Location

Location ANSS

2017/07/08 00:29:29 59.579 -152.660 77.9 3.9 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/07/08 00:29:29:0  59.58 -152.66  77.9 3.9 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.HOM AK.RC01 AK.SWD II.KDAK TA.N18K 
   TA.O18K TA.O22K TA.P18K TA.P19K 
 
 Filtering commands used:
   cut o DIST/3.6 -30 o DIST/3.6 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 7.76e+21 dyne-cm
  Mw = 3.86 
  Z  = 84 km
  Plane   Strike  Dip  Rake
   NP1      339    77   128
   NP2       85    40    20
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.76e+21     44     287
    N   0.00e+00     37     150
    P  -7.76e+21     23      41

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.41e+21
       Mxy    -4.39e+21
       Mxz    -9.46e+20
       Myy     7.94e+20
       Myz    -5.53e+21
       Mzz     2.61e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 #####-----------------              
              ##########------------------           
             ############------------   ---          
           ###############----------- P -----        
          #################----------   ------       
         ###################-------------------      
        #####################-------------------     
        ########   ###########------------------     
       ######### T ###########-------------------    
       #########   ############-----------------#    
       #########################----------------#    
       -########################--------------###    
        -########################------------###     
        ---#######################---------#####     
         ----#####################-------######      
          ------##################---#########       
           -----------##########---##########        
             ----------------------########          
              ---------------------#######           
                 ------------------####              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.61e+21  -9.46e+20   5.53e+21 
 -9.46e+20  -3.41e+21   4.39e+21 
  5.53e+21   4.39e+21   7.94e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170708002929/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 = 40
     RAKE = 20
       MW = 3.86
       HS = 84.0

The NDK file is 20170708002929.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/07/08 00:29:29:0  59.58 -152.66  77.9 3.9 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.HOM AK.RC01 AK.SWD II.KDAK TA.N18K 
   TA.O18K TA.O22K TA.P18K TA.P19K 
 
 Filtering commands used:
   cut o DIST/3.6 -30 o DIST/3.6 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 7.76e+21 dyne-cm
  Mw = 3.86 
  Z  = 84 km
  Plane   Strike  Dip  Rake
   NP1      339    77   128
   NP2       85    40    20
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.76e+21     44     287
    N   0.00e+00     37     150
    P  -7.76e+21     23      41

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.41e+21
       Mxy    -4.39e+21
       Mxz    -9.46e+20
       Myy     7.94e+20
       Myz    -5.53e+21
       Mzz     2.61e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 #####-----------------              
              ##########------------------           
             ############------------   ---          
           ###############----------- P -----        
          #################----------   ------       
         ###################-------------------      
        #####################-------------------     
        ########   ###########------------------     
       ######### T ###########-------------------    
       #########   ############-----------------#    
       #########################----------------#    
       -########################--------------###    
        -########################------------###     
        ---#######################---------#####     
         ----#####################-------######      
          ------##################---#########       
           -----------##########---##########        
             ----------------------########          
              ---------------------#######           
                 ------------------####              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.61e+21  -9.46e+20   5.53e+21 
 -9.46e+20  -3.41e+21   4.39e+21 
  5.53e+21   4.39e+21   7.94e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170708002929/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 +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   135    45   -85   2.99 0.2198
WVFGRD96    4.0   345    80   -50   3.02 0.2729
WVFGRD96    6.0   340    70   -45   3.08 0.3183
WVFGRD96    8.0   340    70   -50   3.16 0.3377
WVFGRD96   10.0   340    65   -45   3.20 0.3383
WVFGRD96   12.0   275    65    60   3.30 0.3413
WVFGRD96   14.0   270    55    15   3.22 0.3435
WVFGRD96   16.0    70    65   -30   3.30 0.3530
WVFGRD96   18.0   270    60   -10   3.29 0.3630
WVFGRD96   20.0   270    60   -15   3.33 0.3792
WVFGRD96   22.0   270    60   -15   3.36 0.3975
WVFGRD96   24.0   270    60   -20   3.40 0.4156
WVFGRD96   26.0    70    70   -25   3.43 0.4328
WVFGRD96   28.0    70    70   -20   3.45 0.4505
WVFGRD96   30.0    70    70   -15   3.46 0.4575
WVFGRD96   32.0    75    60     0   3.47 0.4638
WVFGRD96   34.0    75    60     5   3.49 0.4699
WVFGRD96   36.0    85    50    15   3.50 0.4774
WVFGRD96   38.0    80    55    10   3.53 0.4878
WVFGRD96   40.0    80    40    10   3.63 0.4887
WVFGRD96   42.0    75    45     5   3.64 0.5045
WVFGRD96   44.0    75    45     5   3.66 0.5254
WVFGRD96   46.0    75    45     0   3.68 0.5419
WVFGRD96   48.0    75    45     0   3.69 0.5548
WVFGRD96   50.0    70    50     0   3.72 0.5693
WVFGRD96   52.0    70    50     0   3.74 0.5815
WVFGRD96   54.0    70    50     0   3.75 0.5884
WVFGRD96   56.0    70    50     0   3.76 0.5954
WVFGRD96   58.0    70    50     0   3.78 0.6004
WVFGRD96   60.0    80    45    20   3.78 0.6058
WVFGRD96   62.0    80    45    25   3.79 0.6176
WVFGRD96   64.0    80    45    25   3.80 0.6297
WVFGRD96   66.0    80    45    25   3.81 0.6366
WVFGRD96   68.0    80    45    25   3.82 0.6442
WVFGRD96   70.0    80    45    20   3.82 0.6504
WVFGRD96   72.0    80    40    15   3.83 0.6545
WVFGRD96   74.0    80    40    15   3.84 0.6575
WVFGRD96   76.0    80    40    15   3.85 0.6609
WVFGRD96   78.0    85    40    20   3.84 0.6636
WVFGRD96   80.0    85    40    20   3.85 0.6647
WVFGRD96   82.0    85    40    20   3.85 0.6648
WVFGRD96   84.0    85    40    20   3.86 0.6656
WVFGRD96   86.0    85    40    20   3.87 0.6618
WVFGRD96   88.0    85    40    20   3.87 0.6612
WVFGRD96   90.0    85    40    20   3.88 0.6576
WVFGRD96   92.0    85    40    20   3.88 0.6534
WVFGRD96   94.0    85    40    20   3.89 0.6494
WVFGRD96   96.0    90    35    15   3.89 0.6439
WVFGRD96   98.0    90    40    20   3.88 0.6392

The best solution is

WVFGRD96   84.0    85    40    20   3.86 0.6656

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 +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 Fri Jul 7 21:45:18 CDT 2017