Location

Location ANSS

2023/02/18 12:00:25 62.867 -148.190 66.7 3.8 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2023/02/18 12:00:25:0  62.87 -148.19  66.7 3.8 Alaska
 
 Stations used:
   AK.CUT AK.DHY AK.GHO AK.HDA AK.KLU AK.KTH AK.L22K AK.MCK 
   AK.PAX AK.RND AK.SCM AK.SKN AK.WAT6 AK.WRH AT.PMR AV.STLK 
 
 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 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 1.10e+22 dyne-cm
  Mw = 3.96 
  Z  = 78 km
  Plane   Strike  Dip  Rake
   NP1      251    77   -128
   NP2      145    40   -20
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.10e+22     23       9
    N   0.00e+00     37     260
    P  -1.10e+22     44     123

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     7.44e+21
       Mxy     4.00e+21
       Mxz     6.84e+21
       Myy    -3.75e+21
       Myz    -3.99e+21
       Mzz    -3.69e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ############   #######              
              -############## T ##########           
             --##############   ###########          
           ---###############################        
          ----################################       
         ----##################################      
        -----###########################--------     
        -----###################----------------     
       -------############-----------------------    
       -------#######----------------------------    
       --------##--------------------------------    
       -------#----------------------------------    
        ---#####--------------------   ---------     
        -########------------------- P ---------     
         ##########-----------------   --------      
          ##########--------------------------       
           ###########-----------------------        
             ############------------------          
              ##############--------------           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.69e+21   6.84e+21   3.99e+21 
  6.84e+21   7.44e+21  -4.00e+21 
  3.99e+21  -4.00e+21  -3.75e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230218120025/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 = 145
      DIP = 40
     RAKE = -20
       MW = 3.96
       HS = 78.0

The NDK file is 20230218120025.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2023/02/18 12:00:25:0  62.87 -148.19  66.7 3.8 Alaska
 
 Stations used:
   AK.CUT AK.DHY AK.GHO AK.HDA AK.KLU AK.KTH AK.L22K AK.MCK 
   AK.PAX AK.RND AK.SCM AK.SKN AK.WAT6 AK.WRH AT.PMR AV.STLK 
 
 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 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 1.10e+22 dyne-cm
  Mw = 3.96 
  Z  = 78 km
  Plane   Strike  Dip  Rake
   NP1      251    77   -128
   NP2      145    40   -20
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.10e+22     23       9
    N   0.00e+00     37     260
    P  -1.10e+22     44     123

