Location

Location ANSS

2016/10/23 11:20:10 61.930 -151.717 92.7 3.9 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/10/23 11:20:10:0  61.93 -151.72  92.7 3.9 Alaska
 
 Stations used:
   AK.CAPN AK.CAST AK.DHY AK.KTH AK.MCK AK.RC01 AK.RND AK.SSN 
   AK.TRF TA.L19K TA.M19K TA.M22K TA.N18K TA.N19K TA.O18K 
   TA.O19K TA.O22K 
 
 Filtering commands used:
   cut o DIST/3.9 -40 o DIST/3.9 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.60e+22 dyne-cm
  Mw = 4.21 
  Z  = 134 km
  Plane   Strike  Dip  Rake
   NP1      350    88   115
   NP2       85    25     5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.60e+22     42     284
    N   0.00e+00     25     169
    P  -2.60e+22     38      58

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.62e+21
       Mxy    -1.06e+22
       Mxz    -3.50e+21
       Myy     1.89e+21
       Myz    -2.33e+22
       Mzz     1.74e+21
                                                     
                                                     
                                                     
                                                     
                     ####----------                  
                 ########--------------              
              ############----------------           
             #############-----------------          
           ###############-------------------        
          #################-------------------       
         ##################----------   -------      
        ###################---------- P --------     
        #######   ##########---------   --------     
       ######## T ##########---------------------    
       ########   ##########---------------------    
       ######################-------------------#    
       -#####################------------------##    
        #####################------------------#     
        --####################---------------###     
         --###################--------------###      
          ---#################------------####       
           ----###############----------#####        
             -----#############------######          
              ------------###--###########           
                 --------------########              
                     ----------####                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.74e+21  -3.50e+21   2.33e+22 
 -3.50e+21  -3.62e+21   1.06e+22 
  2.33e+22   1.06e+22   1.89e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20161023112010/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 = 85
      DIP = 25
     RAKE = 5
       MW = 4.21
       HS = 134.0

The NDK file is 20161023112010.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2016/10/23 11:20:10:0  61.93 -151.72  92.7 3.9 Alaska
 
 Stations used:
   AK.CAPN AK.CAST AK.DHY AK.KTH AK.MCK AK.RC01 AK.RND AK.SSN 
   AK.TRF TA.L19K TA.M19K TA.M22K TA.N18K TA.N19K TA.O18K 
   TA.O19K TA.O22K 
 
 Filtering commands used:
   cut o DIST/3.9 -40 o DIST/3.9 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.60e+22 dyne-cm
  Mw = 4.21 
  Z  = 134 km
  Plane   Strike  Dip  Rake
   NP1      350    88   115
   NP2       85    25     5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.60e+22     42     284
    N   0.00e+00     25     169
    P  -2.60e+22     38      58

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.62e+21
       Mxy    -1.06e+22
       Mxz    -3.50e+21
       Myy     1.89e+21
       Myz    -2.33e+22
       Mzz     1.74e+21
                                                     
                                                     
                                                     
