Location

Location ANSS

2018/12/01 05:31:54 61.404 -150.005 42.4 4.8 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2018/12/01 05:31:54:0  61.40 -150.01  42.4 4.8 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.CUT AK.DHY AK.FID AK.GHO AK.GLI AK.HOM 
   AK.KNK AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SLK 
   AK.SSN AV.STLK TA.M19K TA.M20K TA.M22K 
 
 Filtering commands used:
   cut o DIST/3.3 -30 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 = 5.56e+22 dyne-cm
  Mw = 4.43 
  Z  = 49 km
  Plane   Strike  Dip  Rake
   NP1      175    55   -70
   NP2      323    40   -116
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.56e+22      8     251
    N   0.00e+00     16     343
    P  -5.56e+22     72     136

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     3.08e+21
       Mxy     1.96e+22
       Mxz     9.31e+21
       Myy     4.60e+22
       Myz    -1.87e+22
       Mzz    -4.91e+22
                                                     
                                                     
                                                     
                                                     
                     ----##########                  
                 ------################              
              ########-----###############           
             ########----------############          
           ##########-------------###########        
          ##########----------------##########       
         ###########-----------------##########      
        ###########--------------------#########     
        ###########---------------------########     
       ############----------------------########    
       ############-----------------------#######    
       ############----------   ----------#######    
       ############---------- P -----------######    
           #########---------   -----------#####     
         T #########-----------------------#####     
           ##########----------------------####      
          ###########----------------------###       
           ###########--------------------###        
             ##########-------------------#          
              ##########-----------------#           
                 #########-------------              
                     #######-------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.91e+22   9.31e+21   1.87e+22 
  9.31e+21   3.08e+21  -1.96e+22 
  1.87e+22  -1.96e+22   4.60e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201053154/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 = 175
      DIP = 55
     RAKE = -70
       MW = 4.43
       HS = 49.0

The NDK file is 20181201053154.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2018/12/01 05:31:54:0  61.40 -150.01  42.4 4.8 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.CUT AK.DHY AK.FID AK.GHO AK.GLI AK.HOM 
   AK.KNK AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SLK 
   AK.SSN AV.STLK TA.M19K TA.M20K TA.M22K 
 
 Filtering commands used:
   cut o DIST/3.3 -30 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 = 5.56e+22 dyne-cm
  Mw = 4.43 
  Z  = 49 km
  Plane   Strike  Dip  Rake
   NP1      175    55   -70
   NP2      323    40   -116
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.56e+22      8     251
    N   0.00e+00     16     343
    P  -5.56e+22     72     136

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     3.08e+21
       Mxy     1.96e+22
       Mxz     9.31e+21
       Myy     4.60e+22
       Myz    -1.87e+22
       Mzz    -4.91e+22
                                                     
                                                     
                                                     
                                                     
                     ----##########                  
                 ------################              
              ########-----###############           
             ########----------############          
           ##########-------------###########        
          ##########----------------##########       
         ###########-----------------##########      
        ###########--------------------#########     
        ###########---------------------########     
       ############----------------------########    
       ############-----------------------#######    
       ############----------   ----------#######    
       ############---------- P -----------######    
           #########---------   -----------#####     
         T #########-----------------------#####     
           ##########----------------------####      
          ###########----------------------###       
           ###########--------------------###        
             ##########-------------------#          
              ##########-----------------#           
                 #########-------------              
                     #######-------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.91e+22   9.31e+21   1.87e+22 
  9.31e+21   3.08e+21  -1.96e+22 
  1.87e+22  -1.96e+22   4.60e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201053154/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 -30 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    1.0   345    40    90   3.55 0.1850
WVFGRD96    2.0   340    35    85   3.71 0.2411
WVFGRD96    3.0   325    35    60   3.74 0.2328
WVFGRD96    4.0   110    55   -35   3.73 0.2489
WVFGRD96    5.0   110    55   -35   3.76 0.2726
WVFGRD96    6.0   190    90   -50   3.75 0.2897
WVFGRD96    7.0    15    85    50   3.77 0.3103
WVFGRD96    8.0   145    75   -55   3.88 0.3231
WVFGRD96    9.0   150    75   -50   3.89 0.3368
WVFGRD96   10.0   150    75   -50   3.91 0.3481
WVFGRD96   11.0   150    70   -45   3.93 0.3569
WVFGRD96   12.0   150    70   -45   3.95 0.3630
WVFGRD96   13.0   150    70   -45   3.97 0.3665
WVFGRD96   14.0   150    75   -45   3.98 0.3689
WVFGRD96   15.0   345    75    40   3.99 0.3689
WVFGRD96   16.0    10    75    45   3.98 0.3702
WVFGRD96   17.0    10    75    45   3.99 0.3726
WVFGRD96   18.0    10    75    45   4.01 0.3749
WVFGRD96   19.0    20    75    45   4.02 0.3792
WVFGRD96   20.0    20    75    45   4.04 0.3854
WVFGRD96   21.0    20    75    45   4.06 0.3913
WVFGRD96   22.0    25    80    55   4.07 0.3987
WVFGRD96   23.0    25    80    55   4.08 0.4080
WVFGRD96   24.0    25    85    55   4.09 0.4175
WVFGRD96   25.0    25    85    55   4.11 0.4272
WVFGRD96   26.0    20    90    55   4.12 0.4382
WVFGRD96   27.0    20    90    55   4.13 0.4489
WVFGRD96   28.0    20    90    55   4.14 0.4597
WVFGRD96   29.0   200    85   -55   4.15 0.4744
WVFGRD96   30.0   195    80   -55   4.16 0.4889
WVFGRD96   31.0   195    75   -50   4.17 0.5053
WVFGRD96   32.0   195    70   -50   4.18 0.5217
WVFGRD96   33.0   190    65   -55   4.20 0.5403
WVFGRD96   34.0   190    65   -55   4.21 0.5583
WVFGRD96   35.0   190    65   -55   4.21 0.5729
WVFGRD96   36.0   185    60   -55   4.22 0.5914
WVFGRD96   37.0   185    60   -60   4.23 0.6065
WVFGRD96   38.0   180    55   -60   4.25 0.6228
WVFGRD96   39.0   180    55   -60   4.26 0.6371
WVFGRD96   40.0   180    55   -65   4.34 0.6369
WVFGRD96   41.0   180    55   -65   4.36 0.6484
WVFGRD96   42.0   180    55   -65   4.37 0.6586
WVFGRD96   43.0   180    55   -65   4.39 0.6667
WVFGRD96   44.0   180    55   -65   4.40 0.6728
WVFGRD96   45.0   175    55   -70   4.41 0.6782
WVFGRD96   46.0   175    55   -70   4.41 0.6804
WVFGRD96   47.0   175    55   -70   4.42 0.6828
WVFGRD96   48.0   175    55   -70   4.43 0.6833
WVFGRD96   49.0   175    55   -70   4.43 0.6841
WVFGRD96   50.0   175    55   -70   4.44 0.6830
WVFGRD96   51.0   175    55   -70   4.44 0.6817
WVFGRD96   52.0   175    55   -70   4.45 0.6801
WVFGRD96   53.0   175    55   -70   4.45 0.6767
WVFGRD96   54.0   175    55   -70   4.45 0.6742
WVFGRD96   55.0   175    55   -70   4.46 0.6696
WVFGRD96   56.0   175    55   -70   4.46 0.6647
WVFGRD96   57.0   175    55   -70   4.46 0.6603
WVFGRD96   58.0   175    55   -70   4.46 0.6536
WVFGRD96   59.0   175    55   -70   4.46 0.6493

The best solution is

WVFGRD96   49.0   175    55   -70   4.43 0.6841

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 -30 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 Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Iris stations and the Transportable Array of EarthScope.

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 Dec 1 07:26:38 CST 2018