Location

Location ANSS

2019/05/21 03:12:55 61.462 -149.640 43.6 3.6 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2019/05/21 03:12:55:0  61.46 -149.64  43.6 3.6 Alaska
 
 Stations used:
   AK.FID AK.FIRE AK.GHO AK.PPLA AK.PWL AK.RC01 AK.SAW AK.SCM 
   AK.SKN AK.SSN AT.PMR AV.STLK TA.M24K 
 
 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 = 6.53e+21 dyne-cm
  Mw = 3.81 
  Z  = 66 km
  Plane   Strike  Dip  Rake
   NP1      140    90   -155
   NP2       50    65     0
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.53e+21     17     272
    N   0.00e+00     65     140
    P  -6.53e+21     17       8

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -5.83e+21
       Mxy    -1.03e+21
       Mxz    -1.77e+21
       Myy     5.83e+21
       Myz    -2.11e+21
       Mzz     0.00e+00
                                                     
                                                     
                                                     
                                                     
                     --------   ---                  
                 ------------ P -------              
              #--------------   ----------           
             ###---------------------------          
           #######--------------------------#        
          #########------------------------###       
         ############---------------------#####      
        ###############-------------------######     
        ################-----------------#######     
       ##   ##############-------------##########    
       ## T ###############-----------###########    
       ##   #################-------#############    
       #######################----###############    
        ########################################     
        ######################---###############     
         ##################--------############      
          ##############-------------#########       
           #########------------------#######        
             ---------------------------###          
              ---------------------------#           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  0.00e+00  -1.77e+21   2.11e+21 
 -1.77e+21  -5.83e+21   1.03e+21 
  2.11e+21   1.03e+21   5.83e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190521031255/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 = 50
      DIP = 65
     RAKE = 0
       MW = 3.81
       HS = 66.0

The NDK file is 20190521031255.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2019/05/21 03:12:55:0  61.46 -149.64  43.6 3.6 Alaska
 
 Stations used:
   AK.FID AK.FIRE AK.GHO AK.PPLA AK.PWL AK.RC01 AK.SAW AK.SCM 
   AK.SKN AK.SSN AT.PMR AV.STLK TA.M24K 
 
 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 = 6.53e+21 dyne-cm
  Mw = 3.81 
  Z  = 66 km
  Plane   Strike  Dip  Rake
   NP1      140    90   -155
   NP2       50    65     0
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.53e+21     17     272
    N   0.00e+00     65     140
    P  -6.53e+21     17       8

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -5.83e+21
       Mxy    -1.03e+21
       Mxz    -1.77e+21
       Myy     5.83e+21
       Myz    -2.11e+21
       Mzz     0.00e+00
                                                     
                                                     
                                                     
