Location

Location ANSS

2019/03/27 05:06:35 66.313 -156.924 0.0 3.7 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2019/03/27 05:06:35:0  66.31 -156.92   0.0 3.7 Alaska
 
 Stations used:
   AK.COLD AK.GCSA TA.B18K TA.C18K TA.D17K TA.D19K TA.D20K 
   TA.D22K TA.E18K TA.E19K TA.E20K TA.E22K TA.E23K TA.E24K 
   TA.F14K TA.F15K TA.F17K TA.F18K TA.F19K TA.F20K TA.F21K 
   TA.F22K TA.G15K TA.G16K TA.G17K TA.G18K TA.G19K TA.G21K 
   TA.G23K TA.G24K TA.H16K TA.H17K TA.H19K TA.H20K TA.H23K 
   TA.I17K TA.I20K TA.J18K TA.J19K TA.J20K TA.K20K TA.TOLK 
 
 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 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 3.16e+21 dyne-cm
  Mw = 3.60 
  Z  = 9 km
  Plane   Strike  Dip  Rake
   NP1      150    70   -25
   NP2      249    67   -158
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.16e+21      2     200
    N   0.00e+00     58     294
    P  -3.16e+21     32     109

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.55e+21
       Mxy     1.72e+21
       Mxz     3.37e+20
       Myy    -1.69e+21
       Myz    -1.38e+21
       Mzz    -8.59e+20
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 -#####################              
              ----########################           
             -----#########################          
           -------###########################        
          --------############################       
         ----------##################-------###      
        -----------#########--------------------     
        ------------####------------------------     
       -------------#----------------------------    
       ----------#####---------------------------    
       -------#########--------------------------    
       -----############----------------   ------    
        --###############--------------- P -----     
        -#################--------------   -----     
         ##################--------------------      
          ###################-----------------       
           ####################--------------        
             ###################-----------          
              #####################-------           
                 ###   ###############-              
                     T ############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -8.59e+20   3.37e+20   1.38e+21 
  3.37e+20   2.55e+21  -1.72e+21 
  1.38e+21  -1.72e+21  -1.69e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190327050635/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 = 70
     RAKE = -25
       MW = 3.60
       HS = 9.0

The NDK file is 20190327050635.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2019/03/27 05:06:35:0  66.31 -156.92   0.0 3.7 Alaska
 
 Stations used:
   AK.COLD AK.GCSA TA.B18K TA.C18K TA.D17K TA.D19K TA.D20K 
   TA.D22K TA.E18K TA.E19K TA.E20K TA.E22K TA.E23K TA.E24K 
   TA.F14K TA.F15K TA.F17K TA.F18K TA.F19K TA.F20K TA.F21K 
   TA.F22K TA.G15K TA.G16K TA.G17K TA.G18K TA.G19K TA.G21K 
   TA.G23K TA.G24K TA.H16K TA.H17K TA.H19K TA.H20K TA.H23K 
   TA.I17K TA.I20K TA.J18K TA.J19K TA.J20K TA.K20K TA.TOLK 
 
 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 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 3.16e+21 dyne-cm
  Mw = 3.60 
  Z  = 9 km
  Plane   Strike  Dip  Rake
   NP1      150    70   -25
   NP2      249    67   -158
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.16e+21      2     200
    N   0.00e+00     58     294
    P  -3.16e+21     32     109

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.55e+21
       Mxy     1.72e+21
       Mxz     3.37e+20
       Myy    -1.69e+21
       Myz    -1.38e+21
       Mzz    -8.59e+20
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 -#####################              
              ----########################           
             -----#########################          
           -------###########################        
          --------############################       
         ----------##################-------###      
        -----------#########--------------------     
        ------------####------------------------     
       -------------#----------------------------    
       ----------#####---------------------------    
       -------#########--------------------------    
       -----############----------------   ------    
        --###############--------------- P -----     
        -#################--------------   -----     
         ##################--------------------      
          ###################-----------------       
           ####################--------------        
             ###################-----------          
              #####################-------           
                 ###   ###############-              
                     T ############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -8.59e+20   3.37e+20   1.38e+21 
  3.37e+20   2.55e+21  -1.72e+21 
  1.38e+21  -1.72e+21  -1.69e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190327050635/index.html
	

Magnitudes

mLg Magnitude


(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.

