Location

Location ANSS

2017/08/31 07:53:22 63.015 -150.537 103.0 4.6 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/08/31 07:53:22:0  63.01 -150.54 103.0 4.6 Alaska
 
 Stations used:
   AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.GHO AK.GLI AK.HDA 
   AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PPLA AK.PWL 
   AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AK.TRF AK.WRH AT.PMR 
   AT.TTA IM.IL31 IU.COLA TA.J20K TA.K20K TA.L19K TA.M19K 
   TA.M20K TA.M22K TA.O22K TA.POKR TA.TCOL 
 
 Filtering commands used:
   cut o DIST/3.5 -40 o DIST/3.5 +60
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 5.37e+22 dyne-cm
  Mw = 4.42 
  Z  = 96 km
  Plane   Strike  Dip  Rake
   NP1       90    70    80
   NP2      297    22   116
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.37e+22     64     344
    N   0.00e+00      9      93
    P  -5.37e+22     24     188

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.40e+22
       Mxy    -8.76e+21
       Mxz     4.05e+22
       Myy    -7.66e+14
       Myz    -3.19e+21
       Mzz     3.40e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ----###############---------           
             -#######################------          
           -###########################------        
          ###############################-----       
         ################   ###############----      
        ################# T ################----     
        #################   #################---     
       #######################################---    
       ########################################--    
       --########################################    
       ------##############################----##    
        -------------##############------------#     
        ---------------------------------------#     
         --------------------------------------      
          ------------------------------------       
           ----------------------------------        
             -----------   ----------------          
              ---------- P ---------------           
                 -------   ------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  3.40e+22   4.05e+22   3.19e+21 
  4.05e+22  -3.40e+22   8.76e+21 
  3.19e+21   8.76e+21  -7.66e+14 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170831075322/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 = 90
      DIP = 70
     RAKE = 80
       MW = 4.42
       HS = 96.0

The NDK file is 20170831075322.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  2017/08/31 07:53:22:0  63.01 -150.54 103.0 4.6 Alaska
 
 Stations used:
   AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.GHO AK.GLI AK.HDA 
   AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PPLA AK.PWL 
   AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN AK.TRF AK.WRH AT.PMR 
   AT.TTA IM.IL31 IU.COLA TA.J20K TA.K20K TA.L19K TA.M19K 
   TA.M20K TA.M22K TA.O22K TA.POKR TA.TCOL 
 
 Filtering commands used:
   cut o DIST/3.5 -40 o DIST/3.5 +60
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 5.37e+22 dyne-cm
  Mw = 4.42 
  Z  = 96 km
  Plane   Strike  Dip  Rake
   NP1       90    70    80
   NP2      297    22   116
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.37e+22     64     344
    N   0.00e+00      9      93
    P  -5.37e+22     24     188

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.40e+22
       Mxy    -8.76e+21
       Mxz     4.05e+22
       Myy    -7.66e+14
       Myz    -3.19e+21
       Mzz     3.40e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ----###############---------           
             -#######################------          
           -###########################------        
          ###############################-----       
         ################   ###############----      
        ################# T ################----     
        #################   #################---     
       #######################################---    
       ########################################--    
       --########################################    
       ------##############################----##    
        -------------##############------------#     
        ---------------------------------------#     
         --------------------------------------      
          ------------------------------------       
           ----------------------------------        
             -----------   ----------------          
              ---------- P ---------------           
                 -------   ------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  3.40e+22   4.05e+22   3.19e+21 
  4.05e+22  -3.40e+22   8.76e+21 
  3.19e+21   8.76e+21  -7.66e+14 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170831075322/index.html
	
Regional Moment Tensor (Mwr)
Moment	6.759e+15 N-m
Magnitude	4.5 Mwr
Depth	100.0 km
Percent DC	88 %
Half Duration	–
Catalog	US
Data Source	US2
Contributor	US2
Nodal Planes
Plane	Strike	Dip	Rake
NP1	278	21	93
NP2	95	69	89
Principal Axes
Axis	Value	Plunge	Azimuth
T	6.534e+15 N-m	66	3
N	0.429e+15 N-m	1	95
P	-6.963e+15 N-m	24	186

        

