The Nevada Seismological Lab Helicorder webcam (in my righthand column) caught it:

UPDATE:  Happened six minutes ago as I write this. 

Data from USGS says it was a 5.8.  Epicenter is 3 km ( 2 mi) SW of Chino Hills, CA, which is an LA suburb.  Happened at 2:42:15 pm. EST, which is 11:42:15 am Pacific Time.

News reports just coming in….

UPDATE:  USGS says epicenter was 47 km (29 miles) ESE (103°) from Los Angeles Civic Center, CA.  It appears to have been 12.3 km (7.6 miles) deep, which is, as earthquales go, pretty shallow.  Could mean some structural damage.

Looks like many smaller aftershocks, too….

MSNBC reporting only minor structural damage