Location

Location ANSS

2020/12/21 12:16:18 66.343 -135.698 21.5 4.3 Yukon, Canada

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2020/12/21 12:16:18:0  66.34 -135.70  21.5 4.3 Yukon, Canada
 
 Stations used:
   AK.G27K AK.I27K CN.INK TA.E29M TA.EPYK TA.G30M TA.G31M 
   TA.H27K TA.H29M TA.H31M TA.I28M TA.I29M TA.I30M TA.J29N 
   TA.J30M TA.K29M 
 
 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 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 2.79e+22 dyne-cm
  Mw = 4.23 
  Z  = 34 km
  Plane   Strike  Dip  Rake
   NP1       80    90    10
   NP2      350    80   180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.79e+22      7     305
    N   0.00e+00     80      80
    P  -2.79e+22      7     215

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.38e+21
       Mxy    -2.58e+22
       Mxz     4.76e+21
       Myy     9.38e+21
       Myz    -8.40e+20
       Mzz    -4.23e+14
                                                     
                                                     
                                                     
                                                     
                     ####----------                  
                 #########-------------              
              #############---------------           
              #############----------------          
            T ##############-----------------        
          #   ##############------------------       
         ####################------------------      
        #####################-------------------     
        ######################------------------     
       #######################--------------#####    
       #######################---################    
       ################--------##################    
       #####-------------------##################    
        -----------------------#################     
        ------------------------################     
         -----------------------###############      
          ----------------------##############       
           ---------------------#############        
             --   --------------###########          
              - P --------------##########           
                  --------------#######              
                     -----------###                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.23e+14   4.76e+21   8.40e+20 
  4.76e+21  -9.38e+21   2.58e+22 
  8.40e+20   2.58e+22   9.38e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201221121618/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 = 80
      DIP = 90
     RAKE = 10
       MW = 4.23
       HS = 34.0

The NDK file is 20201221121618.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2020/12/21 12:16:18:0  66.34 -135.70  21.5 4.3 Yukon, Canada
 
 Stations used:
   AK.G27K AK.I27K CN.INK TA.E29M TA.EPYK TA.G30M TA.G31M 
   TA.H27K TA.H29M TA.H31M TA.I28M TA.I29M TA.I30M TA.J29N 
   TA.J30M TA.K29M 
 
 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 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 2.79e+22 dyne-cm
  Mw = 4.23 
  Z  = 34 km
  Plane   Strike  Dip  Rake
   NP1       80    90    10
   NP2      350    80   180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.79e+22      7     305
    N   0.00e+00     80      80
    P  -2.79e+22      7     215

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.38e+21
       Mxy    -2.58e+22
       Mxz     4.76e+21
       Myy     9.38e+21
       Myz    -8.40e+20
       Mzz    -4.23e+14
                                                     
                                                     
                                                     
                                                     
                     ####----------                  
                 #########-------------              
              #############---------------           
              #############----------------          
            T ##############-----------------        
          #   ##############------------------       
         ####################------------------      
        #####################-------------------     
        ######################------------------     
       #######################--------------#####    
       #######################---################    
       ################--------##################    
       #####-------------------##################    
        -----------------------#################     
        ------------------------################     
         -----------------------###############      
          ----------------------##############       
           ---------------------#############        
             --   --------------###########          
              - P --------------##########           
                  --------------#######              
                     -----------###                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.23e+14   4.76e+21   8.40e+20 
  4.76e+21  -9.38e+21   2.58e+22 
  8.40e+20   2.58e+22   9.38e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20201221121618/index.html
	

Magnitudes

mLg Magnitude


(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.