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/3.5 -40 o DIST/3.5 +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   280    50   -80   3.59 0.2215
WVFGRD96    4.0   325    55    25   3.61 0.1933
WVFGRD96    6.0   330    55    30   3.67 0.2336
WVFGRD96    8.0   330    55    30   3.75 0.2553
WVFGRD96   10.0   330    55    25   3.79 0.2699
WVFGRD96   12.0   330    55    20   3.82 0.2751
WVFGRD96   14.0   330    60    20   3.85 0.2730
WVFGRD96   16.0   330    55    10   3.87 0.2674
WVFGRD96   18.0   330    55    10   3.89 0.2594
WVFGRD96   20.0   245    65    45   3.91 0.2574
WVFGRD96   22.0   245    75    45   3.94 0.2610
WVFGRD96   24.0   240    75    40   3.97 0.2676
WVFGRD96   26.0   240    75    40   3.99 0.2747
WVFGRD96   28.0   240    75    35   4.02 0.2815
WVFGRD96   30.0   240    75    35   4.03 0.2883
WVFGRD96   32.0   235    75    30   4.05 0.2925
WVFGRD96   34.0   235    75    30   4.07 0.2925
WVFGRD96   36.0   235    70    30   4.08 0.2951
WVFGRD96   38.0   230    75    25   4.11 0.3038
WVFGRD96   40.0   240    65    50   4.20 0.3509
WVFGRD96   42.0   245    65    55   4.24 0.3815
WVFGRD96   44.0   245    65    50   4.26 0.4025
WVFGRD96   46.0   245    65    50   4.28 0.4152
WVFGRD96   48.0   250    55    55   4.29 0.4218
WVFGRD96   50.0   250    55    55   4.30 0.4281
WVFGRD96   52.0    75    55    55   4.32 0.4402
WVFGRD96   54.0    75    55    55   4.33 0.4604
WVFGRD96   56.0    80    55    65   4.33 0.4795
WVFGRD96   58.0    85    55    70   4.34 0.4991
WVFGRD96   60.0    85    55    70   4.34 0.5155
WVFGRD96   62.0    85    60    70   4.36 0.5321
WVFGRD96   64.0    85    60    70   4.36 0.5471
WVFGRD96   66.0    90    60    80   4.36 0.5626
WVFGRD96   68.0    90    60    80   4.37 0.5793
WVFGRD96   70.0    90    60    80   4.37 0.5930
WVFGRD96   72.0    90    65    80   4.38 0.6066
WVFGRD96   74.0    90    65    80   4.39 0.6188
WVFGRD96   76.0    90    65    80   4.39 0.6308
WVFGRD96   78.0    90    65    80   4.39 0.6388
WVFGRD96   80.0    90    65    80   4.40 0.6463
WVFGRD96   82.0    90    65    80   4.40 0.6531
WVFGRD96   84.0    90    70    80   4.41 0.6577
WVFGRD96   86.0    90    70    80   4.42 0.6645
WVFGRD96   88.0    90    70    80   4.42 0.6677
WVFGRD96   90.0    90    70    80   4.42 0.6719
WVFGRD96   92.0    90    70    80   4.42 0.6736
WVFGRD96   94.0    90    70    80   4.42 0.6750
WVFGRD96   96.0    90    70    80   4.42 0.6758
WVFGRD96   98.0    90    70    80   4.42 0.6739
WVFGRD96  100.0    90    70    80   4.43 0.6748
WVFGRD96  102.0    90    70    80   4.43 0.6737
WVFGRD96  104.0    90    70    80   4.43 0.6728
WVFGRD96  106.0    90    70    80   4.43 0.6704
WVFGRD96  108.0    90    70    80   4.43 0.6672
WVFGRD96  110.0    90    70    80   4.43 0.6659
WVFGRD96  112.0    90    75    80   4.45 0.6628
WVFGRD96  114.0    90    75    80   4.45 0.6614
WVFGRD96  116.0    90    75    80   4.45 0.6570
WVFGRD96  118.0   255    15    65   4.45 0.6555

The best solution is

WVFGRD96   96.0    90    70    80   4.42 0.6758

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/3.5 -40 o DIST/3.5 +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 Thu Aug 31 05:54:26 CDT 2017