Location

Location ANSS

2017/02/19 22:17:29 59.787 -153.152 103.5 4.0 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/02/19 22:17:29:0  59.79 -153.15 103.5 4.0 Alaska
 
 Stations used:
   AK.CAPN AK.CNP AK.RC01 AK.SKN AK.SSN AT.SVW2 AV.ILSW 
   TA.M19K TA.O19K TA.O22K TA.P18K TA.P19K TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.5 -40 o DIST/3.5 +60
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.55e+22 dyne-cm
  Mw = 4.06 
  Z  = 120 km
  Plane   Strike  Dip  Rake
   NP1      308    71   159
   NP2       45    70    20
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.55e+22     28     266
    N   0.00e+00     62      88
    P  -1.55e+22      1     357

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.54e+22
       Mxy     1.70e+21
       Mxz    -6.50e+20
       Myy     1.20e+22
       Myz    -6.39e+21
       Mzz     3.41e+21
                                                     
                                                     
                                                     
                                                     
                     ---- P -------                  
                 --------   -----------              
              ----------------------------           
             ------------------------------          
           #####--------------------------###        
          ###########--------------------#####       
         ###############-----------------######      
        ###################-------------########     
        ######################--------##########     
       #########################-----############    
       ####   ####################--#############    
       #### T ####################--#############    
       ####   ##################------###########    
        #######################---------########     
        #####################------------#######     
         #################----------------#####      
          ##############-------------------###       
           ##########-----------------------#        
             ###---------------------------          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  3.41e+21  -6.50e+20   6.39e+21 
 -6.50e+20  -1.54e+22  -1.70e+21 
  6.39e+21  -1.70e+21   1.20e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170219221729/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 = 45
      DIP = 70
     RAKE = 20
       MW = 4.06
       HS = 120.0

The NDK file is 20170219221729.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/02/19 22:17:29:0  59.79 -153.15 103.5 4.0 Alaska
 
 Stations used:
   AK.CAPN AK.CNP AK.RC01 AK.SKN AK.SSN AT.SVW2 AV.ILSW 
   TA.M19K TA.O19K TA.O22K TA.P18K TA.P19K TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.5 -40 o DIST/3.5 +60
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.55e+22 dyne-cm
  Mw = 4.06 
  Z  = 120 km
  Plane   Strike  Dip  Rake
   NP1      308    71   159
   NP2       45    70    20
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.55e+22     28     266
    N   0.00e+00     62      88
    P  -1.55e+22      1     357

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.54e+22
       Mxy     1.70e+21
       Mxz    -6.50e+20
       Myy     1.20e+22
       Myz    -6.39e+21
       Mzz     3.41e+21
                                                     
                                                     
                                                     
