Location

SLU Location

To check the waveform inversion solution, P- and S-wave arrivals were picked as well as unambiguous P-wave first motions. The picks and elcoate solution are given in elocate.txt. The comparison of first motions and the RMT nodal planes are shown below.

Location ANSS

2021/05/22 01:40:20 31.688 -104.298 6.7 3.5 Texas

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2021/05/22 01:40:27:0  31.69 -104.30   6.7 3.5 Texas
 
 Stations used:
   GM.NMP02 GM.NMP25 GM.NMP41 GM.NMP44 GM.NMP53 TX.ODSA 
   TX.PB01 TX.PB11 TX.PB28 TX.PECS 
 
 Filtering commands used:
   cut o DIST/3.3 -20 o DIST/3.3 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.15 n 3 
 
 Best Fitting Double Couple
  Mo = 2.32e+21 dyne-cm
  Mw = 3.51 
  Z  = 9 km
  Plane   Strike  Dip  Rake
   NP1       80    53   -106
   NP2      285    40   -70
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.32e+21      7     181
    N   0.00e+00     13      89
    P  -2.32e+21     76     298

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.26e+21
       Mxy     9.49e+19
       Mxz    -5.22e+20
       Myy    -1.11e+20
       Myz     4.89e+20
       Mzz    -2.14e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ############################           
             ##############################          
           #######-------------##############        
          ####----------------------##########       
         ##----------------------------########      
        #---------------------------------######     
        --------------   ------------------#####     
       --------------- P --------------------###-    
       ---------------   ---------------------#--    
       --------------------------------------##--    
       -------------------------------------####-    
        ---------------------------------#######     
        ###---------------------------##########     
         ######------------------##############      
          ####################################       
           ##################################        
             ##############################          
              ############################           
                 #########   ##########              
                     ##### T ######                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.14e+21  -5.22e+20  -4.89e+20 
 -5.22e+20   2.26e+21  -9.49e+19 
 -4.89e+20  -9.49e+19  -1.11e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210522014027/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 = 285
      DIP = 40
     RAKE = -70
       MW = 3.51
       HS = 9.0

The NDK file is 20210522014027.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
SLUFM
 USGS/SLU Moment Tensor Solution
 ENS  2021/05/22 01:40:27:0  31.69 -104.30   6.7 3.5 Texas
 
 Stations used:
   GM.NMP02 GM.NMP25 GM.NMP41 GM.NMP44 GM.NMP53 TX.ODSA 
   TX.PB01 TX.PB11 TX.PB28 TX.PECS 
 
 Filtering commands used:
   cut o DIST/3.3 -20 o DIST/3.3 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.15 n 3 
 
 Best Fitting Double Couple
  Mo = 2.32e+21 dyne-cm
  Mw = 3.51 
  Z  = 9 km
  Plane   Strike  Dip  Rake
   NP1       80    53   -106
   NP2      285    40   -70
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.32e+21      7     181
    N   0.00e+00     13      89
    P  -2.32e+21     76     298

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.26e+21
       Mxy     9.49e+19
       Mxz    -5.22e+20
       Myy    -1.11e+20
       Myz     4.89e+20
       Mzz    -2.14e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ############################           
             ##############################          
           #######-------------##############        
          ####----------------------##########       
         ##----------------------------########      
        #---------------------------------######     
        --------------   ------------------#####     
       --------------- P --------------------###-    
       ---------------   ---------------------#--    
       --------------------------------------##--    
       -------------------------------------####-    
        ---------------------------------#######     
        ###---------------------------##########     
         ######------------------##############      
          ####################################       
           ##################################        
             ##############################          
              ############################           
                 #########   ##########              
                     ##### T ######                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.14e+21  -5.22e+20  -4.89e+20 
 -5.22e+20   2.26e+21  -9.49e+19 
 -4.89e+20  -9.49e+19  -1.11e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210522014027/index.html
	


First motions and takeoff angles from an elocate run.

Magnitudes

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 using wvfgrd96

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 -20 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.15 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    95    90     5   2.96 0.1685
WVFGRD96    2.0   100    75    15   3.13 0.1965
WVFGRD96    3.0   225    25    30   3.21 0.2424
WVFGRD96    4.0   245    15    55   3.26 0.3102
WVFGRD96    5.0   265    20    80   3.30 0.3586
WVFGRD96    6.0    95    60    85   3.34 0.3894
WVFGRD96    7.0   290    50   -60   3.38 0.4063
WVFGRD96    8.0    95    65   -90   3.46 0.4153
WVFGRD96    9.0   285    40   -70   3.51 0.4258
WVFGRD96   10.0   290    40   -70   3.52 0.4234
WVFGRD96   11.0   290    40   -70   3.54 0.4116
WVFGRD96   12.0   290    40   -70   3.56 0.3928
WVFGRD96   13.0   290    55   -65   3.56 0.3722
WVFGRD96   14.0   290    55   -65   3.57 0.3522
WVFGRD96   15.0   295    55   -60   3.58 0.3288
WVFGRD96   16.0   295    55   -60   3.59 0.3032
WVFGRD96   17.0   295    60   -60   3.59 0.2844
WVFGRD96   18.0   295    55   -65   3.59 0.2691
WVFGRD96   19.0   295    55   -65   3.60 0.2568
WVFGRD96   20.0   295    50   -65   3.60 0.2441
WVFGRD96   21.0   295    50   -65   3.61 0.2329
WVFGRD96   22.0   305    50   -55   3.61 0.2226
WVFGRD96   23.0   310    50   -55   3.61 0.2148
WVFGRD96   24.0   300    50   -60   3.60 0.2059
WVFGRD96   25.0   295    50   -65   3.60 0.2001
WVFGRD96   26.0   295    50   -65   3.60 0.1954
WVFGRD96   27.0   300    55   -60   3.60 0.1906
WVFGRD96   28.0   300    55   -60   3.60 0.1888
WVFGRD96   29.0   295    55   -65   3.60 0.1894

The best solution is

WVFGRD96    9.0   285    40   -70   3.51 0.4258

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 -20 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.15 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 Bureau of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Oklahoma Geological Survey, TexNet, the Iris stations, the Transportable Array of EarthScope and other networks.

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 Mon May 24 13:11:41 CDT 2021