USGS Felt map for this earthquake
USGS Felt reports page for Northern California
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.

Date: Tue, 28 Sep 2004 10:47:52 0700 (PDT) From: BDSN Moment Tensor Analysthttp://earthquake.usgs.gov/recenteqsUS/Quakes/nc51147892.htm Reviewed by: Dolenc UCB Seismological Laboratory Inversion method: complete waveform Stations used: KCC, MHC, PACP, and TIN Berkeley Moment Tensor Solution Best Fitting DoubleCouple: Mo = 9.82E+24 Dynecm Mw = 5.96 Z = 8. km Plane Strike Rake Dip NP1 57 7 85 NP2 147 175 83 Principal Axes: Axis Value Plunge Azimuth T 9.853 1 102 N 0.057 82 198 P 9.796 8 12 Source Composition: Type Percent DC 98.8 CLVD 1.2 Iso 0.0 Event Date/Time: September 28, 2004 at 17:15:24 UTC Event ID: nc51147892 Moment Tensor: Scale = 10**24 Dynecm Component Value Mxx 8.776 Mxy 3.901 Mxz 1.395 Myy 9.034 Myz 0.131 Mzz 0.258   P  ##  #### ####### ########## ############### #################### ######################## ############################# ################################# #################################### ##################################### ################################### T ############################# ########################## #################### ############# ######### ###### ##  
Date: Tue, 28 Sep 2004 15:17:21 0400 (EDT) Here is the solution for the recent event. September 28, 2004, CENTRAL CALIFORNIA, MW=6.0 Natasha Maternovskaya CENTROID, MOMENT TENSOR SOLUTION HARVARD EVENTFILE NAME C092804B DATA USED: GSN L.P. BODY WAVES: 29S, 64C, T= 40 SURFACE WAVES: 40S, 96C, T= 50 CENTROID LOCATION: ORIGIN TIME 17:15:30.4 0.2 LAT 35.95N 0.01;LON 120.55W 0.01 DEP 12.0 FIX;HALFDURATION 2.3 MOMENT TENSOR; SCALE 10**25 DCM MRR=0.19 0.02; MTT=0.92 0.02 MPP= 1.11 0.02; MRT= 0.16 0.06 MRP= 0.17 0.05; MTP= 0.21 0.02 PRINCIPAL AXES: 1.(T) VAL= 1.16;PLG= 8;AZM=276 2.(N) 0.19; 76; 43 3.(P) 0.97; 11; 185 BEST DOUBLE COUPLE:M0=1.1*10**25 NP1:STRIKE=321;DIP=77;SLIP=178 NP2:STRIKE=230;DIP=88;SLIP= 13   ### ######## ################ ##################### ######################### ############################# T ############################ ######################## ######################## ################## ############### ########## ##### #    P 
NODAL PLANES STK= 139.99 DIP= 85.00 RAKE= 155.00 OR STK= 47.66 DIP= 65.10 RAKE= 5.52 DEPTH = 6.0 km Mw = 5.89 Best Fit 0.8335  PT axis plot gives solutions with FIT greater than FIT90
The Pwave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion CMB 360 246 i DAC 78 256 i+
Surface wave analysis was performed using codes from Computer Programs in Seismology, specifically the multiple filter analysis program do_mft and the surfacewave radiation pattern search program srfgrd96.
The velocity model used for the search is a modified Utah model .
Digital data were collected, instrument response removed and traces converted
to Z, R an T components. Multiple filter analysis was applied to the Z and T traces to obtain the Rayleigh and Lovewave spectral amplitudes, respectively.
These were input to the search program which examined all depths between 1 and 25 km
and all possible mechanisms.

Pressuretension axis trends. Since the surfacewave spectra search does not distinguish between P and T axes and since there is a 180 ambiguity in strike, all possible P and T axes are plotted. First motion data and waveforms will be used to select the preferred mechanism. The purpose of this plot is to provide an idea of the possible range of solutions. The P and Taxes for all mechanisms with goodness of fit greater than 0.9 FITMAX (above) are plotted here. 
Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the Love and Rayleigh wave radiation patterns. Each solution is plotted as a vector at a given value of strike and dip with the angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure. A nearly vertical strikeslip fault striking at 75 or 165 degrees is preferred. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0180 degrees are sampled. 
Sta Az(deg) Dist(km) SAO 318 143 CMB 360 246 DAC 78 256 ELK 38 708 MVU 65 784 HWUT 47 999 HLID 29 1000 COR 347 1005 AHID 43 1110 BW06 47 1209 BOZ 31 1317 MSO 22 1336 SDCO 76 1343 OCWA 348 1363 ISCO 67 1371 NEW 10 1409 LAO 40 1688 LTX 110 1722 DGMT 38 1933 KSU1 73 2128 MIAR 86 2437 FVM 76 2669 JFWS 64 2697 PVMO 80 2751 MPH 83 2753 OXF 84 2813 LRAL 86 3069 ACSO 70 3296 ALLY 67 3526 MCWV 71 3568 NHSC 83 3680 DWPF 92 3753 CBN 73 3802 SDMD 71 3828 BINY 66 3866 LOZ 61 3965 NCB 62 3996 BRNJ 68 4001 ACCN 64 4045 PAL 68 4052 HRV 65 4226
Since the analysis of the surfacewave radiation patterns uses only spectral amplitudes and because the surfavewave radiation patterns have a 180 degree symmetry, each surfacewave solution consists of four possible focal mechanisms corresponding to the interchange of the P and Taxes and a roation of the mechanism by 180 degrees. To select one mechanism, Pwave first motion can be used. This was not possible in this case because all the Pwave first motions were emergent ( a feature of the Pwave wave takeoff angle, the station location and the mechanism). The other way to select among the mechanisms is to compute forward synthetics and compare the observed and predicted waveforms.
The velocity model used for the waveform fit is a modified Utah model .
The fits to the waveforms with the given mechanism are show below:
This figure shows the fit to the three components of motion (Z  vertical, Rradial and T  transverse). For each station and component, the observed traces is shown in red and the model predicted trace in blue. The traces represent filtered ground velocity in units of meters/sec (the peak value is printed adjacent to each trace; each pair of traces to plotted to the same scale to emphasize the difference in levels). Both synthetic and observed traces have been filtered using the SAC commands:
hp c 0.01 3 lp c 0.10 3
Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.
Dr. Harley Benz, USGS, provided the USGS USNSN digital data.
The figures below show the observed spectral amplitudes (units of cmsec) at each station and the
theoretical predictions as a function of period for the mechanism given above. The modified Utah model earth model
was used to define the Green's functions. For each station, the Love and Rayleigh wave spectrail amplitudes are plotted with the same scaling so that one can get a sense fo the effects of the effects of the focal mechanism and depth on the excitation of each.
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files: