Overview

This event in southwest Virginia had characteristics that indicated that the event was not an earthquake. It occurred in a coal mining region and may be due to a mine collapse.

Standard data processing was performed as for an earthquake, but the waveform fits were not that good. Next the Zhu - Ben Zion grid search technique was applied to perform a grid search for the best full, deviatoric and double couple solution. After careful waveform QC and selection of the frequency band for inversion, all methods wanted a shallow source.

The shallowness of the source created problems for the inversion because of the large excitation of short-period surface waves, which were very path dependent. The SLU surface-wave tomography http://www.eas.slu.edu/eqc/eqc_research/NATOMO highlights the low velocity surface wave dispersion alon the eastern flank of the Appalachian Mountains.

The source characterization relies on the P-wave first motion data. The program elocate was used to lcoate the event using the CUS velocity model. The results are given in elocate.txt. All of the first motions were unambiguoous (SLUFM in the comparison below) and sampled a large portion of the focal sphere. The first motions were dilatational and were not compatible iwth the double-couple or deviatoric waveform inversion solutions.

Location

The USGS event page is http://earthquake.usgs.gov/earthquakes/eventpage/us20006fi5#executive

Location ANSS

2016/07/18 09:53:40 37.19 -81.86 2.0 4.1 Virginia

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/07/18 09:53:40:0  37.19  -81.86   2.0 4.1 Virginia
 
 Stations used:
   CO.BIRD CO.CASEE CO.HODGE CO.JSC ET.CPCT IM.TKL N4.Q54A 
   N4.R53A N4.R58B N4.S51A N4.S54A N4.S57A N4.T57A N4.T59A 
   N4.U54A N4.U56A N4.U59A N4.V51A N4.V53A N4.V55A N4.V58A 
   N4.W52A N4.W57A N4.X51A N4.Y52A N4.Y57A US.BLA US.CBN 
   US.GOGA US.TZTN 
 
 Filtering commands used:
   cut o DIST/3.3 -10 o DIST/3.3 +40
   rtr
   taper w 0.1
   hp c 0.03 n 4 
   lp c 0.07 n 4 
 
 Best Fitting Double Couple
  Mo = 3.76e+21 dyne-cm
  Mw = 3.65 
  Z  = 1 km
  Plane   Strike  Dip  Rake
   NP1      320    87    94
   NP2       90     5    40
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.76e+21     48     234
    N   0.00e+00      4     140
    P  -3.76e+21     42      46

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.20e+20
       Mxy    -2.51e+20
       Mxz    -2.38e+21
       Myy    -2.19e+13
       Myz    -2.87e+21
       Mzz     4.20e+20
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              #---------------------------           
             ####--------------------------          
           ########--------------------------        
          ##########---------------   --------       
         #############------------- P ---------      
        ################-----------   ----------     
        #################-----------------------     
       ####################----------------------    
       #####################---------------------    
       #######################-------------------    
       -#######################-----------------#    
        ##########   ############---------------     
        -######### T #############-------------#     
         #########   ###############-----------      
          ###########################---------       
           -###########################-----#        
             ###########################---          
              --########################-#           
                 --##################--              
                     ---########---                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.20e+20  -2.38e+21   2.87e+21 
 -2.38e+21  -4.20e+20   2.51e+20 
  2.87e+21   2.51e+20  -2.19e+13 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160718095340/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 = 90
      DIP = 5
     RAKE = 40
       MW = 3.65
       HS = 1.0

The NDK file is 20160718095340.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
MTGRDDC
MTGRDDEV
MTGRD
SLUFM
 USGS/SLU Moment Tensor Solution
 ENS  2016/07/18 09:53:40:0  37.19  -81.86   2.0 4.1 Virginia
 
