Location

SLU Location

Since the moment tensor solution demanded a depth greater than reported, P- and S-wave first arrivals and P-wave first motions were read from the waveforms after appplying the filter .pre> hp c 1 n 2 to the derived ground velocities. The program elocate was used with the WUS model (listed below) to locate the earthquake. The results are in the file elocate.txt. The elocate depth of 43 km is consistent with the moment tensor depth of 44 km (+= 2 km) derived using the same velocity model.

Location ANSS

2017/05/29 08:13:58 61.834 -147.930 23.2 4.2 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/05/29 08:13:59:0  61.83 -147.93  23.2 4.2 Alaska
 
 Stations used:
   AK.CAST AK.CUT AK.DIV AK.EYAK AK.GHO AK.GLI AK.KLU AK.KNK 
   AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AT.PMR TA.M22K 
   TA.M24K TA.N25K 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 1.84e+22 dyne-cm
  Mw = 4.11 
  Z  = 44 km
  Plane   Strike  Dip  Rake
   NP1      215    50   -75
   NP2       12    42   -107
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.84e+22      4     294
    N   0.00e+00     11      25
    P  -1.84e+22     78     186

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.33e+21
       Mxy    -6.98e+21
       Mxz     4.28e+21
       Myy     1.52e+22
       Myz    -7.73e+20
       Mzz    -1.75e+22
                                                     
                                                     
                                                     
                                                     
                     #############-                  
                 ##################----              
              ##################-----#####           
             ###############----------#####          
           ###############-------------######        
           #############---------------#######       
         T ###########------------------#######      
           #########--------------------########     
        ###########---------------------########     
       ###########----------------------#########    
       ##########-----------------------#########    
       #########----------   -----------#########    
       #########---------- P -----------#########    
        #######-----------   ----------#########     
        #######-----------------------##########     
         ######-----------------------#########      
          #####----------------------#########       
           ####--------------------##########        
             ##-------------------#########          
              ##----------------##########           
                 -------------#########              
                     -----#########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.75e+22   4.28e+21   7.73e+20 
  4.28e+21   2.33e+21   6.98e+21 
  7.73e+20   6.98e+21   1.52e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170529081359/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 = 215
      DIP = 50
     RAKE = -75
       MW = 4.11
       HS = 44.0

The NDK file is 20170529081359.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
SLUFM
 USGS/SLU Moment Tensor Solution
 ENS  2017/05/29 08:13:59:0  61.83 -147.93  23.2 4.2 Alaska
 
 Stations used:
   AK.CAST AK.CUT AK.DIV AK.EYAK AK.GHO AK.GLI AK.KLU AK.KNK 
   AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AT.PMR TA.M22K 
   TA.M24K TA.N25K 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 1.84e+22 dyne-cm
  Mw = 4.11 
  Z  = 44 km
  Plane   Strike  Dip  Rake
   NP1      215    50   -75
   NP2       12    42   -107
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.84e+22      4     294
    N   0.00e+00     11      25
    P  -1.84e+22     78     186

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.33e+21
       Mxy    -6.98e+21
       Mxz     4.28e+21
       Myy     1.52e+22
       Myz    -7.73e+20
       Mzz    -1.75e+22
                                                     
                                                     
                                                     
                                                     
                     #############-                  
                 ##################----              
              ##################-----#####           
             ###############----------#####          
           ###############-------------######        
           #############---------------#######       
         T ###########------------------#######      
           #########--------------------########     
        ###########---------------------########     
       ###########----------------------#########    
       ##########-----------------------#########    
       #########----------   -----------#########    
       #########---------- P -----------#########    
        #######-----------   ----------#########     
        #######-----------------------##########     
         ######-----------------------#########      
          #####----------------------#########       
           ####--------------------##########        
             ##-------------------#########          
              ##----------------##########           
                 -------------#########              
                     -----#########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.75e+22   4.28e+21   7.73e+20 
  4.28e+21   2.33e+21   6.98e+21 
  7.73e+20   6.98e+21   1.52e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170529081359/index.html
	


First motions and takeoff angles from an elocate run.

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 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 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    50    85   3.50 0.3503
WVFGRD96    4.0   350    55    65   3.56 0.3186
WVFGRD96    6.0   345    70    55   3.55 0.3256
WVFGRD96    8.0   345    70    60   3.62 0.3492
WVFGRD96   10.0    90    40    25   3.63 0.3740
WVFGRD96   12.0    90    45    20   3.64 0.3938
WVFGRD96   14.0    85    45     5   3.66 0.4105
WVFGRD96   16.0    80    45   -10   3.68 0.4271
WVFGRD96   18.0    70    45   -30   3.71 0.4443
WVFGRD96   20.0    65    50   -35   3.76 0.4631
WVFGRD96   22.0    65    50   -35   3.78 0.4821
WVFGRD96   24.0    60    50   -40   3.81 0.5029
WVFGRD96   26.0    60    50   -40   3.83 0.5233
WVFGRD96   28.0    55    45   -45   3.85 0.5411
WVFGRD96   30.0    40    45   -75   3.88 0.5778
WVFGRD96   32.0    35    45   -80   3.90 0.6127
WVFGRD96   34.0    25    40   -95   3.92 0.6368
WVFGRD96   36.0   210    50   -85   3.94 0.6512
WVFGRD96   38.0   215    50   -80   3.97 0.6602
WVFGRD96   40.0   210    50   -80   4.07 0.6732
WVFGRD96   42.0   215    50   -80   4.10 0.6852
WVFGRD96   44.0   215    50   -75   4.11 0.6899
WVFGRD96   46.0   215    50   -75   4.13 0.6888
WVFGRD96   48.0   215    50   -75   4.14 0.6828
WVFGRD96   50.0   215    50   -75   4.15 0.6739
WVFGRD96   52.0   215    50   -75   4.16 0.6611
WVFGRD96   54.0   215    50   -70   4.16 0.6461
WVFGRD96   56.0   215    50   -70   4.17 0.6306
WVFGRD96   58.0   215    50   -70   4.18 0.6123
WVFGRD96   60.0   215    50   -70   4.18 0.5961
WVFGRD96   62.0   215    50   -70   4.18 0.5792
WVFGRD96   64.0   220    55   -65   4.19 0.5624
WVFGRD96   66.0   215    55   -70   4.19 0.5507
WVFGRD96   68.0   220    55   -65   4.20 0.5373
WVFGRD96   70.0   220    50   -65   4.20 0.5292
WVFGRD96   72.0   220    50   -65   4.20 0.5223
WVFGRD96   74.0   225    50   -65   4.21 0.5173
WVFGRD96   76.0   225    50   -65   4.21 0.5122
WVFGRD96   78.0   230    50   -60   4.22 0.5073
WVFGRD96   80.0   230    50   -60   4.22 0.5021
WVFGRD96   82.0   230    50   -60   4.23 0.4966
WVFGRD96   84.0   230    50   -60   4.23 0.4904
WVFGRD96   86.0   230    50   -55   4.23 0.4842
WVFGRD96   88.0   230    50   -55   4.23 0.4782
WVFGRD96   90.0   230    50   -55   4.23 0.4717
WVFGRD96   92.0   230    50   -55   4.23 0.4652
WVFGRD96   94.0   230    50   -55   4.23 0.4583
WVFGRD96   96.0   230    50   -55   4.23 0.4492
WVFGRD96   98.0   230    50   -55   4.23 0.4375

The best solution is

WVFGRD96   44.0   215    50   -75   4.11 0.6899

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 -30 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 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 Mon May 29 03:59:53 CDT 2017