Location

Location ANSS

2017/11/27 22:18:30 60.563 -147.419 15.7 5.3 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/11/27 22:18:30:0  60.56 -147.42  15.7 5.3 Alaska
 
 Stations used:
   AK.BARN AK.BRLK AK.BWN AK.CAST AK.CNP AK.CUT AK.DIV AK.EYAK 
   AK.GHO AK.GLB AK.GRNC AK.HDA AK.HIN AK.KLU AK.KNK AK.KTH 
   AK.MCAR AK.MCK AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM AK.SWD 
   AK.TGL AK.TRF AK.VRDI AK.WAX AK.WRH AT.MENT AT.PMR TA.L26K 
   TA.M22K TA.M24K TA.M27K TA.N25K TA.O22K TA.Q20K 
 
 Filtering commands used:
   cut o DIST/3.3 -30 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 = 5.07e+23 dyne-cm
  Mw = 5.07 
  Z  = 29 km
  Plane   Strike  Dip  Rake
   NP1      210    85   -70
   NP2      313    21   -166
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.07e+23     37     282
    N   0.00e+00     20      28
    P  -5.07e+23     46     141

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.29e+23
       Mxy     5.05e+22
       Mxz     2.48e+23
       Myy     2.12e+23
       Myz    -3.99e+23
       Mzz    -8.27e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ---#########---------#              
              -###################---#####           
             ######################--######          
           #######################------#####        
          #######################--------#####       
         #######################-----------####      
        ######################--------------####     
        ######   ############----------------###     
       ####### T ###########-----------------####    
       #######   ##########-------------------###    
       ###################--------------------###    
       ##################---------------------###    
        ################----------------------##     
        ###############----------   ----------##     
         #############----------- P ----------#      
          ###########------------   ---------#       
           #########------------------------#        
             ######------------------------          
              #####-----------------------           
                 #---------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -8.27e+22   2.48e+23   3.99e+23 
  2.48e+23  -1.29e+23  -5.05e+22 
  3.99e+23  -5.05e+22   2.12e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171127221830/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 = 210
      DIP = 85
     RAKE = -70
       MW = 5.07
       HS = 29.0

The NDK file is 20171127221830.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSW
 USGS/SLU Moment Tensor Solution
 ENS  2017/11/27 22:18:30:0  60.56 -147.42  15.7 5.3 Alaska
 
 Stations used:
   AK.BARN AK.BRLK AK.BWN AK.CAST AK.CNP AK.CUT AK.DIV AK.EYAK 
   AK.GHO AK.GLB AK.GRNC AK.HDA AK.HIN AK.KLU AK.KNK AK.KTH 
   AK.MCAR AK.MCK AK.PPLA AK.RC01 AK.RND AK.SAW AK.SCM AK.SWD 
   AK.TGL AK.TRF AK.VRDI AK.WAX AK.WRH AT.MENT AT.PMR TA.L26K 
   TA.M22K TA.M24K TA.M27K TA.N25K TA.O22K TA.Q20K 
 
 Filtering commands used:
   cut o DIST/3.3 -30 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 = 5.07e+23 dyne-cm
  Mw = 5.07 
  Z  = 29 km
  Plane   Strike  Dip  Rake
   NP1      210    85   -70
   NP2      313    21   -166
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.07e+23     37     282
    N   0.00e+00     20      28
    P  -5.07e+23     46     141

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.29e+23
       Mxy     5.05e+22
       Mxz     2.48e+23
       Myy     2.12e+23
       Myz    -3.99e+23
       Mzz    -8.27e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ---#########---------#              
              -###################---#####           
             ######################--######          
           #######################------#####        
          #######################--------#####       
         #######################-----------####      
        ######################--------------####     
        ######   ############----------------###     
       ####### T ###########-----------------####    
       #######   ##########-------------------###    
       ###################--------------------###    
       ##################---------------------###    
        ################----------------------##     
        ###############----------   ----------##     
         #############----------- P ----------#      
          ###########------------   ---------#       
           #########------------------------#        
             ######------------------------          
              #####-----------------------           
                 #---------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -8.27e+22   2.48e+23   3.99e+23 
  2.48e+23  -1.29e+23  -5.05e+22 
  3.99e+23  -5.05e+22   2.12e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20171127221830/index.html
	
