Location

Location ANSS

2018/12/01 07:07:38 61.505 -149.941 43.5 5.1 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2018/12/01 07:07:38:0  61.51 -149.94  43.5 5.1 Alaska
 
 Stations used:
   AK.BERG AK.BPAW AK.BRLK AK.CAST AK.CNP AK.CUT AK.DHY AK.DIV 
   AK.EYAK AK.FID AK.GHO AK.GLI AK.HDA AK.HOM AK.KLU AK.KNK 
   AK.KTH AK.PPLA AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN AK.SLK 
   AK.SSN AK.SWD AK.TRF AK.VRDI AK.WRH AT.SVW2 AV.ILSW AV.STLK 
   TA.L18K TA.M19K TA.M20K TA.M22K TA.M24K TA.N18K TA.N19K 
   TA.O18K TA.O19K TA.O22K TA.P18K TA.P19K 
 
 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 = 2.29e+23 dyne-cm
  Mw = 4.84 
  Z  = 50 km
  Plane   Strike  Dip  Rake
   NP1      210    80   -55
   NP2      314    36   -163
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.29e+23     27     273
    N   0.00e+00     34      23
    P  -2.29e+23     44     154

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.60e+22
       Mxy     3.69e+22
       Mxz     1.08e+23
       Myy     1.60e+23
       Myz    -1.41e+23
       Mzz    -6.42e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ---------------------#              
              ---#########----------######           
             ##################---#########          
           ######################-###########        
          ######################-----#########       
         #####################--------#########      
        #####################-----------########     
        ####################-------------#######     
       ####   #############---------------#######    
       #### T ############-----------------######    
       ####   ###########-------------------#####    
       #################--------------------#####    
        ###############---------------------####     
        ##############-----------------------###     
         ############-----------   ---------###      
          ##########------------ P ---------##       
           ########-------------   ---------#        
             ######------------------------          
              ####------------------------           
                 #---------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -6.42e+22   1.08e+23   1.41e+23 
  1.08e+23  -9.60e+22  -3.69e+22 
  1.41e+23  -3.69e+22   1.60e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201070738/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 = 210
      DIP = 80
     RAKE = -55
       MW = 4.84
       HS = 50.0

The NDK file is 20181201070738.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 07:07:28:0  61.40 -150.07  22.8 4.5 Alaska
 
 Stations used:
   AK.BERG AK.BPAW AK.BRLK AK.CAST AK.CNP AK.CUT AK.DHY AK.DIV 
   AK.EYAK AK.FID AK.GHO AK.GLI AK.HDA AK.HOM AK.KLU AK.KNK 
   AK.KTH AK.PPLA AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN AK.SLK 
   AK.SSN AK.SWD AK.TRF AK.VRDI AK.WRH AT.SVW2 AV.ILSW AV.STLK 
   TA.L18K TA.M19K TA.M20K TA.M22K TA.M24K TA.N18K TA.N19K 
   TA.O18K TA.O19K TA.O22K TA.P18K TA.P19K 
 
 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 = 2.29e+23 dyne-cm
  Mw = 4.84 
  Z  = 50 km
  Plane   Strike  Dip  Rake
   NP1      210    80   -55
   NP2      314    36   -163
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.29e+23     27     273
    N   0.00e+00     34      23
    P  -2.29e+23     44     154

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.60e+22
       Mxy     3.69e+22
       Mxz     1.08e+23
       Myy     1.60e+23
       Myz    -1.41e+23
       Mzz    -6.42e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ---------------------#              
              ---#########----------######           
             ##################---#########          
           ######################-###########        
          ######################-----#########       
         #####################--------#########      
        #####################-----------########     
        ####################-------------#######     
       ####   #############---------------#######    
       #### T ############-----------------######    
       ####   ###########-------------------#####    
       #################--------------------#####    
        ###############---------------------####     
        ##############-----------------------###     
         ############-----------   ---------###      
          ##########------------ P ---------##       
           ########-------------   ---------#        
             ######------------------------          
              ####------------------------           
                 #---------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -6.42e+22   1.08e+23   1.41e+23 
  1.08e+23  -9.60e+22  -3.69e+22 
  1.41e+23  -3.69e+22   1.60e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201070728/index.html
 USGS/SLU Moment Tensor Solution
 ENS  2018/12/01 07:07:38:0  61.51 -149.94  43.5 5.1 Alaska
 
 Stations used:
   AK.BERG AK.BPAW AK.BRLK AK.CAST AK.CNP AK.CUT AK.DHY AK.DIV 
   AK.EYAK AK.FID AK.GHO AK.GLI AK.HDA AK.HOM AK.KLU AK.KNK 
   AK.KTH AK.PPLA AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN AK.SLK 
   AK.SSN AK.SWD AK.TRF AK.VRDI AK.WRH AT.SVW2 AV.ILSW AV.STLK 
   TA.L18K TA.M19K TA.M20K TA.M22K TA.M24K TA.N18K TA.N19K 
   TA.O18K TA.O19K TA.O22K TA.P18K TA.P19K 
 
 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 = 2.29e+23 dyne-cm
  Mw = 4.84 
  Z  = 50 km
  Plane   Strike  Dip  Rake
   NP1      210    80   -55
   NP2      314    36   -163
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.29e+23     27     273
    N   0.00e+00     34      23
    P  -2.29e+23     44     154

