Poster of the Greater Los Angeles Area, California Earthquake of 29 July 2008 - Magnitude 5.5

Tectonic Summary

The Chino Hills (greater Los Angeles area) earthquake of 29 July 2008, 11:42 PDT (18:42 UTC), was centered in the hills south of the communities of Diamond Bar and Chino Hills. The characteristics of elastic-waves radiated from the earthquake source imply that the earthquake was caused by oblique-reverse faulting, in which a block of crust is thrust up and over the crust on the opposite side of the fault and also moves sideways with respect to the crust on the opposite side of the fault. This style of faulting is intermediate between pure reverse (thrust) faulting and pure strike-slip faulting. The causative fault is of a type that would be expected to result from the northerly oriented compressive tectonic stresses that affect the Los Angeles region, but the earthquake has not been associated with a specific geologically mapped fault.

The 29 July 2008 earthquake was situated about 30 km east-southeast of the Whittier Narrows earthquake of 1 October 1987, which had a magnitude of 5.9, caused 8 fatalities, and produced over $358 million in damages. The 2008 earthquake was also situated about 20 km south of the Upland earthquake of 28 February 1990, which had a magnitude of 5.5 and caused over $12 million in damages. The Whittier Narrows earthquake occurred as the result of thrust-faulting (reverse-faulting on a shallowly dipping plane) on the Puente Hills thrust. The Upland earthquake occurred as the result of left-lateral strike-slip motion on a northeast-striking fault.