[3] 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.

[5] The main shock was reportedly felt as far south as San Diego, and Tijuana, Mexico,[6][7] and as far east as Las Vegas, Nevada.

Between July 20 and 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.

[14] Amusement rides at Disneyland, Six Flags Magic Mountain, Universal Studios Hollywood and Knott's Berry Farm were evacuated and temporarily shut down.

[16] California Department of Transportation replaced an expansion joint on an Interstate 5 truck overpass at the El Toro Y Interchange.

[16] Los Angeles International Airport reported a ground radar system outage along with a broken water heater, causing flooding in the checked luggage preparation area of Terminal 7.

[21] The earthquake affected candidates who were in the first day of the three-day California Bar Exam in nearby Ontario and Los Angeles.

[26] The Southern California Seismic Network's Aftershock Probability Report, produced minutes after the event, stated that "Most likely, the recent mainshock will be the largest in the sequence.