 Stations used:
   CO.BIRD CO.CASEE CO.HODGE CO.JSC ET.CPCT IM.TKL N4.Q54A 
   N4.R53A N4.R58B N4.S51A N4.S54A N4.S57A N4.T57A N4.T59A 
   N4.U54A N4.U56A N4.U59A N4.V51A N4.V53A N4.V55A N4.V58A 
   N4.W52A N4.W57A N4.X51A N4.Y52A N4.Y57A US.BLA US.CBN 
   US.GOGA US.TZTN 
 
 Filtering commands used:
   cut o DIST/3.3 -10 o DIST/3.3 +40
   rtr
   taper w 0.1
   hp c 0.03 n 4 
   lp c 0.07 n 4 
 
 Best Fitting Double Couple
  Mo = 3.76e+21 dyne-cm
  Mw = 3.65 
  Z  = 1 km
  Plane   Strike  Dip  Rake
   NP1      320    87    94
   NP2       90     5    40
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.76e+21     48     234
    N   0.00e+00      4     140
    P  -3.76e+21     42      46

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.20e+20
       Mxy    -2.51e+20
       Mxz    -2.38e+21
       Myy    -2.19e+13
       Myz    -2.87e+21
       Mzz     4.20e+20
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              #---------------------------           
             ####--------------------------          
           ########--------------------------        
          ##########---------------   --------       
         #############------------- P ---------      
        ################-----------   ----------     
        #################-----------------------     
       ####################----------------------    
       #####################---------------------    
       #######################-------------------    
       -#######################-----------------#    
        ##########   ############---------------     
        -######### T #############-------------#     
         #########   ###############-----------      
          ###########################---------       
           -###########################-----#        
             ###########################---          
              --########################-#           
                 --##################--              
                     ---########---                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.20e+20  -2.38e+21   2.87e+21 
 -2.38e+21  -4.20e+20   2.51e+20 
  2.87e+21   2.51e+20  -2.19e+13 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160718095340/index.html
	
 Moment (dyne-cm)   3.75E+21   dyne-cm
 Magnitude (Mw)    3.65
  
 Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.75E+21     48.    234.
    N  -2.74E+17      4.    140.
    P  -3.75E+21     42.     47.
 Moment Tensor: (dyne-cm) Aki-Richards
    Component   Value
       Mxx   -4.18E+20
       Mxy   -4.18E+20
       Mxz   -2.37E+21
       Myy   -2.50E+20
       Myz   -2.86E+21
       Mzz    4.18E+20
                                                     
                                                     
                                                     
                    --------------                   
                ----------------------               
             #---------------------------            
            ####--------------------------           
          ########--------------------------         
         ##########---------------   --------        
        #############------------- P ---------       
       ################-----------   ----------      
       #################-----------------------      
      ####################----------------------     
      #####################---------------------     
      #######################-------------------     
      -#######################-----------------#     
       ##########   ############---------------      
       -######### T #############-------------#      
        #########   ###############-----------       
         ###########################---------        
          -###########################-----#         
            ###########################---           
             --########################-#            
                --##################--               
                    ---########---                   
                                                     
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor: (dyne-cm)
         R         T         F
  R  4.18E+20 -2.37E+21  2.86E+21
  T -2.37E+21 -4.18E+20  2.50E+20
  F  2.86E+21  2.50E+20  0.00E+00
        
 Moment (dyne-cm)   1.39E+21   dyne-cm
 Magnitude (Mw)    3.36
  
 Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.51E+21     54.     13.
    N  -2.89E+20     10.    117.
    P  -1.23E+21     34.    213.
 Moment Tensor: (dyne-cm) Aki-Richards
    Component   Value
       Mxx   -1.40E+20
       Mxy   -1.40E+20
       Mxz    1.20E+21
       Myy   -1.58E+20
       Myz    4.32E+20
       Mzz    5.92E+20
                                                     
                                                     
                                                     
