Location

Location ANSS

2017/09/13 07:22:18 62.883 -149.962 72.8 4.2 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/09/13 07:22:18:0  62.88 -149.96  72.8 4.2 Alaska
 
 Stations used:
   AK.CAST AK.CUT AK.DHY AK.DIV AK.FID AK.GHO AK.GLI AK.KNK 
   AK.KTH AK.MLY AK.NEA2 AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW 
   AK.SCM AK.SKN AK.SSN AK.TRF AT.PMR TA.I23K TA.J20K TA.K20K 
   TA.M19K TA.M22K TA.M24K 
 
 Filtering commands used:
   cut o DIST/3.5 -30 o DIST/3.5 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.34e+22 dyne-cm
  Mw = 4.18 
  Z  = 78 km
  Plane   Strike  Dip  Rake
   NP1       21    80   102
   NP2      150    15    40
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.34e+22     53     305
    N   0.00e+00     11     199
    P  -2.34e+22     34     101

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.14e+21
       Mxy    -9.38e+20
       Mxz     8.50e+21
       Myy    -9.68e+21
       Myz    -2.00e+22
       Mzz     7.53e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ##################----              
              -####################-------           
             -#####################--------          
           -######################-----------        
          -#######################------------       
         --######################--------------      
        --#########   ###########---------------     
        --######### T ##########----------------     
       ---#########   ##########-----------------    
       ---#####################------------------    
       ---####################----------   ------    
       ---####################---------- P ------    
        ---##################-----------   -----     
        ---#################--------------------     
         ---###############--------------------      
          ---#############--------------------       
           ----##########--------------------        
             ---########-------------------          
              -----####-------------------           
                 ----------------------              
                     ########----##                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  7.53e+21   8.50e+21   2.00e+22 
  8.50e+21   2.14e+21   9.38e+20 
  2.00e+22   9.38e+20  -9.68e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170913072218/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 = 150
      DIP = 15
     RAKE = 40
       MW = 4.18
       HS = 78.0

The NDK file is 20170913072218.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/09/13 07:22:18:0  62.88 -149.96  72.8 4.2 Alaska
 
 Stations used:
   AK.CAST AK.CUT AK.DHY AK.DIV AK.FID AK.GHO AK.GLI AK.KNK 
   AK.KTH AK.MLY AK.NEA2 AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW 
   AK.SCM AK.SKN AK.SSN AK.TRF AT.PMR TA.I23K TA.J20K TA.K20K 
   TA.M19K TA.M22K TA.M24K 
 
 Filtering commands used:
   cut o DIST/3.5 -30 o DIST/3.5 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.34e+22 dyne-cm
  Mw = 4.18 
  Z  = 78 km
  Plane   Strike  Dip  Rake
   NP1       21    80   102
   NP2      150    15    40
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.34e+22     53     305
    N   0.00e+00     11     199
    P  -2.34e+22     34     101

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.14e+21
       Mxy    -9.38e+20
       Mxz     8.50e+21
       Myy    -9.68e+21
       Myz    -2.00e+22
       Mzz     7.53e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ##################----              
              -####################-------           
             -#####################--------          
           -######################-----------        
          -#######################------------       
         --######################--------------      
        --#########   ###########---------------     
        --######### T ##########----------------     
       ---#########   ##########-----------------    
       ---#####################------------------    
       ---####################----------   ------    
       ---####################---------- P ------    
        ---##################-----------   -----     
        ---#################--------------------     
         ---###############--------------------      
          ---#############--------------------       
           ----##########--------------------        
             ---########-------------------          
              -----####-------------------           
                 ----------------------              
                     ########----##                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  7.53e+21   8.50e+21   2.00e+22 
  8.50e+21   2.14e+21   9.38e+20 
  2.00e+22   9.38e+20  -9.68e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170913072218/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/3.5 -30 o DIST/3.5 +70
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    10    70     5   3.21 0.1439
WVFGRD96    4.0    10    70    10   3.32 0.1688
WVFGRD96    6.0    10    70    10   3.39 0.1758
WVFGRD96    8.0   275    75   -15   3.48 0.1854
WVFGRD96   10.0   275    70   -15   3.53 0.1885
WVFGRD96   12.0   100    75    10   3.57 0.1952
WVFGRD96   14.0   100    70    10   3.61 0.2023
WVFGRD96   16.0   100    70    10   3.64 0.2077
WVFGRD96   18.0   100    75     5   3.67 0.2144
WVFGRD96   20.0   100    70     0   3.70 0.2254
WVFGRD96   22.0   100    70     0   3.72 0.2361
WVFGRD96   24.0   100    70     0   3.74 0.2450
WVFGRD96   26.0   100    65     5   3.77 0.2547
WVFGRD96   28.0   105    60    20   3.79 0.2645
WVFGRD96   30.0   105    60    15   3.81 0.2742
WVFGRD96   32.0   105    55    20   3.83 0.2841
WVFGRD96   34.0   105    50    15   3.85 0.2918
WVFGRD96   36.0   100    55     0   3.85 0.2988
WVFGRD96   38.0   100    60     0   3.87 0.3072
WVFGRD96   40.0   155    25    55   4.02 0.3137
WVFGRD96   42.0   150    25    50   4.03 0.3205
WVFGRD96   44.0   150    25    45   4.05 0.3452
WVFGRD96   46.0   150    25    45   4.06 0.3697
WVFGRD96   48.0   150    25    45   4.08 0.3908
WVFGRD96   50.0   150    25    45   4.09 0.4126
WVFGRD96   52.0   150    25    45   4.11 0.4297
WVFGRD96   54.0   150    25    45   4.12 0.4462
WVFGRD96   56.0   150    25    45   4.13 0.4587
WVFGRD96   58.0   155    15    50   4.13 0.4706
WVFGRD96   60.0   150    15    45   4.14 0.4846
WVFGRD96   62.0   145    15    40   4.14 0.4983
WVFGRD96   64.0   145    15    35   4.14 0.5115
WVFGRD96   66.0   145    15    35   4.15 0.5235
WVFGRD96   68.0   145    15    35   4.16 0.5324
WVFGRD96   70.0   145    15    35   4.16 0.5408
WVFGRD96   72.0   145    15    35   4.17 0.5478
WVFGRD96   74.0   150    15    40   4.17 0.5512
WVFGRD96   76.0   150    15    40   4.18 0.5524
WVFGRD96   78.0   150    15    40   4.18 0.5536
WVFGRD96   80.0   145    15    30   4.18 0.5531
WVFGRD96   82.0   145    15    30   4.18 0.5503
WVFGRD96   84.0   145    15    30   4.18 0.5520
WVFGRD96   86.0   140    15    25   4.18 0.5512
WVFGRD96   88.0   140    15    25   4.18 0.5472
WVFGRD96   90.0   140    15    25   4.19 0.5478
WVFGRD96   92.0   140    15    25   4.19 0.5439
WVFGRD96   94.0   140    15    25   4.19 0.5399
WVFGRD96   96.0   140    15    20   4.19 0.5357
WVFGRD96   98.0   145    10    30   4.19 0.5302

The best solution is

WVFGRD96   78.0   150    15    40   4.18 0.5536

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/3.5 -30 o DIST/3.5 +70
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 Wed Sep 13 05:56:08 CDT 2017