Location

2010/10/26 01:24:16 43.645 -110.555 4.6 3.80 Wyoming

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports main page

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2010/10/26 01:24:16:9  43.65 -110.56   4.6 3.8 Wyoming
 
 Stations used:
   IW.FLWY IW.FXWY IW.IMW IW.MOOW IW.REDW IW.SNOW IW.TPAW 
   TA.H17A US.AHID US.BW06 US.HWUT US.LKWY WY.YFT 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
 
 Best Fitting Double Couple
  Mo = 6.31e+21 dyne-cm
  Mw = 3.80 
  Z  = 8 km
  Plane   Strike  Dip  Rake
   NP1      133    51   -124
   NP2        0    50   -55
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.31e+21      1     246
    N   0.00e+00     26     156
    P  -6.31e+21     64     337

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     0.00e+00
       Mxy     2.77e+21
       Mxz    -2.33e+21
       Myy     5.09e+21
       Myz     8.97e+20
       Mzz    -5.09e+21
                                                     
                                                     
                                                     
                                                     
                     ---------#####                  
                 ---------------#######              
              -------------------#########           
             ----------------------########          
           #------------------------#########        
          ##------------------------##########       
         ####-----------   ----------##########      
        #####----------- P ----------###########     
        #####-----------   -----------##########     
       #######------------------------###########    
       ########-----------------------###########    
       #########----------------------###########    
       ##########---------------------###########    
        ###########-------------------##########     
           ##########----------------###########     
         T ############--------------##########      
           ##############-----------##########       
           #################--------#########        
             ####################--########          
              ###################---------           
                 ###############-------              
                     #########-----                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -5.09e+21  -2.33e+21  -8.97e+20 
 -2.33e+21   0.00e+00  -2.77e+21 
 -8.97e+20  -2.77e+21   5.09e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20101026012416/index.html
        

Preferred Solution

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

      STK = 0
      DIP = 50
     RAKE = -55
       MW = 3.80
       HS = 8.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2010/10/26 01:24:16:9  43.65 -110.56   4.6 3.8 Wyoming
 
 Stations used:
   IW.FLWY IW.FXWY IW.IMW IW.MOOW IW.REDW IW.SNOW IW.TPAW 
   TA.H17A US.AHID US.BW06 US.HWUT US.LKWY WY.YFT 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
 
 Best Fitting Double Couple
  Mo = 6.31e+21 dyne-cm
  Mw = 3.80 
  Z  = 8 km
  Plane   Strike  Dip  Rake
   NP1      133    51   -124
   NP2        0    50   -55
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.31e+21      1     246
    N   0.00e+00     26     156
    P  -6.31e+21     64     337

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     0.00e+00
       Mxy     2.77e+21
       Mxz    -2.33e+21
       Myy     5.09e+21
       Myz     8.97e+20
       Mzz    -5.09e+21
                                                     
                                                     
                                                     
                                                     
                     ---------#####                  
                 ---------------#######              
              -------------------#########           
             ----------------------########          
           #------------------------#########        
          ##------------------------##########       
         ####-----------   ----------##########      
        #####----------- P ----------###########     
        #####-----------   -----------##########     
       #######------------------------###########    
       ########-----------------------###########    
       #########----------------------###########    
       ##########---------------------###########    
        ###########-------------------##########     
           ##########----------------###########     
         T ############--------------##########      
           ##############-----------##########       
           #################--------#########        
             ####################--########          
              ###################---------           
                 ###############-------              
                     #########-----                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -5.09e+21  -2.33e+21  -8.97e+20 
 -2.33e+21   0.00e+00  -2.77e+21 
 -8.97e+20  -2.77e+21   5.09e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20101026012416/index.html
	

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
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    90    50    90   3.34 0.2507
WVFGRD96    1.0   195    80   -10   3.33 0.2306
WVFGRD96    2.0   220    50    45   3.55 0.3178
WVFGRD96    3.0   205    50    20   3.59 0.3775
WVFGRD96    4.0   205    50    10   3.61 0.4311
WVFGRD96    5.0     5    55   -45   3.68 0.4778
WVFGRD96    6.0     5    55   -45   3.71 0.5148
WVFGRD96    7.0     5    55   -45   3.73 0.5294
WVFGRD96    8.0     0    50   -55   3.80 0.5375
WVFGRD96    9.0     0    50   -60   3.81 0.5364
WVFGRD96   10.0     0    50   -60   3.82 0.5239
WVFGRD96   11.0     0    55   -60   3.82 0.5060
WVFGRD96   12.0     0    50   -80   3.84 0.4870
WVFGRD96   13.0     0    50   -80   3.85 0.4673
WVFGRD96   14.0    -5    50   -90   3.86 0.4456
WVFGRD96   15.0   215    50   -20   3.78 0.4287
WVFGRD96   16.0   215    50   -10   3.78 0.4195
WVFGRD96   17.0   215    50   -15   3.79 0.4114
WVFGRD96   18.0   220    50   -10   3.80 0.4034
WVFGRD96   19.0   220    50   -10   3.81 0.3964
WVFGRD96   20.0   220    50   -15   3.82 0.3887
WVFGRD96   21.0   220    45   -15   3.83 0.3823
WVFGRD96   22.0   220    45   -15   3.84 0.3755
WVFGRD96   23.0   220    45   -15   3.85 0.3684
WVFGRD96   24.0   220    45   -15   3.86 0.3608
WVFGRD96   25.0   300    70   -50   3.92 0.3537
WVFGRD96   26.0   300    70   -50   3.93 0.3482
WVFGRD96   27.0   295    80   -55   3.92 0.3407
WVFGRD96   28.0   295    80   -55   3.93 0.3375
WVFGRD96   29.0   245    50    45   3.87 0.3347

The best solution is

WVFGRD96    8.0     0    50   -55   3.80 0.5375

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 component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number 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 lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).

The bandpass filter used in the processing and for the display was

hp c 0.02 n 3
lp c 0.10 n 3
Figure 3. Waveform comparison for selected depth
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.

A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:

Assuming only a mislocation, the time shifts are fit to a functional form:

 Time_shift = A + B cos Azimuth + C Sin Azimuth

The time shifts for this inversion lead to the next figure:

The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.

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.

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint L ouis University, Universityof Memphis, Lamont Doehrty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.

Velocity Model

The WUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

MODEL.01
Model after     8 iterations
ISOTROPIC
KGS
FLAT EARTH
1-D
CONSTANT VELOCITY
LINE08
LINE09
LINE10
LINE11
      H(KM)   VP(KM/S)   VS(KM/S) RHO(GM/CC)         QP         QS       ETAP       ETAS      FREFP      FREFS
     1.9000     3.4065     2.0089     2.2150  0.302E-02  0.679E-02   0.00       0.00       1.00       1.00    
     6.1000     5.5445     3.2953     2.6089  0.349E-02  0.784E-02   0.00       0.00       1.00       1.00    
    13.0000     6.2708     3.7396     2.7812  0.212E-02  0.476E-02   0.00       0.00       1.00       1.00    
    19.0000     6.4075     3.7680     2.8223  0.111E-02  0.249E-02   0.00       0.00       1.00       1.00    
     0.0000     7.9000     4.6200     3.2760  0.164E-10  0.370E-10   0.00       0.00       1.00       1.00    

Quality Control

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

The following stations did not have a valid response files:

DATE=Tue Oct 26 01:48:32 MDT 2010

Last Changed 2010/10/26