Location

Location ANSS

2016/10/05 20:08:45 61.466 -147.209 19.6 4.1 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/10/05 20:08:45:0  61.47 -147.21  19.6 4.1 Alaska
 
 Stations used:
   AK.DHY AK.DIV AK.FID AK.FIRE AK.GLI AK.HIN AK.KLU AK.KNK 
   AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AT.PMR TA.M22K 
   TA.M24K TA.N25K 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 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.84e+22 dyne-cm
  Mw = 4.11 
  Z  = 94 km
  Plane   Strike  Dip  Rake
   NP1      160    90   -175
   NP2       70    85     0
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.84e+22      4     295
    N   0.00e+00     85     160
    P  -1.84e+22      4      25

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.18e+22
       Mxy    -1.40e+22
       Mxz    -5.49e+20
       Myy     1.18e+22
       Myz    -1.51e+21
       Mzz     0.00e+00
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 #####------------- P -              
              ########-------------   ----           
             ##########--------------------          
           #############---------------------        
           #############----------------------       
         T ##############----------------------      
           ###############--------------------##     
        ##################-----------------#####     
       ####################------------##########    
       #####################------###############    
       #####################-####################    
       #################-----####################    
        ##########------------##################     
        ###-------------------##################     
         ----------------------################      
          ----------------------##############       
           ---------------------#############        
             --------------------##########          
              --------------------########           
                 -----------------#####              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  0.00e+00  -5.49e+20   1.51e+21 
 -5.49e+20  -1.18e+22   1.40e+22 
  1.51e+21   1.40e+22   1.18e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20161005200845/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 = 70
      DIP = 85
     RAKE = 0
       MW = 4.11
       HS = 94.0

The NDK file is 20161005200845.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2016/10/05 20:08:45:0  61.47 -147.21  19.6 4.1 Alaska
 
 Stations used:
   AK.DHY AK.DIV AK.FID AK.FIRE AK.GLI AK.HIN AK.KLU AK.KNK 
   AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AT.PMR TA.M22K 
   TA.M24K TA.N25K 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 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.84e+22 dyne-cm
  Mw = 4.11 
  Z  = 94 km
  Plane   Strike  Dip  Rake
   NP1      160    90   -175
   NP2       70    85     0
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.84e+22      4     295
    N   0.00e+00     85     160
    P  -1.84e+22      4      25

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.18e+22
       Mxy    -1.40e+22
       Mxz    -5.49e+20
       Myy     1.18e+22
       Myz    -1.51e+21
       Mzz     0.00e+00
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 #####------------- P -              
              ########-------------   ----           
             ##########--------------------          
           #############---------------------        
           #############----------------------       
         T ##############----------------------      
           ###############--------------------##     
        ##################-----------------#####     
       ####################------------##########    
       #####################------###############    
       #####################-####################    
       #################-----####################    
        ##########------------##################     
        ###-------------------##################     
         ----------------------################      
          ----------------------##############       
           ---------------------#############        
             --------------------##########          
              --------------------########           
                 -----------------#####              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  0.00e+00  -5.49e+20   1.51e+21 
 -5.49e+20  -1.18e+22   1.40e+22 
  1.51e+21   1.40e+22   1.18e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20161005200845/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 0.10 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   155    90     5   3.21 0.3083
WVFGRD96    4.0   160    75    20   3.34 0.3739
WVFGRD96    6.0   160    80    20   3.41 0.4141
WVFGRD96    8.0   155    90    25   3.50 0.4440
WVFGRD96   10.0   155    90    20   3.55 0.4510
WVFGRD96   12.0   250    75    10   3.58 0.4444
WVFGRD96   14.0   250    75    10   3.61 0.4554
WVFGRD96   16.0   250    80    10   3.64 0.4596
WVFGRD96   18.0   250    85    10   3.67 0.4614
WVFGRD96   20.0   250    85    10   3.69 0.4624
WVFGRD96   22.0    70    90   -10   3.71 0.4608
WVFGRD96   24.0    70    75    15   3.74 0.4723
WVFGRD96   26.0    70    75    15   3.76 0.4859
WVFGRD96   28.0    70    75    15   3.77 0.4942
WVFGRD96   30.0    70    75    10   3.78 0.5016
WVFGRD96   32.0    65    70     0   3.80 0.5097
WVFGRD96   34.0    65    70    -5   3.82 0.5173
WVFGRD96   36.0    65    70    -5   3.83 0.5187
WVFGRD96   38.0    65    70    -5   3.85 0.5168
WVFGRD96   40.0    65    65    -5   3.91 0.5241
WVFGRD96   42.0    65    65    -5   3.93 0.5261
WVFGRD96   44.0    65    65    -5   3.95 0.5272
WVFGRD96   46.0    65    65    -5   3.96 0.5281
WVFGRD96   48.0    65    70    -5   3.97 0.5297
WVFGRD96   50.0    65    70    -5   3.98 0.5300
WVFGRD96   52.0    65    70     0   3.99 0.5316
WVFGRD96   54.0    65    70     0   4.00 0.5343
WVFGRD96   56.0    65    70     0   4.01 0.5344
WVFGRD96   58.0    65    70     0   4.02 0.5364
WVFGRD96   60.0    65    75     0   4.02 0.5377
WVFGRD96   62.0    65    75     0   4.03 0.5396
WVFGRD96   64.0    65    75     0   4.04 0.5422
WVFGRD96   66.0    65    75     0   4.04 0.5431
WVFGRD96   68.0    65    75     0   4.05 0.5435
WVFGRD96   70.0    65    75     0   4.05 0.5442
WVFGRD96   72.0    65    80     0   4.06 0.5454
WVFGRD96   74.0    65    80     0   4.06 0.5456
WVFGRD96   76.0    65    80     0   4.07 0.5476
WVFGRD96   78.0    65    80     0   4.07 0.5478
WVFGRD96   80.0    65    80     0   4.08 0.5479
WVFGRD96   82.0    65    80     0   4.08 0.5488
WVFGRD96   84.0    65    80     0   4.09 0.5490
WVFGRD96   86.0    65    80     0   4.09 0.5499
WVFGRD96   88.0    65    80     0   4.10 0.5502
WVFGRD96   90.0    65    80     0   4.10 0.5500
WVFGRD96   92.0    65    80     0   4.11 0.5504
WVFGRD96   94.0    70    85     0   4.11 0.5511
WVFGRD96   96.0    70    85     0   4.12 0.5499
WVFGRD96   98.0    65    85     0   4.13 0.5475

The best solution is

WVFGRD96   94.0    70    85     0   4.11 0.5511

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 0.10 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 Wed Oct 5 17:13:47 CDT 2016