Location

Location ANSS

2022/11/23 01:36:42 61.739 -149.608 37.4 4.1 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2022/11/23 01:36:42:0  61.74 -149.61  37.4 4.1 Alaska
 
 Stations used:
   AK.CUT AK.FIRE AK.GHO AK.KNK AK.L22K AK.RC01 AK.SAW AK.SLK 
   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 
 
 Best Fitting Double Couple
  Mo = 2.26e+22 dyne-cm
  Mw = 4.17 
  Z  = 42 km
  Plane   Strike  Dip  Rake
   NP1      330    57   -123
   NP2      200    45   -50
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.26e+22      7      83
    N   0.00e+00     27     349
    P  -2.26e+22     62     186

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.59e+21
       Mxy     2.31e+21
       Mxz     9.67e+21
       Myy     2.19e+22
       Myz     3.52e+21
       Mzz    -1.73e+22
                                                     
                                                     
                                                     
                                                     
                     -------------#                  
                 ###---------##########              
              ##########--################           
             ###########---################          
           ###########-------################        
          ###########----------###############       
         ###########------------###############      
        ###########---------------##############     
        ##########-----------------###########       
       ##########-------------------########## T     
       ##########--------------------#########       
       #########----------------------###########    
       #########----------------------###########    
        ########----------   ----------#########     
        ########---------- P ----------#########     
         #######----------   ----------########      
          ######------------------------######       
           ######-----------------------#####        
             ####----------------------####          
              ####---------------------###           
                 ##--------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.73e+22   9.67e+21  -3.52e+21 
  9.67e+21  -4.59e+21  -2.31e+21 
 -3.52e+21  -2.31e+21   2.19e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20221123013642/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 = 45
     RAKE = -50
       MW = 4.17
       HS = 42.0

The NDK file is 20221123013642.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2022/11/23 01:36:42:0  61.74 -149.61  37.4 4.1 Alaska
 
 Stations used:
   AK.CUT AK.FIRE AK.GHO AK.KNK AK.L22K AK.RC01 AK.SAW AK.SLK 
   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 
 
 Best Fitting Double Couple
  Mo = 2.26e+22 dyne-cm
  Mw = 4.17 
  Z  = 42 km
  Plane   Strike  Dip  Rake
   NP1      330    57   -123
   NP2      200    45   -50
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.26e+22      7      83
    N   0.00e+00     27     349
    P  -2.26e+22     62     186

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.59e+21
       Mxy     2.31e+21
       Mxz     9.67e+21
       Myy     2.19e+22
       Myz     3.52e+21
       Mzz    -1.73e+22
                                                     
                                                     
                                                     
                                                     
                     -------------#                  
                 ###---------##########              
              ##########--################           
             ###########---################          
           ###########-------################        
          ###########----------###############       
         ###########------------###############      
        ###########---------------##############     
        ##########-----------------###########       
       ##########-------------------########## T     
       ##########--------------------#########       
       #########----------------------###########    
       #########----------------------###########    
        ########----------   ----------#########     
        ########---------- P ----------#########     
         #######----------   ----------########      
          ######------------------------######       
           ######-----------------------#####        
             ####----------------------####          
              ####---------------------###           
                 ##--------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.73e+22   9.67e+21  -3.52e+21 
  9.67e+21  -4.59e+21  -2.31e+21 
 -3.52e+21  -2.31e+21   2.19e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20221123013642/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 
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    -5    45    90   3.56 0.2852
WVFGRD96    4.0   155    80    65   3.64 0.2932
WVFGRD96    6.0   160    80    75   3.67 0.3523
WVFGRD96    8.0   345    80   -70   3.74 0.3807
WVFGRD96   10.0   340    70   -70   3.78 0.3950
WVFGRD96   12.0   340    70   -75   3.79 0.3955
WVFGRD96   14.0   200    20   -45   3.83 0.3922
WVFGRD96   16.0   205    25   -40   3.86 0.3857
WVFGRD96   18.0   205    30   -40   3.88 0.3760
WVFGRD96   20.0   190    40   -60   3.89 0.3734
WVFGRD96   22.0   190    45   -60   3.91 0.3789
WVFGRD96   24.0   190    45   -60   3.93 0.3814
WVFGRD96   26.0   225    35   -10   3.97 0.3833
WVFGRD96   28.0   215    35   -25   3.98 0.3980
WVFGRD96   30.0   210    40   -30   4.00 0.4188
WVFGRD96   32.0   205    40   -35   4.02 0.4456
WVFGRD96   34.0   200    40   -45   4.03 0.4699
WVFGRD96   36.0   200    45   -45   4.05 0.4900
WVFGRD96   38.0   200    45   -45   4.07 0.5003
WVFGRD96   40.0   200    45   -50   4.15 0.5170
WVFGRD96   42.0   200    45   -50   4.17 0.5200
WVFGRD96   44.0   200    45   -50   4.18 0.5186
WVFGRD96   46.0   200    45   -50   4.19 0.5153
WVFGRD96   48.0   200    50   -50   4.20 0.5100
WVFGRD96   50.0   180    50   -85   4.20 0.5098
WVFGRD96   52.0   180    50   -85   4.21 0.5089
WVFGRD96   54.0   180    50   -85   4.21 0.5063
WVFGRD96   56.0   180    50   -85   4.22 0.5041
WVFGRD96   58.0   180    50   -85   4.22 0.4996
WVFGRD96   60.0   180    50   -85   4.22 0.4938
WVFGRD96   62.0   185    50   -75   4.24 0.4897
WVFGRD96   64.0   185    50   -70   4.25 0.4839
WVFGRD96   66.0   185    50   -70   4.25 0.4784
WVFGRD96   68.0   185    50   -70   4.25 0.4726
WVFGRD96   70.0   195    55   -55   4.28 0.4673
WVFGRD96   72.0   195    55   -50   4.29 0.4629
WVFGRD96   74.0   195    55   -50   4.30 0.4584
WVFGRD96   76.0   195    55   -50   4.30 0.4535
WVFGRD96   78.0   195    55   -50   4.31 0.4478

The best solution is

WVFGRD96   42.0   200    45   -50   4.17 0.5200

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 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 Nov 22 08:09:14 PM CST 2022