Location

Location SLU

2016/08/30 12:27:58 61.138 -149.176 45. 4.0 Alaska

The waveform fit was not very good. We then picked first arrival times and relocated using elocate and the WUS model (below). The output is given in the file elocate.txt. This hypocenter was used to plot the picked first motions together with the waveform inversion mechanism. The agreement is OK given that many observations lie near the nodal planes. The relocated depth is similar to that of the waveform inversion.

With the new depth, the mechanism changed and fits the context of previous earthquakes with the same epicenter. In addition the relocation figure based on the inversion time shifts now indicates significantly better agreement with the given epicenter.

Location ANSS

2016/08/30 12:27:58 61.085 -149.205 19.5 4.0 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/08/30 12:27:58:0  61.14 -149.18  45.0 4.0 Alaska
 
 Stations used:
   AK.BPAW AK.FID AK.FIRE AK.GHO AK.GLI AK.KLU AK.KNK AK.PWL 
   AK.RC01 AK.SAW AK.SCM AT.PMR AT.SVW2 TA.M22K 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.17e+22 dyne-cm
  Mw = 3.98 
  Z  = 50 km
  Plane   Strike  Dip  Rake
   NP1      217    50   -86
   NP2       30    40   -95
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.17e+22      5     304
    N   0.00e+00      3      34
    P  -1.17e+22     84     156

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     3.45e+21
       Mxy    -5.32e+21
       Mxz     1.70e+21
       Myy     8.07e+21
       Myz    -1.37e+21
       Mzz    -1.15e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ###################-------##           
             ################------------##          
             #############---------------####        
           T ###########------------------####       
         #   #########--------------------#####      
        ############----------------------######     
        ###########-----------------------######     
       ###########------------------------#######    
       ##########----------   ------------#######    
       #########----------- P -----------########    
       ########------------   ----------#########    
        #######-------------------------########     
        #######------------------------#########     
         #####-----------------------##########      
          ####----------------------##########       
           ###--------------------###########        
             ##-----------------###########          
              #--------------#############           
                 -------###############              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.15e+22   1.70e+21   1.37e+21 
  1.70e+21   3.45e+21   5.32e+21 
  1.37e+21   5.32e+21   8.07e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160830122758/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 = 30
      DIP = 40
     RAKE = -95
       MW = 3.98
       HS = 50.0

The NDK file is 20160830122758.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  2016/08/30 12:27:58:0  61.14 -149.18  45.0 4.0 Alaska
 
 Stations used:
   AK.BPAW AK.FID AK.FIRE AK.GHO AK.GLI AK.KLU AK.KNK AK.PWL 
   AK.RC01 AK.SAW AK.SCM AT.PMR AT.SVW2 TA.M22K 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.17e+22 dyne-cm
  Mw = 3.98 
  Z  = 50 km
  Plane   Strike  Dip  Rake
   NP1      217    50   -86
   NP2       30    40   -95
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.17e+22      5     304
    N   0.00e+00      3      34
    P  -1.17e+22     84     156

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     3.45e+21
       Mxy    -5.32e+21
       Mxz     1.70e+21
       Myy     8.07e+21
       Myz    -1.37e+21
       Mzz    -1.15e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ###################-------##           
             ################------------##          
             #############---------------####        
           T ###########------------------####       
         #   #########--------------------#####      
        ############----------------------######     
        ###########-----------------------######     
       ###########------------------------#######    
       ##########----------   ------------#######    
       #########----------- P -----------########    
       ########------------   ----------#########    
        #######-------------------------########     
        #######------------------------#########     
         #####-----------------------##########      
          ####----------------------##########       
           ###--------------------###########        
             ##-----------------###########          
              #--------------#############           
                 -------###############              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.15e+22   1.70e+21   1.37e+21 
  1.70e+21   3.45e+21   5.32e+21 
  1.37e+21   5.32e+21   8.07e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160830122758/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 +70
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   165    50    95   3.29 0.1971
WVFGRD96    4.0   110    15    35   3.36 0.2333
WVFGRD96    6.0   105    20    30   3.38 0.2987
WVFGRD96    8.0     0    75    70   3.48 0.3282
WVFGRD96   10.0     0    75    65   3.50 0.3450
WVFGRD96   12.0     5    70    65   3.53 0.3454
WVFGRD96   14.0   125    40    40   3.56 0.3419
WVFGRD96   16.0   130    45    45   3.60 0.3373
WVFGRD96   18.0   130    45    45   3.62 0.3300
WVFGRD96   20.0   125    45    40   3.64 0.3189
WVFGRD96   22.0   120    45    35   3.67 0.3045
WVFGRD96   24.0   255    35     0   3.64 0.2954
WVFGRD96   26.0   255    35     0   3.66 0.3019
WVFGRD96   28.0   250    35   -10   3.69 0.3089
WVFGRD96   30.0   245    35   -15   3.71 0.3132
WVFGRD96   32.0   245    35   -15   3.73 0.3133
WVFGRD96   34.0   230    50   -70   3.77 0.3506
WVFGRD96   36.0   225    50   -70   3.79 0.3787
WVFGRD96   38.0   225    50   -75   3.80 0.3946
WVFGRD96   40.0   225    50   -75   3.91 0.4188
WVFGRD96   42.0   220    50   -80   3.93 0.4260
WVFGRD96   44.0   220    50   -80   3.95 0.4305
WVFGRD96   46.0   220    50   -80   3.96 0.4347
WVFGRD96   48.0   215    50   -85   3.97 0.4383
WVFGRD96   50.0    30    40   -95   3.98 0.4396
WVFGRD96   52.0   210    50   -90   3.99 0.4390
WVFGRD96   54.0    25    45   -95   3.99 0.4379
WVFGRD96   56.0    30    45   -90   4.00 0.4365
WVFGRD96   58.0    30    45   -90   4.00 0.4343
WVFGRD96   60.0    30    45   -85   4.01 0.4314
WVFGRD96   62.0    30    45   -85   4.01 0.4287
WVFGRD96   64.0   205    45   -95   4.02 0.4243
WVFGRD96   66.0    30    45   -85   4.02 0.4203
WVFGRD96   68.0    30    45   -85   4.02 0.4144
WVFGRD96   70.0    30    45   -85   4.03 0.4078
WVFGRD96   72.0    30    45   -85   4.03 0.4013
WVFGRD96   74.0    30    45   -85   4.03 0.3944
WVFGRD96   76.0    30    45   -85   4.03 0.3864
WVFGRD96   78.0    30    45   -80   4.04 0.3797
WVFGRD96   80.0    30    45   -80   4.04 0.3726
WVFGRD96   82.0    30    50   -75   4.04 0.3655
WVFGRD96   84.0    30    50   -75   4.04 0.3589
WVFGRD96   86.0    30    50   -75   4.04 0.3525
WVFGRD96   88.0    30    50   -75   4.04 0.3443
WVFGRD96   90.0    30    50   -75   4.04 0.3371
WVFGRD96   92.0    30    50   -75   4.05 0.3301
WVFGRD96   94.0    30    55   -75   4.04 0.3243
WVFGRD96   96.0    30    55   -80   4.04 0.3181
WVFGRD96   98.0    30    55   -75   4.04 0.3123

The best solution is

WVFGRD96   50.0    30    40   -95   3.98 0.4396

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 +70
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 Aug 30 08:46:37 CDT 2016