                    -##########---                   
                -##################---               
             --#######################---            
            -###########################--           
          --#############################---         
         ---###############   ############---        
        ----############### T #############---       
       ------##############   ##############---      
       -------##############################---      
      ----------############################----     
      ------------##########################----     
      ---------------######################-----     
      -------------------#################------     
       ------------------------########--------      
       ----------------------------------------      
        --------------------------------------       
         ---------   ------------------------        
          -------- P -----------------------         
            ------   ---------------------           
             ----------------------------            
                ----------------------               
                    --------------                   
                                                     
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor: (dyne-cm)
         R         T         F
  R  5.92E+20  1.20E+21 -4.32E+20
  T  1.20E+21 -1.40E+20  1.58E+20
  F -4.32E+20  1.58E+20 -4.52E+20
        
 Moment (dyne-cm)   3.19E+21   dyne-cm
 Magnitude (Mw)    3.60
  
 Principal Axes:
   Axis    Value   Plunge  Azimuth
    T  -5.68E+20     20.    284.
    N  -1.54E+21      5.    192.
    P  -4.20E+21     70.     90.
 Moment Tensor: (dyne-cm) Aki-Richards
    Component   Value
       Mxx   -1.49E+21
       Mxy   -1.49E+21
       Mxz    7.09E+19
       Myy   -1.99E+20
       Myz   -1.17E+21
       Mzz   -3.77E+21
                                                     
                                                     
                                                     
                    --------------                   
                ----------------------               
             ----------------------------            
            ------------------------------           
          ----------------------------------         
         ------------------------------------        
        --------------------------------------       
       --   -----------------------------------      
       -- T -----------------------------------      
      ---   -------------------   --------------     
      ------------------------- P --------------     
      -------------------------   --------------     
      ------------------------------------------     
       ----------------------------------------      
       ----------------------------------------      
        --------------------------------------       
         ------------------------------------        
          ----------------------------------         
            ------------------------------           
             ----------------------------            
                ----------------------               
                    --------------                   
                                                     
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor: (dyne-cm)
         R         T         F
  R -3.77E+21  7.09E+19  1.17E+21
  T  7.09E+19 -1.49E+21  1.99E+20
  F  1.17E+21  1.99E+20 -1.05E+21
        


First motions and takeoff angles from an elocate run.

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 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 -10 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 4 
lp c 0.07 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    1.0    90     5    40   3.65 0.5467
WVFGRD96    2.0    90    10    45   3.44 0.5432
WVFGRD96    3.0   120    80    80   3.35 0.5399
WVFGRD96    4.0   125    75    80   3.31 0.5359
WVFGRD96    5.0   125    70    80   3.29 0.5266
WVFGRD96    6.0   130    70    75   3.27 0.5172
WVFGRD96    7.0   110    65    95   3.29 0.5059
WVFGRD96    8.0   305    25    95   3.26 0.4985
WVFGRD96    9.0   120    65    85   3.24 0.4888
WVFGRD96   10.0   120    70    85   3.23 0.4806
WVFGRD96   11.0   120    70    80   3.22 0.4709
WVFGRD96   12.0   180    50   -70   3.32 0.4710
WVFGRD96   13.0   180    50   -70   3.31 0.4687
WVFGRD96   14.0   120    45   -65   3.30 0.4697
WVFGRD96   15.0   125    45   -65   3.29 0.4676
WVFGRD96   16.0   125    45   -65   3.28 0.4637
WVFGRD96   17.0   125    45   -65   3.28 0.4590
WVFGRD96   18.0   130    50   -60   3.28 0.4532
WVFGRD96   19.0   130    50   -55   3.29 0.4446
WVFGRD96   20.0   130    50   -55   3.31 0.4358
WVFGRD96   21.0   130    50   -55   3.31 0.4327
WVFGRD96   22.0   130    50   -55   3.31 0.4281
WVFGRD96   23.0   130    50   -55   3.31 0.4224
WVFGRD96   24.0   130    50   -55   3.31 0.4161
WVFGRD96   25.0    25    45   -20   3.32 0.4138
WVFGRD96   26.0    25    45   -20   3.32 0.4110
WVFGRD96   27.0    25    45   -20   3.33 0.4081
WVFGRD96   28.0    25    45   -20   3.34 0.4049
WVFGRD96   29.0    25    45   -15   3.34 0.4016

The best solution is

WVFGRD96    1.0    90     5    40   3.65 0.5467

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 -10 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 4 
lp c 0.07 n 4 
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.


