Location

2006/10/08 02:48:27 46.85N 121.60W 4 4.5 Washington

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Pacific Northwest US

Focal Mechanism

 SLU Moment Tensor Solution
 2006/10/08 02:48:27 46.85N 121.60W 4 4.5 Washington
 
 Best Fitting Double Couple
    Mo = 3.67e+22 dyne-cm
    Mw = 4.31 
    Z  = 12 km
     Plane   Strike  Dip  Rake
      NP1      145    74   127
      NP2      255    40    25
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   3.67e+22     47      94
     N   0.00e+00     36     314
     P  -3.67e+22     20     208



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -2.50e+22
       Mxy    -1.47e+22
       Mxz     9.20e+21
       Myy     9.67e+21
       Myz     2.39e+22
       Mzz     1.53e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ##--------------------------           
             ####--------------------------          
           ######--------#############-------        
          ########--######################----       
         ########--##########################--      
        ######------###########################-     
        #####-------############################     
       ####----------############################    
       ###-------------##############   #########    
       ##---------------############# T #########    
       #-----------------############   #########    
        ------------------######################     
        --------------------####################     
         --------------------##################      
          ---------------------###############       
           --------   -----------############        
             ------ P -------------########          
              -----   ---------------#####           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  1.53e+22   9.20e+21  -2.39e+22 
  9.20e+21  -2.50e+22   1.47e+22 
 -2.39e+22   1.47e+22   9.67e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20061008024827/index.html
        
University of Washington First Motion Solution http://www.ess.washington.edu/SEIS/EQ_Special/WEBDIR_06100802482c/foc.html

Fault Plane Parameters for 06100802482c    
            Fault Choice 1 Fault Choice 2
Strike(deg) 120.0          277.0
Dip(deg)     65.0           27.0
Rake(deg)   100.2           69.3
Fault Type reverse          reverse
	

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is

      STK = 255
      DIP = 40
     RAKE = 25
       MW = 4.31
       HS = 12

Both techniques give similar results in terms of source depth and moment magnitude. The common feature is a very shallowly dipping nodal plane to the SE. The waveform inversion is used for defining the source parameters. Much processing time was required to define an appropriate pass band for the wave form inversion. In this case the surface wave technique was more robust. Using higher frequencies for the waveform inversion introduced ringing surface waves into the data, which was hard to model, in that the orientations of the P and T axes were very sensitive to the time shifts in the fitting process. The final solution is consistent with short distance first motions.

Waveform Inversion

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.
Location of broadband stations used for waveform inversion

The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.

The observed and predicted traces are filtered using the following gsac commands:

hp c 0.02 n 3
lp c 0.05 n 3
The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   115    55    65   3.99 0.1253
WVFGRD96    1.0    95    60    45   3.98 0.1181
WVFGRD96    2.0   270    55    35   4.06 0.1385
WVFGRD96    3.0   255    45    15   4.12 0.1409
WVFGRD96    4.0   250    30    10   4.21 0.1556
WVFGRD96    5.0   250    30    10   4.23 0.1740
WVFGRD96    6.0   250    30    10   4.25 0.1886
WVFGRD96    7.0   255    30    20   4.26 0.2028
WVFGRD96    8.0   260    30    30   4.30 0.2129
WVFGRD96    9.0   255    35    25   4.30 0.2222
WVFGRD96   10.0   260    35    35   4.31 0.2287
WVFGRD96   11.0   260    35    35   4.31 0.2310
WVFGRD96   12.0   255    40    25   4.31 0.2331
WVFGRD96   13.0   255    40    25   4.31 0.2325
WVFGRD96   14.0   245    45     0   4.30 0.2316
WVFGRD96   15.0   245    45     0   4.31 0.2312
WVFGRD96   16.0   240    50   -15   4.31 0.2307
WVFGRD96   17.0   240    55   -15   4.31 0.2300
WVFGRD96   18.0   240    55   -15   4.32 0.2288
WVFGRD96   19.0   240    55   -15   4.32 0.2271
WVFGRD96   20.0   240    60   -15   4.33 0.2257
WVFGRD96   21.0   240    55   -15   4.36 0.2214
WVFGRD96   22.0   240    55   -15   4.37 0.2192
WVFGRD96   23.0   240    55   -15   4.37 0.2167
WVFGRD96   24.0   240    55   -15   4.38 0.2142
WVFGRD96   25.0   240    55   -10   4.38 0.2113
WVFGRD96   26.0   240    55   -10   4.39 0.2084
WVFGRD96   27.0   240    55   -10   4.39 0.2053
WVFGRD96   28.0   240    55   -10   4.40 0.2022
WVFGRD96   29.0   240    60   -10   4.41 0.1990

The best solution is

WVFGRD96   12.0   255    40    25   4.31 0.2331

The mechanism correspond to the best fit is
Figure 1. Waveform inversion focal mechanism

The best fit as a function of depth is given in the following figure:

Figure 2. Depth sensitivity for waveform mechanism

The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted. Each observed-predicted componnet is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. The number in black at the rightr of each predicted traces it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed and because the velocity model used in the predictions may not be perfect. A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The bandpass filter used in the processing and for the display was

hp c 0.02 n 3
lp c 0.05 n 3
Figure 3. Waveform comparison for depth of 8 km
Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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.

