Location

Location ANSS

2016/10/22 01:06:33 62.518 -151.236 94.4 3.9 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/10/22 01:06:33:0  62.52 -151.24  94.4 3.9 Alaska
 
 Stations used:
   AK.BPAW AK.BWN AK.DHY AK.KLU AK.KNK AK.KTH AK.MCK AK.NEA2 
   AK.RC01 AK.SAW AK.SCM AK.SKN AK.TRF AK.WRH AT.PMR AT.TTA 
   TA.I21K TA.I23K TA.L19K TA.M19K TA.M20K TA.N19K 
 
 Filtering commands used:
   cut o DIST/3.6 -30 o DIST/3.6 +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 = 1.60e+22 dyne-cm
  Mw = 4.07 
  Z  = 102 km
  Plane   Strike  Dip  Rake
   NP1      349    58   138
   NP2      105    55    40
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.60e+22     51     315
    N   0.00e+00     39     139
    P  -1.60e+22      2      48

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.01e+21
       Mxy    -1.11e+22
       Mxz     5.23e+21
       Myy    -5.68e+21
       Myz    -5.89e+21
       Mzz     9.68e+21
                                                     
                                                     
                                                     
                                                     
                     #####---------                  
                 ###########-----------              
              ################-----------            
             ##################---------- P          
           #####################---------   -        
          #######################-------------       
         ###########   ###########-------------      
        ############ T ############-------------     
        ############   ############-------------     
       --###########################-------------    
       ---##########################-------------    
       -----########################-------------    
       -------######################-------------    
        ---------####################-----------     
        ------------#################----------#     
         ----------------###########-------####      
          --------------------------##########       
           -------------------------#########        
             ----------------------########          
              ---------------------#######           
                 -----------------#####              
                     ------------##                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  9.68e+21   5.23e+21   5.89e+21 
  5.23e+21  -4.01e+21   1.11e+22 
  5.89e+21   1.11e+22  -5.68e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20161022010633/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 = 105
      DIP = 55
     RAKE = 40
       MW = 4.07
       HS = 102.0

The NDK file is 20161022010633.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2016/10/22 01:06:33:0  62.52 -151.24  94.4 3.9 Alaska
 
 Stations used:
   AK.BPAW AK.BWN AK.DHY AK.KLU AK.KNK AK.KTH AK.MCK AK.NEA2 
   AK.RC01 AK.SAW AK.SCM AK.SKN AK.TRF AK.WRH AT.PMR AT.TTA 
   TA.I21K TA.I23K TA.L19K TA.M19K TA.M20K TA.N19K 
 
 Filtering commands used:
   cut o DIST/3.6 -30 o DIST/3.6 +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 = 1.60e+22 dyne-cm
  Mw = 4.07 
  Z  = 102 km
  Plane   Strike  Dip  Rake
   NP1      349    58   138
   NP2      105    55    40
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.60e+22     51     315
    N   0.00e+00     39     139
    P  -1.60e+22      2      48

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.01e+21
       Mxy    -1.11e+22
       Mxz     5.23e+21
       Myy    -5.68e+21
       Myz    -5.89e+21
       Mzz     9.68e+21
                                                     
                                                     
                                                     
