Location

2007/02/25 03:52:21 42.47N 110.67W 1 3.7 Wyoming

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Intermountain Western US

Focal Mechanism

 SLU Moment Tensor Solution
 2007/02/25 03:52:21 42.47N 110.67W 1 3.7 Wyoming
 
 Best Fitting Double Couple
    Mo = 9.77e+21 dyne-cm
    Mw = 3.96 
    Z  = 11 km
     Plane   Strike  Dip  Rake
      NP1      325    70   -75
      NP2      107    25   -125
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   9.77e+21     24      43
     N   0.00e+00     14     140
     P  -9.77e+21     62     258



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     4.23e+21
       Mxy     3.66e+21
       Mxz     3.44e+21
       Myy     1.84e+21
       Myz     6.42e+21
       Mzz    -6.07e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              -----#######################           
             ---------################   ##          
           -------------############## T ####        
          ----------------############   #####       
         -------------------###################      
        ---------------------###################     
        -----------------------#################     
       #------------------------#################    
       #------------   -----------###############    
       ##----------- P ------------##############    
       ###----------   -------------#############    
        ##---------------------------###########     
        ####-------------------------###########     
         ####-------------------------########-      
          #####------------------------######-       
           ######----------------------####--        
             #######-----------------------          
              ############--------######--           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -6.07e+21   3.44e+21  -6.42e+21 
  3.44e+21   4.23e+21  -3.66e+21 
 -6.42e+21  -3.66e+21   1.84e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070225035221/index.html
        

      STK = 325
      DIP = 70
     RAKE = -75
       MW = 3.96
       HS = 11

Both the surface-wave and waveform inversion give the same solution.

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 0.10 n 3
br c 0.12 0.2 n 4 p 2
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   140    40   -90   3.60 0.1913
WVFGRD96    1.0   185    90     5   3.48 0.1430
WVFGRD96    2.0   140    40   -90   3.73 0.1944
WVFGRD96    3.0   360    70   -30   3.68 0.1660
WVFGRD96    4.0   185    30   -10   3.73 0.1699
WVFGRD96    5.0   180    20   -25   3.81 0.2136
WVFGRD96    6.0   165    15   -45   3.86 0.2525
WVFGRD96    7.0   155    20   -60   3.89 0.2823
WVFGRD96    8.0   320    70   -80   3.96 0.3002
WVFGRD96    9.0   320    70   -80   3.96 0.3161
WVFGRD96   10.0   320    70   -80   3.97 0.3226
WVFGRD96   11.0   325    70   -75   3.96 0.3230
WVFGRD96   12.0   325    70   -75   3.96 0.3213
WVFGRD96   13.0   325    70   -75   3.96 0.3173
WVFGRD96   14.0   325    70   -70   3.96 0.3129
WVFGRD96   15.0   325    70   -70   3.96 0.3073
WVFGRD96   16.0   120    65    60   3.96 0.3022
WVFGRD96   17.0   120    65    60   3.96 0.3006
WVFGRD96   18.0   120    65    60   3.97 0.2975
WVFGRD96   19.0   120    65    60   3.98 0.2932
WVFGRD96   20.0   115    70    55   3.98 0.2878
WVFGRD96   21.0   115    65    55   4.04 0.2837
WVFGRD96   22.0   115    65    55   4.05 0.2799
WVFGRD96   23.0   115    65    55   4.06 0.2750
WVFGRD96   24.0   115    65    55   4.07 0.2695
WVFGRD96   25.0   115    65    55   4.08 0.2633
WVFGRD96   26.0   115    65    50   4.08 0.2565
WVFGRD96   27.0   115    65    50   4.09 0.2508
WVFGRD96   28.0   115    60    50   4.10 0.2444
WVFGRD96   29.0   110    60    45   4.11 0.2386

The best solution is

WVFGRD96   11.0   325    70   -75   3.96 0.3230

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 0.10 n 3
br c 0.12 0.2 n 4 p 2
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=     309.98
  DIP=      64.99
 RAKE=    -105.00
  
             OR
  
  STK=     162.35
  DIP=      28.91
 RAKE=     -60.97
 
 
DEPTH = 10.0 km
 
Mw = 4.05
Best Fit 0.7607 - 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
AHID      313   48 eP_-
BW06       70   97 iP_C
SPU       229  196 eP_-
CTU       205  217 eP_X
JLU       198  217 iP_-
YFT       357  221 eP_+
NOQ       211  235 eP_X
YMR       355  246 eP_X
YNR       360  250 iP_+
BGU       230  261 eP_X
MPU       197  284 eP_X
NLU       203  303 eP_X
DUG       216  309 eP_X
RLMT       20  316 eP_X
HLID      293  328 eP_X
TMU       187  355 eP_X
BOZ       348  362 eP_X
SRU       178  373 eP_X
ELK       245  426 eP_X
ISCO      123  518 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.

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)   
AHID	  314	   48
BW06	   70	   97
SPU	  230	  196
CTU	  205	  217
JLU	  198	  217
YFT	  357	  221
LKWY	    5	  234
NOQ	  211	  235
YUF	    3	  250
BGU	  230	  261
MPU	  197	  284
NLU	  203	  303
DUG	  216	  309
RLMT	   20	  316
HLID	  293	  328
TMU	  188	  355
BOZ	  348	  362
SRU	  178	  373
ELK	  245	  426
HMU	  181	  503
ISCO	  123	  518
MSO	  333	  550
LAO	   36	  587
CCUT	  204	  592
BMO	  299	  597
MVCO	  162	  613
EGMT	    6	  622
WVOR	  272	  655
SDCO	  138	  685
WUAZ	  185	  774
TPNV	  220	  777
NEW	  324	  818
HAWA	  305	  829
DGMT	   35	  837

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
br c 0.12 0.2 n 4 p 2

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 Sun Feb 25 11:43:18 CST 2007