter account said.