Seismotectonic Characterization of the Yorba Linda Trend and La Habra Earthquake Sequence Through Earthquake Relocation

The Yorba Linda Trend is a northeast-southwest trending system of earthquakes extending from the central Chino Hills into the southeastern corner of the Los Angeles Basin, transecting the Whittier Fault. Most earthquakes along this Trend exhibit a strike-slip mechanism, but they do not fall on or near any mapped fault traces. Because a majority of these earthquakes are small, they are also relatively poorly located. Relocation using waveform cross-correlation allows for these small events to be relocated with a relatively high certainty and for possible fault planes to be revealed. Two major earthquake clusters, the 2002 Yorba Linda earthquakes and the 2008 Chino Hills earthquake sequence are commonly associated with the Yorba Linda Trend. The 2014 La Habra earthquake sequence, located just west of the Trend, follows a parallel orientation and contains similar mechanisms.

In this study, we utilized the GISMO cross correlation MATLAB toolbox and the Growclust relocation algorithm to relocate approximately 1300 of the total 4400 earthquakes comprising the Yorba Linda Trend and the La Habra sequence from the Southern California Earthquake Data Center catalog from 1932 to 2018. We utilized numerous 1-D velocity models associated with the area and arrival times from 20 close-in permanent network stations. Preliminary relocation results show a tighter series of clusters associated with both the La Habra and the Yorba Linda earthquake sequences. A subset of the M_w 5.1 La Habra aftershock sequence is relocated on or in close proximity to the Coyote Hills Fault System. The 2002 Yorba Linda sequence reveals a small northeast-southwest trending series of events located just north of the surface expression of the Whittier Fault. The aftershock sequence associated with the 2008 M_w5.4 Chino Hills earthquake appears more scattered, occurring at shallower depths than the mainshock.

After earthquake relocation, we do not see a clear linear trend of events that would correspond to the Yorba Linda Trend, as it was defined in the early 1990s. Our future research plans include the implementation of the Bayesloc3 relocation code that utilizes the Markov-Chain Monte Carlo method. Earthquake locations from the Bayesloc3 algorithm will be compared to our Growclust results.