Location

Location ANSS

2020/01/11 11:36:33 63.209 -151.465 10.9 4.1 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2020/01/11 11:36:33:0  63.21 -151.46  10.9 4.1 Alaska
 
 Stations used:
   AK.BPAW AK.CCB AK.CUT AK.DHY AK.DIV AK.DOT AK.EYAK AK.GHO 
   AK.GLB AK.GLI AK.HDA AK.HIN AK.I23K AK.J20K AK.K24K AK.KNK 
   AK.KTH AK.L18K AK.L19K AK.M20K AK.MCK AK.N18K AK.N19K 
   AK.O18K AK.PAX AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM 
   AK.SCRK AK.SKN AK.SLK AK.SSN AK.SWD AK.TRF AK.WRH AT.PMR 
   AV.ILSW IM.IL31 IU.COLA TA.G23K TA.H18K TA.M22K TA.M24K 
   TA.P19K TA.POKR 
 
 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 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 6.76e+21 dyne-cm
  Mw = 3.82 
  Z  = 14 km
  Plane   Strike  Dip  Rake
   NP1       60    55   120
   NP2      195    45    55
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.76e+21     65      28
    N   0.00e+00     24     221
    P  -6.76e+21      5     129

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.74e+21
       Mxy     3.76e+21
       Mxz     2.65e+21
       Myy    -3.80e+21
       Myz     7.10e+20
       Mzz     5.54e+21
                                                     
                                                     
                                                     
                                                     
                     ----------####                  
                 ----------############              
              -----------#################           
             ----------####################          
           ----------########################        
          ----------##########################       
         ----------###########################-      
        ----------###########   #############---     
        ----------########### T ############----     
       ----------############   ###########------    
       ----------#########################-------    
       ---------#########################--------    
       ---------#######################----------    
        --------#####################-----------     
        --------##################--------------     
         --------##############----------------      
          -------##########---------------   -       
           ######------------------------- P         
             #####------------------------           
              #####-----------------------           
                 ###-------------------              
                     #-------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  5.54e+21   2.65e+21  -7.10e+20 
  2.65e+21  -1.74e+21  -3.76e+21 
 -7.10e+20  -3.76e+21  -3.80e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200111113633/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 = 195
      DIP = 45
     RAKE = 55
       MW = 3.82
       HS = 14.0

The NDK file is 20200111113633.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2020/01/11 11:36:33:0  63.21 -151.46  10.9 4.1 Alaska
 
 Stations used:
   AK.BPAW AK.CCB AK.CUT AK.DHY AK.DIV AK.DOT AK.EYAK AK.GHO 
   AK.GLB AK.GLI AK.HDA AK.HIN AK.I23K AK.J20K AK.K24K AK.KNK 
   AK.KTH AK.L18K AK.L19K AK.M20K AK.MCK AK.N18K AK.N19K 
   AK.O18K AK.PAX AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM 
   AK.SCRK AK.SKN AK.SLK AK.SSN AK.SWD AK.TRF AK.WRH AT.PMR 
   AV.ILSW IM.IL31 IU.COLA TA.G23K TA.H18K TA.M22K TA.M24K 
   TA.P19K TA.POKR 
 
 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 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 6.76e+21 dyne-cm
  Mw = 3.82 
  Z  = 14 km
  Plane   Strike  Dip  Rake
   NP1       60    55   120
   NP2      195    45    55
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.76e+21     65      28
    N   0.00e+00     24     221
    P  -6.76e+21      5     129

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.74e+21
       Mxy     3.76e+21
       Mxz     2.65e+21
       Myy    -3.80e+21
       Myz     7.10e+20
       Mzz     5.54e+21
                                                     
                                                     
                                                     
                                                     
                     ----------####                  
                 ----------############              
              -----------#################           
             ----------####################          
           ----------########################        
          ----------##########################       
         ----------###########################-      
        ----------###########   #############---     
        ----------########### T ############----     
       ----------############   ###########------    
       ----------#########################-------    
       ---------#########################--------    
       ---------#######################----------    
        --------#####################-----------     
        --------##################--------------     
         --------##############----------------      
          -------##########---------------   -       
           ######------------------------- P         
             #####------------------------           
              #####-----------------------           
                 ###-------------------              
                     #-------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  5.54e+21   2.65e+21  -7.10e+20 
  2.65e+21  -1.74e+21  -3.76e+21 
 -7.10e+20  -3.76e+21  -3.80e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200111113633/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 0.10 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    1.0   120    45   -90   3.37 0.3181
WVFGRD96    2.0   295    45   -90   3.55 0.4856
WVFGRD96    3.0   170    60   -25   3.56 0.4195
WVFGRD96    4.0   180    35     0   3.65 0.4493
WVFGRD96    5.0   185    30    10   3.69 0.5370
WVFGRD96    6.0   190    30    10   3.70 0.6025
WVFGRD96    7.0   195    35    25   3.71 0.6408
WVFGRD96    8.0   190    25     5   3.75 0.6532
WVFGRD96    9.0   200    35    45   3.79 0.6724
WVFGRD96   10.0   200    40    55   3.81 0.7018
WVFGRD96   11.0   200    40    60   3.81 0.7240
WVFGRD96   12.0   195    45    55   3.81 0.7381
WVFGRD96   13.0   195    45    55   3.82 0.7461
WVFGRD96   14.0   195    45    55   3.82 0.7491
WVFGRD96   15.0   195    45    55   3.83 0.7488
WVFGRD96   16.0   190    50    50   3.83 0.7467
WVFGRD96   17.0   190    50    50   3.84 0.7429
WVFGRD96   18.0   190    50    50   3.84 0.7372
WVFGRD96   19.0   190    50    50   3.85 0.7296
WVFGRD96   20.0   190    50    50   3.86 0.7204
WVFGRD96   21.0   190    50    50   3.87 0.7084
WVFGRD96   22.0   190    50    50   3.88 0.6961
WVFGRD96   23.0   190    50    50   3.89 0.6817
WVFGRD96   24.0   190    50    50   3.90 0.6654
WVFGRD96   25.0   190    50    50   3.90 0.6477
WVFGRD96   26.0   190    50    50   3.91 0.6290
WVFGRD96   27.0   190    50    50   3.91 0.6096
WVFGRD96   28.0   190    50    45   3.92 0.5899
WVFGRD96   29.0   190    50    45   3.92 0.5696

The best solution is

WVFGRD96   14.0   195    45    55   3.82 0.7491

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 
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.
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 Jan 11 06:11:11 CST 2020