Location

Location ANSS

2021/01/26 22:23:25 59.176 -151.660 41.9 4.3 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2021/01/26 22:23:25:0  59.18 -151.66  41.9 4.3 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.DIV AK.GHO AK.GLI AK.KLU AK.KNK AK.M20K 
   AK.N19K AK.O18K AK.O19K AK.PWL AK.SAW AK.SLK AT.PMR AV.ILS 
   AV.RED II.KDAK 
 
 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 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 3.80e+22 dyne-cm
  Mw = 4.32 
  Z  = 50 km
  Plane   Strike  Dip  Rake
   NP1      200    75   -50
   NP2      307    42   -157
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.80e+22     20     261
    N   0.00e+00     38       8
    P  -3.80e+22     45     150

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.35e+22
       Mxy     1.34e+22
       Mxz     1.46e+22
       Myy     2.80e+22
       Myz    -2.15e+22
       Mzz    -1.46e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------######              
              ------------------##########           
             --###########----#############          
           ##################--##############        
          ##################------############       
         ##################----------##########      
        ##################-------------#########     
        ##################--------------########     
       ##################----------------########    
       #################------------------#######    
       ###   ###########-------------------######    
       ### T ##########---------------------#####    
        ##   ##########----------------------###     
        ##############-----------------------###     
         #############----------   ----------##      
          ###########----------- P ----------#       
           ##########-----------   ----------        
             ########----------------------          
              #######---------------------           
                 ####------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.46e+22   1.46e+22   2.15e+22 
  1.46e+22  -1.35e+22  -1.34e+22 
  2.15e+22  -1.34e+22   2.80e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210126222325/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 = 200
      DIP = 75
     RAKE = -50
       MW = 4.32
       HS = 50.0

The NDK file is 20210126222325.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMWR
 USGS/SLU Moment Tensor Solution
 ENS  2021/01/26 22:23:25:0  59.18 -151.66  41.9 4.3 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.DIV AK.GHO AK.GLI AK.KLU AK.KNK AK.M20K 
   AK.N19K AK.O18K AK.O19K AK.PWL AK.SAW AK.SLK AT.PMR AV.ILS 
   AV.RED II.KDAK 
 
 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 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 3.80e+22 dyne-cm
  Mw = 4.32 
  Z  = 50 km
  Plane   Strike  Dip  Rake
   NP1      200    75   -50
   NP2      307    42   -157
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.80e+22     20     261
    N   0.00e+00     38       8
    P  -3.80e+22     45     150

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.35e+22
       Mxy     1.34e+22
       Mxz     1.46e+22
       Myy     2.80e+22
       Myz    -2.15e+22
       Mzz    -1.46e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------######              
              ------------------##########           
             --###########----#############          
           ##################--##############        
          ##################------############       
         ##################----------##########      
        ##################-------------#########     
        ##################--------------########     
       ##################----------------########    
       #################------------------#######    
       ###   ###########-------------------######    
       ### T ##########---------------------#####    
        ##   ##########----------------------###     
        ##############-----------------------###     
         #############----------   ----------##      
          ###########----------- P ----------#       
           ##########-----------   ----------        
             ########----------------------          
              #######---------------------           
                 ####------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.46e+22   1.46e+22   2.15e+22 
  1.46e+22  -1.35e+22  -1.34e+22 
  2.15e+22  -1.34e+22   2.80e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210126222325/index.html
	
Regional Moment Tensor (Mwr)
Moment 4.818e+15 N-m
Magnitude 4.39 Mwr
Depth 50.0 km
Percent DC 67%
Half Duration -
Catalog US
Data Source US 3
Contributor US 3

Nodal Planes
Plane Strike Dip Rake
NP1 347 26 -116
NP2 195 66 -78

Principal Axes
Axis Value Plunge Azimuth
T 5.184e+15 N-m 20 276
N -0.843e+15 N-m 11 10
P -4.340e+15 N-m 66 127


        

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 0.07 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   355    45    95   3.72 0.3037
WVFGRD96    4.0    15    85     0   3.68 0.3539
WVFGRD96    6.0    15    75   -15   3.75 0.3874
WVFGRD96    8.0   200    65   -10   3.80 0.4233
WVFGRD96   10.0   200    65   -15   3.84 0.4537
WVFGRD96   12.0   200    65   -20   3.88 0.4794
WVFGRD96   14.0   200    65   -20   3.91 0.5026
WVFGRD96   16.0   205    70   -20   3.93 0.5251
WVFGRD96   18.0   205    70   -25   3.96 0.5485
WVFGRD96   20.0   205    70   -25   3.99 0.5718
WVFGRD96   22.0   205    70   -25   4.01 0.5962
WVFGRD96   24.0   205    75   -25   4.03 0.6180
WVFGRD96   26.0   205    75   -30   4.05 0.6460
WVFGRD96   28.0   205    75   -30   4.07 0.6727
WVFGRD96   30.0   205    75   -35   4.10 0.6955
WVFGRD96   32.0   200    75   -35   4.12 0.7156
WVFGRD96   34.0   200    75   -35   4.14 0.7348
WVFGRD96   36.0   200    75   -35   4.15 0.7496
WVFGRD96   38.0   200    75   -35   4.17 0.7612
WVFGRD96   40.0   200    75   -45   4.25 0.7698
WVFGRD96   42.0   200    75   -50   4.28 0.7786
WVFGRD96   44.0   200    75   -50   4.29 0.7836
WVFGRD96   46.0   200    75   -50   4.30 0.7891
WVFGRD96   48.0   200    75   -50   4.31 0.7930
WVFGRD96   50.0   200    75   -50   4.32 0.7936
WVFGRD96   52.0   200    75   -55   4.34 0.7928
WVFGRD96   54.0   200    75   -55   4.35 0.7905
WVFGRD96   56.0   200    75   -55   4.35 0.7872
WVFGRD96   58.0   200    75   -55   4.36 0.7826
WVFGRD96   60.0   200    75   -55   4.36 0.7798
WVFGRD96   62.0   200    75   -55   4.36 0.7755
WVFGRD96   64.0   200    75   -55   4.37 0.7693
WVFGRD96   66.0   200    75   -55   4.37 0.7616
WVFGRD96   68.0   200    75   -55   4.37 0.7566
WVFGRD96   70.0   200    75   -55   4.38 0.7522
WVFGRD96   72.0   200    75   -55   4.38 0.7463
WVFGRD96   74.0   200    75   -55   4.38 0.7415
WVFGRD96   76.0   200    75   -55   4.39 0.7377
WVFGRD96   78.0   200    75   -55   4.39 0.7320
WVFGRD96   80.0   200    75   -55   4.39 0.7289
WVFGRD96   82.0   200    75   -60   4.40 0.7247
WVFGRD96   84.0   200    75   -60   4.40 0.7198
WVFGRD96   86.0   200    75   -60   4.41 0.7172
WVFGRD96   88.0   200    75   -60   4.41 0.7113
WVFGRD96   90.0   205    75   -60   4.41 0.7087
WVFGRD96   92.0   205    75   -60   4.41 0.7026
WVFGRD96   94.0   230    90   -70   4.43 0.6982
WVFGRD96   96.0    55    85    70   4.44 0.6993
WVFGRD96   98.0    55    85    70   4.44 0.6992

The best solution is

WVFGRD96   50.0   200    75   -50   4.32 0.7936

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 0.07 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 Bureau of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Oklahoma Geological Survey, TexNet, the Iris stations, the Transportable Array of EarthScope and other networks.

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 Jan 26 18:38:14 CST 2021