2008 Chino Hills Earthquake - Tectonics

Tectonics

The Chino Hills earthquake was caused by oblique slip faulting, with components of both thrust and sinistral strike-slip displacement. Preliminary reports cited the Whittier fault as the active cause, but the quake was later determined to have been generated by the "Yorba Linda trend," as identified by Caltech seismologist Egill Hauksson. Its epicenter was within 3 miles (4.8 km) of Chino Hills and its hypocenter was approximately 9.1 miles (14.6 km) deep. Initial estimations of the moderate main shock reported it as magnitude 5.8, but this was later revised to magnitude 5.4 and in later months to a 5.5. The main shock was reportedly felt as far south as San Diego, and Tijuana, Mexico, and as far east as Las Vegas, Nevada. It was the strongest earthquake to occur in the greater Los Angeles area since the 1994 Northridge earthquake.

As reported by The Orange County Register, three microquakes, all less than magnitude 3.0, occurred in Anaheim Hills, 7 miles (11 km) southwest of Chino Hills, two months before the Chino Hills earthquake. There was an unusually low amount of seismic activity in Southern California in the week prior to the quake. Between July 20 and July 26, 2008, there were no earthquakes in Southern California exceeding magnitude 3.0, thus there was speculation that the wane in seismic activity was a precursor to a possible larger event.