Location

Location ANSS

2018/12/14 15:36:57 61.438 -151.622 86.5 3.8 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2018/12/14 15:36:57:0  61.44 -151.62  86.5 3.8 Alaska
 
 Stations used:
   AK.CUT AK.PWL AK.SKN AK.SLK AK.SSN IU.COLA TA.M22K TA.O22K 
 
 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 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 2.43e+22 dyne-cm
  Mw = 4.19 
  Z  = 90 km
  Plane   Strike  Dip  Rake
   NP1      215    85   -35
   NP2      308    55   -174
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.43e+22     20     267
    N   0.00e+00     55      28
    P  -2.43e+22     28     166

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.78e+22
       Mxy     5.64e+21
       Mxz     9.28e+21
       Myy     2.02e+22
       Myz    -1.02e+22
       Mzz    -2.42e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              -------------------------###           
             ------------------------######          
           #############-----------##########        
          ###################-----############       
         #######################-##############      
        #######################---##############     
        ######################------############     
       #####################----------###########    
       ###   ##############------------##########    
       ### T #############---------------########    
       ###   ############-----------------#######    
        ###############--------------------#####     
        ##############---------------------#####     
         ############-----------------------###      
          ##########------------------------##       
           ########-----------   -----------#        
             #####------------ P ----------          
              ###-------------   ---------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.42e+21   9.28e+21   1.02e+22 
  9.28e+21  -1.78e+22  -5.64e+21 
  1.02e+22  -5.64e+21   2.02e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181214153657/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 = 215
      DIP = 85
     RAKE = -35
       MW = 4.19
       HS = 90.0

The NDK file is 20181214153657.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2018/12/14 15:36:57:0  61.44 -151.62  86.5 3.8 Alaska
 
 Stations used:
   AK.CUT AK.PWL AK.SKN AK.SLK AK.SSN IU.COLA TA.M22K TA.O22K 
 
 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 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 2.43e+22 dyne-cm
  Mw = 4.19 
  Z  = 90 km
  Plane   Strike  Dip  Rake
   NP1      215    85   -35
   NP2      308    55   -174
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.43e+22     20     267
    N   0.00e+00     55      28
    P  -2.43e+22     28     166

