Location

Location ANSS

2016/05/01 20:38:47 60.140 -152.905 119.4 4.7 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/05/01 20:38:47:0  60.14 -152.90 119.4 4.7 Alaska
 
 Stations used:
   AK.BRLK AK.CAST AK.CNP AK.CUT AK.GHO AK.GLI AK.HOM AK.KNK 
   AK.KTH AK.PPLA AK.PWL AK.RC01 AK.SAW AT.PMR AT.SVW2 AV.ILSW 
   II.KDAK TA.K20K TA.L19K TA.M19K TA.M22K TA.N18K TA.N19K 
   TA.O18K TA.O19K TA.O22K TA.P19K TA.Q19K 
 
 Filtering commands used:
   cut a -10 a 90
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 1.32e+23 dyne-cm
  Mw = 4.68 
  Z  = 134 km
  Plane   Strike  Dip  Rake
   NP1       80    90   -35
   NP2      170    55   -180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.32e+23     24     131
    N   0.00e+00     55     260
    P  -1.32e+23     24      29

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.69e+22
       Mxy    -1.01e+23
       Mxz    -7.45e+22
       Myy     3.69e+22
       Myz     1.31e+22
       Mzz     6.61e+15
                                                     
                                                     
                                                     
                                                     
                     ##------------                  
                 #####-----------------              
              #######-------------   -----           
             #######-------------- P ------          
           #########--------------   --------        
          #########---------------------------       
         ##########----------------------------      
        ###########-----------------------------     
        ###########-----------------------------     
       ############-------------------------#####    
       ############--------------################    
       ############----##########################    
       #####--------#############################    
        ------------############################     
        -------------###########################     
         ------------##################   #####      
          ------------################# T ####       
           ------------################   ###        
             -----------###################          
              ------------################           
                 ----------############              
                     ---------#####                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  6.61e+15  -7.45e+22  -1.31e+22 
 -7.45e+22  -3.69e+22   1.01e+23 
 -1.31e+22   1.01e+23   3.69e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160501203847/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 = 80
      DIP = 90
     RAKE = -35
       MW = 4.68
       HS = 134.0

The NDK file is 20160501203847.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2016/05/01 20:38:47:0  60.14 -152.90 119.4 4.7 Alaska
 
 Stations used:
   AK.BRLK AK.CAST AK.CNP AK.CUT AK.GHO AK.GLI AK.HOM AK.KNK 
   AK.KTH AK.PPLA AK.PWL AK.RC01 AK.SAW AT.PMR AT.SVW2 AV.ILSW 
   II.KDAK TA.K20K TA.L19K TA.M19K TA.M22K TA.N18K TA.N19K 
   TA.O18K TA.O19K TA.O22K TA.P19K TA.Q19K 
 
 Filtering commands used:
   cut a -10 a 90
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 1.32e+23 dyne-cm
  Mw = 4.68 
  Z  = 134 km
  Plane   Strike  Dip  Rake
   NP1       80    90   -35
   NP2      170    55   -180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.32e+23     24     131
    N   0.00e+00     55     260
    P  -1.32e+23     24      29

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.69e+22
       Mxy    -1.01e+23
       Mxz    -7.45e+22
       Myy     3.69e+22
       Myz     1.31e+22
       Mzz     6.61e+15
                                                     
                                                     
                                                     
                                                     
                     ##------------                  
                 #####-----------------              
              #######-------------   -----           
             #######-------------- P ------          
           #########--------------   --------        
          #########---------------------------       
         ##########----------------------------      
        ###########-----------------------------     
        ###########-----------------------------     
       ############-------------------------#####    
       ############--------------################    
       ############----##########################    
       #####--------#############################    
        ------------############################     
        -------------###########################     
         ------------##################   #####      
          ------------################# T ####       
           ------------################   ###        
             -----------###################          
              ------------################           
                 ----------############              
                     ---------#####                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  6.61e+15  -7.45e+22  -1.31e+22 
 -7.45e+22  -3.69e+22   1.01e+23 
 -1.31e+22   1.01e+23   3.69e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160501203847/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

