Location

Location ANSS

2017/01/31 09:38:37 63.073 -150.949 134.8 5.2 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/01/31 09:38:37:0  63.07 -150.95 134.8 5.2 Alaska
 
 Stations used:
   AK.BPAW AK.BRLK AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV 
   AK.DOT AK.FIRE AK.GHO AK.GLB AK.HDA AK.HOM AK.KLU AK.KNK 
   AK.KTH AK.MDM AK.MLY AK.NEA2 AK.PAX AK.PPD AK.PWL AK.RC01 
   AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.SSN AK.SWD 
   AK.TRF AK.WRH AT.MENT AT.PMR AT.TTA IM.IL31 IU.COLA TA.H21K 
   TA.H24K TA.I21K TA.I23K TA.K20K TA.L19K TA.L26K TA.M20K 
   TA.M26K TA.N25K TA.POKR TA.TCOL 
 
 Filtering commands used:
   cut o DIST/4.0 -60 o DIST/4.0 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 6.68e+23 dyne-cm
  Mw = 5.15 
  Z  = 130 km
  Plane   Strike  Dip  Rake
   NP1       45    80    70
   NP2      289    22   153
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.68e+23     51     292
    N   0.00e+00     20      49
    P  -6.68e+23     32     152

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.33e+23
       Mxy     1.07e+23
       Mxz     3.89e+23
       Myy     1.18e+23
       Myz    -4.45e+23
       Mzz     2.15e+23
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ------#############---------           
             ---#####################-----#          
           --###########################-####        
          -#############################--####       
         ##############################----####      
        #############################--------###     
        #########   ################----------##     
       ########## T ##############------------###    
       ##########   ############---------------##    
       ########################----------------##    
       ######################-------------------#    
        ###################---------------------     
        #################-----------------------     
         ##############------------------------      
          ##########-------------   ----------       
           #######--------------- P ---------        
             ##------------------   -------          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.15e+23   3.89e+23   4.45e+23 
  3.89e+23  -3.33e+23  -1.07e+23 
  4.45e+23  -1.07e+23   1.18e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170131093837/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 = 45
      DIP = 80
     RAKE = 70
       MW = 5.15
       HS = 130.0

The NDK file is 20170131093837.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/01/31 09:38:37:0  63.07 -150.95 134.8 5.2 Alaska
 
 Stations used:
   AK.BPAW AK.BRLK AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV 
   AK.DOT AK.FIRE AK.GHO AK.GLB AK.HDA AK.HOM AK.KLU AK.KNK 
   AK.KTH AK.MDM AK.MLY AK.NEA2 AK.PAX AK.PPD AK.PWL AK.RC01 
   AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.SSN AK.SWD 
   AK.TRF AK.WRH AT.MENT AT.PMR AT.TTA IM.IL31 IU.COLA TA.H21K 
   TA.H24K TA.I21K TA.I23K TA.K20K TA.L19K TA.L26K TA.M20K 
   TA.M26K TA.N25K TA.POKR TA.TCOL 
 
 Filtering commands used:
   cut o DIST/4.0 -60 o DIST/4.0 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 6.68e+23 dyne-cm
  Mw = 5.15 
  Z  = 130 km
  Plane   Strike  Dip  Rake
   NP1       45    80    70
   NP2      289    22   153
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.68e+23     51     292
    N   0.00e+00     20      49
    P  -6.68e+23     32     152

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.33e+23
       Mxy     1.07e+23
       Mxz     3.89e+23
       Myy     1.18e+23
       Myz    -4.45e+23
       Mzz     2.15e+23
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ------#############---------           
             ---#####################-----#          
           --###########################-####        
          -#############################--####       
         ##############################----####      
        #############################--------###     
        #########   ################----------##     
       ########## T ##############------------###    
       ##########   ############---------------##    
       ########################----------------##    
       ######################-------------------#    
        ###################---------------------     
        #################-----------------------     
         ##############------------------------      
          ##########-------------   ----------       
           #######--------------- P ---------        
             ##------------------   -------          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.15e+23   3.89e+23   4.45e+23 
  3.89e+23  -3.33e+23  -1.07e+23 
  4.45e+23  -1.07e+23   1.18e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170131093837/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/4.0 -60 o DIST/4.0 +40
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   70.0    45    65   -45   5.01 0.2420
WVFGRD96   72.0    50    75   -40   5.02 0.2466
WVFGRD96   74.0    45    55    45   4.94 0.2669
WVFGRD96   76.0    45    60    50   4.96 0.3227
WVFGRD96   78.0    40    65    45   5.00 0.3882
WVFGRD96   80.0    40    70    50   5.02 0.4550
WVFGRD96   82.0    40    70    50   5.04 0.5153
WVFGRD96   84.0    45    70    50   5.05 0.5641
WVFGRD96   86.0    45    70    55   5.07 0.5970
WVFGRD96   88.0    45    70    55   5.07 0.6102
WVFGRD96   90.0    45    70    55   5.07 0.6194
WVFGRD96   92.0    45    70    55   5.08 0.6249
WVFGRD96   94.0    45    75    60   5.09 0.6328
WVFGRD96   96.0    50    75    65   5.09 0.6385
WVFGRD96   98.0    50    75    65   5.10 0.6464
WVFGRD96  100.0    50    75    65   5.10 0.6528
WVFGRD96  102.0    50    75    65   5.10 0.6584
WVFGRD96  104.0    50    75    65   5.11 0.6642
WVFGRD96  106.0    50    75    65   5.11 0.6690
WVFGRD96  108.0    50    75    65   5.11 0.6737
WVFGRD96  110.0    50    75    65   5.11 0.6771
WVFGRD96  112.0    45    80    70   5.13 0.6804
WVFGRD96  114.0    45    80    70   5.13 0.6841
WVFGRD96  116.0    45    80    70   5.13 0.6861
WVFGRD96  118.0    45    80    70   5.14 0.6896
WVFGRD96  120.0    45    80    70   5.14 0.6906
WVFGRD96  122.0    45    80    70   5.14 0.6933
WVFGRD96  124.0    45    80    70   5.14 0.6930
WVFGRD96  126.0    45    80    70   5.14 0.6959
WVFGRD96  128.0    45    80    70   5.14 0.6951
WVFGRD96  130.0    45    80    70   5.15 0.6963
WVFGRD96  132.0    45    80    70   5.15 0.6954
WVFGRD96  134.0    45    80    70   5.15 0.6942
WVFGRD96  136.0    50    80    70   5.15 0.6950
WVFGRD96  138.0    45    80    70   5.15 0.6922
WVFGRD96  140.0    50    80    70   5.15 0.6922
WVFGRD96  142.0    45    80    70   5.15 0.6901
WVFGRD96  144.0    45    80    70   5.16 0.6894
WVFGRD96  146.0    45    80    70   5.16 0.6877
WVFGRD96  148.0    45    80    70   5.16 0.6843
WVFGRD96  150.0    45    80    70   5.16 0.6834
WVFGRD96  152.0    45    80    70   5.16 0.6793
WVFGRD96  154.0    45    80    70   5.16 0.6774
WVFGRD96  156.0    50    75    70   5.15 0.6759
WVFGRD96  158.0    50    75    70   5.16 0.6719

The best solution is

WVFGRD96  130.0    45    80    70   5.15 0.6963

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/4.0 -60 o DIST/4.0 +40
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 Tue Jan 31 08:01:34 CST 2017