Location

Location ANSS

2017/06/05 20:51:09 63.046 -149.661 81.3 3.4 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/06/05 20:51:09:0  63.05 -149.66  81.3 3.4 Alaska
 
 Stations used:
   AK.BPAW AK.CUT AK.KNK AK.MCK AK.RND AK.SAW AK.SCM AT.PMR 
   TA.J25K TA.M20K 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 5.13e+21 dyne-cm
  Mw = 3.74 
  Z  = 104 km
  Plane   Strike  Dip  Rake
   NP1      314    79   134
   NP2       55    45    15
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.13e+21     39     263
    N   0.00e+00     43     124
    P  -5.13e+21     22      12

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.18e+21
       Mxy    -5.74e+20
       Mxz    -2.01e+21
       Myy     2.85e+21
       Myz    -2.87e+21
       Mzz     1.33e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 -------------   ------              
              ---------------- P ---------           
             #----------------   ----------          
           #######---------------------------        
          ##########-------------------------#       
         ##############----------------------##      
        #################--------------------###     
        ###################------------------###     
       ######################---------------#####    
       #######   ##############------------######    
       ####### T ################---------#######    
       #######   #################-------########    
        ############################----########     
        ########################################     
         ###########################---########      
          #######################-------######       
           ##################-------------###        
             ----######--------------------          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.33e+21  -2.01e+21   2.87e+21 
 -2.01e+21  -4.18e+21   5.74e+20 
  2.87e+21   5.74e+20   2.85e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170605205109/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 = 55
      DIP = 45
     RAKE = 15
       MW = 3.74
       HS = 104.0

The NDK file is 20170605205109.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/06/05 20:51:09:0  63.05 -149.66  81.3 3.4 Alaska
 
 Stations used:
   AK.BPAW AK.CUT AK.KNK AK.MCK AK.RND AK.SAW AK.SCM AT.PMR 
   TA.J25K TA.M20K 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 5.13e+21 dyne-cm
  Mw = 3.74 
  Z  = 104 km
  Plane   Strike  Dip  Rake
   NP1      314    79   134
   NP2       55    45    15
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.13e+21     39     263
    N   0.00e+00     43     124
    P  -5.13e+21     22      12

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.18e+21
       Mxy    -5.74e+20
       Mxz    -2.01e+21
       Myy     2.85e+21
       Myz    -2.87e+21
       Mzz     1.33e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 -------------   ------              
              ---------------- P ---------           
             #----------------   ----------          
           #######---------------------------        
          ##########-------------------------#       
         ##############----------------------##      
        #################--------------------###     
        ###################------------------###     
       ######################---------------#####    
       #######   ##############------------######    
       ####### T ################---------#######    
       #######   #################-------########    
        ############################----########     
        ########################################     
         ###########################---########      
          #######################-------######       
           ##################-------------###        
             ----######--------------------          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.33e+21  -2.01e+21   2.87e+21 
 -2.01e+21  -4.18e+21   5.74e+20 
  2.87e+21   5.74e+20   2.85e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170605205109/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 +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   340    85     0   2.77 0.2649
WVFGRD96    4.0   345    55    25   2.91 0.2966
WVFGRD96    6.0   340    60    20   2.96 0.3302
WVFGRD96    8.0   340    60    15   3.04 0.3547
WVFGRD96   10.0   335    55     0   3.09 0.3639
WVFGRD96   12.0   335    60    -5   3.12 0.3617
WVFGRD96   14.0   335    60    -5   3.15 0.3461
WVFGRD96   16.0   335    65    -5   3.17 0.3222
WVFGRD96   18.0   220    25   -45   3.28 0.3069
WVFGRD96   20.0   220    25   -45   3.32 0.3170
WVFGRD96   22.0   225    30   -40   3.34 0.3297
WVFGRD96   24.0   230    35   -30   3.34 0.3420
WVFGRD96   26.0   235    40   -25   3.36 0.3578
WVFGRD96   28.0   240    45   -15   3.36 0.3761
WVFGRD96   30.0   245    50   -10   3.39 0.3984
WVFGRD96   32.0   245    55   -10   3.40 0.4197
WVFGRD96   34.0   250    60    -5   3.44 0.4397
WVFGRD96   36.0   250    60    -5   3.46 0.4533
WVFGRD96   38.0   250    60     0   3.49 0.4633
WVFGRD96   40.0   250    50    -5   3.59 0.4813
WVFGRD96   42.0   250    55    -5   3.60 0.4829
WVFGRD96   44.0   250    55    -5   3.62 0.4838
WVFGRD96   46.0   250    55     5   3.62 0.4862
WVFGRD96   48.0   250    55     5   3.64 0.4874
WVFGRD96   50.0   250    60     5   3.65 0.4881
WVFGRD96   52.0   250    65    15   3.66 0.4916
WVFGRD96   54.0   250    65    15   3.68 0.4994
WVFGRD96   56.0   250    65    15   3.69 0.5059
WVFGRD96   58.0   250    65    10   3.70 0.5119
WVFGRD96   60.0   250    70    10   3.70 0.5166
WVFGRD96   62.0   245    75    20   3.70 0.5242
WVFGRD96   64.0   245    80    15   3.69 0.5342
WVFGRD96   66.0   245    80    15   3.70 0.5446
WVFGRD96   68.0   245    80    15   3.71 0.5536
WVFGRD96   70.0   240    85    20   3.72 0.5599
WVFGRD96   72.0   240    85    15   3.71 0.5693
WVFGRD96   74.0    60    90   -15   3.71 0.5767
WVFGRD96   76.0   240    90    15   3.71 0.5851
WVFGRD96   78.0    60    90   -15   3.72 0.5913
WVFGRD96   80.0   240    90    15   3.72 0.5966
WVFGRD96   82.0   235    90    15   3.73 0.5996
WVFGRD96   84.0    55    85   -15   3.73 0.6075
WVFGRD96   86.0   235    90    15   3.74 0.6071
WVFGRD96   88.0    55    85   -15   3.74 0.6129
WVFGRD96   90.0    55    80   -10   3.72 0.6152
WVFGRD96   92.0    55    80   -10   3.72 0.6159
WVFGRD96   94.0    55    80   -10   3.73 0.6197
WVFGRD96   96.0    55    40    10   3.74 0.6217
WVFGRD96   98.0    60    40    15   3.73 0.6236
WVFGRD96  100.0    60    40    15   3.73 0.6234
WVFGRD96  102.0    60    40    15   3.74 0.6254
WVFGRD96  104.0    55    45    15   3.74 0.6273
WVFGRD96  106.0    55    45    15   3.74 0.6248
WVFGRD96  108.0    60    45    20   3.73 0.6246
WVFGRD96  110.0    60    45    20   3.73 0.6226
WVFGRD96  112.0    60    45    20   3.73 0.6227
WVFGRD96  114.0    60    50    20   3.72 0.6207
WVFGRD96  116.0    60    50    20   3.72 0.6209
WVFGRD96  118.0    60    50    20   3.72 0.6194

The best solution is

WVFGRD96  104.0    55    45    15   3.74 0.6273

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 +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 Tue Jun 6 05:17:46 CDT 2017