Location

Location ANSS

2019/07/28 10:18:57 63.196 -150.500 114.8 4.0 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2019/07/28 10:18:57:0  63.20 -150.50 114.8 4.0 Alaska
 
 Stations used:
   AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.GHO AK.HDA AK.KNK 
   AK.KTH AK.MCK AK.MLY AK.NEA2 AK.PPLA AK.RND AK.SCM AK.SKN 
   AK.SLK AK.SSN AK.TRF AK.WRH AT.PMR AV.STLK IM.IL31 IU.COLA 
   TA.H21K TA.H23K TA.H24K TA.I23K TA.J19K TA.J20K TA.J25K 
   TA.K20K TA.L19K TA.M19K TA.M22K TA.POKR 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 5.19e+22 dyne-cm
  Mw = 4.41 
  Z  = 126 km
  Plane   Strike  Dip  Rake
   NP1      200    70   -70
   NP2      333    28   -133
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.19e+22     22     275
    N   0.00e+00     19      13
    P  -5.19e+22     60     139

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -7.05e+21
       Mxy     2.70e+21
       Mxz     1.85e+22
       Myy     3.84e+22
       Myz    -3.30e+22
       Mzz    -3.13e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 #############---######              
              #################--#########           
             #################------#######          
           #################----------#######        
          #################------------#######       
         #################---------------######      
        #################-----------------######     
        ################------------------######     
       ###   ###########-------------------######    
       ### T ##########---------------------#####    
       ###   #########----------------------#####    
       ###############----------------------#####    
        #############-----------   ---------####     
        #############----------- P ---------####     
         ############-----------   --------####      
          ##########-----------------------###       
           #########----------------------###        
             #######---------------------##          
              ######--------------------##           
                 ###------------------#              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.13e+22   1.85e+22   3.30e+22 
  1.85e+22  -7.05e+21  -2.70e+21 
  3.30e+22  -2.70e+21   3.84e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190728101857/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 = 200
      DIP = 70
     RAKE = -70
       MW = 4.41
       HS = 126.0

The NDK file is 20190728101857.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2019/07/28 10:18:57:0  63.20 -150.50 114.8 4.0 Alaska
 
 Stations used:
   AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.GHO AK.HDA AK.KNK 
   AK.KTH AK.MCK AK.MLY AK.NEA2 AK.PPLA AK.RND AK.SCM AK.SKN 
   AK.SLK AK.SSN AK.TRF AK.WRH AT.PMR AV.STLK IM.IL31 IU.COLA 
   TA.H21K TA.H23K TA.H24K TA.I23K TA.J19K TA.J20K TA.J25K 
   TA.K20K TA.L19K TA.M19K TA.M22K TA.POKR 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 5.19e+22 dyne-cm
  Mw = 4.41 
  Z  = 126 km
  Plane   Strike  Dip  Rake
   NP1      200    70   -70
   NP2      333    28   -133
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.19e+22     22     275
    N   0.00e+00     19      13
    P  -5.19e+22     60     139

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -7.05e+21
       Mxy     2.70e+21
       Mxz     1.85e+22
       Myy     3.84e+22
       Myz    -3.30e+22
       Mzz    -3.13e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 #############---######              
              #################--#########           
             #################------#######          
           #################----------#######        
          #################------------#######       
         #################---------------######      
        #################-----------------######     
        ################------------------######     
       ###   ###########-------------------######    
       ### T ##########---------------------#####    
       ###   #########----------------------#####    
       ###############----------------------#####    
        #############-----------   ---------####     
        #############----------- P ---------####     
         ############-----------   --------####      
          ##########-----------------------###       
           #########----------------------###        
             #######---------------------##          
              ######--------------------##           
                 ###------------------#              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.13e+22   1.85e+22   3.30e+22 
  1.85e+22  -7.05e+21  -2.70e+21 
  3.30e+22  -2.70e+21   3.84e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190728101857/index.html
	