Grid Search Full Moment Tensor Inversion using wvfmtgrd96

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 wvfmtgrd96 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 -10 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 4 
lp c 0.07 n 4 
The results of this grid search over depth are as follow:

MT Program  H(km) Mxx(dyne-cm)   Myy        Mxy        Mxz        Myz        Mzz       Mw      Fit
WVFMTGRD96    1.0 -0.215E+22 -0.199E+22 -0.176E+21  0.214E+21 -0.226E+22 -0.566E+22  3.7356  0.5740
WVFMTGRD96    2.0 -0.149E+22 -0.105E+22 -0.199E+21  0.709E+20 -0.117E+22 -0.377E+22  3.6025  0.5796
WVFMTGRD96    3.0 -0.117E+22 -0.739E+21 -0.217E+21  0.783E+20 -0.738E+21 -0.319E+22  3.5407  0.5747
WVFMTGRD96    4.0 -0.946E+21 -0.593E+21 -0.210E+21  0.182E+21 -0.391E+21 -0.287E+22  3.4986  0.5686
WVFMTGRD96    5.0 -0.763E+21 -0.189E+21 -0.244E+21  0.202E+20 -0.442E+21 -0.247E+22  3.4528  0.5609
WVFMTGRD96    6.0 -0.660E+21 -0.164E+21 -0.208E+21  0.129E+21 -0.355E+21 -0.216E+22  3.4134  0.5528
WVFMTGRD96    7.0 -0.504E+21  0.111E+21 -0.190E+21  0.221E+21 -0.149E+21 -0.201E+22  3.3851  0.5417
WVFMTGRD96    8.0 -0.735E+21 -0.140E+22 -0.210E+21  0.614E+21  0.245E+21  0.318E+21  3.3502  0.5306
WVFMTGRD96    9.0 -0.977E+21 -0.160E+22 -0.234E+21  0.579E+21  0.223E+21  0.215E+21  3.3823  0.5257

The best solution is

WVFMTGRD96    2.0 -0.149E+22 -0.105E+22 -0.199E+21  0.709E+20 -0.117E+22 -0.377E+22  3.6025  0.5796

The complete moment tensor decomposition using the program mtinfo is given in the text file MTGRDinfo.txt. (Jost, M. L., and R. B. Herrmann (1989). A student's guide to and review of moment tensors, Seism. Res. Letters 60, 37-57. SRL_60_2_37-57.pdf.

The P-wave first motion mechanism corresponding 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 -10 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 4 
lp c 0.07 n 4 
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.

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.


Grid Search Double Couple Inversion using wvfmtgrd96

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 wvfmtgrd96 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 -10 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 4 
lp c 0.07 n 4 
The results of this grid search over depth are as follow:

