Hi All. The 3.6Mw quake on12/24/2002 is the second strongest quake to have centered with a 10 mile radius of 37.6030N/122.4832W. This is about 1 mile from Pacifica. The strongest quake to have occurred in the area was a 4.0Md on 04/28/1979. There has been only one other quake M>3.0 and larger in this area for the year 2002. This was a 3.1Ml on 03/20/2002.

The “first motion mechanism” shows this quake to have occurred on a “pure normal fault.” I’m not 100% sure, but I don’t think this quake is associated with the San Andreas fault.

This quake may have occurred on the Pilarcitos fault and appears to have an important role in the movement of the San Andreas fault.

“The total offset on the San Andreas fault system here is further complicated by movement on branch faults to the east (Calaveras and Hayward fault systems of unknown offset) and, more importantly, by past movement along the Pilarcitos fault, the presently inactive fault strand branching westward from the San Andreas fault on the San Francisco peninsula. Probably most of the 300 to 320 km of displacement on the San Andreas fault has taken place along this strand because the presently active strand of the San Andreas fault that lies directly east of the Pilarcitos fault demonstrates only about 26 km of offset of a characteristic limestone belt within the Permanente terrane of the Franciscan assemblage. The Pilarcitos fault (now truncated to the northwest by the San Gregorio fault) may have its former extension on the ocean side of the San Gregorio-San Andreas fault at about lat 38°30' N, as proposed by Graham and Dickinson (1978).”

You will have to read the link to see just how complicated this area is. It’s the kind of stuff that keeps a seismologist up at nights. Take Care…Don in creepy town

• Pilarcitos Fault