Location

Location ANSS

2017/09/21 22:15:21 62.781 -148.845 19.3 4.2 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/09/21 22:15:21:0  62.78 -148.85  19.3 4.2 Alaska
 
 Stations used:
   AK.BMR AK.BPAW AK.BWN AK.CAST AK.CCB AK.CRQ AK.CTG AK.CUT 
   AK.DHY AK.DIV AK.FID AK.GHO AK.GLB AK.GLI AK.HDA AK.HIN 
   AK.KAI AK.KLU AK.KNK AK.KTH AK.MCAR AK.MCK AK.MDM AK.MLY 
   AK.NEA2 AK.PPD AK.PPLA AK.PWL AK.RC01 AK.RIDG AK.RND AK.SAW 
   AK.SCM AK.SCRK AK.SKN AK.SSN AK.TRF AK.VRDI AK.WRH AT.MENT 
   AT.MID AT.PMR AT.SVW2 AT.TTA IM.IL31 TA.H21K TA.H23K 
   TA.H24K TA.J20K TA.J25K TA.J26L TA.K20K TA.L19K TA.L26K 
   TA.L27K TA.M19K TA.M22K TA.M24K TA.M26K TA.M27K TA.N19K 
   TA.N25K TA.O22K TA.POKR TA.Q23K 
 
 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 = 1.72e+22 dyne-cm
  Mw = 4.09 
  Z  = 18 km
  Plane   Strike  Dip  Rake
   NP1      195    60    60
   NP2       64    41   131
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.72e+22     62      56
    N   0.00e+00     26     211
    P  -1.72e+22     10     306

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.58e+21
       Mxy     9.66e+21
       Mxz     2.22e+21
       Myy    -8.30e+21
       Myz     8.30e+21
       Mzz     1.29e+22
                                                     
                                                     
                                                     
                                                     
                     -----------###                  
                 -------------#########              
              --------------##############           
               -----------#################          
           - P ----------####################        
          --   ---------######################       
         --------------########################      
        --------------###########   ###########-     
        -------------############ T ##########--     
       --------------############   ##########---    
       -------------#########################----    
       -------------########################-----    
       -------------#######################------    
        -----------######################-------     
        -----------#####################--------     
         ----------##################----------      
          #--------################-----------       
           ###------###########--------------        
             #######-----------------------          
              #######---------------------           
                 ####------------------              
                     #-------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.29e+22   2.22e+21  -8.30e+21 
  2.22e+21  -4.58e+21  -9.66e+21 
 -8.30e+21  -9.66e+21  -8.30e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170921221521/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 = 195
      DIP = 60
     RAKE = 60
       MW = 4.09
       HS = 18.0

The NDK file is 20170921221521.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/09/21 22:15:21:0  62.78 -148.85  19.3 4.2 Alaska
 
 Stations used:
   AK.BMR AK.BPAW AK.BWN AK.CAST AK.CCB AK.CRQ AK.CTG AK.CUT 
   AK.DHY AK.DIV AK.FID AK.GHO AK.GLB AK.GLI AK.HDA AK.HIN 
   AK.KAI AK.KLU AK.KNK AK.KTH AK.MCAR AK.MCK AK.MDM AK.MLY 
   AK.NEA2 AK.PPD AK.PPLA AK.PWL AK.RC01 AK.RIDG AK.RND AK.SAW 
   AK.SCM AK.SCRK AK.SKN AK.SSN AK.TRF AK.VRDI AK.WRH AT.MENT 
   AT.MID AT.PMR AT.SVW2 AT.TTA IM.IL31 TA.H21K TA.H23K 
   TA.H24K TA.J20K TA.J25K TA.J26L TA.K20K TA.L19K TA.L26K 
   TA.L27K TA.M19K TA.M22K TA.M24K TA.M26K TA.M27K TA.N19K 
   TA.N25K TA.O22K TA.POKR TA.Q23K 
 
 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 = 1.72e+22 dyne-cm
  Mw = 4.09 
  Z  = 18 km
  Plane   Strike  Dip  Rake
   NP1      195    60    60
   NP2       64    41   131
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.72e+22     62      56
    N   0.00e+00     26     211
    P  -1.72e+22     10     306

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.58e+21
       Mxy     9.66e+21
       Mxz     2.22e+21
       Myy    -8.30e+21
       Myz     8.30e+21
       Mzz     1.29e+22
                                                     
                                                     
                                                     
                                                     
                     -----------###                  
                 -------------#########              
              --------------##############           
               -----------#################          
           - P ----------####################        
          --   ---------######################       
         --------------########################      
        --------------###########   ###########-     
        -------------############ T ##########--     
       --------------############   ##########---    
       -------------#########################----    
       -------------########################-----    
       -------------#######################------    
        -----------######################-------     
        -----------#####################--------     
         ----------##################----------      
          #--------################-----------       
           ###------###########--------------        
             #######-----------------------          
              #######---------------------           
                 ####------------------              
                     #-------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.29e+22   2.22e+21  -8.30e+21 
  2.22e+21  -4.58e+21  -9.66e+21 
 -8.30e+21  -9.66e+21  -8.30e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170921221521/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 -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   225    45   -85   3.62 0.2969
WVFGRD96    2.0    40    45   -95   3.76 0.3871
WVFGRD96    3.0   245    45   -65   3.77 0.2939
WVFGRD96    4.0     0    40     0   3.72 0.2788
WVFGRD96    5.0    -5    40    -5   3.74 0.3128
WVFGRD96    6.0   200    85   -65   3.79 0.3446
WVFGRD96    7.0    65    25   -45   3.81 0.3810
WVFGRD96    8.0    65    25   -50   3.90 0.4088
WVFGRD96    9.0    65    30   -50   3.93 0.4407
WVFGRD96   10.0    65    35   -50   3.95 0.4679
WVFGRD96   11.0   195    65    65   3.97 0.5075
WVFGRD96   12.0   195    60    60   3.99 0.5461
WVFGRD96   13.0   195    60    60   4.01 0.5781
WVFGRD96   14.0   195    60    60   4.03 0.6033
WVFGRD96   15.0   195    60    60   4.05 0.6217
WVFGRD96   16.0   195    60    60   4.06 0.6342
WVFGRD96   17.0   195    60    60   4.08 0.6409
WVFGRD96   18.0   195    60    60   4.09 0.6420
WVFGRD96   19.0   195    60    60   4.10 0.6382
WVFGRD96   20.0   195    60    60   4.11 0.6297
WVFGRD96   21.0   195    60    60   4.13 0.6167
WVFGRD96   22.0   190    60    60   4.14 0.6021
WVFGRD96   23.0   190    60    60   4.14 0.5854
WVFGRD96   24.0   190    60    60   4.15 0.5670
WVFGRD96   25.0   190    60    60   4.15 0.5472
WVFGRD96   26.0   190    60    60   4.16 0.5261
WVFGRD96   27.0   190    60    60   4.16 0.5044
WVFGRD96   28.0   190    60    60   4.17 0.4817
WVFGRD96   29.0   190    60    60   4.17 0.4581

The best solution is

WVFGRD96   18.0   195    60    60   4.09 0.6420

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 Thu Sep 21 18:00:29 CDT 2017