Location

2004/08/26 23:11:37 64.76N 86.28W 18 5.0 Canada

Arrival Times (from NRCAN)

http://www.seismo.nrcan.gc.ca/nedb/bull_sol_e.php?solid=20040826.2311004 Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Central and Southeastern US

Focal Mechanism

 SLU Moment Tensor Solution
 2004/08/26 23:11:37 64.76N 86.28W 18 5.0 Canada
 
 Best Fitting Double Couple
    Mo = 2.57e+22 dyne-cm
    Mw = 4.24 
    Z  = 23 km
     Plane   Strike  Dip  Rake
      NP1      155    60    80
      NP2      354    31   107
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   2.57e+22     73      40
     N   0.00e+00      9     160
     P  -2.57e+22     14     252



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -9.54e+20
       Mxy    -5.91e+21
       Mxz     7.37e+21
       Myy    -2.10e+22
       Myz     1.05e+22
       Mzz     2.19e+22
                                                     
                                                     
                                                     
                                                     
                     #########-----                  
                 --##############------              
              ----#################-------           
             -----###################------          
           -------#####################------        
          --------######################------       
         ---------#######################------      
        ----------#######################-------     
        ----------###########   ##########------     
       ------------########## T ##########-------    
       ------------##########   ##########-------    
       -------------######################-------    
       --   ---------#####################-------    
        - P ----------####################------     
        -   -----------###################------     
         ---------------#################------      
          ---------------###############------       
           ----------------#############-----        
             ----------------#########-----          
              -----------------######-----           
                 -----------------#----              
                     -----------###                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
  2.19e+22   7.37e+21  -1.05e+22 
  7.37e+21  -9.54e+20   5.91e+21 
 -1.05e+22   5.91e+21  -2.10e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/NEW/20040826231137/index.html
        

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
Figure 1. Location of broadband stations used to obtain focal mechanism

Preferred Solution

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

      STK = 155
      DIP = 60
     RAKE = 80
       MW = 4.24
       HS = 23

The waveform solution is preferred. The surface-wave solution admitted two likely possibilities differing in strike by about 90 degrees. The surface-wave soltuions elected provides a better fit to the waveforms

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 3
lp c 0.10 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   335    50   -90   3.91 0.4943
WVFGRD96    1.0   155    40   -85   3.96 0.5241
WVFGRD96    2.0   150    45   -85   4.07 0.5530
WVFGRD96    3.0   150    50   -80   4.11 0.4384
WVFGRD96    4.0   320    65   -55   4.07 0.4175
WVFGRD96    5.0   340    80    50   4.02 0.4317
WVFGRD96    6.0   305    10   -25   4.14 0.4605
WVFGRD96    7.0   305    10   -25   4.14 0.4866
WVFGRD96    8.0   300    10   -30   4.14 0.5093
WVFGRD96    9.0   300    10   -30   4.14 0.5276
WVFGRD96   10.0   300    10   -30   4.18 0.5423
WVFGRD96   11.0   300    10   -30   4.18 0.5597
WVFGRD96   12.0   305    10   -25   4.19 0.5743
WVFGRD96   13.0   305    10   -25   4.20 0.5859
WVFGRD96   14.0   240    30   -10   4.09 0.5992
WVFGRD96   15.0   240    30   -10   4.10 0.6165
WVFGRD96   16.0   240    30   -10   4.12 0.6309
WVFGRD96   17.0   240    30   -10   4.13 0.6424
WVFGRD96   18.0   245    30     0   4.14 0.6515
WVFGRD96   19.0   320    20    70   4.15 0.6606
WVFGRD96   20.0   335    20    85   4.19 0.6669
WVFGRD96   21.0   335    20    85   4.20 0.6713
WVFGRD96   22.0   155    65    85   4.22 0.6741
WVFGRD96   23.0   155    60    80   4.24 0.6757
WVFGRD96   24.0   155    60    80   4.24 0.6743
WVFGRD96   25.0   155    55    80   4.26 0.6710
WVFGRD96   26.0   155    55    80   4.27 0.6658
WVFGRD96   27.0   155    55    80   4.27 0.6580
WVFGRD96   28.0   155    55    80   4.28 0.6481
WVFGRD96   29.0   155    55    80   4.29 0.6371
WVFGRD96   30.0   150    50    70   4.30 0.6266
WVFGRD96   31.0   150    50    70   4.31 0.6169
WVFGRD96   32.0   150    50    70   4.32 0.6065
WVFGRD96   33.0   145    50    70   4.32 0.5956