                                                     
                     ####----------                  
                 ########--------------              
              ############----------------           
             #############-----------------          
           ###############-------------------        
          #################-------------------       
         ##################----------   -------      
        ###################---------- P --------     
        #######   ##########---------   --------     
       ######## T ##########---------------------    
       ########   ##########---------------------    
       ######################-------------------#    
       -#####################------------------##    
        #####################------------------#     
        --####################---------------###     
         --###################--------------###      
          ---#################------------####       
           ----###############----------#####        
             -----#############------######          
              ------------###--###########           
                 --------------########              
                     ----------####                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.74e+21  -3.50e+21   2.33e+22 
 -3.50e+21  -3.62e+21   1.06e+22 
  2.33e+22   1.06e+22   1.89e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20161023112010/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/3.9 -40 o DIST/3.9 +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    2.0   160    90     0   3.27 0.2805
WVFGRD96    4.0   160    85   -15   3.38 0.3282
WVFGRD96    6.0   160    85   -15   3.46 0.3494
WVFGRD96    8.0   160    80   -10   3.53 0.3560
WVFGRD96   10.0   160    85   -10   3.57 0.3425
WVFGRD96   12.0   160    80   -10   3.60 0.3176
WVFGRD96   14.0   160    85   -15   3.62 0.2885
WVFGRD96   16.0   245    70     0   3.62 0.2704
WVFGRD96   18.0   245    70     0   3.65 0.2757
WVFGRD96   20.0   245    70     0   3.68 0.2876
WVFGRD96   22.0   250    70     0   3.73 0.3057
WVFGRD96   24.0   250    75     0   3.76 0.3256
WVFGRD96   26.0   250    75     0   3.78 0.3435
WVFGRD96   28.0   250    80     0   3.80 0.3583
WVFGRD96   30.0   250    80     0   3.82 0.3723
WVFGRD96   32.0   245    75     5   3.82 0.3878
WVFGRD96   34.0   245    75     5   3.84 0.4062
WVFGRD96   36.0   250    80     5   3.88 0.4189
WVFGRD96   38.0   250    80     5   3.92 0.4325
WVFGRD96   40.0   250    85     0   3.97 0.4459
WVFGRD96   42.0   250    80     5   4.00 0.4487
WVFGRD96   44.0   250    80     0   4.03 0.4534
WVFGRD96   46.0   250    80     0   4.04 0.4544
WVFGRD96   48.0   250    85     0   4.06 0.4555
WVFGRD96   50.0   250    85     0   4.07 0.4577
WVFGRD96   52.0    70    85    -5   4.07 0.4641
WVFGRD96   54.0    70    80    -5   4.08 0.4707
WVFGRD96   56.0    70    80    -5   4.08 0.4791
WVFGRD96   58.0    70    75     0   4.09 0.4872
WVFGRD96   60.0    70    75     0   4.09 0.4934
WVFGRD96   62.0    70    75     0   4.09 0.4973
WVFGRD96   64.0    70    70     0   4.09 0.5022
WVFGRD96   66.0    70    70     0   4.10 0.5070
WVFGRD96   68.0    70    70     5   4.10 0.5098
WVFGRD96   70.0    70    70     5   4.10 0.5150
WVFGRD96   72.0    70    65     5   4.10 0.5174
WVFGRD96   74.0    70    65     5   4.10 0.5219
WVFGRD96   76.0    70    65    15   4.11 0.5229
WVFGRD96   78.0    70    65    15   4.11 0.5288
WVFGRD96   80.0    70    65    15   4.12 0.5320
WVFGRD96   82.0    70    60    15   4.11 0.5353
WVFGRD96   84.0    70    60    15   4.12 0.5390
WVFGRD96   86.0    70    60    15   4.12 0.5402
WVFGRD96   88.0    70    55    10   4.11 0.5396
WVFGRD96   90.0    75    50    15   4.13 0.5426
WVFGRD96   92.0    75    50    15   4.13 0.5463
WVFGRD96   94.0    75    50    15   4.13 0.5474
WVFGRD96   96.0    75    45    10   4.13 0.5469
WVFGRD96   98.0    75    45    10   4.13 0.5480
WVFGRD96  100.0    75    45    10   4.13 0.5518
WVFGRD96  102.0    75    40     5   4.14 0.5533
WVFGRD96  104.0    75    40     5   4.14 0.5547
WVFGRD96  106.0    75    40     5   4.14 0.5588
WVFGRD96  108.0    75    40     5   4.14 0.5599
WVFGRD96  110.0    80    30     5   4.17 0.5628
WVFGRD96  112.0    80    30     5   4.17 0.5670
WVFGRD96  114.0    80    30     5   4.17 0.5679
WVFGRD96  116.0    80    30     5   4.17 0.5716
WVFGRD96  118.0    85    25     5   4.20 0.5734
WVFGRD96  120.0    85    25     5   4.20 0.5739
WVFGRD96  122.0    85    25     5   4.20 0.5771
WVFGRD96  124.0    85    25     5   4.20 0.5756
WVFGRD96  126.0    85    25     5   4.20 0.5783
WVFGRD96  128.0    85    25     5   4.20 0.5773
WVFGRD96  130.0    85    25     5   4.21 0.5789
WVFGRD96  132.0    85    25     5   4.21 0.5791
WVFGRD96  134.0    85    25     5   4.21 0.5795
WVFGRD96  136.0    85    25     5   4.21 0.5790
WVFGRD96  138.0    85    25     5   4.21 0.5775

The best solution is

WVFGRD96  134.0    85    25     5   4.21 0.5795

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/3.9 -40 o DIST/3.9 +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 Sun Oct 23 10:05:38 CDT 2016