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Perilous fault might be causing quake swarm near O.C.

April 25th, 2009, 9:41 am · 5 Comments · posted by Gary Robbins, science writer-editor

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A magnitude 4.0 earthquake occurred 1 mile north of Yorba Linda at 8:27 p.m. on Thursday. Image courtesy of USGS.

Scientists have yet to precisely determine which fault is responsible for the swarm of earthquakes that’s occurring just  north of Yorba Linda. But there’s evidence that the shaking is associated with the dangerous Whittier fault, which produced 5.9 and 5.3 quakes over a 3 day period in 1987, says Caltech.

The new Chino Hills swarm perked up on Thursday with quick, back-to-back quakes measuring 3.8 and 4.0. The latter event, occurring at 8:27 p.m., was widely felt across much of Orange County.

In a technical review of the events, Caltech researchers Kate Hutton and Anthony Guarino write:  “The tensor solutions show mostly strike-slip for both of these large events, with one plane similar in strike to the nearby Whittier Fault.

“The hypocenters are relatively shallow, located to the southwest of the Whittier Fault trace, which dips to the northeast. There is a slight hint of a northeast-southwest trend in the locations. One can speculate that the current sequence is either on a parallel strand to the Whittier or on a conjugate fault.

“There have only been six members of the sequence so far, with the aftershock  sequence including a 2.6, a 2.5, a 1.9 and a 1.8 …

“These events are only about 6 km (3.7 miles) from the Chino Hills sequence, which began last summer, on 28 July, with a (magnitude) 5.4. The aftershocks of the Chino Hills sequence decayed as expected for the first few months up until about February 2009, at which time it started to exhibit swarm-like behavior.”

It should also be noted that the swarm is located not far from the Chino and Elsinore faults, and that seismologists cannot predict when any earthquake will occur.

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     5 Comments

    • Eric Steadmond says:
      April 25, 2009 at 1:14 pm

      Well, whatever is happening in this little area of Yorba Linda has virtually silenced the “swarm” over in Chino Hills during the last week. I don’t see one quake registered on USGS since these Yorba Linda quakes have happened over in the Chino Hills area. This is the only time I can recall in over 3 months where this much time has passed between microquakes on the Western Hills GC in Chino Hills. Strange.

    • boom laka says:
      April 25, 2009 at 11:25 pm

      this looks like a good case to be handled by geotechnical engineering department at UCLA. There are extremely competent professors there that can give great expert analysis on this situation.

      On another note, 5.9-6 is not that large of magnitude, with respect to the quality of building standards in the US.

      The most important thing to consider are the local soil properties in different regions in orange county, because depending on type of soil, there will be magnification of shaking intensity.

    • Mr. C says:
      April 26, 2009 at 1:11 am

      I moved to downtown Los Angeles, and I felt the quake that occurred at 8:27PM but it was a two second shake. I did not know there was a quake at 5p until I called my father in Anaheim Hills. I got on the OCR website and read the story of the quakes.

    • ocbear says:
      April 26, 2009 at 10:21 am

      It would be interesting to see a 3D view of this fault in the Yorba Linda area. I’d like to find a good map of the fault system in Southern CA including the Puente Fault, that can be overlaid on a detailed map. It would be nice to have a 3D fault map that you could fly through in Google Earth (kind of like the undersea terrain map that they recently introduced).