Location

Location ANSS

2017/04/18 11:59:33 61.505 -145.866 29.1 3.8 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/04/18 11:59:33:0  61.51 -145.87  29.1 3.8 Alaska
 
 Stations used:
   AK.BARN AK.GHO AK.GLB AK.KNK AK.MCAR AK.PAX AK.PWL AK.SAW 
   AK.SCM AK.VRDI AT.MENT AT.PMR TA.M24K TA.M26K TA.N25K 
 
 Filtering commands used:
   cut o DIST/3.3 -30 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 = 8.04e+21 dyne-cm
  Mw = 3.87 
  Z  = 41 km
  Plane   Strike  Dip  Rake
   NP1      321    58   -138
   NP2      205    55   -40
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   8.04e+21      2      82
    N   0.00e+00     39     351
    P  -8.04e+21     51     175

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.00e+21
       Mxy     1.38e+21
       Mxz     3.95e+21
       Myy     7.85e+21
       Myz    -1.09e+20
       Mzz    -4.85e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------######              
              ####------------############           
             ###########---################          
           ##############--##################        
          ##############-----#################       
         ##############--------################      
        #############------------###############     
        ############---------------############      
       #############----------------########### T    
       ############------------------##########      
       ###########--------------------###########    
       ###########---------------------##########    
        #########-----------------------########     
        #########------------------------#######     
         ########----------   -----------######      
          #######---------- P -----------#####       
           ######----------   -----------####        
             ####------------------------##          
              ####-----------------------#           
                 ##--------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.85e+21   3.95e+21   1.09e+20 
  3.95e+21  -3.00e+21  -1.38e+21 
  1.09e+20  -1.38e+21   7.85e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170418115933/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 = 205
      DIP = 55
     RAKE = -40
       MW = 3.87
       HS = 41.0

The NDK file is 20170418115933.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/04/18 11:59:33:0  61.51 -145.87  29.1 3.8 Alaska
 
 Stations used:
   AK.BARN AK.GHO AK.GLB AK.KNK AK.MCAR AK.PAX AK.PWL AK.SAW 
   AK.SCM AK.VRDI AT.MENT AT.PMR TA.M24K TA.M26K TA.N25K 
 
 Filtering commands used:
   cut o DIST/3.3 -30 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 = 8.04e+21 dyne-cm
  Mw = 3.87 
  Z  = 41 km
  Plane   Strike  Dip  Rake
   NP1      321    58   -138
   NP2      205    55   -40
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   8.04e+21      2      82
    N   0.00e+00     39     351
    P  -8.04e+21     51     175

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.00e+21
       Mxy     1.38e+21
       Mxz     3.95e+21
       Myy     7.85e+21
       Myz    -1.09e+20
       Mzz    -4.85e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------######              
              ####------------############           
             ###########---################          
           ##############--##################        
          ##############-----#################       
         ##############--------################      
        #############------------###############     
        ############---------------############      
       #############----------------########### T    
       ############------------------##########      
       ###########--------------------###########    
       ###########---------------------##########    
        #########-----------------------########     
        #########------------------------#######     
         ########----------   -----------######      
          #######---------- P -----------#####       
           ######----------   -----------####        
             ####------------------------##          
              ####-----------------------#           
                 ##--------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.85e+21   3.95e+21   1.09e+20 
  3.95e+21  -3.00e+21  -1.38e+21 
  1.09e+20  -1.38e+21   7.85e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170418115933/index.html
	

Magnitudes

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 -30 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   135    60    25   3.00 0.1747
WVFGRD96    2.0   145    45    55   3.18 0.2358
WVFGRD96    3.0   155    45    70   3.29 0.2834
WVFGRD96    4.0   140    60    40   3.28 0.2711
WVFGRD96    5.0   295    55   -30   3.28 0.2905
WVFGRD96    6.0   295    60   -35   3.32 0.3122
WVFGRD96    7.0   295    60   -35   3.34 0.3283
WVFGRD96    8.0   290    55   -45   3.42 0.3364
WVFGRD96    9.0   295    60   -40   3.43 0.3434
WVFGRD96   10.0   295    60   -35   3.43 0.3467
WVFGRD96   11.0   295    60   -35   3.45 0.3468
WVFGRD96   12.0   295    65   -35   3.47 0.3451
WVFGRD96   13.0   295    65   -35   3.48 0.3429
WVFGRD96   14.0   295    65   -35   3.49 0.3388
WVFGRD96   15.0   295    65   -35   3.51 0.3333
WVFGRD96   16.0   300    70   -40   3.53 0.3264
WVFGRD96   17.0   300    70   -40   3.54 0.3186
WVFGRD96   18.0   215    75    50   3.52 0.3215
WVFGRD96   19.0    15    65   -40   3.54 0.3326
WVFGRD96   20.0    15    60   -35   3.56 0.3460
WVFGRD96   21.0    15    60   -35   3.58 0.3582
WVFGRD96   22.0    15    60   -35   3.60 0.3684
WVFGRD96   23.0     0    40   -55   3.60 0.3785
WVFGRD96   24.0     0    40   -55   3.62 0.3963
WVFGRD96   25.0     0    40   -55   3.63 0.4132
WVFGRD96   26.0   200    65   -40   3.64 0.4303
WVFGRD96   27.0   200    65   -40   3.65 0.4480
WVFGRD96   28.0   200    65   -45   3.66 0.4649
WVFGRD96   29.0   200    65   -45   3.68 0.4776
WVFGRD96   30.0   200    65   -45   3.69 0.4890
WVFGRD96   31.0    45    60   -15   3.75 0.5004
WVFGRD96   32.0    45    60   -15   3.76 0.5099
WVFGRD96   33.0    45    60   -15   3.77 0.5158
WVFGRD96   34.0    45    60   -15   3.78 0.5186
WVFGRD96   35.0   215    65   -25   3.77 0.5177
WVFGRD96   36.0   215    65   -25   3.77 0.5196
WVFGRD96   37.0   215    65   -25   3.78 0.5199
WVFGRD96   38.0   215    65   -25   3.79 0.5183
WVFGRD96   39.0   215    65   -25   3.81 0.5174
WVFGRD96   40.0   210    60   -35   3.86 0.5301
WVFGRD96   41.0   205    55   -40   3.87 0.5318
WVFGRD96   42.0   205    55   -40   3.88 0.5291
WVFGRD96   43.0   195    50   -50   3.89 0.5266
WVFGRD96   44.0   195    50   -50   3.90 0.5221
WVFGRD96   45.0   195    50   -50   3.91 0.5160
WVFGRD96   46.0   195    50   -50   3.91 0.5102
WVFGRD96   47.0   195    50   -50   3.91 0.5036
WVFGRD96   48.0   195    50   -50   3.92 0.4958
WVFGRD96   49.0   195    50   -50   3.92 0.4891

The best solution is

WVFGRD96   41.0   205    55   -40   3.87 0.5318

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 -30 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 Tue Apr 18 07:46:31 CDT 2017