Location

2010/12/19 05:05:28 35.824 -96.822 5.0 3.60

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  2010/12/19 05:05:28:0  35.82  -96.82   5.0 3.6 
 
 Stations used:
   TA.234A TA.M31A TA.M33A TA.M35A TA.N38A TA.O33A TA.O35A 
   TA.P31A TA.P32A TA.P34A TA.P35A TA.P37A TA.Q30A TA.Q31A 
   TA.Q32A TA.Q35A TA.Q37A TA.R29A TA.R32A TA.R33A TA.R34A 
   TA.R35A TA.R36A TA.R37A TA.S33A TA.S34A TA.S35A TA.S36A 
   TA.S37A TA.T32A TA.T33A TA.T34A TA.T35A TA.T36A TA.T37A 
   TA.U34A TA.U35A TA.U36A TA.V36A TA.W33A TA.W35A TA.W36A 
   TA.WHTX US.WMOK 
 
 Filtering commands used:
   hp c 0.03 n 4
   lp c 0.06 n 4
 
 Best Fitting Double Couple
  Mo = 1.12e+21 dyne-cm
  Mw = 3.30 
  Z  = 2 km
  Plane   Strike  Dip  Rake
   NP1       93    45   -95
   NP2      280    45   -85
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.12e+21      0     186
    N   0.00e+00      4      96
    P  -1.12e+21     86     278

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.11e+21
       Mxy     1.26e+20
       Mxz    -1.20e+19
       Myy     1.01e+19
       Myz     6.81e+19
       Mzz    -1.12e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ############################           
             ##############################          
           ##################################        
          #######-----------------############       
         ####-------------------------#########      
        ##-------------------------------#######     
        -----------------------------------#####     
       ----------------   -------------------####    
       ---------------- P --------------------###    
       #---------------   ---------------------##    
       ###---------------------------------------    
        ###-----------------------------------##     
        ######------------------------------####     
         ########------------------------######      
          ###########---------------##########       
           ##################################        
             ##############################          
              ############################           
                 #######   ############              
                     ### T ########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.12e+21  -1.20e+19  -6.81e+19 
 -1.20e+19   1.11e+21  -1.26e+20 
 -6.81e+19  -1.26e+20   1.01e+19 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20101219050528/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 = 280
      DIP = 45
     RAKE = -85
       MW = 3.30
       HS = 2.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2010/12/19 05:05:28:0  35.82  -96.82   5.0 3.6 
 
 Stations used:
   TA.234A TA.M31A TA.M33A TA.M35A TA.N38A TA.O33A TA.O35A 
   TA.P31A TA.P32A TA.P34A TA.P35A TA.P37A TA.Q30A TA.Q31A 
   TA.Q32A TA.Q35A TA.Q37A TA.R29A TA.R32A TA.R33A TA.R34A 
   TA.R35A TA.R36A TA.R37A TA.S33A TA.S34A TA.S35A TA.S36A 
   TA.S37A TA.T32A TA.T33A TA.T34A TA.T35A TA.T36A TA.T37A 
   TA.U34A TA.U35A TA.U36A TA.V36A TA.W33A TA.W35A TA.W36A 
   TA.WHTX US.WMOK 
 
 Filtering commands used:
   hp c 0.03 n 4
   lp c 0.06 n 4
 
 Best Fitting Double Couple
  Mo = 1.12e+21 dyne-cm
  Mw = 3.30 
  Z  = 2 km
  Plane   Strike  Dip  Rake
   NP1       93    45   -95
   NP2      280    45   -85
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.12e+21      0     186
    N   0.00e+00      4      96
    P  -1.12e+21     86     278

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.11e+21
       Mxy     1.26e+20
       Mxz    -1.20e+19
       Myy     1.01e+19
       Myz     6.81e+19
       Mzz    -1.12e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ############################           
             ##############################          
           ##################################        
          #######-----------------############       
         ####-------------------------#########      
        ##-------------------------------#######     
        -----------------------------------#####     
       ----------------   -------------------####    
       ---------------- P --------------------###    
       #---------------   ---------------------##    
       ###---------------------------------------    
        ###-----------------------------------##     
        ######------------------------------####     
         ########------------------------######      
          ###########---------------##########       
           ##################################        
             ##############################          
              ############################           
                 #######   ############              
                     ### T ########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.12e+21  -1.20e+19  -6.81e+19 
 -1.20e+19   1.11e+21  -1.26e+20 
 -6.81e+19  -1.26e+20   1.01e+19 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20101219050528/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 0.03 n 4
lp c 0.06 n 4
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   3.22 0.4532
WVFGRD96    1.0   285    40   -75   3.26 0.4680
WVFGRD96    2.0   280    45   -85   3.30 0.4815
WVFGRD96    3.0   100    45   -85   3.36 0.4778
WVFGRD96    4.0   100    40   -85   3.40 0.4323
WVFGRD96    5.0   295    25   -65   3.44 0.3938
WVFGRD96    6.0    85    70  -105   3.42 0.3809
WVFGRD96    7.0   305    25   -55   3.40 0.3739
WVFGRD96    8.0   310    25   -50   3.45 0.3849
WVFGRD96    9.0   310    25   -50   3.44 0.3821
WVFGRD96   10.0   315    25   -40   3.41 0.3810
WVFGRD96   11.0   325    30   -30   3.41 0.3806
WVFGRD96   12.0   335    35   -15   3.40 0.3820
WVFGRD96   13.0   345    45    10   3.42 0.3851
WVFGRD96   14.0   345    45    15   3.43 0.3894
WVFGRD96   15.0   345    45    20   3.43 0.3920
WVFGRD96   16.0   345    45    20   3.43 0.3943
WVFGRD96   17.0   345    50    25   3.45 0.3960
WVFGRD96   18.0   345    50    25   3.45 0.3970
WVFGRD96   19.0   345    50    25   3.46 0.3971
WVFGRD96   20.0   345    50    25   3.46 0.3965
WVFGRD96   21.0   345    50    25   3.48 0.3931
WVFGRD96   22.0   345    50    30   3.48 0.3910
WVFGRD96   23.0   345    50    30   3.49 0.3885
WVFGRD96   24.0   345    50    30   3.49 0.3853
WVFGRD96   25.0   345    50    30   3.50 0.3817
WVFGRD96   26.0   345    50    30   3.50 0.3776
WVFGRD96   27.0   350    50    35   3.51 0.3732
WVFGRD96   28.0   345    55    35   3.52 0.3686
WVFGRD96   29.0   345    55    35   3.53 0.3637

The best solution is

WVFGRD96    2.0   280    45   -85   3.30 0.4815

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 0.03 n 4
lp c 0.06 n 4
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 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=Sat Jan 22 10:03:34 CST 2011

Last Changed 2010/12/19