Location

Location ANSS

2018/12/03 12:18:58 61.421 -149.983 37.0 3.8 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2018/12/03 12:18:58:0  61.42 -149.98  37.0 3.8 Alaska
 
 Stations used:
   AK.GHO AK.KNK AK.RC01 AK.SAW AK.SKN AK.SSN TA.M20K TA.M22K 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.10e+22 dyne-cm
  Mw = 3.96 
  Z  = 50 km
  Plane   Strike  Dip  Rake
   NP1      175    50   -85
   NP2      347    40   -96
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.10e+22      5     261
    N   0.00e+00      4     352
    P  -1.10e+22     84     120

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.09e+20
       Mxy     1.65e+21
       Mxz     4.47e+20
       Myy     1.05e+22
       Myz    -1.94e+21
       Mzz    -1.08e+22
                                                     
                                                     
                                                     
                                                     
                     ####-#########                  
                 ######-------#########              
              ########-----------#########           
             ########-------------#########          
           #########----------------#########        
          #########------------------#########       
         ##########-------------------#########      
        ##########---------------------#########     
        ##########----------------------########     
       ###########----------------------#########    
       ###########----------   ---------#########    
          ########---------- P ----------########    
        T ########----------   ----------########    
          #########----------------------#######     
        ###########---------------------########     
         ###########--------------------#######      
          ##########--------------------######       
           ##########------------------######        
             #########----------------#####          
              ##########-------------#####           
                 ########-----------###              
                     #######------#                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.08e+22   4.47e+20   1.94e+21 
  4.47e+20   2.09e+20  -1.65e+21 
  1.94e+21  -1.65e+21   1.05e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181203121858/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 = 50
     RAKE = -85
       MW = 3.96
       HS = 50.0

The NDK file is 20181203121858.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2018/12/03 12:18:58:0  61.42 -149.98  37.0 3.8 Alaska
 
 Stations used:
   AK.GHO AK.KNK AK.RC01 AK.SAW AK.SKN AK.SSN TA.M20K TA.M22K 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.10e+22 dyne-cm
  Mw = 3.96 
  Z  = 50 km
  Plane   Strike  Dip  Rake
   NP1      175    50   -85
   NP2      347    40   -96
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.10e+22      5     261
    N   0.00e+00      4     352
    P  -1.10e+22     84     120

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.09e+20
       Mxy     1.65e+21
       Mxz     4.47e+20
       Myy     1.05e+22
       Myz    -1.94e+21
       Mzz    -1.08e+22
                                                     
                                                     
                                                     
                                                     
                     ####-#########                  
                 ######-------#########              
              ########-----------#########           
             ########-------------#########          
           #########----------------#########        
          #########------------------#########       
         ##########-------------------#########      
        ##########---------------------#########     
        ##########----------------------########     
       ###########----------------------#########    
       ###########----------   ---------#########    
          ########---------- P ----------########    
        T ########----------   ----------########    
          #########----------------------#######     
        ###########---------------------########     
         ###########--------------------#######      
          ##########--------------------######       
           ##########------------------######        
             #########----------------#####          
              ##########-------------#####           
                 ########-----------###              
                     #######------#                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.08e+22   4.47e+20   1.94e+21 
  4.47e+20   2.09e+20  -1.65e+21 
  1.94e+21  -1.65e+21   1.05e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181203121858/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 +40
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   185    45    95   3.10 0.1741
WVFGRD96    2.0    10    45    90   3.27 0.2306
WVFGRD96    3.0   340    25    65   3.33 0.2025
WVFGRD96    4.0     5    85    65   3.37 0.2583
WVFGRD96    5.0    15    80    75   3.37 0.3020
WVFGRD96    6.0   160    15    50   3.37 0.3348
WVFGRD96    7.0   145    20    35   3.38 0.3589
WVFGRD96    8.0   170    15    60   3.47 0.3762
WVFGRD96    9.0   155    20    45   3.48 0.3909
WVFGRD96   10.0   165    20    55   3.49 0.4001
WVFGRD96   11.0   165    25    55   3.50 0.4067
WVFGRD96   12.0   175    25    65   3.52 0.4122
WVFGRD96   13.0   175    25    65   3.53 0.4144
WVFGRD96   14.0   170    25    60   3.54 0.4139
WVFGRD96   15.0   170    25    60   3.55 0.4110
WVFGRD96   16.0   170    25    60   3.56 0.4063
WVFGRD96   17.0   170    25    60   3.57 0.4002
WVFGRD96   18.0   170    20    60   3.57 0.3934
WVFGRD96   19.0   170    20    60   3.58 0.3861
WVFGRD96   20.0   165    20    55   3.59 0.3772
WVFGRD96   21.0    10    80    45   3.68 0.3718
WVFGRD96   22.0    10    80    45   3.69 0.3670
WVFGRD96   23.0    25    85    65   3.65 0.3615
WVFGRD96   24.0   200    90   -65   3.65 0.3575
WVFGRD96   25.0    25    85    65   3.67 0.3564
WVFGRD96   26.0   200    90   -65   3.68 0.3534
WVFGRD96   27.0    25    85    60   3.69 0.3521
WVFGRD96   28.0   200    75   -65   3.69 0.3560
WVFGRD96   29.0   205    75   -65   3.69 0.3625
WVFGRD96   30.0   200    75   -65   3.71 0.3685
WVFGRD96   31.0   200    70   -65   3.72 0.3788
WVFGRD96   32.0     5    30   -85   3.74 0.3927
WVFGRD96   33.0   180    60   -95   3.75 0.4129
WVFGRD96   34.0     5    30   -85   3.76 0.4329
WVFGRD96   35.0     5    30   -85   3.76 0.4473
WVFGRD96   36.0   185    50   -80   3.77 0.4671
WVFGRD96   37.0   185    50   -80   3.77 0.4831
WVFGRD96   38.0   185    50   -80   3.78 0.4987
WVFGRD96   39.0   185    50   -80   3.80 0.5109
WVFGRD96   40.0   180    50   -80   3.88 0.5142
WVFGRD96   41.0   180    50   -80   3.90 0.5225
WVFGRD96   42.0   180    50   -80   3.91 0.5303
WVFGRD96   43.0   180    50   -80   3.92 0.5387
WVFGRD96   44.0   180    50   -80   3.93 0.5455
WVFGRD96   45.0   180    50   -80   3.93 0.5528
WVFGRD96   46.0   180    50   -80   3.94 0.5563
WVFGRD96   47.0   180    50   -80   3.94 0.5612
WVFGRD96   48.0   175    50   -80   3.96 0.5632
WVFGRD96   49.0   175    50   -85   3.96 0.5657
WVFGRD96   50.0   175    50   -85   3.96 0.5665
WVFGRD96   51.0   175    50   -85   3.96 0.5657
WVFGRD96   52.0   345    40  -100   3.97 0.5663
WVFGRD96   53.0   175    50   -85   3.97 0.5645
WVFGRD96   54.0   175    50   -85   3.97 0.5658
WVFGRD96   55.0   350    40   -95   3.97 0.5646
WVFGRD96   56.0   170    50   -95   3.98 0.5642
WVFGRD96   57.0     0    40   -80   3.99 0.5639
WVFGRD96   58.0     0    40   -80   3.99 0.5627
WVFGRD96   59.0     5    40   -75   4.00 0.5634

The best solution is

WVFGRD96   50.0   175    50   -85   3.96 0.5665

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 +40
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 Mon Dec 3 12:23:30 CST 2018