                                                     
                     ---- P -------                  
                 --------   -----------              
              ----------------------------           
             ------------------------------          
           #####--------------------------###        
          ###########--------------------#####       
         ###############-----------------######      
        ###################-------------########     
        ######################--------##########     
       #########################-----############    
       ####   ####################--#############    
       #### T ####################--#############    
       ####   ##################------###########    
        #######################---------########     
        #####################------------#######     
         #################----------------#####      
          ##############-------------------###       
           ##########-----------------------#        
             ###---------------------------          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  3.41e+21  -6.50e+20   6.39e+21 
 -6.50e+20  -1.54e+22  -1.70e+21 
  6.39e+21  -1.70e+21   1.20e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170219221729/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/3.5 -40 o DIST/3.5 +60
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    2.0   225    70   -20   3.03 0.1106
WVFGRD96    4.0   220    55   -25   3.17 0.1311
WVFGRD96    6.0   220    55   -15   3.22 0.1521
WVFGRD96    8.0   325    70    35   3.32 0.1637
WVFGRD96   10.0   320    65    25   3.35 0.1734
WVFGRD96   12.0   320    60    15   3.39 0.1732
WVFGRD96   14.0   315    55   -10   3.41 0.1699
WVFGRD96   16.0   315    55   -10   3.43 0.1598
WVFGRD96   18.0   315    50   -10   3.43 0.1469
WVFGRD96   20.0   315    50   -10   3.44 0.1337
WVFGRD96   22.0   315    50   -10   3.45 0.1212
WVFGRD96   24.0    50    80    -5   3.48 0.1160
WVFGRD96   26.0    50    80    -5   3.51 0.1269
WVFGRD96   28.0    50    85    -5   3.54 0.1398
WVFGRD96   30.0   230    90    10   3.56 0.1537
WVFGRD96   32.0    50    85   -10   3.59 0.1645
WVFGRD96   34.0    50    85   -10   3.61 0.1708
WVFGRD96   36.0    50    85   -10   3.64 0.1729
WVFGRD96   38.0   230    90     5   3.67 0.1750
WVFGRD96   40.0    50    90    -5   3.73 0.1788
WVFGRD96   42.0    50    90    -5   3.75 0.1777
WVFGRD96   44.0   230    90     5   3.78 0.1760
WVFGRD96   46.0   230    80    -5   3.80 0.1746
WVFGRD96   48.0   230    80    -5   3.82 0.1761
WVFGRD96   50.0   230    80    -5   3.84 0.1784
WVFGRD96   52.0   230    80    -5   3.85 0.1820
WVFGRD96   54.0   230    80    -5   3.87 0.1904
WVFGRD96   56.0    50    80    25   3.89 0.2142
WVFGRD96   58.0    50    80    25   3.91 0.2383
WVFGRD96   60.0    50    75    25   3.92 0.2551
WVFGRD96   62.0    50    75    25   3.94 0.2660
WVFGRD96   64.0    50    75    25   3.95 0.2758
WVFGRD96   66.0    50    75    25   3.96 0.2859
WVFGRD96   68.0    50    75    25   3.97 0.2953
WVFGRD96   70.0    50    75    25   3.98 0.3035
WVFGRD96   72.0    50    75    25   3.98 0.3125
WVFGRD96   74.0    50    75    25   3.99 0.3193
WVFGRD96   76.0    50    75    25   4.00 0.3256
WVFGRD96   78.0    50    75    30   4.00 0.3323
WVFGRD96   80.0    50    75    25   4.01 0.3357
WVFGRD96   82.0    50    70    30   4.01 0.3415
WVFGRD96   84.0    50    70    30   4.01 0.3451
WVFGRD96   86.0    50    70    30   4.01 0.3496
WVFGRD96   88.0    50    65    30   4.01 0.3540
WVFGRD96   90.0    50    65    30   4.02 0.3565
WVFGRD96   92.0    50    65    30   4.02 0.3612
WVFGRD96   94.0    50    65    30   4.02 0.3643
WVFGRD96   96.0    50    65    30   4.02 0.3653
WVFGRD96   98.0    50    65    30   4.03 0.3676
WVFGRD96  100.0    45    70    25   4.04 0.3694
WVFGRD96  102.0    45    70    25   4.04 0.3706
WVFGRD96  104.0    45    70    25   4.05 0.3715
WVFGRD96  106.0    45    70    25   4.05 0.3723
WVFGRD96  108.0    45    70    25   4.05 0.3744
WVFGRD96  110.0    45    70    25   4.05 0.3758
WVFGRD96  112.0    45    70    25   4.05 0.3766
WVFGRD96  114.0    45    70    25   4.06 0.3765
WVFGRD96  116.0    45    70    25   4.06 0.3769
WVFGRD96  118.0    45    70    20   4.06 0.3773
WVFGRD96  120.0    45    70    20   4.06 0.3778
WVFGRD96  122.0    45    70    20   4.07 0.3776
WVFGRD96  124.0    45    70    20   4.07 0.3771
WVFGRD96  126.0    45    70    20   4.07 0.3773
WVFGRD96  128.0    45    70    20   4.07 0.3772
WVFGRD96  130.0    45    70    20   4.07 0.3765
WVFGRD96  132.0    55    65    25   4.05 0.3769
WVFGRD96  134.0    55    65    25   4.05 0.3777
WVFGRD96  136.0    55    65    25   4.05 0.3774
WVFGRD96  138.0    55    65    25   4.06 0.3774

The best solution is

WVFGRD96  120.0    45    70    20   4.06 0.3778

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/3.5 -40 o DIST/3.5 +60
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 Sun Feb 19 18:15:32 CST 2017