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.60e+22
       Mxy     3.69e+22
       Mxz     1.08e+23
       Myy     1.60e+23
       Myz    -1.41e+23
       Mzz    -6.42e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ---------------------#              
              ---#########----------######           
             ##################---#########          
           ######################-###########        
          ######################-----#########       
         #####################--------#########      
        #####################-----------########     
        ####################-------------#######     
       ####   #############---------------#######    
       #### T ############-----------------######    
       ####   ###########-------------------#####    
       #################--------------------#####    
        ###############---------------------####     
        ##############-----------------------###     
         ############-----------   ---------###      
          ##########------------ P ---------##       
           ########-------------   ---------#        
             ######------------------------          
              ####------------------------           
                 #---------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -6.42e+22   1.08e+23   1.41e+23 
  1.08e+23  -9.60e+22  -3.69e+22 
  1.41e+23  -3.69e+22   1.60e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201070738/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   320    75    25   3.88 0.1850
WVFGRD96    2.0   330    60    45   4.06 0.2355
WVFGRD96    3.0   305    50     0   4.09 0.2353
WVFGRD96    4.0   305    55   -15   4.12 0.2477
WVFGRD96    5.0   305    55   -15   4.15 0.2588
WVFGRD96    6.0   300    50   -10   4.17 0.2705
WVFGRD96    7.0   305    60   -25   4.20 0.2829
WVFGRD96    8.0   300    50   -15   4.25 0.2888
WVFGRD96    9.0   300    45    -5   4.26 0.2936
WVFGRD96   10.0   305    45     5   4.27 0.2979
WVFGRD96   11.0   305    45     5   4.29 0.3007
WVFGRD96   12.0   305    45     5   4.30 0.3022
WVFGRD96   13.0   305    45    10   4.32 0.3022
WVFGRD96   14.0   305    45    10   4.33 0.3013
WVFGRD96   15.0   305    45    10   4.34 0.2990
WVFGRD96   16.0   305    45    10   4.35 0.2952
WVFGRD96   17.0   305    45    10   4.36 0.2904
WVFGRD96   18.0   305    45    10   4.37 0.2843
WVFGRD96   19.0   305    45    10   4.38 0.2777
WVFGRD96   20.0   305    45    10   4.39 0.2706
WVFGRD96   21.0   245    50    30   4.42 0.2664
WVFGRD96   22.0   245    50    30   4.43 0.2679
WVFGRD96   23.0   245    50    30   4.44 0.2695
WVFGRD96   24.0   240    50    25   4.46 0.2715
WVFGRD96   25.0   215    50   -45   4.46 0.2747
WVFGRD96   26.0   220    55   -35   4.47 0.2783
WVFGRD96   27.0    35    90    40   4.48 0.2871
WVFGRD96   28.0    30    90    45   4.49 0.3011
WVFGRD96   29.0   210    85   -45   4.51 0.3195
WVFGRD96   30.0    30    90    45   4.52 0.3366
WVFGRD96   31.0    30    90    45   4.54 0.3532
WVFGRD96   32.0    30    90    50   4.55 0.3684
WVFGRD96   33.0   205    80   -55   4.56 0.3894
WVFGRD96   34.0   205    80   -55   4.57 0.4044
WVFGRD96   35.0   205    80   -55   4.58 0.4168
WVFGRD96   36.0   205    80   -55   4.59 0.4304
WVFGRD96   37.0   205    80   -55   4.60 0.4499
WVFGRD96   38.0   205    80   -55   4.61 0.4686
WVFGRD96   39.0   205    80   -50   4.62 0.4876
WVFGRD96   40.0   205    80   -65   4.74 0.5085
WVFGRD96   41.0   205    80   -60   4.75 0.5355
WVFGRD96   42.0   205    80   -60   4.77 0.5605
WVFGRD96   43.0   205    80   -60   4.78 0.5851
WVFGRD96   44.0   205    80   -60   4.79 0.6070
WVFGRD96   45.0   205    80   -55   4.80 0.6263
WVFGRD96   46.0   210    80   -55   4.81 0.6419
WVFGRD96   47.0   210    80   -55   4.82 0.6538
WVFGRD96   48.0   210    80   -55   4.82 0.6615
WVFGRD96   49.0   210    80   -55   4.83 0.6643
WVFGRD96   50.0   210    80   -55   4.84 0.6643
WVFGRD96   51.0   205    75   -55   4.84 0.6629
WVFGRD96   52.0   205    75   -55   4.84 0.6624
WVFGRD96   53.0   205    75   -60   4.85 0.6605
WVFGRD96   54.0   205    75   -60   4.85 0.6589
WVFGRD96   55.0   205    75   -60   4.86 0.6560
WVFGRD96   56.0   205    75   -60   4.86 0.6519
WVFGRD96   57.0   205    75   -60   4.86 0.6499
WVFGRD96   58.0   205    75   -60   4.87 0.6452
WVFGRD96   59.0   205    75   -60   4.87 0.6401

The best solution is

WVFGRD96   50.0   210    80   -55   4.84 0.6643

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 07:58:15 CST 2018