LOS ANGELES (CBSLA) – After Los Angeles County residents failed to receive warnings from the fledgling ShakeAlert mobile app during the two massive earthquakes that rattled the Southland last month, officials have made some tweaks to the the app that they hope will ensure that doesn't happen again.
L.A. Mayor Eric Garcetti and the U.S. Geological Survey held a news conference California Institute of Technology in Pasadena to discuss updates to the ShakeAlert app, which was first released back in January. One of those involves lowering the threshold that will trigger an alert in the event of an earthquake.
Last month, the region of Ridgecrest and Searles Valley, which is located about 160 miles northeast of L.A., was struck by two large earthquakes in consecutive days. On the morning of July 4, the region was hit by a magnitude 6.4 quake, the largest to hit Southern California in 20 years.
However, that turned out to only be a foreshock to the main shock, the magnitude 7.1 earthquake which struck on the night of July 5, ten times larger than the 6.4 quake.
However, L.A. residents were not alerted to either one through the app. Although the notification system worked correctly, USGS had designed the app so that an alert would only be sent out if shaking registered at a threshold of magnitude 5.0 within L.A. County.
Since neither earthquake registered at a 5.0, residents did not receive an alert.
"Many people were disappointed and surprised that the app didn't go off for an earthquake that they felt," USGS Early Warning System Coordinator Doug Given said Wednesday. "But the threshold for alerting was set to a level of shaking that might start to cause some damage. But in the L.A. County area, the ground motion didn't reach that level. It was felt, but it didn't do any damage."
Based on the criticism the city and USGS received, on July 24 the threshold for notifications was lowered to magnitude 4.5.
Garcetti explained that scientists had initially set the threshold higher because they wanted to avoid earthquake fatigue or causing unneeded panic among Angelenos.
"If we decide over time it's too often, and we're getting that feedback, we can always change that again," Garcetti said. "But I think, overwhelmingly, the public comments -- and we always try to be responsive to what people actually want -- was, 'please lower it so we know.' And that's a great opportunity for us to practice, for us to be reminded that we're in earthquake country. And for us to know as well, that even though this may not cause damage, this is coming."
The ShakeAlert system is based on a network of in-ground sensors developed by USGS to detect seismic activity. Depending on where the quake hits, an alert could arrive before, during or after the quake. The alert could potentially give the public precious seconds to take cover, or give first responders and hospitals time to react and prepare.
USGS researchers stressed Wednesday that the system indeed worked last month, it's just the two earthquakes didn't meet the previous threshold for an alert. Under the new threshold, both earthquakes would have prompted alerts.
The app has been downloaded more than 800,000 times. It is the nation's first publicly available earthquake early warning mobile app, and was developed as part of a pilot program with USGS, AT&T and The Annenberg Foundation. To download the app, click here.
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