MT Program  H(km) Mxx(dyne-cm)   Myy        Mxy        Mxz        Myz        Mzz       Mw      Fit
WVFMTGRD96    1.0 -0.418E+21  0.000E+00 -0.250E+21 -0.237E+22 -0.286E+22  0.418E+21  3.6487  0.5468
WVFMTGRD96    2.0 -0.434E+21  0.000E+00 -0.221E+21 -0.119E+22 -0.125E+22  0.434E+21  3.4362  0.5432
WVFMTGRD96    3.0 -0.138E+21 -0.307E+21 -0.306E+21  0.108E+22  0.577E+21  0.445E+21  3.3475  0.5399
WVFMTGRD96    4.0 -0.164E+21 -0.402E+21 -0.361E+21  0.846E+21  0.472E+21  0.566E+21  3.3072  0.5351
WVFMTGRD96    5.0 -0.243E+21 -0.344E+21 -0.341E+21  0.745E+21  0.425E+21  0.587E+21  3.2823  0.5272
WVFMTGRD96    6.0 -0.126E+21 -0.503E+21 -0.353E+21  0.632E+21  0.413E+21  0.629E+21  3.2705  0.5172
WVFMTGRD96    7.0 -0.777E+21 -0.413E+20 -0.198E+21  0.632E+21  0.272E+21  0.818E+21  3.2869  0.5059
WVFMTGRD96    8.0 -0.529E+21 -0.209E+21 -0.335E+21  0.551E+21  0.293E+21  0.739E+21  3.2572  0.4985
WVFMTGRD96    9.0 -0.460E+21 -0.237E+21 -0.338E+21  0.523E+21  0.263E+21  0.697E+21  3.2405  0.4888
WVFMTGRD96   10.0 -0.409E+21 -0.163E+21 -0.260E+21  0.600E+21  0.331E+21  0.572E+21  3.2339  0.4804
WVFMTGRD96   11.0 -0.287E+21 -0.258E+21 -0.306E+21  0.588E+21  0.280E+21  0.545E+21  3.2233  0.4709
WVFMTGRD96   12.0 -0.542E+14  0.112E+22  0.318E+21  0.267E+21 -0.198E+21 -0.112E+22  3.3225  0.4710
WVFMTGRD96   13.0  0.107E+22  0.000E+00  0.302E+21  0.188E+21 -0.254E+21 -0.107E+22  3.3082  0.4695
WVFMTGRD96   14.0  0.103E+22  0.000E+00  0.293E+21  0.182E+21 -0.246E+21 -0.103E+22  3.2988  0.4697
WVFMTGRD96   15.0  0.968E+21  0.000E+00  0.351E+21  0.171E+21 -0.295E+21 -0.968E+21  3.2902  0.4679
WVFMTGRD96   16.0  0.951E+21  0.000E+00  0.345E+21  0.168E+21 -0.290E+21 -0.951E+21  3.2851  0.4637
WVFMTGRD96   17.0  0.941E+21  0.000E+00  0.341E+21  0.166E+21 -0.286E+21 -0.941E+21  3.2819  0.4594
WVFMTGRD96   18.0  0.921E+21 -0.260E+20  0.371E+21  0.959E+20 -0.360E+21 -0.895E+21  3.2807  0.4532
WVFMTGRD96   19.0  0.972E+21 -0.107E+21  0.344E+21  0.137E+21 -0.401E+21 -0.866E+21  3.2871  0.4446
WVFMTGRD96   20.0  0.106E+22 -0.116E+21  0.374E+21  0.149E+21 -0.436E+21 -0.941E+21  3.3112  0.4358
WVFMTGRD96   21.0  0.106E+22 -0.116E+21  0.374E+21  0.149E+21 -0.436E+21 -0.941E+21  3.3111  0.4327
WVFMTGRD96   22.0  0.106E+22 -0.116E+21  0.375E+21  0.149E+21 -0.436E+21 -0.942E+21  3.3115  0.4281
WVFMTGRD96   23.0  0.106E+22 -0.116E+21  0.376E+21  0.150E+21 -0.438E+21 -0.945E+21  3.3123  0.4224
WVFMTGRD96   24.0  0.107E+22 -0.117E+21  0.378E+21  0.151E+21 -0.440E+21 -0.950E+21  3.3139  0.4161
WVFMTGRD96   25.0 -0.530E+21  0.935E+21  0.350E+21 -0.712E+21 -0.332E+21 -0.405E+21  3.3154  0.4138
WVFMTGRD96   26.0 -0.542E+21  0.956E+21  0.358E+21 -0.729E+21 -0.340E+21 -0.414E+21  3.3219  0.4110
WVFMTGRD96   27.0 -0.555E+21  0.979E+21  0.367E+21 -0.746E+21 -0.348E+21 -0.424E+21  3.3288  0.4081
WVFMTGRD96   28.0 -0.569E+21  0.100E+22  0.376E+21 -0.765E+21 -0.357E+21 -0.434E+21  3.3358  0.4049
WVFMTGRD96   29.0 -0.624E+21  0.962E+21  0.444E+21 -0.809E+21 -0.377E+21 -0.338E+21  3.3443  0.4016