                                                     
                     #####---------                  
                 ###########-----------              
              ################-----------            
             ##################---------- P          
           #####################---------   -        
          #######################-------------       
         ###########   ###########-------------      
        ############ T ############-------------     
        ############   ############-------------     
       --###########################-------------    
       ---##########################-------------    
       -----########################-------------    
       -------######################-------------    
        ---------####################-----------     
        ------------#################----------#     
         ----------------###########-------####      
          --------------------------##########       
           -------------------------#########        
             ----------------------########          
              ---------------------#######           
                 -----------------#####              
                     ------------##                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  9.68e+21   5.23e+21   5.89e+21 
  5.23e+21  -4.01e+21   1.11e+22 
  5.89e+21   1.11e+22  -5.68e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20161022010633/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/3.6 -30 o DIST/3.6 +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    2.0   170    60   -30   3.26 0.3508
WVFGRD96    4.0     0    45    -5   3.36 0.3536
WVFGRD96    6.0   275    90   -50   3.39 0.3919
WVFGRD96    8.0   320    40   -85   3.53 0.4288
WVFGRD96   10.0   155    50   -65   3.51 0.4221
WVFGRD96   12.0   105    65    45   3.48 0.4276
WVFGRD96   14.0   100    80    40   3.49 0.4351
WVFGRD96   16.0   100    80    40   3.51 0.4415
WVFGRD96   18.0   100    80    40   3.53 0.4451
WVFGRD96   20.0   100    80    40   3.55 0.4460
WVFGRD96   22.0   100    80    40   3.58 0.4427
WVFGRD96   24.0   100    85    45   3.59 0.4366
WVFGRD96   26.0   275    80   -45   3.62 0.4432
WVFGRD96   28.0   275    80   -45   3.65 0.4446
WVFGRD96   30.0   270    80   -45   3.67 0.4467
WVFGRD96   32.0   270    80   -45   3.69 0.4504
WVFGRD96   34.0   270    80   -45   3.71 0.4529
WVFGRD96   36.0   270    75   -45   3.74 0.4563
WVFGRD96   38.0   275    85   -35   3.75 0.4683
WVFGRD96   40.0    95    90    45   3.84 0.4816
WVFGRD96   42.0    95    80    40   3.85 0.4951
WVFGRD96   44.0    95    75    40   3.87 0.5124
WVFGRD96   46.0    95    75    40   3.88 0.5249
WVFGRD96   48.0   100    70    40   3.89 0.5379
WVFGRD96   50.0   100    60    40   3.91 0.5535
WVFGRD96   52.0   105    55    45   3.92 0.5696
WVFGRD96   54.0   105    55    45   3.94 0.5851
WVFGRD96   56.0   100    55    40   3.95 0.6001
WVFGRD96   58.0   100    55    40   3.96 0.6135
WVFGRD96   60.0   100    55    40   3.97 0.6238
WVFGRD96   62.0   100    55    40   3.97 0.6321
WVFGRD96   64.0   100    55    35   3.98 0.6388
WVFGRD96   66.0   100    50    35   3.99 0.6483
WVFGRD96   68.0   100    50    35   4.00 0.6575
WVFGRD96   70.0   100    50    35   4.00 0.6662
WVFGRD96   72.0   100    50    35   4.01 0.6733
WVFGRD96   74.0   100    50    35   4.01 0.6827
WVFGRD96   76.0   100    55    40   4.02 0.6933
WVFGRD96   78.0   100    55    40   4.02 0.7036
WVFGRD96   80.0   100    55    40   4.03 0.7117
WVFGRD96   82.0   110    50    40   4.03 0.7193
WVFGRD96   84.0   110    50    40   4.04 0.7300
WVFGRD96   86.0   110    50    40   4.04 0.7381
WVFGRD96   88.0   110    50    40   4.05 0.7426
WVFGRD96   90.0   110    50    40   4.05 0.7494
WVFGRD96   92.0   110    50    40   4.05 0.7548
WVFGRD96   94.0   105    55    40   4.06 0.7561
WVFGRD96   96.0   105    55    40   4.06 0.7611
WVFGRD96   98.0   105    55    40   4.07 0.7637
WVFGRD96  100.0   105    55    40   4.07 0.7640
WVFGRD96  102.0   105    55    40   4.07 0.7668
WVFGRD96  104.0   105    55    40   4.08 0.7647
WVFGRD96  106.0   105    50    35   4.08 0.7656
WVFGRD96  108.0   105    50    35   4.08 0.7640
WVFGRD96  110.0   105    50    35   4.08 0.7623
WVFGRD96  112.0   105    50    35   4.08 0.7595
WVFGRD96  114.0   105    50    35   4.09 0.7559
WVFGRD96  116.0   105    50    35   4.09 0.7533
WVFGRD96  118.0   105    50    35   4.09 0.7490

The best solution is

WVFGRD96  102.0   105    55    40   4.07 0.7668

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/3.6 -30 o DIST/3.6 +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 Sat Oct 22 09:49:35 CDT 2016