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     7.44e+21
       Mxy     4.00e+21
       Mxz     6.84e+21
       Myy    -3.75e+21
       Myz    -3.99e+21
       Mzz    -3.69e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ############   #######              
              -############## T ##########           
             --##############   ###########          
           ---###############################        
          ----################################       
         ----##################################      
        -----###########################--------     
        -----###################----------------     
       -------############-----------------------    
       -------#######----------------------------    
       --------##--------------------------------    
       -------#----------------------------------    
        ---#####--------------------   ---------     
        -########------------------- P ---------     
         ##########-----------------   --------      
          ##########--------------------------       
           ###########-----------------------        
             ############------------------          
              ##############--------------           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.69e+21   6.84e+21   3.99e+21 
  6.84e+21   7.44e+21  -4.00e+21 
  3.99e+21  -4.00e+21  -3.75e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230218120025/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/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 n 3 
br c 0.12 0.25 n 4 p 2
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    2.0    10    50    65   3.28 0.3817
WVFGRD96    4.0   165    40     0   3.33 0.4226
WVFGRD96    6.0   160    40    -5   3.36 0.4779
WVFGRD96    8.0   165    40    10   3.40 0.4986
WVFGRD96   10.0   165    45    10   3.41 0.5109
WVFGRD96   12.0   165    50    10   3.42 0.5198
WVFGRD96   14.0   165    50    10   3.44 0.5275
WVFGRD96   16.0   160    50    -5   3.46 0.5342
WVFGRD96   18.0   160    55     0   3.48 0.5417
WVFGRD96   20.0   155    55   -20   3.52 0.5492
WVFGRD96   22.0   160    50    -5   3.53 0.5563
WVFGRD96   24.0   160    50    -5   3.55 0.5644
WVFGRD96   26.0   160    50    -5   3.57 0.5722
WVFGRD96   28.0   160    55    -5   3.59 0.5800
WVFGRD96   30.0   160    55    -5   3.61 0.5853
WVFGRD96   32.0   160    55    -5   3.62 0.5907
WVFGRD96   34.0   160    55    -5   3.64 0.5940
WVFGRD96   36.0   160    55   -10   3.67 0.5959
WVFGRD96   38.0   160    60   -10   3.69 0.6007
WVFGRD96   40.0   160    45    -5   3.77 0.6024
WVFGRD96   42.0   160    50    -5   3.78 0.6043
WVFGRD96   44.0   160    50    -5   3.80 0.6057
WVFGRD96   46.0   160    50    -5   3.81 0.6071
WVFGRD96   48.0   160    50    -5   3.82 0.6083
WVFGRD96   50.0   160    50    -5   3.84 0.6101
WVFGRD96   52.0   155    45   -10   3.86 0.6129
WVFGRD96   54.0   140    45   -25   3.88 0.6170
WVFGRD96   56.0   140    45   -25   3.89 0.6260
WVFGRD96   58.0   140    45   -25   3.90 0.6339
WVFGRD96   60.0   140    45   -25   3.90 0.6426
WVFGRD96   62.0   140    45   -25   3.91 0.6508
WVFGRD96   64.0   140    40   -25   3.92 0.6562
WVFGRD96   66.0   140    40   -25   3.93 0.6639
WVFGRD96   68.0   140    40   -25   3.94 0.6681
WVFGRD96   70.0   140    40   -25   3.94 0.6726
WVFGRD96   72.0   140    40   -20   3.95 0.6750
WVFGRD96   74.0   145    40   -20   3.95 0.6771
WVFGRD96   76.0   145    40   -20   3.96 0.6781
WVFGRD96   78.0   145    40   -20   3.96 0.6782
WVFGRD96   80.0   145    40   -20   3.97 0.6781
WVFGRD96   82.0   145    40   -20   3.97 0.6761
WVFGRD96   84.0   140    40   -25   3.97 0.6733
WVFGRD96   86.0   140    40   -25   3.97 0.6705
WVFGRD96   88.0   140    40   -25   3.98 0.6669
WVFGRD96   90.0   140    40   -25   3.98 0.6637
WVFGRD96   92.0   140    40   -30   3.98 0.6597
WVFGRD96   94.0   140    45   -30   3.98 0.6559
WVFGRD96   96.0   140    45   -30   3.98 0.6525
WVFGRD96   98.0   140    45   -30   3.98 0.6488
WVFGRD96  100.0   140    45   -35   3.99 0.6455
WVFGRD96  102.0   140    45   -35   3.99 0.6424
WVFGRD96  104.0   140    45   -35   3.99 0.6390
WVFGRD96  106.0   140    45   -35   4.00 0.6348
WVFGRD96  108.0   140    45   -40   4.00 0.6314
WVFGRD96  110.0   140    45   -40   4.01 0.6281
WVFGRD96  112.0   140    45   -40   4.01 0.6242
WVFGRD96  114.0   135    45   -45   4.01 0.6206
WVFGRD96  116.0   135    45   -45   4.02 0.6174
WVFGRD96  118.0   135    45   -50   4.03 0.6147
WVFGRD96  120.0   135    45   -50   4.03 0.6104
WVFGRD96  122.0   135    45   -50   4.03 0.5999
WVFGRD96  124.0   135    45   -50   4.03 0.5848
WVFGRD96  126.0   135    45   -50   4.03 0.5689
WVFGRD96  128.0   135    45   -50   4.03 0.5511
WVFGRD96  130.0   130    45   -60   4.05 0.5383
WVFGRD96  132.0   130    45   -60   4.05 0.5324
WVFGRD96  134.0   135    50   -55   4.04 0.5261
WVFGRD96  136.0   135    50   -55   4.04 0.5171
WVFGRD96  138.0   135    50   -60   4.06 0.5043
WVFGRD96  140.0   135    50   -60   4.06 0.4895
WVFGRD96  142.0   125    45   -65   4.06 0.4747
WVFGRD96  144.0   125    45   -70   4.07 0.4580
WVFGRD96  146.0   125    45   -70   4.07 0.4371
WVFGRD96  148.0   125    45   -70   4.07 0.4202

The best solution is

WVFGRD96   78.0   145    40   -20   3.96 0.6782

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 
br c 0.12 0.25 n 4 p 2
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. The time scale is based on the last trace plotted. It is not used to represent travel time or absolute time, but rather to indicate the umebr os seconds plotted. If there is not a trace directly above the scale, then a default, meaningless scale is plotted.
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 Sat Feb 18 09:29:04 CST 2023