The best solution is

WVFMTGRD96    1.0 -0.418E+21  0.000E+00 -0.250E+21 -0.237E+22 -0.286E+22  0.418E+21  3.6487  0.5468

The complete moment tensor decomposition using the program mtinfo is given in the text file MTGRDDCinfo.txt. (Jost, M. L., and R. B. Herrmann (1989). A student's guide to and review of moment tensors, Seism. Res. Letters 60, 37-57. SRL_60_2_37-57.pdf.

The P-wave first motion mechanism corresponding 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 -10 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 4 
lp c 0.07 n 4 
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.

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.


Grid Search Deviatoric Moment Tensor Inversion using wvfmtdgrd96

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 wvfmtgrd96 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 -10 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 4 
lp c 0.07 n 4 
The results of this grid search over depth are as follow:

MT Program  H(km) Mxx(dyne-cm)   Myy        Mxy        Mxz        Myz        Mzz       Mw      Fit
WVFMTGRD96    1.0 -0.277E+21 -0.344E+21 -0.127E+21 -0.269E+22 -0.153E+22  0.620E+21  3.5981  0.5359
WVFMTGRD96    2.0 -0.463E+21 -0.109E+21 -0.117E+21 -0.105E+22 -0.137E+22  0.572E+21  3.4381  0.5496
WVFMTGRD96    3.0 -0.140E+21 -0.452E+21 -0.158E+21  0.120E+22  0.432E+21  0.592E+21  3.3620  0.5503
WVFMTGRD96    4.0 -0.287E+21 -0.528E+21 -0.195E+21  0.911E+21  0.436E+21  0.816E+21  3.3320  0.5474
WVFMTGRD96    5.0 -0.442E+21 -0.683E+21 -0.241E+21  0.734E+21  0.384E+21  0.112E+22  3.3441  0.5406
WVFMTGRD96    6.0 -0.382E+21 -0.721E+21 -0.231E+21  0.684E+21  0.283E+21  0.110E+22  3.3294  0.5308
WVFMTGRD96    7.0 -0.339E+21 -0.651E+21 -0.230E+21  0.615E+21  0.263E+21  0.990E+21  3.3002  0.5136
WVFMTGRD96    8.0 -0.260E+21 -0.438E+21 -0.258E+21  0.569E+21  0.312E+21  0.698E+21  3.2449  0.4978
WVFMTGRD96    9.0 -0.316E+21 -0.505E+21 -0.287E+21  0.534E+21  0.203E+21  0.821E+21  3.2551  0.4889
WVFMTGRD96   10.0 -0.283E+21 -0.549E+21 -0.263E+21  0.604E+21  0.193E+21  0.832E+21  3.2678  0.4824
WVFMTGRD96   11.0  0.543E+21  0.733E+21  0.223E+21  0.230E+21 -0.309E+21 -0.128E+22  3.3182  0.4802
WVFMTGRD96   12.0  0.511E+21  0.689E+21  0.210E+21  0.216E+21 -0.291E+21 -0.120E+22  3.3004  0.4808
WVFMTGRD96   13.0  0.805E+21  0.283E+21  0.231E+21  0.226E+21 -0.322E+21 -0.109E+22  3.2886  0.4794
WVFMTGRD96   14.0  0.809E+21  0.238E+21  0.248E+21  0.215E+21 -0.298E+21 -0.105E+22  3.2806  0.4769
WVFMTGRD96   15.0  0.818E+21  0.198E+21  0.265E+21  0.207E+21 -0.277E+21 -0.102E+22  3.2752  0.4730
WVFMTGRD96   16.0  0.767E+21  0.186E+21  0.309E+21  0.211E+21 -0.335E+21 -0.953E+21  3.2692  0.4671
WVFMTGRD96   17.0  0.772E+21  0.169E+21  0.319E+21  0.207E+21 -0.324E+21 -0.941E+21  3.2673  0.4619
WVFMTGRD96   18.0  0.768E+21  0.168E+21  0.317E+21  0.205E+21 -0.322E+21 -0.936E+21  3.2657  0.4558
WVFMTGRD96   19.0  0.768E+21  0.168E+21  0.317E+21  0.205E+21 -0.322E+21 -0.936E+21  3.2657  0.4468
WVFMTGRD96   20.0  0.863E+21  0.111E+21  0.341E+21  0.196E+21 -0.436E+21 -0.974E+21  3.2929  0.4389
WVFMTGRD96   21.0  0.863E+21  0.111E+21  0.341E+21  0.196E+21 -0.436E+21 -0.973E+21  3.2927  0.4349
WVFMTGRD96   22.0  0.864E+21  0.111E+21  0.342E+21  0.196E+21 -0.437E+21 -0.975E+21  3.2931  0.4297
WVFMTGRD96   23.0  0.896E+21  0.800E+20  0.369E+21  0.189E+21 -0.418E+21 -0.976E+21  3.2964  0.4238
WVFMTGRD96   24.0  0.901E+21  0.804E+20  0.371E+21  0.191E+21 -0.420E+21 -0.982E+21  3.2981  0.4175
WVFMTGRD96   25.0  0.932E+21  0.545E+20  0.342E+21  0.125E+21 -0.447E+21 -0.987E+21  3.2995  0.4109
WVFMTGRD96   26.0  0.921E+21  0.330E+20  0.410E+21  0.212E+21 -0.489E+21 -0.954E+21  3.3082  0.4041
WVFMTGRD96   27.0  0.104E+21  0.614E+21  0.265E+21 -0.642E+21 -0.283E+21 -0.718E+21  3.2687  0.3979
WVFMTGRD96   28.0  0.150E+21  0.522E+21  0.267E+21 -0.656E+21 -0.381E+21 -0.672E+21  3.2694  0.3938
WVFMTGRD96   29.0  0.153E+21  0.531E+21  0.272E+21 -0.668E+21 -0.388E+21 -0.684E+21  3.2748  0.3892

