Location

Location ANSS

2017/12/19 00:10:15 59.860 -136.624 1.2 4.2 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/12/19 00:10:15:0  59.86 -136.62   1.2 4.2 Alaska
 
 Stations used:
   AK.BARN AK.BCP AK.CTG AK.GLB AK.HMT AK.ISLE AK.JIS AK.LOGN 
   AK.MCAR AK.MESA AK.PIN AK.SSP AK.TABL AK.VRDI AK.WAX AT.SIT 
   AT.SKAG AT.YKU2 CN.DLBC CN.HYT CN.WHY NY.FARO NY.MAYO 
   NY.WTLY TA.M30M TA.M31M TA.N31M TA.O30N TA.P32M TA.P33M 
   TA.R32K TA.R33M TA.S31K TA.S34M 
 
 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 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 1.26e+22 dyne-cm
  Mw = 4.00 
  Z  = 10 km
  Plane   Strike  Dip  Rake
   NP1      155    60    90
   NP2      335    30    90
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.26e+22     75      65
    N   0.00e+00     -0     335
    P  -1.26e+22     15     245

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.95e+21
       Mxy    -4.18e+21
       Mxz     2.66e+21
       Myy    -8.96e+21
       Myz     5.70e+21
       Mzz     1.09e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 --###########---------              
              ----###############---------           
             -----##################-------          
           -------###################--------        
          --------#####################-------       
         ---------######################-------      
        ----------#######################-------     
        ----------########################------     
       ------------##########   ###########------    
       ------------########## T ###########------    
       -------------#########   ###########------    
       --------------######################------    
        --   ---------#####################-----     
        -- P ----------####################-----     
         -   -----------###################----      
          ---------------#################----       
           ----------------###############---        
             ----------------############--          
              -----------------#########--           
                 -----------------####-              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.09e+22   2.66e+21  -5.70e+21 
  2.66e+21  -1.95e+21   4.18e+21 
 -5.70e+21   4.18e+21  -8.96e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171219001015/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 = 335
      DIP = 30
     RAKE = 90
       MW = 4.00
       HS = 10.0

The NDK file is 20171219001015.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/12/19 00:10:15:0  59.86 -136.62   1.2 4.2 Alaska
 
 Stations used:
   AK.BARN AK.BCP AK.CTG AK.GLB AK.HMT AK.ISLE AK.JIS AK.LOGN 
   AK.MCAR AK.MESA AK.PIN AK.SSP AK.TABL AK.VRDI AK.WAX AT.SIT 
   AT.SKAG AT.YKU2 CN.DLBC CN.HYT CN.WHY NY.FARO NY.MAYO 
   NY.WTLY TA.M30M TA.M31M TA.N31M TA.O30N TA.P32M TA.P33M 
   TA.R32K TA.R33M TA.S31K TA.S34M 
 
 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 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 1.26e+22 dyne-cm
  Mw = 4.00 
  Z  = 10 km
  Plane   Strike  Dip  Rake
   NP1      155    60    90
   NP2      335    30    90
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.26e+22     75      65
    N   0.00e+00     -0     335
    P  -1.26e+22     15     245

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.95e+21
       Mxy    -4.18e+21
       Mxz     2.66e+21
       Myy    -8.96e+21
       Myz     5.70e+21
       Mzz     1.09e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 --###########---------              
              ----###############---------           
             -----##################-------          
           -------###################--------        
          --------#####################-------       
         ---------######################-------      
        ----------#######################-------     
        ----------########################------     
       ------------##########   ###########------    
       ------------########## T ###########------    
       -------------#########   ###########------    
       --------------######################------    
        --   ---------#####################-----     
        -- P ----------####################-----     
         -   -----------###################----      
          ---------------#################----       
           ----------------###############---        
             ----------------############--          
              -----------------#########--           
                 -----------------####-              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.09e+22   2.66e+21  -5.70e+21 
  2.66e+21  -1.95e+21   4.18e+21 
 -5.70e+21   4.18e+21  -8.96e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171219001015/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 
br c 0.12 0.25 n 4 p 2
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   335    45    85   3.64 0.3224
WVFGRD96    2.0   335    45    85   3.80 0.4812
WVFGRD96    3.0   130    55    40   3.79 0.4343
WVFGRD96    4.0   295    40     5   3.83 0.4596
WVFGRD96    5.0   290    25    30   3.91 0.5122
WVFGRD96    6.0   315    25    65   3.93 0.5575
WVFGRD96    7.0   325    25    80   3.94 0.5917
WVFGRD96    8.0   330    25    85   4.00 0.6168
WVFGRD96    9.0   330    25    85   4.00 0.6279
WVFGRD96   10.0   335    30    90   4.00 0.6334
WVFGRD96   11.0   335    30    90   4.00 0.6312
WVFGRD96   12.0   335    30    90   3.99 0.6250
WVFGRD96   13.0   310    35    55   3.99 0.6187
WVFGRD96   14.0   305    40    45   3.99 0.6156
WVFGRD96   15.0   300    45    35   3.99 0.6114
WVFGRD96   16.0   280    55   -30   3.99 0.6052
WVFGRD96   17.0   300    45    35   4.01 0.6001
WVFGRD96   18.0   300    45    35   4.01 0.5925
WVFGRD96   19.0   295    50    30   4.03 0.5841
WVFGRD96   20.0   295    50    30   4.03 0.5741
WVFGRD96   21.0   300    50    35   4.05 0.5641
WVFGRD96   22.0   300    50    35   4.06 0.5516
WVFGRD96   23.0   300    50    35   4.06 0.5378
WVFGRD96   24.0   125    55    40   4.06 0.5264
WVFGRD96   25.0   125    50    40   4.07 0.5136
WVFGRD96   26.0   125    50    40   4.07 0.4999
WVFGRD96   27.0   125    50    40   4.08 0.4856
WVFGRD96   28.0   125    50    40   4.08 0.4707
WVFGRD96   29.0   125    50    40   4.09 0.4561

The best solution is

WVFGRD96   10.0   335    30    90   4.00 0.6334

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 
br c 0.12 0.25 n 4 p 2
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 18 18:50:44 CST 2017