                                                     
                     --------   ---                  
                 ------------ P -------              
              #--------------   ----------           
             ###---------------------------          
           #######--------------------------#        
          #########------------------------###       
         ############---------------------#####      
        ###############-------------------######     
        ################-----------------#######     
       ##   ##############-------------##########    
       ## T ###############-----------###########    
       ##   #################-------#############    
       #######################----###############    
        ########################################     
        ######################---###############     
         ##################--------############      
          ##############-------------#########       
           #########------------------#######        
             ---------------------------###          
              ---------------------------#           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  0.00e+00  -1.77e+21   2.11e+21 
 -1.77e+21  -5.83e+21   1.03e+21 
  2.11e+21   1.03e+21   5.83e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190521031255/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    1.0   310    85     5   2.79 0.1849
WVFGRD96    2.0   130    90    -5   2.94 0.2537
WVFGRD96    3.0   130    90   -25   3.02 0.2736
WVFGRD96    4.0   315    80    25   3.07 0.2914
WVFGRD96    5.0   130    90   -25   3.11 0.3039
WVFGRD96    6.0   130    85   -25   3.14 0.3163
WVFGRD96    7.0   130    85   -20   3.18 0.3288
WVFGRD96    8.0   310    90    25   3.23 0.3406
WVFGRD96    9.0   130    90   -25   3.25 0.3483
WVFGRD96   10.0   310    90    20   3.27 0.3515
WVFGRD96   11.0   130    90   -20   3.29 0.3520
WVFGRD96   12.0   130    90   -20   3.31 0.3491
WVFGRD96   13.0   125    80   -20   3.33 0.3453
WVFGRD96   14.0   220    75   -10   3.34 0.3416
WVFGRD96   15.0   220    75   -10   3.35 0.3438
WVFGRD96   16.0   220    75   -10   3.37 0.3449
WVFGRD96   17.0   220    80   -10   3.38 0.3459
WVFGRD96   18.0   235    80   -10   3.38 0.3466
WVFGRD96   19.0   235    80   -10   3.39 0.3478
WVFGRD96   20.0   235    80   -10   3.40 0.3495
WVFGRD96   21.0    60    75    20   3.43 0.3508
WVFGRD96   22.0    60    75    20   3.44 0.3579
WVFGRD96   23.0    60    75    20   3.45 0.3640
WVFGRD96   24.0    60    75    15   3.45 0.3713
WVFGRD96   25.0    60    75    15   3.46 0.3775
WVFGRD96   26.0    60    70    15   3.47 0.3861
WVFGRD96   27.0    55    75    10   3.47 0.3948
WVFGRD96   28.0    55    75    10   3.48 0.4035
WVFGRD96   29.0    55    75     5   3.49 0.4120
WVFGRD96   30.0    55    70     5   3.50 0.4208
WVFGRD96   31.0    55    70     0   3.51 0.4286
WVFGRD96   32.0    55    70     0   3.52 0.4349
WVFGRD96   33.0    55    70     0   3.52 0.4401
WVFGRD96   34.0    55    70    -5   3.53 0.4447
WVFGRD96   35.0    55    70    -5   3.54 0.4475
WVFGRD96   36.0    55    75     0   3.55 0.4492
WVFGRD96   37.0    55    75     0   3.56 0.4506
WVFGRD96   38.0    55    75     5   3.58 0.4562
WVFGRD96   39.0    55    75     5   3.60 0.4631
WVFGRD96   40.0    55    65    10   3.64 0.4746
WVFGRD96   41.0    55    65    10   3.66 0.4762
WVFGRD96   42.0    55    65     5   3.67 0.4777
WVFGRD96   43.0    55    70     5   3.67 0.4786
WVFGRD96   44.0    55    65     5   3.69 0.4793
WVFGRD96   45.0    55    65     5   3.70 0.4803
WVFGRD96   46.0    55    65     5   3.70 0.4823
WVFGRD96   47.0    55    65     5   3.71 0.4841
WVFGRD96   48.0    55    65     5   3.72 0.4856
WVFGRD96   49.0    55    65     5   3.73 0.4882
WVFGRD96   50.0    55    65     5   3.73 0.4884
WVFGRD96   51.0    55    65     5   3.74 0.4918
WVFGRD96   52.0    55    65     5   3.74 0.4932
WVFGRD96   53.0    55    65     5   3.75 0.4949
WVFGRD96   54.0    55    65     5   3.76 0.4954
WVFGRD96   55.0    55    65     5   3.76 0.4965
WVFGRD96   56.0    55    65     5   3.77 0.4978
WVFGRD96   57.0    55    65     5   3.77 0.4976
WVFGRD96   58.0    50    60     0   3.79 0.5004
WVFGRD96   59.0    50    60     0   3.79 0.5005
WVFGRD96   60.0    55    70     0   3.78 0.4991
WVFGRD96   61.0    50    60     0   3.80 0.5010
WVFGRD96   62.0    50    65     0   3.80 0.5011
WVFGRD96   63.0    50    65     0   3.80 0.5013
WVFGRD96   64.0    50    65     0   3.80 0.5009
WVFGRD96   65.0    50    65     0   3.81 0.5007
WVFGRD96   66.0    50    65     0   3.81 0.5013
WVFGRD96   67.0    50    65     0   3.81 0.4996
WVFGRD96   68.0    50    65     0   3.82 0.5005
WVFGRD96   69.0    50    65     0   3.82 0.4999
WVFGRD96   70.0    50    65     0   3.82 0.4983
WVFGRD96   71.0    50    65     0   3.83 0.4985
WVFGRD96   72.0    50    65     0   3.83 0.4964
WVFGRD96   73.0    50    65     0   3.83 0.4965
WVFGRD96   74.0    50    65     0   3.83 0.4954
WVFGRD96   75.0    50    65     0   3.84 0.4937
WVFGRD96   76.0    50    65     0   3.84 0.4932
WVFGRD96   77.0    50    65     0   3.84 0.4915
WVFGRD96   78.0    50    65     0   3.84 0.4914
WVFGRD96   79.0    55    75    -5   3.84 0.4895

The best solution is

WVFGRD96   66.0    50    65     0   3.81 0.5013

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 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 Mon May 20 23:46:59 CDT 2019