Tag Archives: psychological aftershocks

After string of small quakes: Expert attention is directed at some of LA’s oldest neighborhoods where casualties would be higher than a San Andres Megaquake

Puente-Hills-Thrust-fault

Important Takeaways:

  • Southern California was recently rattled by several small earthquakes. They produced minor shaking but nonetheless left psychological aftershocks in a region whose seismic vulnerabilities are matched by our willingness to put the dangers out of our minds.
  • For many, it all added to one question: Is this the beginning of something bigger?
  • First, a magnitude 3.6 earthquake in the Ojai Valley sent weak shaking from Santa Barbara to Los Angeles on May 31. Then came two small quakes under the eastern L.A. neighborhood of El Sereno, the most powerful a 3.4. Finally, a trio of tremors hit the Costa Mesa-Newport Beach border, topping out at a magnitude 3.6 Thursday.
  • Having half a dozen earthquakes with a magnitude over 2.5 in a week, hitting three distinct parts of Southern California, all in highly populated areas, is not a common occurrence.
  • But experts say these smaller quakes have no predictive power over the next major, destructive earthquake in urban Southern California, the last of which came 30 years ago.
  • Generally speaking, there is a 1 in 20 chance any earthquake in California will be followed by one that’s larger, said Susan Hough, a seismologist with the U.S. Geological Survey. Those odds aren’t high, and typically, the subsequent, larger quake would occur in the same area within a week. Plus, if something bigger did happen, the odds are a new temblor would be only a little bigger, Hough said.
  • In contrast, last week’s earthquakes highlighted nearby fault systems directly under our most populated cities and could produce even worse death tolls than a San Andreas megaquake, targeting our oldest neighborhoods with many unretrofitted buildings when they rupture.
  • “All three sets of these earthquakes occurred near large, potentially dangerous faults,” said James Dolan, an earth sciences professor at USC. “The L.A. urban fault network has been in a seismic lull for the entire historic period, and this lull likely extends back on the order of the last 1,000 years. We know at some point this lull we’re in will end.”

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