Location

Location ANSS

2017/09/01 03:21:20 63.041 -150.970 131.6 4.0 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/09/01 03:21:20:0  63.04 -150.97 131.6 4.0 Alaska
 
 Stations used:
   AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.GHO AK.GLI AK.HDA 
   AK.KNK AK.KTH AK.MDM AK.MLY AK.NEA2 AK.PPD AK.PWL AK.RC01 
   AK.RND AK.SAW AK.SCM AK.SCRK AK.SSN AK.TRF AK.WRH AT.PMR 
   AT.TTA IM.IL31 IU.COLA TA.H21K TA.J20K TA.J25K TA.J26L 
   TA.K20K TA.L26K TA.M19K TA.M20K TA.M22K TA.N19K TA.N25K 
   TA.O22K TA.POKR TA.TCOL 
 
 Filtering commands used:
   cut o DIST/3.3 -60 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.88e+22 dyne-cm
  Mw = 4.24 
  Z  = 136 km
  Plane   Strike  Dip  Rake
   NP1       60    80    45
   NP2      320    46   166
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.88e+22     38     291
    N   0.00e+00     44      70
    P  -2.88e+22     22     183

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.26e+22
       Mxy    -7.02e+21
       Mxz     1.48e+22
       Myy     1.56e+22
       Myz    -1.26e+22
       Mzz     6.97e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              #########-------------------           
             ###############---------------          
           ####################--------------        
          #######################-------------       
         ##########################---------###      
        ######   ####################-----######     
        ###### T #####################--########     
       #######   ####################--##########    
       ###########################------#########    
       ########################----------########    
       #####################--------------#######    
        #################-----------------######     
        #############----------------------#####     
         ########--------------------------####      
          #--------------------------------###       
           -------------------------------###        
             -------------   -------------#          
              ------------ P ------------#           
                 ---------   ----------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  6.97e+21   1.48e+22   1.26e+22 
  1.48e+22  -2.26e+22   7.02e+21 
  1.26e+22   7.02e+21   1.56e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170901032120/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 = 60
      DIP = 80
     RAKE = 45
       MW = 4.24
       HS = 136.0

The NDK file is 20170901032120.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/09/01 03:21:20:0  63.04 -150.97 131.6 4.0 Alaska
 
 Stations used:
   AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.GHO AK.GLI AK.HDA 
   AK.KNK AK.KTH AK.MDM AK.MLY AK.NEA2 AK.PPD AK.PWL AK.RC01 
   AK.RND AK.SAW AK.SCM AK.SCRK AK.SSN AK.TRF AK.WRH AT.PMR 
   AT.TTA IM.IL31 IU.COLA TA.H21K TA.J20K TA.J25K TA.J26L 
   TA.K20K TA.L26K TA.M19K TA.M20K TA.M22K TA.N19K TA.N25K 
   TA.O22K TA.POKR TA.TCOL 
 
 Filtering commands used:
   cut o DIST/3.3 -60 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.88e+22 dyne-cm
  Mw = 4.24 
  Z  = 136 km
  Plane   Strike  Dip  Rake
   NP1       60    80    45
   NP2      320    46   166
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.88e+22     38     291
    N   0.00e+00     44      70
    P  -2.88e+22     22     183