W-phase Moment Tensor (Mww)
Moment	1.059e+17 N-m
Magnitude	5.3 Mww
Depth	23.5 km
Percent DC	87 %
Half Duration	1.38 s
Catalog	US
Data Source	US3
Contributor	US3
Nodal Planes
Plane	Strike	Dip	Rake
NP1	334	21	-151
NP2	216	80	-71
Principal Axes
Axis	Value	Plunge	Azimuth
T	1.023e+17 N-m	33	291
N	0.069e+17 N-m	18	33
P	-1.092e+17 N-m	51	148

        

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 +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    1.0    30    45    85   4.40 0.1888
WVFGRD96    2.0    35    40    90   4.55 0.2543
WVFGRD96    3.0    35    40    90   4.59 0.2105
WVFGRD96    4.0   300    35    -5   4.55 0.2061
WVFGRD96    5.0   315    25    20   4.58 0.2408
WVFGRD96    6.0   310    30    15   4.59 0.2697
WVFGRD96    7.0   315    30    20   4.61 0.2922
WVFGRD96    8.0    40    85    60   4.69 0.3148
WVFGRD96    9.0    40    85    60   4.71 0.3465
WVFGRD96   10.0    40    85    55   4.74 0.3766
WVFGRD96   11.0    40    85    55   4.76 0.4047
WVFGRD96   12.0    35    90    55   4.78 0.4310
WVFGRD96   13.0   215    90   -55   4.80 0.4560
WVFGRD96   14.0   215    85   -55   4.82 0.4804
WVFGRD96   15.0   215    85   -55   4.85 0.5050
WVFGRD96   16.0   215    85   -55   4.87 0.5278
WVFGRD96   17.0   215    85   -60   4.88 0.5497
WVFGRD96   18.0   210    80   -60   4.90 0.5705
WVFGRD96   19.0   210    80   -65   4.91 0.5897
WVFGRD96   20.0   210    80   -65   4.93 0.6081
WVFGRD96   21.0   210    80   -65   4.96 0.6244
WVFGRD96   22.0   210    80   -65   4.97 0.6397
WVFGRD96   23.0   210    80   -65   4.99 0.6540
WVFGRD96   24.0   210    80   -65   5.01 0.6667
WVFGRD96   25.0   210    80   -65   5.02 0.6782
WVFGRD96   26.0   215    85   -65   5.04 0.6880
WVFGRD96   27.0   215    85   -65   5.05 0.6955
WVFGRD96   28.0   210    85   -70   5.06 0.7009
WVFGRD96   29.0   210    85   -70   5.07 0.7032
WVFGRD96   30.0   210    85   -70   5.09 0.7030
WVFGRD96   31.0   210    85   -65   5.10 0.6998
WVFGRD96   32.0   210    85   -65   5.10 0.6946
WVFGRD96   33.0   210    85   -65   5.11 0.6871
WVFGRD96   34.0    35    90    65   5.12 0.6712
WVFGRD96   35.0    35    90    65   5.12 0.6637
WVFGRD96   36.0    35    90    65   5.13 0.6548
WVFGRD96   37.0    35    90    60   5.13 0.6443
WVFGRD96   38.0    35    90    60   5.14 0.6333
WVFGRD96   39.0    35    90    60   5.14 0.6198
WVFGRD96   40.0    35    90    70   5.27 0.6037
WVFGRD96   41.0    35    90    65   5.26 0.5879
WVFGRD96   42.0    35    90    65   5.26 0.5736
WVFGRD96   43.0   215    90   -65   5.26 0.5576
WVFGRD96   44.0    35    90    60   5.26 0.5433
WVFGRD96   45.0    40    85    60   5.27 0.5304
WVFGRD96   46.0    40    85    60   5.27 0.5180
WVFGRD96   47.0    45    80    60   5.27 0.5077
WVFGRD96   48.0    45    80    55   5.27 0.4986
WVFGRD96   49.0    45    80    55   5.28 0.4887

The best solution is

WVFGRD96   29.0   210    85   -70   5.07 0.7032

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 +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 Mon Nov 27 17:06:13 CST 2017