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 0.08 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   340    75   -15   3.23 0.3250
WVFGRD96    2.0   340    75   -15   3.35 0.4290
WVFGRD96    3.0   340    70   -20   3.42 0.4771
WVFGRD96    4.0   335    65   -30   3.47 0.5107
WVFGRD96    5.0   340    70   -20   3.49 0.5344
WVFGRD96    6.0   150    70   -25   3.52 0.5568
WVFGRD96    7.0   150    70   -20   3.54 0.5723
WVFGRD96    8.0   150    70   -25   3.58 0.5814
WVFGRD96    9.0   150    70   -25   3.60 0.5815
WVFGRD96   10.0   150    70   -20   3.61 0.5777
WVFGRD96   11.0   150    70   -20   3.62 0.5726
WVFGRD96   12.0   345    70    20   3.64 0.5716
WVFGRD96   13.0   345    70    15   3.65 0.5694
WVFGRD96   14.0   345    70    15   3.67 0.5656
WVFGRD96   15.0   345    70    15   3.68 0.5602
WVFGRD96   16.0   345    70    15   3.69 0.5533
WVFGRD96   17.0   345    70    15   3.70 0.5453
WVFGRD96   18.0   345    70    15   3.71 0.5369
WVFGRD96   19.0   345    70    15   3.72 0.5282
WVFGRD96   20.0   340    65    10   3.72 0.5199
WVFGRD96   21.0   340    65    10   3.73 0.5118
WVFGRD96   22.0   340    65    10   3.74 0.5031
WVFGRD96   23.0   340    65     5   3.75 0.4942
WVFGRD96   24.0   340    65     5   3.75 0.4847
WVFGRD96   25.0   340    60     5   3.76 0.4751
WVFGRD96   26.0   340    60    10   3.77 0.4652
WVFGRD96   27.0   340    60    10   3.78 0.4556
WVFGRD96   28.0   340    60    10   3.78 0.4457
WVFGRD96   29.0   340    60     5   3.78 0.4358
WVFGRD96   30.0   340    60     5   3.79 0.4258
WVFGRD96   31.0   335    70   -15   3.79 0.4206
WVFGRD96   32.0   335    70   -15   3.79 0.4142
WVFGRD96   33.0   335    70   -15   3.80 0.4076
WVFGRD96   34.0   335    70   -15   3.80 0.4010
WVFGRD96   35.0   335    70   -15   3.81 0.3959
WVFGRD96   36.0   335    70   -15   3.82 0.3919
WVFGRD96   37.0   335    70   -20   3.83 0.3891
WVFGRD96   38.0   330    70   -20   3.85 0.3876
WVFGRD96   39.0   330    70   -20   3.86 0.3882
WVFGRD96   40.0   330    65   -25   3.91 0.3828
WVFGRD96   41.0   330    70   -30   3.92 0.3837
WVFGRD96   42.0   330    70   -30   3.93 0.3836
WVFGRD96   43.0   330    70   -25   3.94 0.3836
WVFGRD96   44.0   330    70   -25   3.95 0.3838
WVFGRD96   45.0   330    70   -25   3.95 0.3833
WVFGRD96   46.0   330    70   -25   3.96 0.3828
WVFGRD96   47.0   330    70   -25   3.97 0.3825
WVFGRD96   48.0   330    70   -25   3.98 0.3816
WVFGRD96   49.0   330    70   -25   3.98 0.3802
WVFGRD96   50.0   330    70   -25   3.99 0.3789
WVFGRD96   51.0   330    70   -25   3.99 0.3774
WVFGRD96   52.0   330    70   -25   4.00 0.3758
WVFGRD96   53.0   330    70   -25   4.00 0.3741
WVFGRD96   54.0   330    70   -25   4.01 0.3722
WVFGRD96   55.0   335    75   -20   4.00 0.3707
WVFGRD96   56.0   335    75   -20   4.01 0.3694
WVFGRD96   57.0   335    75   -20   4.01 0.3677
WVFGRD96   58.0   335    75   -20   4.01 0.3662
WVFGRD96   59.0   335    75   -20   4.02 0.3649

The best solution is

WVFGRD96    9.0   150    70   -25   3.60 0.5815

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 0.08 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 Mar 27 07:10:35 CDT 2019