Location

Location ANSS

2017/09/30 21:15:06 59.629 -152.234 81.3 4.1 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/09/30 21:15:06:0  59.63 -152.23  81.3 4.1 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.HOM AK.SSN AV.ILSW II.KDAK TA.N18K 
   TA.N19K TA.O18K TA.O19K TA.O22K TA.P18K TA.P19K TA.Q19K 
   TA.Q20K 
 
 Filtering commands used:
   cut o DIST/3.4 -30 o DIST/3.4 +60
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.69e+22 dyne-cm
  Mw = 4.22 
  Z  = 82 km
  Plane   Strike  Dip  Rake
   NP1       34    46   100
   NP2      200    45    80
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.69e+22     83      24
    N   0.00e+00      7     207
    P  -2.69e+22      0     117

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -5.23e+21
       Mxy     1.11e+22
       Mxz     3.11e+21
       Myy    -2.13e+22
       Myz     1.13e+21
       Mzz     2.65e+22
                                                     
                                                     
                                                     
                                                     
                     -------------#                  
                 ------------##########              
              ------------##############--           
             -----------#################--          
           -----------###################----        
          -----------#####################----       
         -----------######################-----      
        -----------#######################------     
        ----------#######################-------     
       ----------##########   ###########--------    
       ----------########## T ###########--------    
       ---------###########   ##########---------    
       ---------#######################----------    
        --------######################----------     
        --------#####################--------        
         -------####################--------- P      
          ------##################-----------        
           ------###############-------------        
             ----############--------------          
              ----########----------------           
                 ##--------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.65e+22   3.11e+21  -1.13e+21 
  3.11e+21  -5.23e+21  -1.11e+22 
 -1.13e+21  -1.11e+22  -2.13e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170930211506/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 = 200
      DIP = 45
     RAKE = 80
       MW = 4.22
       HS = 82.0

The NDK file is 20170930211506.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMWR
 USGS/SLU Moment Tensor Solution
 ENS  2017/09/30 21:15:06:0  59.63 -152.23  81.3 4.1 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.HOM AK.SSN AV.ILSW II.KDAK TA.N18K 
   TA.N19K TA.O18K TA.O19K TA.O22K TA.P18K TA.P19K TA.Q19K 
   TA.Q20K 
 
 Filtering commands used:
   cut o DIST/3.4 -30 o DIST/3.4 +60
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.69e+22 dyne-cm
  Mw = 4.22 
  Z  = 82 km
  Plane   Strike  Dip  Rake
   NP1       34    46   100
   NP2      200    45    80
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.69e+22     83      24
    N   0.00e+00      7     207
    P  -2.69e+22      0     117

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -5.23e+21
       Mxy     1.11e+22
       Mxz     3.11e+21
       Myy    -2.13e+22
       Myz     1.13e+21
       Mzz     2.65e+22
                                                     
                                                     
                                                     
                                                     
                     -------------#                  
                 ------------##########              
              ------------##############--           
             -----------#################--          
           -----------###################----        
          -----------#####################----       
         -----------######################-----      
        -----------#######################------     
        ----------#######################-------     
       ----------##########   ###########--------    
       ----------########## T ###########--------    
       ---------###########   ##########---------    
       ---------#######################----------    
        --------######################----------     
        --------#####################--------        
         -------####################--------- P      
          ------##################-----------        
           ------###############-------------        
             ----############--------------          
              ----########----------------           
                 ##--------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.65e+22   3.11e+21  -1.13e+21 
  3.11e+21  -5.23e+21  -1.11e+22 
 -1.13e+21  -1.11e+22  -2.13e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170930211506/index.html
	
Moment	3.376e+15 N-m
Magnitude	4.3 Mwr
Depth	88.0 km
Percent DC	94 %
Half Duration	–
Catalog	US
Data Source	US3
Contributor	US3
Nodal Planes
Plane	Strike	Dip	Rake
NP1	210	39	88
NP2	32	51	91
Principal Axes
Axis	Value	Plunge	Azimuth
T	3.429e+15 N-m	84	311
N	-0.110e+15 N-m	1	211
P	-3.319e+15 N-m	6	121

        

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/3.4 -30 o DIST/3.4 +60
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    20    50   -90   3.49 0.3142
WVFGRD96    4.0    65    30   -15   3.48 0.2448
WVFGRD96    6.0    75    30     0   3.50 0.2985
WVFGRD96    8.0    75    25    -5   3.59 0.3407
WVFGRD96   10.0    65    30   -25   3.63 0.3810
WVFGRD96   12.0    60    30   -40   3.66 0.4113
WVFGRD96   14.0    50    35   -50   3.71 0.4332
WVFGRD96   16.0    45    35   -60   3.74 0.4484
WVFGRD96   18.0    45    40   -55   3.77 0.4573
WVFGRD96   20.0    45    40   -55   3.80 0.4610
WVFGRD96   22.0    45    40   -50   3.82 0.4525
WVFGRD96   24.0    45    40   -50   3.84 0.4300
WVFGRD96   26.0    50    45   -40   3.86 0.3998
WVFGRD96   28.0   170    80   -65   3.84 0.3745
WVFGRD96   30.0   170    80   -65   3.86 0.3628
WVFGRD96   32.0   180    80   -65   3.87 0.3511
WVFGRD96   34.0   180    80   -65   3.88 0.3448
WVFGRD96   36.0   185    80   -65   3.89 0.3411
WVFGRD96   38.0    25    50    85   3.91 0.3413
WVFGRD96   40.0    25    50    90   4.03 0.4307
WVFGRD96   42.0    25    50    85   4.07 0.4551
WVFGRD96   44.0    25    50    85   4.09 0.4760
WVFGRD96   46.0   200    40    80   4.11 0.4994
WVFGRD96   48.0   200    40    80   4.13 0.5239
WVFGRD96   50.0   200    45    80   4.14 0.5484
WVFGRD96   52.0   200    45    80   4.16 0.5741
WVFGRD96   54.0   200    45    80   4.16 0.5991
WVFGRD96   56.0   200    45    80   4.17 0.6210
WVFGRD96   58.0   200    45    80   4.18 0.6432
WVFGRD96   60.0   200    45    80   4.18 0.6622
WVFGRD96   62.0   200    45    80   4.19 0.6781
WVFGRD96   64.0   200    45    80   4.19 0.6939
WVFGRD96   66.0   200    45    80   4.20 0.7047
WVFGRD96   68.0   200    45    80   4.20 0.7145
WVFGRD96   70.0   200    45    80   4.20 0.7234
WVFGRD96   72.0   200    45    80   4.21 0.7322
WVFGRD96   74.0   200    45    80   4.21 0.7378
WVFGRD96   76.0   205    40    85   4.22 0.7422
WVFGRD96   78.0   205    40    85   4.23 0.7444
WVFGRD96   80.0    35    50    95   4.23 0.7485
WVFGRD96   82.0   200    45    80   4.22 0.7486
WVFGRD96   84.0    30    50    95   4.23 0.7465
WVFGRD96   86.0   200    45    80   4.23 0.7470
WVFGRD96   88.0    30    50    95   4.24 0.7463
WVFGRD96   90.0   205    40    85   4.24 0.7408
WVFGRD96   92.0    30    45    95   4.24 0.7406
WVFGRD96   94.0   190    45    75   4.24 0.7361
WVFGRD96   96.0    30    45    95   4.25 0.7308
WVFGRD96   98.0    30    45    95   4.25 0.7277

The best solution is

WVFGRD96   82.0   200    45    80   4.22 0.7486

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/3.4 -30 o DIST/3.4 +60
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 Sat Sep 30 21:06:27 CDT 2017