Location

Location ANSS

2022/01/21 05:18:26 60.321 -152.372 117.3 5.1 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2022/01/21 05:18:26:0  60.32 -152.37 117.3 5.1 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CNP AK.CUT AK.DHY AK.FIRE AK.GHO AK.GLI 
   AK.HIN AK.HOM AK.K20K AK.KNK AK.L18K AK.L20K AK.L22K 
   AK.M20K AK.MCK AK.N18K AK.N19K AK.O18K AK.O19K AK.P17K 
   AK.P23K AK.R18K AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SLK 
   AK.SWD AT.PMR AV.ILS AV.STLK 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 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 4.27e+23 dyne-cm
  Mw = 5.02 
  Z  = 96 km
  Plane   Strike  Dip  Rake
   NP1       55    85    25
   NP2      323    65   174
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.27e+23     21     282
    N   0.00e+00     65      66
    P  -4.27e+23     14     186

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.83e+23
       Mxy    -1.17e+23
       Mxz     1.26e+23
       Myy     3.52e+23
       Myz    -1.29e+23
       Mzz     3.13e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ####------------------------           
             #########---------------------          
           ##############--------------------        
          #################----------------###       
         ####################------------######      
        #######################--------#########     
        ##   ###################----############     
       ### T ####################################    
       ###   ###################---##############    
       ######################-------#############    
       ###################-----------############    
        ###############---------------##########     
        #############------------------#########     
         #########----------------------#######      
          ####--------------------------######       
           -----------------------------#####        
             ----------------------------##          
              ----------   --------------#           
                 ------- P ------------              
                     ---   --------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  3.13e+22   1.26e+23   1.29e+23 
  1.26e+23  -3.83e+23   1.17e+23 
  1.29e+23   1.17e+23   3.52e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20220121051826/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 = 55
      DIP = 85
     RAKE = 25
       MW = 5.02
       HS = 96.0

The NDK file is 20220121051826.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSW
 USGS/SLU Moment Tensor Solution
 ENS  2022/01/21 05:18:26:0  60.32 -152.37 117.3 5.1 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CNP AK.CUT AK.DHY AK.FIRE AK.GHO AK.GLI 
   AK.HIN AK.HOM AK.K20K AK.KNK AK.L18K AK.L20K AK.L22K 
   AK.M20K AK.MCK AK.N18K AK.N19K AK.O18K AK.O19K AK.P17K 
   AK.P23K AK.R18K AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SLK 
   AK.SWD AT.PMR AV.ILS AV.STLK 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 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 4.27e+23 dyne-cm
  Mw = 5.02 
  Z  = 96 km
  Plane   Strike  Dip  Rake
   NP1       55    85    25
   NP2      323    65   174
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.27e+23     21     282
    N   0.00e+00     65      66
    P  -4.27e+23     14     186

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.83e+23
       Mxy    -1.17e+23
       Mxz     1.26e+23
       Myy     3.52e+23
       Myz    -1.29e+23
       Mzz     3.13e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ####------------------------           
             #########---------------------          
           ##############--------------------        
          #################----------------###       
         ####################------------######      
        #######################--------#########     
        ##   ###################----############     
       ### T ####################################    
       ###   ###################---##############    
       ######################-------#############    
       ###################-----------############    
        ###############---------------##########     
        #############------------------#########     
         #########----------------------#######      
          ####--------------------------######       
           -----------------------------#####        
             ----------------------------##          
              ----------   --------------#           
                 ------- P ------------              
                     ---   --------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  3.13e+22   1.26e+23   1.29e+23 
  1.26e+23  -3.83e+23   1.17e+23 
  1.29e+23   1.17e+23   3.52e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20220121051826/index.html
	
W-phase Moment Tensor (Mww)
Moment 5.174e+16 N-m
Magnitude 5.08 Mww
Depth 100.5 km
Percent DC 97%
Half Duration 0.89 s
Catalog US
Data Source US 3
Contributor US 3

Nodal Planes
Plane Strike Dip Rake
NP1 325° 86° 177°
NP2 55° 87° 4°

Principal Axes
Axis Value Plunge Azimuth
T 5.208e+16 N-m 5° 280°
N -0.070e+16 N-m 85° 93°
P -5.138e+16 N-m 1° 190°

        

