A little history on this one - there were 2 rather similar quakes in that same area - Last night's was the largest though - Here is a remark from Kate Hutton on the last notable one in that same area:

17:06 PST, Monday, March 06 2000

Orange County and eastern Los Angeles County felt a quake this afternoon: a M4.0 located in the Santa Ana Mountains 7 miles southeast of Yorba Linda. The time was 4:20 pm. It was felt as far away as the Pasadena area.
Interestingly, although today/s quake was not a member of any of our on-going aftershock sequences, it was very similar in location and magnitude to a M3.9 on December 7, 1999.

Here are the two prior ones:

   date              UTC Time   Mag     Lat            Long     Depth  ID 
1999/12/07 21:58:16.16 L 3.81 l  33.811 -117.711  11.3 A 3321584  97 455  0  0
2000/03/07 00:20:28.26 L 3.93 l  33.809 -117.711  10.7 A 9141946 127 610  0  0
And today's:
  Date                    UTC          Lat        Long              Mag
2002 09 03  07 08 51.82  33 55.17-117 45.86 A 4.6  

Canie