Location

Location SLU

Because the depth from the regional MT solution was greater than the posted automatic AEIC value, the progran elocate was used to determine the hypocenter. The results are in the file elocate.txt. The latitude, longitude and origin time are essentailly the same as the value posted on ANSS.

Location ANSS

2016/07/20 14:04:28 61.728 -148.612 18.5 3.5 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/07/20 14:04:28:0  61.73 -148.61  18.5 3.5 Alaska
 
 Stations used:
   AK.GHO AK.KNK AK.RC01 AK.SAW AK.SCM AT.PMR TA.M22K TA.O22K 
 
 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 = 1.43e+21 dyne-cm
  Mw = 3.37 
  Z  = 37 km
  Plane   Strike  Dip  Rake
   NP1      316    52   -102
   NP2      155    40   -75
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.43e+21      6      54
    N   0.00e+00     10     323
    P  -1.43e+21     79     176

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.25e+20
       Mxy     6.73e+20
       Mxz     3.58e+20
       Myy     9.34e+20
       Myz     9.75e+19
       Mzz    -1.36e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 -#####################              
              ---#########################           
             ###--------##################           
           ####-------------############## T         
          #####----------------###########   #       
         ######------------------##############      
        ######---------------------#############     
        ######-----------------------###########     
       #######------------------------###########    
       ########------------------------##########    
       ########------------   ----------#########    
       #########----------- P -----------########    
        ########-----------   ------------######     
        #########-------------------------######     
         #########-------------------------####      
          ##########-----------------------###       
           ##########----------------------##        
             ##########--------------------          
              ############----------------           
                 ############----------              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.36e+21   3.58e+20  -9.75e+19 
  3.58e+20   4.25e+20  -6.73e+20 
 -9.75e+19  -6.73e+20   9.34e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160720140428/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 = 155
      DIP = 40
     RAKE = -75
       MW = 3.37
       HS = 37.0

The NDK file is 20160720140428.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  2016/07/20 14:04:28:0  61.73 -148.61  18.5 3.5 Alaska
 
 Stations used:
   AK.GHO AK.KNK AK.RC01 AK.SAW AK.SCM AT.PMR TA.M22K TA.O22K 
 
 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 = 1.43e+21 dyne-cm
  Mw = 3.37 
  Z  = 37 km
  Plane   Strike  Dip  Rake
   NP1      316    52   -102
   NP2      155    40   -75
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.43e+21      6      54
    N   0.00e+00     10     323
    P  -1.43e+21     79     176

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.25e+20
       Mxy     6.73e+20
       Mxz     3.58e+20
       Myy     9.34e+20
       Myz     9.75e+19
       Mzz    -1.36e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 -#####################              
              ---#########################           
             ###--------##################           
           ####-------------############## T         
          #####----------------###########   #       
         ######------------------##############      
        ######---------------------#############     
        ######-----------------------###########     
       #######------------------------###########    
       ########------------------------##########    
       ########------------   ----------#########    
       #########----------- P -----------########    
        ########-----------   ------------######     
        #########-------------------------######     
         #########-------------------------####      
          ##########-----------------------###       
           ##########----------------------##        
             ##########--------------------          
              ############----------------           
                 ############----------              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.36e+21   3.58e+20  -9.75e+19 
  3.58e+20   4.25e+20  -6.73e+20 
 -9.75e+19  -6.73e+20   9.34e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160720140428/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    1.0   150    50   -90   2.68 0.2181
WVFGRD96    2.0   155    45   -85   2.84 0.2771
WVFGRD96    3.0   355    50   -60   2.87 0.2636
WVFGRD96    4.0   130    90    70   2.91 0.2889
WVFGRD96    5.0   125    90    75   2.91 0.3140
WVFGRD96    6.0   265    30   -25   2.93 0.3302
WVFGRD96    7.0   260    30   -35   2.95 0.3414
WVFGRD96    8.0   250    25    35   3.03 0.3475
WVFGRD96    9.0   300    40    50   3.03 0.3554
WVFGRD96   10.0   135    60    70   3.05 0.3619
WVFGRD96   11.0   130    60    65   3.07 0.3649
WVFGRD96   12.0   130    60    65   3.09 0.3621
WVFGRD96   13.0   290    45    30   3.08 0.3555
WVFGRD96   14.0   285    35     5   3.08 0.3504
WVFGRD96   15.0    15    85    50   3.09 0.3486
WVFGRD96   16.0   190    90   -45   3.11 0.3465
WVFGRD96   17.0   275    35   -15   3.12 0.3461
WVFGRD96   18.0   275    35   -20   3.14 0.3444
WVFGRD96   19.0   165    45   -60   3.17 0.3451
WVFGRD96   20.0   165    45   -60   3.19 0.3653
WVFGRD96   21.0   165    45   -60   3.21 0.3818
WVFGRD96   22.0   165    45   -60   3.23 0.3991
WVFGRD96   23.0   165    45   -60   3.24 0.4120
WVFGRD96   24.0   170    45   -60   3.25 0.4216
WVFGRD96   25.0   150    40   -80   3.26 0.4301
WVFGRD96   26.0   150    40   -80   3.27 0.4344
WVFGRD96   27.0   145    45   -90   3.28 0.4452
WVFGRD96   28.0   145    45   -90   3.29 0.4597
WVFGRD96   29.0   330    50   -85   3.30 0.4786
WVFGRD96   30.0   325    50   -90   3.31 0.5012
WVFGRD96   31.0   145    40   -90   3.32 0.5207
WVFGRD96   32.0   150    40   -85   3.33 0.5384
WVFGRD96   33.0   325    50   -95   3.34 0.5554
WVFGRD96   34.0   155    40   -80   3.35 0.5685
WVFGRD96   35.0   155    40   -80   3.36 0.5793
WVFGRD96   36.0   155    40   -80   3.36 0.5838
WVFGRD96   37.0   155    40   -75   3.37 0.5848
WVFGRD96   38.0   155    40   -75   3.38 0.5810
WVFGRD96   39.0   155    40   -75   3.39 0.5731
WVFGRD96   40.0   160    40   -75   3.47 0.5614
WVFGRD96   41.0   160    40   -75   3.48 0.5601
WVFGRD96   42.0   160    40   -75   3.49 0.5564
WVFGRD96   43.0   160    40   -75   3.50 0.5517
WVFGRD96   44.0   160    40   -75   3.51 0.5451
WVFGRD96   45.0   165    45   -65   3.51 0.5406
WVFGRD96   46.0   165    45   -65   3.52 0.5329
WVFGRD96   47.0   165    45   -65   3.53 0.5273
WVFGRD96   48.0   165    45   -65   3.53 0.5198
WVFGRD96   49.0   165    45   -65   3.53 0.5154

The best solution is

WVFGRD96   37.0   155    40   -75   3.37 0.5848

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 Wed Jul 20 10:52:26 CDT 2016