LOS ANGELES – A magnitude 4.2 earthquake gave the Los Angeles region a predawn wake-up call Thursday, triggering local alerts from the state's new quake warning system but resulting in no reports of significant damage.
The 4:29 a.m. jolt was centered in the northern San Fernando Valley and occurred at a depth of 5.5 miles (8.9 kilometers), the U.S. Geological Survey said. Dozens of aftershocks followed.
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The Los Angeles Fire Department found no damage or injuries in a survey by ground and air units, a standard procedure after earthquakes in the nation's second-largest city.
The Police Department also reported no problems.
“Good morning Los Angeles. Yes, we felt it too," Los Angeles police tweeted, noting that 911 and other systems were not impacted.
The quake was mostly felt in the Los Angeles metropolitan area, according to thousands of contributions to the USGS citizen reporting system. A few reports came from more distant locations.
The ShakeAlert earthquake early warning system at one point estimated a magnitude greater than 4.5, the threshold for alerts, and between 3.6 seconds and 12.8 seconds after detection it sent messages to areas where shaking was predicted to reach at least a certain intensity level.
The alerts that reach the public through apps are intended at giving people time to protect themselves or allow systems such as manufacturing operations and rail lines to shut down before harmful shaking arrives.
More than 10,000 alerts went out Thursday in the Southern California region through two of the apps, said Robert de Groot, the USGS ShakeAlert coordinator.
“Really good results from this particular event," he said in an online news conference organized by the California Institute of Technology.
USGS research seismologist Susan Hough said that while the quake was not large enough to cause damage, the shaking was stronger than average for a 4.2-magnitude earthquake and will need to be analyzed.
The quake occurred in the vicinity of destructive and deadly earthquakes that were centered in the San Fernando Valley in 1971 and 1994.
“It's not possible for us to say if today's event is related to those but it's not surprising that we're having seismic activity in this area, said Jen Andrews, a Caltech Seismology Laboratory seismologist. “We've got a number of fault zones nearby."
Hough said that the USGS usually does not issue an aftershock forecast for quakes under magnitude 5 but will do so for Thursday's quake because it affected a populated area.
The initial aftershock forecast following the earthquake was about a 1 in 10 chance of an earthquake of a quake equal to or greater than magnitude 4.2 over the next month, Hough said.
Thursday's quakes were “garden variety," according to former USGS seismologist Lucy Jones, who now leads a center focusing on making communities disaster-resilient.
“What does it mean? It means you live in California," she tweeted.