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 0.10 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   325    85     0   4.07 0.2649
WVFGRD96    4.0   325    80     5   4.19 0.3204
WVFGRD96    6.0   325    80   -10   4.26 0.3459
WVFGRD96    8.0   325    80   -10   4.34 0.3643
WVFGRD96   10.0   325    80   -10   4.39 0.3609
WVFGRD96   12.0   325    85    -5   4.43 0.3505
WVFGRD96   14.0   325    85    -5   4.45 0.3333
WVFGRD96   16.0   325    85     0   4.47 0.3093
WVFGRD96   18.0   325    85     5   4.48 0.2810
WVFGRD96   20.0   145    80    -5   4.49 0.2511
WVFGRD96   22.0   235    75    10   4.51 0.2492
WVFGRD96   24.0   235    80     5   4.52 0.2515
WVFGRD96   26.0   240    80     5   4.54 0.2521
WVFGRD96   28.0   240    85   -10   4.55 0.2508
WVFGRD96   30.0   240    90   -10   4.56 0.2485
WVFGRD96   32.0    60    90    15   4.57 0.2470
WVFGRD96   34.0    60    90    15   4.58 0.2440
WVFGRD96   36.0   240    90   -20   4.61 0.2429
WVFGRD96   38.0   240    90   -20   4.63 0.2470
WVFGRD96   40.0   240    90   -30   4.70 0.2592
WVFGRD96   42.0    60    85    20   4.72 0.2659
WVFGRD96   44.0    60    85    20   4.74 0.2750
WVFGRD96   46.0    60    80    20   4.76 0.2857
WVFGRD96   48.0    60    80    15   4.78 0.2989
WVFGRD96   50.0    60    80    15   4.80 0.3145
WVFGRD96   52.0    60    80    20   4.82 0.3357
WVFGRD96   54.0    60    80    15   4.83 0.3598
WVFGRD96   56.0    60    80    20   4.86 0.3892
WVFGRD96   58.0    55    85    20   4.88 0.4221
WVFGRD96   60.0    55    85    20   4.89 0.4586
WVFGRD96   62.0    55    85    20   4.91 0.4957
WVFGRD96   64.0    55    85    25   4.93 0.5343
WVFGRD96   66.0    55    90    25   4.95 0.5712
WVFGRD96   68.0    55    90    25   4.96 0.6056
WVFGRD96   70.0   235    90   -25   4.97 0.6300
WVFGRD96   72.0    55    90    25   4.98 0.6413
WVFGRD96   74.0   235    90   -25   4.98 0.6464
WVFGRD96   76.0    55    90    25   4.99 0.6514
WVFGRD96   78.0    55    85    25   4.99 0.6547
WVFGRD96   80.0    55    85    25   4.99 0.6572
WVFGRD96   82.0    55    85    25   5.00 0.6590
WVFGRD96   84.0    55    85    25   5.00 0.6613
WVFGRD96   86.0    55    85    25   5.01 0.6619
WVFGRD96   88.0    55    85    25   5.01 0.6622
WVFGRD96   90.0    55    85    25   5.01 0.6621
WVFGRD96   92.0    55    85    25   5.02 0.6634
WVFGRD96   94.0    55    85    25   5.02 0.6642
WVFGRD96   96.0    55    85    25   5.02 0.6642
WVFGRD96   98.0    55    85    25   5.03 0.6625
WVFGRD96  100.0    55    85    25   5.03 0.6594
WVFGRD96  102.0    55    85    25   5.03 0.6591
WVFGRD96  104.0    55    85    25   5.03 0.6584
WVFGRD96  106.0    55    85    25   5.04 0.6560
WVFGRD96  108.0    55    85    25   5.04 0.6523
WVFGRD96  110.0    55    85    25   5.04 0.6515
WVFGRD96  112.0    55    85    25   5.05 0.6486
WVFGRD96  114.0    55    85    25   5.05 0.6434
WVFGRD96  116.0    55    85    25   5.05 0.6422
WVFGRD96  118.0    55    80    20   5.05 0.6399
WVFGRD96  120.0    55    80    20   5.05 0.6353
WVFGRD96  122.0    55    80    20   5.05 0.6343
WVFGRD96  124.0    55    80    20   5.06 0.6312
WVFGRD96  126.0    55    80    20   5.06 0.6269
WVFGRD96  128.0    55    80    20   5.06 0.6260
WVFGRD96  130.0    55    80    20   5.06 0.6216
WVFGRD96  132.0    55    80    20   5.06 0.6187
WVFGRD96  134.0    55    80    20   5.07 0.6161
WVFGRD96  136.0    55    80    20   5.07 0.6114
WVFGRD96  138.0    55    80    20   5.07 0.6091
WVFGRD96  140.0    55    80    25   5.07 0.6033
WVFGRD96  142.0    55    80    25   5.08 0.6022
WVFGRD96  144.0    55    80    25   5.08 0.5968
WVFGRD96  146.0    55    80    25   5.08 0.5932
WVFGRD96  148.0    55    80    25   5.08 0.5893

The best solution is

WVFGRD96   96.0    55    85    25   5.02 0.6642

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 0.10 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 Fri Jan 21 06:38:27 CST 2022