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 a -10 a 90
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.06 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     5    75    20   3.75 0.2616
WVFGRD96    4.0   185    90   -20   3.81 0.2840
WVFGRD96    6.0     5    55   -10   3.89 0.3007
WVFGRD96    8.0     0    50   -15   3.97 0.3264
WVFGRD96   10.0   255    80     5   3.98 0.3511
WVFGRD96   12.0   255    80     5   4.01 0.3800
WVFGRD96   14.0   255    80     0   4.05 0.3945
WVFGRD96   16.0    75    90     5   4.06 0.4055
WVFGRD96   18.0   255    85    -5   4.10 0.4283
WVFGRD96   20.0    75    90     5   4.11 0.4421
WVFGRD96   22.0    75    90     0   4.13 0.4552
WVFGRD96   24.0    75    90     0   4.15 0.4650
WVFGRD96   26.0   255    90     5   4.17 0.4707
WVFGRD96   28.0    75    90    -5   4.18 0.4728
WVFGRD96   30.0    75    90    -5   4.20 0.4717
WVFGRD96   32.0    75    90    -5   4.22 0.4719
WVFGRD96   34.0    75    90    -5   4.25 0.4725
WVFGRD96   36.0   255    90    10   4.27 0.4751
WVFGRD96   38.0   255    90    10   4.30 0.4839
WVFGRD96   40.0   255    85    15   4.36 0.5114
WVFGRD96   42.0   255    85    15   4.38 0.5175
WVFGRD96   44.0   255    85    15   4.40 0.5234
WVFGRD96   46.0   255    85    15   4.41 0.5294
WVFGRD96   48.0   255    90    15   4.43 0.5351
WVFGRD96   50.0    75    90   -15   4.44 0.5430
WVFGRD96   52.0    75    90   -15   4.46 0.5505
WVFGRD96   54.0    75    90   -15   4.47 0.5585
WVFGRD96   56.0    75    85   -20   4.47 0.5668
WVFGRD96   58.0    75    85   -20   4.49 0.5763
WVFGRD96   60.0    75    85   -20   4.50 0.5860
WVFGRD96   62.0    75    85   -20   4.51 0.5964
WVFGRD96   64.0    75    85   -20   4.52 0.6067
WVFGRD96   66.0    75    85   -20   4.53 0.6181
WVFGRD96   68.0    75    85   -20   4.54 0.6297
WVFGRD96   70.0    75    85   -20   4.54 0.6419
WVFGRD96   72.0    75    85   -20   4.55 0.6532
WVFGRD96   74.0    75    85   -20   4.56 0.6651
WVFGRD96   76.0    75    85   -20   4.57 0.6756
WVFGRD96   78.0    75    85   -20   4.57 0.6866
WVFGRD96   80.0    75    85   -20   4.58 0.6967
WVFGRD96   82.0    75    85   -20   4.59 0.7055
WVFGRD96   84.0    75    85   -20   4.59 0.7147
WVFGRD96   86.0    75    85   -20   4.60 0.7225
WVFGRD96   88.0    75    85   -25   4.60 0.7298
WVFGRD96   90.0    75    85   -25   4.60 0.7379
WVFGRD96   92.0    75    85   -25   4.61 0.7459
WVFGRD96   94.0   255    90    30   4.61 0.7440
WVFGRD96   96.0    75    85   -25   4.61 0.7588
WVFGRD96   98.0   255    90    30   4.62 0.7575
WVFGRD96  100.0    75    85   -25   4.62 0.7694
WVFGRD96  102.0    75    85   -25   4.63 0.7746
WVFGRD96  104.0    75    85   -25   4.63 0.7784
WVFGRD96  106.0    75    85   -25   4.63 0.7822
WVFGRD96  108.0   255    90    30   4.64 0.7858
WVFGRD96  110.0   255    90    30   4.64 0.7897
WVFGRD96  112.0   255    90    35   4.64 0.7929
WVFGRD96  114.0    75    90   -35   4.65 0.7972
WVFGRD96  116.0    80    90   -35   4.65 0.8008
WVFGRD96  118.0    80    90   -35   4.65 0.8027
WVFGRD96  120.0   260    90    35   4.66 0.8069
WVFGRD96  122.0    80    90   -35   4.66 0.8086
WVFGRD96  124.0    80    90   -35   4.66 0.8103
WVFGRD96  126.0    80    90   -35   4.67 0.8124
WVFGRD96  128.0   260    90    35   4.67 0.8133
WVFGRD96  130.0   260    90    35   4.67 0.8145
WVFGRD96  132.0    80    90   -35   4.68 0.8152
WVFGRD96  134.0    80    90   -35   4.68 0.8152
WVFGRD96  136.0    80    90   -35   4.68 0.8145
WVFGRD96  138.0   260    90    35   4.69 0.8138
WVFGRD96  140.0    75    85   -35   4.68 0.8131
WVFGRD96  142.0    75    85   -35   4.68 0.8121
WVFGRD96  144.0    75    85   -35   4.69 0.8114
WVFGRD96  146.0   260    90    35   4.70 0.8081
WVFGRD96  148.0    75    85   -35   4.69 0.8079

The best solution is

WVFGRD96  134.0    80    90   -35   4.68 0.8152

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 a -10 a 90
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.06 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 May 1 18:50:17 CDT 2016