The best solution is

WVFGRD96   23.0   155    60    80   4.24 0.6757

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 3
lp c 0.10 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


  NODAL PLANES 

  
  STK=     149.99
  DIP=      54.99
 RAKE=      64.99
  
             OR
  
  STK=       9.11
  DIP=      42.07
 RAKE=     121.11
 
 
DEPTH = 19.0 km
 
Mw = 4.29
Best Fit 0.8234 - 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
FCC       215  785 eP_X
FRB        89  866 eP_X

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.

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

The distribution of broadband stations with azimuth and distance is

Sta Az(deg)    Dist(km)   
FCC	  215	  785
FRB	   89	  866
RES	  347	 1156
YKW1	  273	 1411
YKW3	  273	 1414
YKW4	  273	 1423
SCHQ	  127	 1544
ULM	  204	 1713
KAPO	  171	 1721
VLDQ	  160	 1917
EDM	  243	 1994
FNBB	  268	 2013
INK	  302	 2086
DGMT	  219	 2107
BMBC	  260	 2165
SADO	  165	 2271
MNT	  154	 2283
KGNO	  160	 2368
DLBC	  273	 2376
DRLN	  122	 2420
NCB	  156	 2436
JFWS	  188	 2447
WHY	  282	 2468
LMN	  138	 2483
DAWY	  293	 2490
ACCN	  156	 2510
ERPA	  168	 2554
LLLB	  250	 2591
BESE	  278	 2596
NEW	  240	 2596
BINY	  160	 2597
ALLY	  168	 2604
MSO	  233	 2605
PNT	  245	 2606
BOZ	  228	 2635
HRV	  152	 2649
DCPH	  281	 2720
PNL	  283	 2739
PAL	  157	 2763
FOR	  157	 2779
COLD	  304	 2792
HARP	  292	 2834
TTW	  245	 2842
BLO	  180	 2850
BW06	  222	 2855
LTH	  241	 2859
SQM	  247	 2866
HAWA	  241	 2870
OPC	  248	 2880
SDMD	  163	 2889
BMR	  289	 2896
AHID	  225	 2906
MCK	  297	 2921
SLM	  187	 2921
LON	  244	 2933
KSU1	  198	 2936
DIV	  290	 2938
HLID	  230	 2942
OCWA	  248	 2946
WCI	  180	 2954
EYAK	  289	 2974
USIN	  182	 2985
BPAW	  298	 2987
FVM	  187	 2994
TRF	  297	 2995
SAW	  293	 3010
CBN	  164	 3014
KTH	  297	 3016
SIUC	  185	 3018
CBKS	  204	 3020
HWUT	  224	 3039
BLA	  170	 3092
PPLA	  297	 3109
RC01	  292	 3117
PVMO	  186	 3164
UTMT	  184	 3168
SWD	  290	 3169
WVT	  183	 3189
WVOR	  235	 3213
DUG	  225	 3225
SDCO	  212	 3263
MPH	  186	 3308
PLAL	  183	 3317
DBO	  242	 3334
BMN	  231	 3357
UALR	  190	 3363
OXF	  185	 3374
MIAR	  192	 3399
WMOK	  200	 3447
LRAL	  181	 3531
NHSC	  170	 3547
WDC	  238	 3550
MNV	  231	 3592
TPH	  229	 3594
TPNV	  227	 3674
CMB	  234	 3716
NATX	  193	 3717
SAO	  234	 3884
JCT	  200	 3928
PAS	  228	 4028

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 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 3
lp c 0.10 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.

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 Tue Feb 21 20:22:58 CST 2006