Location

NEIC Location

2010/09/20 00:47:22 43.597 -110.416 5.0 3.60 Wyoming

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports main page

SLU Location

First arrival times of the P and sharp S arrivals were measured. The program elocate was run using the same WUS model used for the inversion. The output is the file elocate.txt. The source depth from the location is essentially the same as that from the waveform inversion. The SLU location is 4.9 km away from the NEIC location in a direction of 287 degrees. This shift is in the same direction as that given by the analysis of the azimuthal dependence of the time shift required for waveform matching.

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2010/09/20 00:47:22:0  43.60 -110.42   5.0 3.6 Wyoming
 
 Stations used:
   IW.FLWY IW.IMW IW.LOHW IW.MOOW IW.REDW IW.SNOW IW.TPAW 
   TA.H17A 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
 
 Best Fitting Double Couple
  Mo = 1.76e+21 dyne-cm
  Mw = 3.43 
  Z  = 7 km
  Plane   Strike  Dip  Rake
   NP1      146    76   164
   NP2      240    75    15
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.76e+21     21     103
    N   0.00e+00     69     284
    P  -1.76e+21      0     193

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.59e+21
       Mxy    -7.22e+20
       Mxz    -1.21e+20
       Myy     1.36e+21
       Myz     5.78e+20
       Mzz     2.27e+20
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ###-------------------------           
             ####--------------------------          
           #######---------------------------        
          ########--------------------------##       
         ##########------------------##########      
        ############------------################     
        #############-------####################     
       ###############---########################    
       ###############-##########################    
       #############----###################   ###    
       ##########--------################## T ###    
        #######------------################   ##     
        #####---------------####################     
         ##-------------------#################      
          ---------------------###############       
           ----------------------############        
             ----------------------########          
              -----------------------#####           
                 -----   --------------              
                     - P ----------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.27e+20  -1.21e+20  -5.78e+20 
 -1.21e+20  -1.59e+21   7.22e+20 
 -5.78e+20   7.22e+20   1.36e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100920004722/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 = 240
      DIP = 75
     RAKE = 15
       MW = 3.43
       HS = 7.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
SLUFM
 USGS/SLU Moment Tensor Solution
 ENS  2010/09/20 00:47:22:0  43.60 -110.42   5.0 3.6 Wyoming
 
 Stations used:
   IW.FLWY IW.IMW IW.LOHW IW.MOOW IW.REDW IW.SNOW IW.TPAW 
   TA.H17A 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
 
 Best Fitting Double Couple
  Mo = 1.76e+21 dyne-cm
  Mw = 3.43 
  Z  = 7 km
  Plane   Strike  Dip  Rake
   NP1      146    76   164
   NP2      240    75    15
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.76e+21     21     103
    N   0.00e+00     69     284
    P  -1.76e+21      0     193

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.59e+21
       Mxy    -7.22e+20
       Mxz    -1.21e+20
       Myy     1.36e+21
       Myz     5.78e+20
       Mzz     2.27e+20
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ###-------------------------           
             ####--------------------------          
           #######---------------------------        
          ########--------------------------##       
         ##########------------------##########      
        ############------------################     
        #############-------####################     
       ###############---########################    
       ###############-##########################    
       #############----###################   ###    
       ##########--------################## T ###    
        #######------------################   ##     
        #####---------------####################     
         ##-------------------#################      
          ---------------------###############       
           ----------------------############        
             ----------------------########          
              -----------------------#####           
                 -----   --------------              
                     - P ----------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.27e+20  -1.21e+20  -5.78e+20 
 -1.21e+20  -1.59e+21   7.22e+20 
 -5.78e+20   7.22e+20   1.36e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100920004722/index.html
	
First motions plot using the waveform inversion nodal planes and the elcoate
takeoff angles and azimuths. Symbols: o strong compression, + weak compression, Delta 
strong dilatation, - weak dilatation, X undetermined polarity.

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   220    35   -45   3.17 0.3173
WVFGRD96    1.0   235    50   -20   3.13 0.3219
WVFGRD96    2.0    45    70   -50   3.31 0.4598
WVFGRD96    3.0    45    70   -50   3.36 0.5320
WVFGRD96    4.0   240    80    25   3.35 0.5765
WVFGRD96    5.0   240    75    20   3.38 0.6085
WVFGRD96    6.0   240    75    20   3.41 0.6252
WVFGRD96    7.0   240    75    15   3.43 0.6291
WVFGRD96    8.0   240    70    20   3.48 0.6235
WVFGRD96    9.0   240    70    20   3.49 0.6139
WVFGRD96   10.0   240    70    15   3.51 0.5978
WVFGRD96   11.0   240    65    15   3.52 0.5775
WVFGRD96   12.0   240    65    15   3.53 0.5585
WVFGRD96   13.0   240    65    20   3.54 0.5353
WVFGRD96   14.0   240    65    20   3.55 0.5150
WVFGRD96   15.0   240    60    25   3.56 0.4929
WVFGRD96   16.0   240    60    25   3.57 0.4744
WVFGRD96   17.0   240    60    25   3.58 0.4542
WVFGRD96   18.0   245    55    35   3.59 0.4386
WVFGRD96   19.0   105    40    95   3.69 0.4251
WVFGRD96   20.0   100    40    90   3.71 0.4167
WVFGRD96   21.0    95    40    85   3.73 0.4062
WVFGRD96   22.0    95    45    80   3.75 0.3981
WVFGRD96   23.0   290    50   105   3.77 0.3903
WVFGRD96   24.0    90    45    75   3.77 0.3873
WVFGRD96   25.0    85    45    70   3.78 0.3833
WVFGRD96   26.0    85    50    70   3.79 0.3797
WVFGRD96   27.0    90    45    75   3.78 0.3795
WVFGRD96   28.0    85    45    70   3.80 0.3812
WVFGRD96   29.0    80    45    65   3.81 0.3820

The best solution is

WVFGRD96    7.0   240    75    15   3.43 0.6291

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=Mon Sep 20 13:43:06 CDT 2010

Last Changed 2010/09/20