Location

Location ANSS

2017/06/21 17:44:32 57.795 -154.319 48.8 4.1 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/06/21 17:44:32:0  57.79 -154.32  48.8 4.1 Alaska
 
 Stations used:
   AK.CNP AK.SII AT.OHAK AV.ILSW II.KDAK TA.O18K TA.O19K 
   TA.P19K TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.5 -30 o DIST/3.5 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 5.01e+22 dyne-cm
  Mw = 4.40 
  Z  = 62 km
  Plane   Strike  Dip  Rake
   NP1      236    71   -83
   NP2       35    20   -110
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.01e+22     26     321
    N   0.00e+00      7      54
    P  -5.01e+22     63     157

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.55e+22
       Mxy    -1.62e+22
       Mxz     3.39e+22
       Myy     1.48e+22
       Myz    -2.03e+22
       Mzz    -3.03e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ###########################-           
             ####   ######################-          
           ###### T ########################-        
          #######   ##################------##       
         ########################------------##      
        #####################----------------###     
        ##################--------------------##     
       #################----------------------###    
       ##############-------------------------###    
       ############--------------------------####    
       ##########----------------------------####    
        #######--------------   ------------####     
        ######--------------- P ------------####     
         ###-----------------   -----------####      
          #------------------------------#####       
           -----------------------------#####        
             -------------------------#####          
              ---------------------#######           
                 ---------------#######              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.03e+22   3.39e+22   2.03e+22 
  3.39e+22   1.55e+22   1.62e+22 
  2.03e+22   1.62e+22   1.48e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170621174432/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 = 35
      DIP = 20
     RAKE = -110
       MW = 4.40
       HS = 62.0

The NDK file is 20170621174432.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/06/21 17:44:32:0  57.79 -154.32  48.8 4.1 Alaska
 
 Stations used:
   AK.CNP AK.SII AT.OHAK AV.ILSW II.KDAK TA.O18K TA.O19K 
   TA.P19K TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.5 -30 o DIST/3.5 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 5.01e+22 dyne-cm
  Mw = 4.40 
  Z  = 62 km
  Plane   Strike  Dip  Rake
   NP1      236    71   -83
   NP2       35    20   -110
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.01e+22     26     321
    N   0.00e+00      7      54
    P  -5.01e+22     63     157

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.55e+22
       Mxy    -1.62e+22
       Mxz     3.39e+22
       Myy     1.48e+22
       Myz    -2.03e+22
       Mzz    -3.03e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ###########################-           
             ####   ######################-          
           ###### T ########################-        
          #######   ##################------##       
         ########################------------##      
        #####################----------------###     
        ##################--------------------##     
       #################----------------------###    
       ##############-------------------------###    
       ############--------------------------####    
       ##########----------------------------####    
        #######--------------   ------------####     
        ######--------------- P ------------####     
         ###-----------------   -----------####      
          #------------------------------#####       
           -----------------------------#####        
             -------------------------#####          
              ---------------------#######           
                 ---------------#######              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.03e+22   3.39e+22   2.03e+22 
  3.39e+22   1.55e+22   1.62e+22 
  2.03e+22   1.62e+22   1.48e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170621174432/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 +40
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   155    50   -75   3.54 0.2209
WVFGRD96    4.0   205    70    55   3.58 0.2724
WVFGRD96    6.0   205    65    50   3.62 0.3200
WVFGRD96    8.0   200    65    50   3.70 0.3407
WVFGRD96   10.0   275    55    35   3.76 0.3577
WVFGRD96   12.0   275    55    35   3.79 0.3706
WVFGRD96   14.0   270    60    30   3.83 0.3760
WVFGRD96   16.0   270    60    30   3.86 0.3797
WVFGRD96   18.0   270    60    30   3.90 0.3830
WVFGRD96   20.0   270    60    30   3.93 0.3905
WVFGRD96   22.0   270    60    30   3.96 0.3956
WVFGRD96   24.0    90    65    25   3.98 0.4095
WVFGRD96   26.0    90    65    25   4.00 0.4337
WVFGRD96   28.0    90    60    25   4.02 0.4547
WVFGRD96   30.0    90    60    30   4.05 0.4701
WVFGRD96   32.0    95    60    35   4.06 0.4782
WVFGRD96   34.0    95    60    35   4.07 0.4821
WVFGRD96   36.0    85    70    30   4.10 0.4949
WVFGRD96   38.0    90    65    30   4.12 0.5195
WVFGRD96   40.0    90    60    30   4.20 0.5302
WVFGRD96   42.0    90    65    35   4.23 0.5529
WVFGRD96   44.0    90    65    35   4.25 0.5652
WVFGRD96   46.0    90    70    35   4.26 0.5683
WVFGRD96   48.0    90    70    35   4.27 0.5680
WVFGRD96   50.0   250    80   -55   4.31 0.5798
WVFGRD96   52.0   245    75   -60   4.32 0.5920
WVFGRD96   54.0   245    75   -60   4.33 0.5986
WVFGRD96   56.0   245    75   -60   4.34 0.6019
WVFGRD96   58.0   240    75   -65   4.36 0.6070
WVFGRD96   60.0   240    75   -70   4.37 0.6097
WVFGRD96   62.0    35    20  -110   4.40 0.6102
WVFGRD96   64.0    45    20   -95   4.41 0.6091
WVFGRD96   66.0    50    20   -90   4.42 0.6024
WVFGRD96   68.0   230    75   -75   4.41 0.6050
WVFGRD96   70.0   235    80   -70   4.41 0.6036
WVFGRD96   72.0   235    80   -75   4.42 0.5991
WVFGRD96   74.0   230    80   -75   4.43 0.5970
WVFGRD96   76.0   230    80   -80   4.44 0.5917
WVFGRD96   78.0   230    80   -80   4.45 0.5846
WVFGRD96   80.0    90    10   -45   4.46 0.5873
WVFGRD96   82.0    95    10   -40   4.46 0.5881
WVFGRD96   84.0    80     5   -60   4.47 0.5897
WVFGRD96   86.0    90     5   -50   4.47 0.5908
WVFGRD96   88.0   100     5   -40   4.47 0.5898
WVFGRD96   90.0   110     5   -30   4.48 0.5872
WVFGRD96   92.0    35     0  -110   4.47 0.5792
WVFGRD96   94.0    50    90    95   4.48 0.5781
WVFGRD96   96.0    40     0  -105   4.48 0.5701
WVFGRD96   98.0   145     5     5   4.48 0.5701

The best solution is

WVFGRD96   62.0    35    20  -110   4.40 0.6102

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 +40
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 Thu Jun 22 05:31:44 CDT 2017