Location

Location ANSS

2017/07/07 07:58:17 60.470 -151.748 71.1 4.4 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/07/07 07:58:17:0  60.47 -151.75  71.1 4.4 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CNP AK.CUT AK.FIRE AK.GHO AK.HOM AK.KNK 
   AK.RC01 AK.SSN AK.SWD AT.PMR AT.SVW2 AV.ILSW TA.M19K 
   TA.M20K TA.M22K TA.N18K TA.N19K TA.O18K TA.O19K TA.O22K 
   TA.P19K TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.5 -40 o DIST/3.5 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 3.94e+22 dyne-cm
  Mw = 4.33 
  Z  = 72 km
  Plane   Strike  Dip  Rake
   NP1      205    80   -30
   NP2      301    61   -168
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.94e+22     13     256
    N   0.00e+00     59       8
    P  -3.94e+22     28     159

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.45e+22
       Mxy     1.90e+22
       Mxz     1.32e+22
       Myy     3.12e+22
       Myz    -1.43e+22
       Mzz    -6.73e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 -------------------###              
              --------------------########           
             --------------------##########          
           ---------------------#############        
          #############-------################       
         ###################-##################      
        ####################----################     
        ###################--------#############     
       ###################-----------############    
       ###################-------------##########    
       ##################----------------########    
       ##   ############------------------#######    
        # T ###########--------------------#####     
        #   ##########----------------------####     
         #############-----------------------##      
          ###########-------------------------       
           ##########-----------   ----------        
             #######------------ P --------          
              ######------------   -------           
                 ##--------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -6.73e+21   1.32e+22   1.43e+22 
  1.32e+22  -2.45e+22  -1.90e+22 
  1.43e+22  -1.90e+22   3.12e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170707075817/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 = 205
      DIP = 80
     RAKE = -30
       MW = 4.33
       HS = 72.0

The NDK file is 20170707075817.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/07/07 07:58:17:0  60.47 -151.75  71.1 4.4 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CNP AK.CUT AK.FIRE AK.GHO AK.HOM AK.KNK 
   AK.RC01 AK.SSN AK.SWD AT.PMR AT.SVW2 AV.ILSW TA.M19K 
   TA.M20K TA.M22K TA.N18K TA.N19K TA.O18K TA.O19K TA.O22K 
   TA.P19K TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.5 -40 o DIST/3.5 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 3.94e+22 dyne-cm
  Mw = 4.33 
  Z  = 72 km
  Plane   Strike  Dip  Rake
   NP1      205    80   -30
   NP2      301    61   -168
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.94e+22     13     256
    N   0.00e+00     59       8
    P  -3.94e+22     28     159

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.45e+22
       Mxy     1.90e+22
       Mxz     1.32e+22
       Myy     3.12e+22
       Myz    -1.43e+22
       Mzz    -6.73e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 -------------------###              
              --------------------########           
             --------------------##########          
           ---------------------#############        
          #############-------################       
         ###################-##################      
        ####################----################     
        ###################--------#############     
       ###################-----------############    
       ###################-------------##########    
       ##################----------------########    
       ##   ############------------------#######    
        # T ###########--------------------#####     
        #   ##########----------------------####     
         #############-----------------------##      
          ###########-------------------------       
           ##########-----------   ----------        
             #######------------ P --------          
              ######------------   -------           
                 ##--------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -6.73e+21   1.32e+22   1.43e+22 
  1.32e+22  -2.45e+22  -1.90e+22 
  1.43e+22  -1.90e+22   3.12e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170707075817/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 +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   120    80   -15   3.43 0.2150
WVFGRD96    4.0   305    90   -20   3.53 0.2473
WVFGRD96    6.0   125    85    20   3.60 0.2639
WVFGRD96    8.0   215    70     5   3.68 0.2871
WVFGRD96   10.0   215    70     5   3.73 0.2990
WVFGRD96   12.0    35    75    -5   3.77 0.3085
WVFGRD96   14.0    35    75    -5   3.81 0.3125
WVFGRD96   16.0    35    75   -10   3.84 0.3145
WVFGRD96   18.0    35    75   -10   3.87 0.3169
WVFGRD96   20.0    35    80   -10   3.90 0.3224
WVFGRD96   22.0    35    75    -5   3.92 0.3322
WVFGRD96   24.0    35    75    -5   3.94 0.3471
WVFGRD96   26.0    35    80     0   3.96 0.3649
WVFGRD96   28.0    35    80     0   3.98 0.3856
WVFGRD96   30.0    35    80     0   4.00 0.4017
WVFGRD96   32.0    35    80     0   4.02 0.4138
WVFGRD96   34.0    35    80     0   4.03 0.4189
WVFGRD96   36.0   215    85   -10   4.05 0.4208
WVFGRD96   38.0   215    85   -10   4.08 0.4286
WVFGRD96   40.0   210    80   -20   4.14 0.4431
WVFGRD96   42.0   210    80   -20   4.17 0.4495
WVFGRD96   44.0   210    80   -20   4.19 0.4532
WVFGRD96   46.0   210    80   -20   4.21 0.4579
WVFGRD96   48.0   210    85   -20   4.22 0.4663
WVFGRD96   50.0   210    85   -25   4.24 0.4763
WVFGRD96   52.0   210    85   -25   4.25 0.4860
WVFGRD96   54.0   210    85   -25   4.26 0.4955
WVFGRD96   56.0   210    85   -25   4.27 0.5044
WVFGRD96   58.0   210    85   -25   4.28 0.5112
WVFGRD96   60.0   210    85   -25   4.29 0.5177
WVFGRD96   62.0   205    80   -30   4.30 0.5241
WVFGRD96   64.0   205    80   -30   4.31 0.5312
WVFGRD96   66.0   205    80   -30   4.31 0.5344
WVFGRD96   68.0   205    80   -30   4.32 0.5367
WVFGRD96   70.0   205    80   -30   4.32 0.5380
WVFGRD96   72.0   205    80   -30   4.33 0.5402
WVFGRD96   74.0   205    80   -30   4.33 0.5388
WVFGRD96   76.0   205    80   -30   4.33 0.5380
WVFGRD96   78.0   205    80   -30   4.33 0.5365
WVFGRD96   80.0    30    90    25   4.33 0.5311
WVFGRD96   82.0    30    90    25   4.33 0.5298
WVFGRD96   84.0   205    85   -30   4.34 0.5280
WVFGRD96   86.0    30    85    25   4.34 0.5245
WVFGRD96   88.0   205    85   -30   4.35 0.5209
WVFGRD96   90.0   205    85   -30   4.35 0.5161
WVFGRD96   92.0    30    85    25   4.34 0.5135
WVFGRD96   94.0   210    90   -25   4.34 0.5078
WVFGRD96   96.0   210    90   -25   4.35 0.5044
WVFGRD96   98.0   210    90   -30   4.35 0.4994

The best solution is

WVFGRD96   72.0   205    80   -30   4.33 0.5402

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 +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 Fri Jul 7 07:58:35 CDT 2017