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California

ShakeAlertLA will drop threshold for earthquake alerts, amid gripes people not alerted

PASADENA, CA -- OCTOBER 17, 2018: An earthquake simulation shows shockwaves radiating outwards. Offi
During their 2018 rollout of the ShakeAlertLA, Los Angeles officials and Caltech scientists showed this simulation of earthquake shock waves radiating outwards. Some users of the app complained they should have been notified of Thursday’s quake in the Mojave Desert.
(Myung J. Chun / Los Angeles Times)

Following the 6.4 magnitude earthquake that struck Southern California, Los Angeles will lower the notification threshold on its ShakeAlertLA app, alerting users when temblors of at least a 4.5 magnitude occur, the United States Geological Survey and Los Angeles city officials said Friday.

The City of Los Angeles has been in conversations with USGS about lowering the threshold for several months, and began procedures to implement the change about two weeks ago, said Jeanne Holm, senior technology advisor to Mayor Eric Garcetti. Holm expects that the app will be updated by the end of the month.

“We have already done the planning to lower the threshold,” Holm said. “We will continue with thorough testing to make sure that the threshold is set correctly.”

The app, released in early 2019, is meant to give residents in Los Angeles County a warning when an earthquake reaches a magnitude of 5, and a level 4 or higher on the Modified Mercalli Intensity Scale. Depending how far a user is from the epicenter of the earthquake, the app has the ability to send out a push notification before the user feels the shaking.

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On Thursday, many users were worried that the application was not working. People on social media denounced the app as a failure — but in fact, ShakeAlert was functioning normally, as city officials and USGS soon pointed out.

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Thursday’s quake damaged homes and rattled nerves in Ridgecrest and other Mojave Desert communities, and could be felt as far away as Ensenada, Mexico. The shaking that hit Los Angeles, 125 miles away, did not reach level 4 on the Modified Mercalli Intensity Scale, and therefore no push notification was sent to ShakeAlert users, said Robert de Groot from the Earthquake Science Center at USGS.

That’s because “shaking levels under 4 on the MMI scale are not considered high enough to cause any damage,” said de Groot. At level 4, objects may fall of shelves and the shaking can become dangerous, USGS said. This did occur close to the epicenter of the earthquake, which was near Ridgecrest in the Mojave Desert, but Los Angeles County overall did not experience any significant damage.

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Even if the magnitude threshold had been lowered for Thursday’s earthquake, a ShakeAlert notification still would not have been activated, Holm said. The threshold for the Modified Mercalli Intensity Scale on ShakeAlert will not change.

The conversations about testing the threshold are always ongoing, said de Groot, as the application is recent and modifications are still being made.

RELATED: Ridgecrest earthquake was intense. Exactly how strong was it? »

When an earthquake meets the criteria for a notification, users get a push that starts with “earthquake, earthquake, expect shaking.”

City officials did not want people to become apathetic and stop responding to the alerts if the threshold was set too low, they said, which is why they initially set it for earthquakes that were at magnitudes of 5.0 and higher. But they decided to change it, realizing that people would likely prefer too many notifications than too few, Holm said.

“We are really interested in making sure that people are as safe as possible before during and after the earthquake,” she said.


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