Location

Location ANSS

2023/02/16 23:32:34 61.534 -150.555 61.5 3.9 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2023/02/16 23:32:34:0  61.53 -150.55  61.5 3.9 Alaska
 
 Stations used:
   AK.CAPN AK.CUT AK.FIRE AK.GHO AK.GLI AK.KNK AK.L22K AK.PWL 
   AK.RC01 AK.SCM AK.SKN 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 0.06 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 3.43e+22 dyne-cm
  Mw = 4.29 
  Z  = 82 km
  Plane   Strike  Dip  Rake
   NP1      222    78   -118
   NP2      110    30   -25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.43e+22     28     334
    N   0.00e+00     27     228
    P  -3.43e+22     50     102

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.11e+22
       Mxy    -7.87e+21
       Mxz     1.60e+22
       Myy    -8.52e+21
       Myz    -2.28e+22
       Mzz    -1.25e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ######   ###################           
             ####### T ################----          
           #########   #############---------        
          ########################------------       
         #######################---------------      
        ######################------------------     
        ####################--------------------     
       --#################-----------------------    
       --################------------------------    
       ---#############-------------   ----------    
       ---############-------------- P ----------    
        ---#########----------------   ---------     
        -----######-----------------------------     
         -----####-----------------------------      
          ------#----------------------------#       
           -----###-----------------------###        
             --########---------------#####          
              ############################           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.25e+22   1.60e+22   2.28e+22 
  1.60e+22   2.11e+22   7.87e+21 
  2.28e+22   7.87e+21  -8.52e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230216233234/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 = 110
      DIP = 30
     RAKE = -25
       MW = 4.29
       HS = 82.0

The NDK file is 20230216233234.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2023/02/16 23:32:34:0  61.53 -150.55  61.5 3.9 Alaska
 
 Stations used:
   AK.CAPN AK.CUT AK.FIRE AK.GHO AK.GLI AK.KNK AK.L22K AK.PWL 
   AK.RC01 AK.SCM AK.SKN 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 0.06 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 3.43e+22 dyne-cm
  Mw = 4.29 
  Z  = 82 km
  Plane   Strike  Dip  Rake
   NP1      222    78   -118
   NP2      110    30   -25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.43e+22     28     334
    N   0.00e+00     27     228
    P  -3.43e+22     50     102

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.11e+22
       Mxy    -7.87e+21
       Mxz     1.60e+22
       Myy    -8.52e+21
       Myz    -2.28e+22
       Mzz    -1.25e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ######   ###################           
             ####### T ################----          
           #########   #############---------        
          ########################------------       
         #######################---------------      
        ######################------------------     
        ####################--------------------     
       --#################-----------------------    
       --################------------------------    
       ---#############-------------   ----------    
       ---############-------------- P ----------    
        ---#########----------------   ---------     
        -----######-----------------------------     
         -----####-----------------------------      
          ------#----------------------------#       
           -----###-----------------------###        
             --########---------------#####          
              ############################           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.25e+22   1.60e+22   2.28e+22 
  1.60e+22   2.11e+22   7.87e+21 
  2.28e+22   7.87e+21  -8.52e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230216233234/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 0.06 n 3 
br c 0.12 0.25 n 4 p 2
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    95    50   -45   3.60 0.2991
WVFGRD96    4.0    95    70   -60   3.72 0.3303
WVFGRD96    6.0   125    55    40   3.72 0.3611
WVFGRD96    8.0    85    50   -65   3.81 0.3836
WVFGRD96   10.0   130    55    45   3.78 0.4059
WVFGRD96   12.0   125    60    40   3.78 0.4122
WVFGRD96   14.0   120    65    35   3.77 0.4164
WVFGRD96   16.0   120    60    30   3.78 0.4202
WVFGRD96   18.0   120    60    30   3.79 0.4235
WVFGRD96   20.0   120    60    25   3.80 0.4258
WVFGRD96   22.0   105    65   -30   3.83 0.4390
WVFGRD96   24.0   105    65   -30   3.84 0.4464
WVFGRD96   26.0   105    65   -30   3.86 0.4517
WVFGRD96   28.0   105    60   -30   3.87 0.4544
WVFGRD96   30.0   105    60   -30   3.89 0.4560
WVFGRD96   32.0   105    60   -25   3.90 0.4555
WVFGRD96   34.0   105    60   -25   3.92 0.4524
WVFGRD96   36.0   105    60   -25   3.94 0.4457
WVFGRD96   38.0   120    55     5   3.93 0.4396
WVFGRD96   40.0   100    50   -35   4.05 0.4526
WVFGRD96   42.0   115    40   -15   4.05 0.4586
WVFGRD96   44.0   110    35   -25   4.08 0.4687
WVFGRD96   46.0   105    35   -30   4.10 0.4823
WVFGRD96   48.0   105    35   -30   4.12 0.4977
WVFGRD96   50.0   100    35   -40   4.14 0.5145
WVFGRD96   52.0   100    35   -40   4.16 0.5314
WVFGRD96   54.0   100    35   -40   4.17 0.5467
WVFGRD96   56.0   100    35   -35   4.18 0.5610
WVFGRD96   58.0   100    35   -35   4.19 0.5745
WVFGRD96   60.0   105    35   -30   4.20 0.5860
WVFGRD96   62.0   105    35   -30   4.21 0.5970
WVFGRD96   64.0   105    35   -30   4.22 0.6055
WVFGRD96   66.0   105    35   -30   4.23 0.6141
WVFGRD96   68.0   105    35   -30   4.24 0.6203
WVFGRD96   70.0   105    35   -25   4.24 0.6254
WVFGRD96   72.0   110    35   -25   4.25 0.6305
WVFGRD96   74.0   110    35   -25   4.26 0.6343
WVFGRD96   76.0   110    35   -25   4.27 0.6363
WVFGRD96   78.0   110    30   -25   4.27 0.6382
WVFGRD96   80.0   110    30   -25   4.28 0.6394
WVFGRD96   82.0   110    30   -25   4.29 0.6395
WVFGRD96   84.0   110    30   -25   4.29 0.6381
WVFGRD96   86.0   115    30   -20   4.30 0.6362
WVFGRD96   88.0   115    30   -20   4.30 0.6328
WVFGRD96   90.0   115    30   -20   4.31 0.6284
WVFGRD96   92.0   115    35   -20   4.31 0.6231
WVFGRD96   94.0   115    35   -20   4.31 0.6174
WVFGRD96   96.0   115    35   -20   4.31 0.6112
WVFGRD96   98.0   115    35   -20   4.31 0.6048

The best solution is

WVFGRD96   82.0   110    30   -25   4.29 0.6395

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 0.06 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 Thu Feb 16 06:05:14 PM CST 2023