Surface-Wave Focal Mechanism

The following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
Location of broadband stations used to obtain focal mechanism from surface-wave spectral amplitudes

The surface-wave determined focal mechanism is shown here.


  NODAL PLANES 

  
  STK=     138.98
  DIP=      71.25
 RAKE=     113.86
  
             OR
  
  STK=     264.98
  DIP=      30.00
 RAKE=      39.99
 
 
DEPTH = 9.0 km
 
Mw = 4.36
Best Fit 0.8325 - P-T axis plot gives solutions with FIT greater than FIT90

First motion data

The P-wave first motion data for focal mechanism studies are as follow:

Sta Az(deg)    Dist(km)   First motion
HOOD      181  170 eP_-
MEGW      250  186 iP_D
OFR       301  243 eP_X
PGC       326  244 eP_+
HEBO      223  247 eP_-
HNB       345  279 eP_X
HOPB        3  283 eP_X
COR       208  285 iP_D
TOLO      217  307 eP_X
PNT        28  312 eP_-
EUO       201  334 eP_-
TAKO      210  397 iP_D
BMO       122  401 eP_-
LLLB      357  419 eP_X
SLEB       27  543 eP_X
DLMT      100  715 iP_D

Surface-wave analysis

Surface wave analysis was performed using codes from Computer Programs in Seismology, specifically the multiple filter analysis program do_mft and the surface-wave radiation pattern search program srfgrd96.

The velocity model used for the search is a modified Utah model .

Data preparation

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 Love-wave spectral amplitudes, respectively. These were input to the search program which examined all depths between 1 and 25 km and all possible mechanisms.
Best mechanism fit as a function of depth. The preferred depth is given above. Lower hemisphere projection

Pressure-tension axis trends. Since the surface-wave 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 T-axes 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. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0-180 degrees are sampled.

Love-wave radiation patterns

Rayleigh-wave radiation patterns

Broadband station distributiuon

Sta Az(deg)    Dist(km)   
HOOD	  181	  170
MEGW	  250	  186
OFR	  301	  243
PGC	  326	  244
HEBO	  223	  247
HNB	  345	  279
HOPB	    3	  283
COR	  208	  285
TOLO	  217	  307
PNT	   28	  312
EUO	  201	  334
TAKO	  210	  397
BMO	  122	  401
LLLB	  357	  419
DBO	  198	  435
SLEB	   27	  543
MOD	  169	  560
YBH	  189	  576
WALA	   64	  625
JCC	  197	  698
WDC	  186	  701
DLMT	  100	  715
ORV	  179	  811
HOPS	  188	  881
IMW	  108	  897
RRI2	  112	  897
HVU	  126	  901
FLWY	  106	  906
TPAW	  110	  916
MOOW	  108	  918
EDM	   37	  922
REDW	  111	  930
SNOW	  110	  932
LOHW	  109	  935
AHID	  115	  946
CMB	  174	  985
DUG	  133	 1025
CTU	  127	 1046
JRSC	  183	 1051
BW06	  111	 1054
MPU	  130	 1107
PKD	  176	 1215
SRU	  130	 1245
RWWY	  111	 1282
FNBB	  356	 1344
DLBC	  339	 1409
ULM	   69	 1928
CBKS	  109	 1990
FCC	   44	 2256
UALR	  108	 2787
USIN	   97	 2937

Waveform comparison for this mechanism

Since the analysis of the surface-wave radiation patterns uses only spectral amplitudes and because the surfave-wave radiation patterns have a 180 degree symmetry, each surface-wave solution consists of four possible focal mechanisms corresponding to the interchange of the P- and T-axes and a roation of the mechanism by 180 degrees. To select one mechanism, P-wave first motion can be used. This was not possible in this case because all the P-wave first motions were emergent ( a feature of the P-wave 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, R-radial 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 0.02 n 3
lp c 0.10 n 3

Discussion

The Future

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 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.

Acknowledgements

Dr. Harley Benz, USGS, provided the USGS USNSN digital data. The digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface

Appendix A

The figures below show the observed spectral amplitudes (units of cm-sec) 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.

Quality Control

Here we tabulate the reasons for not using certain digital data sets

The following stations did not have a valid response files:

Last Changed Mon Oct 9 10:07:25 CDT 2006