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Yorba Linda Earthquake Cluster on 4/23/09
by Kate Hutton, Anthony Guarino, California Institute of Technology Seismological Laboratory, Bob Dollar, USGS Pasadena Field Office 4/24/09 12:17:30 PDT
A small cluster of earthquakes occurred overnight about 1 mile north of Yorba Linda , include a ML3.8 at 4:56 pm and a ML4.0 at 8:27 pm. The moment 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 (~4 km), 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.
This Google Earth map shows the current Yorba Linda cluster (red), with background seismicity (yellow) and the M5.4 Chino Hills Earthquake sequence (orange).
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 & a 1.8. This suggests a low b-value for the sequence. More aftershocks are needed for good statistics.
These events are only about 6 km from the Chino Hills sequence, which began last summer, on 28 July, with a Mw5.4. 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. The following plot, from Robert Dollar of the U.S. Geological Survey, shows the time development of the combined Chino Hills and Yorba Linda sequences since that time.
The histogram above shows earthquake magnitude as a function of time.