The focal mechanism shown below graphically portrays the orientation of the fault plane on which the earthquake occurred and the direction of relative motion on that plane. This information was automatically calculated from the first motion directions of the compressional (P) waves. A more detailed explanation describes how to interpret a focal mechanism. If more than one mechanism exists for this earthquake, then any of the solutions fit the first-motion data equally well. An index of other focal mechanisms is also available.

USGS NORTHERN CALIFORNIA SEISMIC NETWORK 
P FIRST-MOTION FOCAL MECHANISM 
(double-couple source assumed) 
 
Event Date & Time          : 09/18/2004 23:02:18 GMT = Sat Sep 18 16:02:18 PDT 2004

Location                   : 38.0090 N, 118.6793 W 
                           : (38 deg.  0.54 min. N, 118.deg. 40.76 min. W) 
Depth                      :  7.6 km. deep (  4.7 miles) 
Magnitude                  : 5.3 MC 
# P First motions          :  40 
Strike uncertainty         :  25 deg. 
Dip    uncertainty         :  35 deg. 
Rake   uncertainty         :  25 deg.