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Incident Shakes Things Up

Posted by on Sep 3rd, 2015 and filed under News. You can follow any responses to this entry through the RSS 2.0. You can skip to the end and leave a response. Pinging is currently not allowed.

By Charly SHELTON

On Monday, a 2.9 earthquake occurred just outside Compton. The epicenter was about a mile southwest of Compton proper and about eight miles below the surface. The shaking was felt in the surrounding areas, with an intensity reported as weak to light. Social media was abuzz with people saying they were awoken by the quake, some claiming to have been shaken out of bed, when it hit just after 9 a.m.

The Los Angeles basin is always under some measure of stress whether due to moving plates or settling of the Earth.

“The earthquake was quite small and is more a reflection of the local stress state and likely to influence other faults,” said JPL Geophysicist Andrea Donnellan.

Although it was felt in the area, 2.9 is rather small on the scale of what California can expect to experience in the future. The most recent quake to have a lasting impression on the public mindset was the La Habra quake in April 2014. This was a magnitude of 5.1, “so the fault rupture would have been about 1.5 by 1.5 miles with about one foot of slip on it. [The Compton earthquake had about] 0.2 by 0.2 miles with about 0.4 inches of slip on it,” said Donnellan.

This epicenter of the Compton quake was 22 miles from the Crescenta Valley area, yet it has ramifications for all of Los Angeles. Below the L.A. basin is a network of smaller faults that are all interconnected in one way or another and connect to the San Andreas Fault. A small earthquake like Monday’s can put more stress on some other faults in the area and cause slipping, which is another earthquake. This small quake can, as Donnellan said, “influence other faults” in this way.

“The earthquake is a reflection of the surrounding stress,” said Donnellan. “We know that the L.A. basin is shortening or contracting and that Southern California is shearing with the western side moving northwest.

“Earthquakes are a fact of life in California. We have many small ones and less frequent large ones. We should always be prepared for them.”

In addition to the smaller faults that crisscross the L.A. basin, the San Andreas is overdue for a quake. “The Big One” is a constant threat in California that, if unprepared for, can be devastating. Before a quake hits, get prepared and have supplies ready. Putting Down Roots in Earthquake Country is a pamphlet made and distributed by U.S. Geological Survey, FEMA, Cal EMA and several other organizations which, Donnellan says, “has excellent recommendations for preparing for earthquakes.” The pamphlet covers topics relating to earthquakes from geology and seismology to home preparedness and disaster kit contents.

The pamphlet details earthquake frequencies and where The Big One may originate. There are three sections of the San Andreas: Northern through Central California, Central California through San Bernardino, and San Bernardino through Coachella Valley. The northern section was active most recently, producing a 7.8 earthquake in 1906 that killed over 3,000 people and left 225,000 homeless. The Central section was last active in 1857 with a 7.9 quake, but did less damage because there were fewer people in the area at the time. (By comparison to all these, the Northridge earthquake of 1994 was magnitude 6.7.) The southernmost section hasn’t been active since about 1680.

While there is no foolproof way of predicting the exact when and where of an earthquake, the pamphlet explains, “The most comprehensive statewide analysis of earthquake probabilities determined that the chance of having one or more magnitude 6.7 or larger earthquakes in California over the next 30 years is 99.7%. The fault with the highest probability of such earthquakes is the southern San Andreas – 59% in the next 30 years. For powerful quakes of magnitude 7.5 or greater, there is a 37% chance that one or more will occur in the next 30 years in Southern California.”

For more information and to get a digital copy of the Putting Down Roots in Earthquake Country pamphlet, scan the QR code or visit earthquakecountry.org/roots.

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1 Response for “Incident Shakes Things Up”

  1. Liz Nelson says:

    Charly, you always do a great job of reporting but I am a little concerned about your reference to the pamphlet estimating a 7.9 in 30 years. I thought that estimate was made 2 or 3 years ago which would make the projection a 7.9 in 26 or 27 years. You didn’t give the date of the pamphlet. If the projected 7.9 is getting closer, we probably ought to wake folks up with that thought.
    Thank you, Charley.
    Liz

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