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.26e+22
       Mxy    -7.02e+21
       Mxz     1.48e+22
       Myy     1.56e+22
       Myz    -1.26e+22
       Mzz     6.97e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              #########-------------------           
             ###############---------------          
           ####################--------------        
          #######################-------------       
         ##########################---------###      
        ######   ####################-----######     
        ###### T #####################--########     
       #######   ####################--##########    
       ###########################------#########    
       ########################----------########    
       #####################--------------#######    
        #################-----------------######     
        #############----------------------#####     
         ########--------------------------####      
          #--------------------------------###       
           -------------------------------###        
             -------------   -------------#          
              ------------ P ------------#           
                 ---------   ----------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  6.97e+21   1.48e+22   1.26e+22 
  1.48e+22  -2.26e+22   7.02e+21 
  1.26e+22   7.02e+21   1.56e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170901032120/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 -60 o DIST/3.3 +50
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   135    45   -45   3.29 0.1476
WVFGRD96    4.0   155    55   -10   3.32 0.1572
WVFGRD96    6.0   160    65    15   3.37 0.1709
WVFGRD96    8.0   160    65    15   3.45 0.1825
WVFGRD96   10.0   160    65    20   3.50 0.1899
WVFGRD96   12.0   160    65    25   3.54 0.1954
WVFGRD96   14.0   160    70    25   3.58 0.1980
WVFGRD96   16.0   160    65    25   3.60 0.1950
WVFGRD96   18.0   160    65    30   3.62 0.1884
WVFGRD96   20.0   160    65    35   3.64 0.1793
WVFGRD96   22.0   160    70    35   3.66 0.1692
WVFGRD96   24.0   250    70    30   3.66 0.1723
WVFGRD96   26.0   245    80    25   3.69 0.1762
WVFGRD96   28.0   245    80    25   3.70 0.1786
WVFGRD96   30.0   245    80    25   3.72 0.1802
WVFGRD96   32.0   245    80    25   3.74 0.1796
WVFGRD96   34.0   245    85    20   3.75 0.1776
WVFGRD96   36.0    70    80    15   3.78 0.1773
WVFGRD96   38.0    75    75    20   3.81 0.1754
WVFGRD96   40.0    75    75    25   3.87 0.1755
WVFGRD96   42.0    75    75    20   3.89 0.1768
WVFGRD96   44.0    75    75    20   3.92 0.1780
WVFGRD96   46.0    75    75    20   3.94 0.1804
WVFGRD96   48.0    80    70    30   3.96 0.1844
WVFGRD96   50.0    80    70    30   3.98 0.1899
WVFGRD96   52.0    80    70    30   3.99 0.1960
WVFGRD96   54.0    80    70    30   4.00 0.2024
WVFGRD96   56.0    80    70    30   4.01 0.2087
WVFGRD96   58.0    60    75   -25   4.05 0.2171
WVFGRD96   60.0    60    75   -20   4.06 0.2298
WVFGRD96   62.0    60    75   -20   4.07 0.2432
WVFGRD96   64.0    60    75   -20   4.09 0.2560
WVFGRD96   66.0    60    75   -20   4.10 0.2679
WVFGRD96   68.0    55    75   -25   4.12 0.2847
WVFGRD96   70.0    55    75   -25   4.14 0.3013
WVFGRD96   72.0    55    75   -20   4.15 0.3242
WVFGRD96   74.0    55    75   -15   4.16 0.3627
WVFGRD96   76.0    60    65    20   4.13 0.4235
WVFGRD96   78.0    60    65    20   4.16 0.4934
WVFGRD96   80.0    60    70    25   4.17 0.5557
WVFGRD96   82.0    60    70    25   4.18 0.6036
WVFGRD96   84.0    60    70    30   4.18 0.6311
WVFGRD96   86.0    60    70    30   4.19 0.6390
WVFGRD96   88.0    60    75    30   4.19 0.6447
WVFGRD96   90.0    60    75    30   4.19 0.6484
WVFGRD96   92.0    60    75    35   4.19 0.6547
WVFGRD96   94.0    60    75    35   4.19 0.6593
WVFGRD96   96.0    60    75    35   4.20 0.6653
WVFGRD96   98.0    60    75    40   4.20 0.6699
WVFGRD96  100.0    60    75    40   4.20 0.6754
WVFGRD96  102.0    60    75    40   4.20 0.6809
WVFGRD96  104.0    60    75    40   4.20 0.6834
WVFGRD96  106.0    60    75    40   4.21 0.6889
WVFGRD96  108.0    60    75    40   4.21 0.6902
WVFGRD96  110.0    60    75    40   4.21 0.6950
WVFGRD96  112.0    60    75    40   4.21 0.6965
WVFGRD96  114.0    60    75    40   4.22 0.7010
WVFGRD96  116.0    60    75    40   4.22 0.7018
WVFGRD96  118.0    60    80    45   4.22 0.7060
WVFGRD96  120.0    60    80    45   4.23 0.7060
WVFGRD96  122.0    60    80    45   4.23 0.7098
WVFGRD96  124.0    60    80    45   4.23 0.7094
WVFGRD96  126.0    60    80    45   4.23 0.7135
WVFGRD96  128.0    60    80    45   4.23 0.7139
WVFGRD96  130.0    60    80    45   4.24 0.7152
WVFGRD96  132.0    60    80    45   4.24 0.7152
WVFGRD96  134.0    60    80    45   4.24 0.7156
WVFGRD96  136.0    60    80    45   4.24 0.7166
WVFGRD96  138.0    60    80    45   4.24 0.7148
WVFGRD96  140.0    60    80    45   4.24 0.7149
WVFGRD96  142.0    60    80    45   4.25 0.7137
WVFGRD96  144.0    60    80    45   4.25 0.7133
WVFGRD96  146.0    60    80    45   4.25 0.7115
WVFGRD96  148.0    60    80    45   4.25 0.7084

The best solution is

WVFGRD96  136.0    60    80    45   4.24 0.7166

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 -60 o DIST/3.3 +50
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 Fri Sep 1 06:09:50 CDT 2017