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 0.07 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    1.0   255    75    -5   3.90 0.5538
WVFGRD96    2.0   255    90     5   3.92 0.5913
WVFGRD96    3.0   255    90    10   3.95 0.6081
WVFGRD96    4.0    75    90    15   3.97 0.6187
WVFGRD96    5.0   255    85   -15   3.98 0.6367
WVFGRD96    6.0    75    90    15   3.99 0.6461
WVFGRD96    7.0    75    90    15   4.00 0.6563
WVFGRD96    8.0   255    85   -15   4.01 0.6677
WVFGRD96    9.0   255    85   -15   4.02 0.6749
WVFGRD96   10.0    75    90    15   4.03 0.6808
WVFGRD96   11.0    75    90    15   4.04 0.6870
WVFGRD96   12.0    75    90    15   4.05 0.6938
WVFGRD96   13.0    75    90    10   4.05 0.7006
WVFGRD96   14.0    75    90    10   4.06 0.7075
WVFGRD96   15.0   255    90   -10   4.07 0.7138
WVFGRD96   16.0   255    90   -10   4.08 0.7204
WVFGRD96   17.0    75    90    10   4.08 0.7270
WVFGRD96   18.0    75    90    10   4.09 0.7328
WVFGRD96   19.0    75    90    10   4.10 0.7393
WVFGRD96   20.0   255    90   -10   4.11 0.7472
WVFGRD96   21.0    75    90    10   4.12 0.7535
WVFGRD96   22.0   260    90   -10   4.13 0.7595
WVFGRD96   23.0    80    90    10   4.14 0.7660
WVFGRD96   24.0   260    90   -10   4.15 0.7716
WVFGRD96   25.0   260    90   -10   4.15 0.7764
WVFGRD96   26.0    80    90    10   4.16 0.7810
WVFGRD96   27.0   260    90   -10   4.17 0.7853
WVFGRD96   28.0   260    90   -10   4.18 0.7903
WVFGRD96   29.0    80    90    10   4.19 0.7946
WVFGRD96   30.0    80    90    10   4.19 0.7971
WVFGRD96   31.0    80    90    10   4.20 0.7993
WVFGRD96   32.0   260    90   -10   4.21 0.8013
WVFGRD96   33.0    80    90    10   4.22 0.8023
WVFGRD96   34.0    80    90    10   4.23 0.8036
WVFGRD96   35.0    80    90    10   4.24 0.8031
WVFGRD96   36.0   260    85   -10   4.25 0.8029
WVFGRD96   37.0    80    90    10   4.27 0.8017
WVFGRD96   38.0    80    90    10   4.28 0.8001
WVFGRD96   39.0   260    90   -10   4.30 0.7977
WVFGRD96   40.0   260    90   -10   4.32 0.7948
WVFGRD96   41.0    80    90    10   4.33 0.7910
WVFGRD96   42.0    80    90    10   4.34 0.7882
WVFGRD96   43.0    80    90    10   4.34 0.7839
WVFGRD96   44.0    80    90    10   4.35 0.7788
WVFGRD96   45.0    80    90    10   4.36 0.7728
WVFGRD96   46.0    80    90    10   4.37 0.7665
WVFGRD96   47.0    80    90    10   4.37 0.7614
WVFGRD96   48.0   260    90   -10   4.38 0.7555
WVFGRD96   49.0   260    90   -10   4.39 0.7487
WVFGRD96   50.0    80    90    10   4.39 0.7421

The best solution is

WVFGRD96   34.0    80    90    10   4.23 0.8036

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 0.07 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 CUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

MODEL.01
CUS Model with Q from simple gamma values
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
  1.0000  5.0000  2.8900  2.5000 0.172E-02 0.387E-02 0.00  0.00  1.00  1.00 
  9.0000  6.1000  3.5200  2.7300 0.160E-02 0.363E-02 0.00  0.00  1.00  1.00 
 10.0000  6.4000  3.7000  2.8200 0.149E-02 0.336E-02 0.00  0.00  1.00  1.00 
 20.0000  6.7000  3.8700  2.9020 0.000E-04 0.000E-04 0.00  0.00  1.00  1.00 
  0.0000  8.1500  4.7000  3.3640 0.194E-02 0.431E-02 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 Mon Dec 21 11:30:57 CST 2020