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.78e+22
       Mxy     5.64e+21
       Mxz     9.28e+21
       Myy     2.02e+22
       Myz    -1.02e+22
       Mzz    -2.42e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              -------------------------###           
             ------------------------######          
           #############-----------##########        
          ###################-----############       
         #######################-##############      
        #######################---##############     
        ######################------############     
       #####################----------###########    
       ###   ##############------------##########    
       ### T #############---------------########    
       ###   ############-----------------#######    
        ###############--------------------#####     
        ##############---------------------#####     
         ############-----------------------###      
          ##########------------------------##       
           ########-----------   -----------#        
             #####------------ P ----------          
              ###-------------   ---------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.42e+21   9.28e+21   1.02e+22 
  9.28e+21  -1.78e+22  -5.64e+21 
  1.02e+22  -5.64e+21   2.02e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181214153657/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 0.08 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    2.0   125    70     5   3.38 0.2116
WVFGRD96    4.0   295    65   -35   3.51 0.2434
WVFGRD96    6.0   325    75   -40   3.54 0.2621
WVFGRD96    8.0   295    70   -40   3.63 0.2782
WVFGRD96   10.0   130    50    20   3.68 0.2792
WVFGRD96   12.0   130    50    20   3.71 0.2809
WVFGRD96   14.0   215    75    30   3.72 0.2819
WVFGRD96   16.0   215    75    30   3.75 0.2885
WVFGRD96   18.0   215    80    25   3.76 0.2933
WVFGRD96   20.0   215    75    20   3.78 0.3004
WVFGRD96   22.0   215    75    20   3.80 0.3065
WVFGRD96   24.0   215    80    20   3.83 0.3137
WVFGRD96   26.0   215    80    20   3.85 0.3216
WVFGRD96   28.0   215    85    25   3.88 0.3315
WVFGRD96   30.0   215    85    20   3.89 0.3441
WVFGRD96   32.0   215    85    15   3.91 0.3643
WVFGRD96   34.0    35    90   -10   3.93 0.3823
WVFGRD96   36.0    35    90    -5   3.95 0.3970
WVFGRD96   38.0    35    90    -5   3.99 0.4128
WVFGRD96   40.0   215    85    10   4.04 0.4313
WVFGRD96   42.0    35    90   -10   4.06 0.4315
WVFGRD96   44.0   215    85     5   4.08 0.4322
WVFGRD96   46.0   215    85     5   4.09 0.4347
WVFGRD96   48.0   215    85     5   4.11 0.4370
WVFGRD96   50.0   215    90     5   4.12 0.4406
WVFGRD96   52.0   215    90     0   4.13 0.4448
WVFGRD96   54.0   215    90     0   4.14 0.4509
WVFGRD96   56.0   215    90     0   4.15 0.4581
WVFGRD96   58.0   215    90     0   4.16 0.4651
WVFGRD96   60.0   215    90     0   4.17 0.4694
WVFGRD96   62.0   215    90     0   4.18 0.4736
WVFGRD96   64.0    35    90     5   4.18 0.4778
WVFGRD96   66.0   215    85   -25   4.16 0.4831
WVFGRD96   68.0    40    90    25   4.16 0.4847
WVFGRD96   70.0    40    90    30   4.16 0.4891
WVFGRD96   72.0   215    85   -30   4.17 0.4995
WVFGRD96   74.0   215    85   -30   4.18 0.5023
WVFGRD96   76.0   215    85   -30   4.18 0.5067
WVFGRD96   78.0   215    85   -30   4.18 0.5075
WVFGRD96   80.0   215    85   -30   4.18 0.5113
WVFGRD96   82.0   215    85   -35   4.18 0.5130
WVFGRD96   84.0   215    85   -35   4.19 0.5145
WVFGRD96   86.0   215    85   -35   4.19 0.5152
WVFGRD96   88.0   215    85   -35   4.19 0.5173
WVFGRD96   90.0   215    85   -35   4.19 0.5181
WVFGRD96   92.0   215    85   -35   4.20 0.5178
WVFGRD96   94.0   215    85   -35   4.20 0.5175
WVFGRD96   96.0   215    85   -35   4.20 0.5169
WVFGRD96   98.0   215    85   -35   4.20 0.5174
WVFGRD96  100.0   215    85   -35   4.20 0.5180
WVFGRD96  102.0   215    85   -40   4.20 0.5165
WVFGRD96  104.0   215    85   -40   4.21 0.5168
WVFGRD96  106.0   215    90   -40   4.21 0.5147
WVFGRD96  108.0   215    85   -40   4.21 0.5152
WVFGRD96  110.0   215    85   -40   4.21 0.5139
WVFGRD96  112.0   215    90   -40   4.21 0.5129
WVFGRD96  114.0   215    90   -40   4.22 0.5113
WVFGRD96  116.0   215    90   -40   4.22 0.5108
WVFGRD96  118.0   215    90   -40   4.22 0.5096
WVFGRD96  120.0   215    90   -40   4.22 0.5077
WVFGRD96  122.0   215    90   -40   4.22 0.5076
WVFGRD96  124.0   210    85   -45   4.23 0.5059
WVFGRD96  126.0   215    90   -40   4.23 0.5049
WVFGRD96  128.0   210    85   -45   4.23 0.5055
WVFGRD96  130.0   210    85   -45   4.24 0.5047
WVFGRD96  132.0   210    85   -45   4.24 0.5050
WVFGRD96  134.0   210    85   -45   4.24 0.5044
WVFGRD96  136.0   210    85   -45   4.24 0.5027
WVFGRD96  138.0   210    85   -45   4.25 0.5023
WVFGRD96  140.0   210    85   -45   4.25 0.5014
WVFGRD96  142.0   210    85   -45   4.25 0.5019
WVFGRD96  144.0   210    85   -45   4.25 0.5011
WVFGRD96  146.0   210    85   -45   4.25 0.4893
WVFGRD96  148.0   210    80   -40   4.26 0.4553

The best solution is

WVFGRD96   90.0   215    85   -35   4.19 0.5181

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 0.08 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 Fri Dec 14 10:14:34 CST 2018