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 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 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    2.0   185    45    85   3.50 0.2084
WVFGRD96    4.0   340    75    55   3.49 0.1904
WVFGRD96    6.0   160    80    50   3.54 0.2280
WVFGRD96    8.0     5    70    70   3.66 0.2600
WVFGRD96   10.0   155    80    45   3.67 0.2746
WVFGRD96   12.0   325    80   -45   3.70 0.2837
WVFGRD96   14.0   325    80   -45   3.74 0.2876
WVFGRD96   16.0    65    35   -25   3.78 0.2858
WVFGRD96   18.0   340    75   -50   3.80 0.2820
WVFGRD96   20.0    65    35   -25   3.83 0.2770
WVFGRD96   22.0    65    35   -25   3.86 0.2738
WVFGRD96   24.0    65    35   -25   3.89 0.2682
WVFGRD96   26.0    40    50   -40   3.89 0.2638
WVFGRD96   28.0    40    50   -35   3.91 0.2611
WVFGRD96   30.0    40    55   -35   3.92 0.2579
WVFGRD96   32.0    40    55   -35   3.93 0.2493
WVFGRD96   34.0    40    55   -35   3.94 0.2381
WVFGRD96   36.0   120    50    40   3.96 0.2350
WVFGRD96   38.0   120    55    35   3.99 0.2391
WVFGRD96   40.0   125    50    40   4.07 0.2385
WVFGRD96   42.0   125    55    45   4.09 0.2382
WVFGRD96   44.0    40    50   -35   4.10 0.2406
WVFGRD96   46.0    40    55   -35   4.12 0.2437
WVFGRD96   48.0    40    55   -35   4.14 0.2478
WVFGRD96   50.0    40    55   -30   4.15 0.2525
WVFGRD96   52.0    40    55   -30   4.17 0.2593
WVFGRD96   54.0    45    60   -15   4.17 0.2673
WVFGRD96   56.0    45    60   -15   4.19 0.2781
WVFGRD96   58.0   220    65   -45   4.15 0.2932
WVFGRD96   60.0   220    65   -45   4.17 0.3199
WVFGRD96   62.0   215    60   -50   4.19 0.3442
WVFGRD96   64.0   215    60   -50   4.21 0.3671
WVFGRD96   66.0   215    60   -50   4.22 0.3865
WVFGRD96   68.0   215    60   -50   4.23 0.4051
WVFGRD96   70.0   215    60   -55   4.24 0.4233
WVFGRD96   72.0   215    65   -55   4.25 0.4488
WVFGRD96   74.0   215    65   -55   4.27 0.4860
WVFGRD96   76.0   215    65   -55   4.28 0.5243
WVFGRD96   78.0   215    70   -55   4.30 0.5685
WVFGRD96   80.0   200    70   -65   4.32 0.6118
WVFGRD96   82.0   200    70   -65   4.33 0.6386
WVFGRD96   84.0   200    70   -65   4.33 0.6556
WVFGRD96   86.0   200    70   -65   4.34 0.6698
WVFGRD96   88.0   200    70   -65   4.35 0.6844
WVFGRD96   90.0   200    70   -65   4.35 0.6967
WVFGRD96   92.0   200    70   -65   4.35 0.7091
WVFGRD96   94.0   200    70   -70   4.36 0.7201
WVFGRD96   96.0   200    70   -70   4.36 0.7304
WVFGRD96   98.0   200    70   -70   4.37 0.7418
WVFGRD96  100.0   200    70   -70   4.37 0.7515
WVFGRD96  102.0   200    70   -70   4.38 0.7600
WVFGRD96  104.0   200    70   -70   4.38 0.7679
WVFGRD96  106.0   200    70   -70   4.38 0.7750
WVFGRD96  108.0   200    70   -70   4.39 0.7823
WVFGRD96  110.0   200    70   -70   4.39 0.7869
WVFGRD96  112.0   200    70   -70   4.39 0.7897
WVFGRD96  114.0   200    70   -70   4.40 0.7966
WVFGRD96  116.0   200    70   -70   4.40 0.8001
WVFGRD96  118.0   200    70   -70   4.40 0.8010
WVFGRD96  120.0   200    70   -70   4.40 0.8049
WVFGRD96  122.0   200    70   -70   4.41 0.8063
WVFGRD96  124.0   200    70   -70   4.41 0.8057
WVFGRD96  126.0   200    70   -70   4.41 0.8086
WVFGRD96  128.0   205    70   -70   4.41 0.8071
WVFGRD96  130.0   205    70   -70   4.41 0.8067
WVFGRD96  132.0   205    70   -70   4.42 0.8069
WVFGRD96  134.0   205    70   -70   4.42 0.8032
WVFGRD96  136.0   205    70   -70   4.42 0.8022
WVFGRD96  138.0   205    70   -70   4.42 0.7985
WVFGRD96  140.0   205    70   -70   4.42 0.7954
WVFGRD96  142.0   200    65   -75   4.43 0.7922
WVFGRD96  144.0   200    65   -75   4.43 0.7883
WVFGRD96  146.0   200    65   -75   4.43 0.7863
WVFGRD96  148.0   200    65   -75   4.43 0.7815

The best solution is

WVFGRD96  126.0   200    70   -70   4.41 0.8086

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 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 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 Sun Jul 28 06:43:27 CDT 2019