Location

Location ANSS

2017/08/31 17:46:02 58.441 -153.642 63.2 5.4 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/08/31 17:46:02:0  58.44 -153.64  63.2 5.4 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.HOM AK.SII AT.OHAK AV.ILSW II.KDAK 
   TA.N18K TA.N19K TA.O18K TA.O19K TA.P18K TA.P19K TA.Q19K 
   TA.Q20K TA.R18K 
 
 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 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 2.16e+24 dyne-cm
  Mw = 5.49 
  Z  = 72 km
  Plane   Strike  Dip  Rake
   NP1      225    80   -70
   NP2      340    22   -153
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.16e+24     32     298
    N   0.00e+00     20      41
    P  -2.16e+24     51     158

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.81e+23
       Mxy    -3.48e+23
       Mxz     1.44e+24
       Myy     1.08e+24
       Myz    -1.26e+24
       Mzz    -6.95e+23
                                                     
                                                     
                                                     
                                                     
                     ######--------                  
                 ################------              
              ######################------           
             #########################---##          
           ###########################--#####        
          ##########################-----#####       
         #####   ################---------#####      
        ###### T ##############------------#####     
        ######   ############---------------####     
       ####################-----------------#####    
       ##################--------------------####    
       #################---------------------####    
       ###############-----------------------####    
        ############-------------------------###     
        ###########-----------   -----------####     
         ########------------- P -----------###      
          ######--------------   ----------###       
           ####---------------------------###        
             #---------------------------##          
              --------------------------##           
                 ---------------------#              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -6.95e+23   1.44e+24   1.26e+24 
  1.44e+24  -3.81e+23   3.48e+23 
  1.26e+24   3.48e+23   1.08e+24 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170831174602/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 = 225
      DIP = 80
     RAKE = -70
       MW = 5.49
       HS = 72.0

The NDK file is 20170831174602.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 17:46:02:0  58.44 -153.64  63.2 5.4 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.HOM AK.SII AT.OHAK AV.ILSW II.KDAK 
   TA.N18K TA.N19K TA.O18K TA.O19K TA.P18K TA.P19K TA.Q19K 
   TA.Q20K TA.R18K 
 
 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 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 2.16e+24 dyne-cm
  Mw = 5.49 
  Z  = 72 km
  Plane   Strike  Dip  Rake
   NP1      225    80   -70
   NP2      340    22   -153
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.16e+24     32     298
    N   0.00e+00     20      41
    P  -2.16e+24     51     158

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.81e+23
       Mxy    -3.48e+23
       Mxz     1.44e+24
       Myy     1.08e+24
       Myz    -1.26e+24
       Mzz    -6.95e+23
                                                     
                                                     
                                                     
                                                     
                     ######--------                  
                 ################------              
              ######################------           
             #########################---##          
           ###########################--#####        
          ##########################-----#####       
         #####   ################---------#####      
        ###### T ##############------------#####     
        ######   ############---------------####     
       ####################-----------------#####    
       ##################--------------------####    
       #################---------------------####    
       ###############-----------------------####    
        ############-------------------------###     
        ###########-----------   -----------####     
         ########------------- P -----------###      
          ######--------------   ----------###       
           ####---------------------------###        
             #---------------------------##          
              --------------------------##           
                 ---------------------#              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -6.95e+23   1.44e+24   1.26e+24 
  1.44e+24  -3.81e+23   3.48e+23 
  1.26e+24   3.48e+23   1.08e+24 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170831174602/index.html
	
Regional Moment Tensor (Mwr)
Moment	2.286e+17 N-m
Magnitude	5.5 Mwr
Depth	69.0 km
Percent DC	79 %
Half Duration	–
Catalog	US
Data Source	US3
Contributor	US3
Nodal Planes
Plane	Strike	Dip	Rake
NP1	3	20	-124
NP2	219	74	-79
Principal Axes
Axis	Value	Plunge	Azimuth
T	2.400e+17 N-m	28	300
N	-0.247e+17 N-m	11	35
P	-2.153e+17 N-m	60	145

        

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 0.08 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    30    40   -90   4.68 0.2368
WVFGRD96    4.0   325    45   -25   4.70 0.2559
WVFGRD96    6.0   330    55   -20   4.73 0.2901
WVFGRD96    8.0   330    50   -10   4.80 0.3135
WVFGRD96   10.0   340    50    15   4.83 0.3291
WVFGRD96   12.0    65    90    35   4.86 0.3505
WVFGRD96   14.0    70    80    35   4.89 0.3694
WVFGRD96   16.0    65    90    35   4.92 0.3858
WVFGRD96   18.0   245    85   -35   4.94 0.4016
WVFGRD96   20.0   240    80   -35   4.97 0.4194
WVFGRD96   22.0   240    80   -35   5.00 0.4377
WVFGRD96   24.0   240    80   -40   5.02 0.4563
WVFGRD96   26.0   240    80   -40   5.05 0.4750
WVFGRD96   28.0   240    75   -40   5.07 0.4927
WVFGRD96   30.0   240    75   -40   5.09 0.5124
WVFGRD96   32.0   240    75   -40   5.11 0.5323
WVFGRD96   34.0   235    70   -45   5.14 0.5490
WVFGRD96   36.0   240    75   -40   5.15 0.5591
WVFGRD96   38.0   240    75   -40   5.17 0.5673
WVFGRD96   40.0   235    75   -55   5.28 0.5906
WVFGRD96   42.0   230    70   -55   5.30 0.5972
WVFGRD96   44.0   235    75   -55   5.31 0.6081
WVFGRD96   46.0   235    75   -55   5.33 0.6200
WVFGRD96   48.0   235    75   -50   5.34 0.6309
WVFGRD96   50.0   235    75   -50   5.36 0.6407
WVFGRD96   52.0   235    75   -50   5.37 0.6497
WVFGRD96   54.0   220    75   -65   5.40 0.6583
WVFGRD96   56.0   220    75   -65   5.41 0.6711
WVFGRD96   58.0   220    75   -65   5.43 0.6830
WVFGRD96   60.0   220    75   -65   5.44 0.6926
WVFGRD96   62.0   220    75   -65   5.45 0.7006
WVFGRD96   64.0   220    75   -75   5.46 0.7100
WVFGRD96   66.0   220    75   -75   5.47 0.7184
WVFGRD96   68.0   220    75   -75   5.48 0.7233
WVFGRD96   70.0   220    75   -75   5.49 0.7252
WVFGRD96   72.0   225    80   -70   5.49 0.7263
WVFGRD96   74.0   225    80   -70   5.49 0.7253
WVFGRD96   76.0   220    80   -75   5.50 0.7227
WVFGRD96   78.0   220    80   -75   5.51 0.7177
WVFGRD96   80.0   220    80   -75   5.51 0.7102
WVFGRD96   82.0   220    80   -75   5.51 0.7003
WVFGRD96   84.0   220    80   -75   5.51 0.6898
WVFGRD96   86.0   225    85   -75   5.51 0.6770
WVFGRD96   88.0   225    85   -70   5.51 0.6659
WVFGRD96   90.0   225    85   -70   5.51 0.6554
WVFGRD96   92.0    50    90    70   5.50 0.6346
WVFGRD96   94.0    50    90    70   5.50 0.6253
WVFGRD96   96.0   225    85   -70   5.51 0.6169
WVFGRD96   98.0    50    90    70   5.50 0.6028

The best solution is

WVFGRD96   72.0   225    80   -70   5.49 0.7263

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 0.08 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 15:52:49 CDT 2017