Location

Location ANSS

2016/02/27 05:05:49 36.286 -97.520 2.1 4.0 Oklahoma

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/02/27 05:05:49:0  36.29  -97.52   2.1 4.0 Oklahoma
 
 Stations used:
   GS.KAN06 GS.KAN10 GS.KAN12 GS.KAN14 GS.KAN17 GS.OK025 
   GS.OK029 GS.OK030 GS.OK031 GS.OK032 GS.OK033 GS.OK034 
   GS.OK035 GS.OK036 GS.OK040 GS.OK041 N4.R32B N4.T35B OK.CROK 
   OK.FNO OK.OKCFA OK.X37A 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 4.95e+21 dyne-cm
  Mw = 3.73 
  Z  = 7 km
  Plane   Strike  Dip  Rake
   NP1      140    90   -30
   NP2      230    60   -180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.95e+21     21     189
    N   0.00e+00     60     320
    P  -4.95e+21     21      91

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.23e+21
       Mxy     7.45e+20
       Mxz    -1.59e+21
       Myy    -4.23e+21
       Myz    -1.90e+21
       Mzz     2.17e+14
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              -###########################           
             ---###########################          
           -------################-----------        
          ---------##########-----------------       
         ------------#####---------------------      
        ---------------#------------------------     
        --------------##------------------------     
       -------------######-----------------------    
       -----------#########----------------   ---    
       ----------############-------------- P ---    
       ---------##############-------------   ---    
        -------#################----------------     
        ------###################---------------     
         ----######################------------      
          --#########################---------       
           -##########################-------        
             ###########   #############---          
              ########## T ##############-           
                 #######   ############              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.17e+14  -1.59e+21   1.90e+21 
 -1.59e+21   4.23e+21  -7.45e+20 
  1.90e+21  -7.45e+20  -4.23e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160227050549/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 = 140
      DIP = 90
     RAKE = -30
       MW = 3.73
       HS = 7.0

The NDK file is 20160227050549.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMT
 USGS/SLU Moment Tensor Solution
 ENS  2016/02/27 05:05:49:0  36.29  -97.52   2.1 4.0 Oklahoma
 
 Stations used:
   GS.KAN06 GS.KAN10 GS.KAN12 GS.KAN14 GS.KAN17 GS.OK025 
   GS.OK029 GS.OK030 GS.OK031 GS.OK032 GS.OK033 GS.OK034 
   GS.OK035 GS.OK036 GS.OK040 GS.OK041 N4.R32B N4.T35B OK.CROK 
   OK.FNO OK.OKCFA OK.X37A 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 4.95e+21 dyne-cm
  Mw = 3.73 
  Z  = 7 km
  Plane   Strike  Dip  Rake
   NP1      140    90   -30
   NP2      230    60   -180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.95e+21     21     189
    N   0.00e+00     60     320
    P  -4.95e+21     21      91

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.23e+21
       Mxy     7.45e+20
       Mxz    -1.59e+21
       Myy    -4.23e+21
       Myz    -1.90e+21
       Mzz     2.17e+14
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              -###########################           
             ---###########################          
           -------################-----------        
          ---------##########-----------------       
         ------------#####---------------------      
        ---------------#------------------------     
        --------------##------------------------     
       -------------######-----------------------    
       -----------#########----------------   ---    
       ----------############-------------- P ---    
       ---------##############-------------   ---    
        -------#################----------------     
        ------###################---------------     
         ----######################------------      
          --#########################---------       
           -##########################-------        
             ###########   #############---          
              ########## T ##############-           
                 #######   ############              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.17e+14  -1.59e+21   1.90e+21 
 -1.59e+21   4.23e+21  -7.45e+20 
  1.90e+21  -7.45e+20  -4.23e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160227050549/index.html
	
Regional Moment Tensor (Mwr)
Moment	5.573e+14 N-m
Magnitude	3.76
Depth	6.0 km
Percent DC	68%
Half Duration	–
Catalog	US (us10004t3j)
Data Source	US1
Contributor	US1
Nodal Planes
Plane	Strike	Dip	Rake
NP1	140	84	7
NP2	49	83	174
Principal Axes
Axis	Value	Plunge	Azimuth
T	5.994	9	5
N	-0.968	81	184
P	-5.026	0	275

        

Magnitudes

mLg Magnitude


(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.

ML Magnitude


(a) ML computed using the IASPEI formula for Horizontal components; (b) ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.


(a) ML computed using the IASPEI formula for Vertical components (research); (b) ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.

Context

The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors).

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:

cut o DIST/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 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    1.0   140    90    10   3.39 0.3668
WVFGRD96    2.0   320    85    -5   3.52 0.4727
WVFGRD96    3.0   140    85   -10   3.57 0.5128
WVFGRD96    4.0   140    90   -20   3.62 0.5348
WVFGRD96    5.0   140    90   -15   3.65 0.5453
WVFGRD96    6.0   140    90   -30   3.70 0.5486
WVFGRD96    7.0   140    90   -30   3.73 0.5490
WVFGRD96    8.0   135    80   -40   3.79 0.5474
WVFGRD96    9.0   320    90    45   3.82 0.5386
WVFGRD96   10.0   320    90    50   3.86 0.5299
WVFGRD96   11.0   140    90   -50   3.88 0.5199
WVFGRD96   12.0   140    90   -50   3.89 0.5091
WVFGRD96   13.0   140    90   -45   3.89 0.4979
WVFGRD96   14.0   320    90    45   3.90 0.4876
WVFGRD96   15.0   320    90    45   3.91 0.4774
WVFGRD96   16.0   140    90   -35   3.88 0.4683
WVFGRD96   17.0   320    90    35   3.89 0.4597
WVFGRD96   18.0   320    90    35   3.90 0.4531
WVFGRD96   19.0   140    90   -35   3.91 0.4476
WVFGRD96   20.0   140    90   -30   3.90 0.4431
WVFGRD96   21.0   140    90   -30   3.91 0.4396
WVFGRD96   22.0   140    90   -30   3.92 0.4368
WVFGRD96   23.0   140    90   -30   3.93 0.4349
WVFGRD96   24.0   320    90    30   3.93 0.4332
WVFGRD96   25.0   320    90    25   3.93 0.4329
WVFGRD96   26.0   320    90    25   3.93 0.4326
WVFGRD96   27.0   140    85   -20   3.93 0.4334
WVFGRD96   28.0   140    85   -20   3.93 0.4333
WVFGRD96   29.0   320    90    20   3.94 0.4324

The best solution is

WVFGRD96    7.0   140    90   -30   3.73 0.5490

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

cut o DIST/3.3 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 n 3 
Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated.
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

Acknowledgements

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

Velocity Model

The WUS.model 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:

Last Changed Sat Feb 27 07:09:14 CST 2016