The best solution is

WVFMTGRD96    3.0 -0.140E+21 -0.452E+21 -0.158E+21  0.120E+22  0.432E+21  0.592E+21  3.3620  0.5503

The complete moment tensor decomposition using the program mtinfo is given in the text file MTGRDDEVinfo.txt. (Jost, M. L., and R. B. Herrmann (1989). A student's guide to and review of moment tensors, Seism. Res. Letters 60, 37-57. SRL_60_2_37-57.pdf.

The P-wave first motion mechanism corresponding 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 -10 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 4 
lp c 0.07 n 4 
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.

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 CUS model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

MODEL.01
CUS Model with Q from simple gamma values
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
  1.0000  5.0000  2.8900  2.5000 0.172E-02 0.387E-02 0.00  0.00  1.00  1.00 
  9.0000  6.1000  3.5200  2.7300 0.160E-02 0.363E-02 0.00  0.00  1.00  1.00 
 10.0000  6.4000  3.7000  2.8200 0.149E-02 0.336E-02 0.00  0.00  1.00  1.00 
 20.0000  6.7000  3.8700  2.9020 0.000E-04 0.000E-04 0.00  0.00  1.00  1.00 
  0.0000  8.1500  4.7000  3.3640 0.194E-02 0.431E-02 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 Tue Jul 19 08:35:45 CDT 2016