Location

Location ANSS

2017/05/02 13:13:54 63.147 -151.127 12.5 4.2 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/05/02 13:13:54:0  63.15 -151.13  12.5 4.2 Alaska
 
 Stations used:
   AK.BARN AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.DIV AK.DOT 
   AK.EYAK AK.FIRE AK.FYU AK.GHO AK.GLB AK.HDA AK.KLU AK.KNK 
   AK.KTH AK.MCAR AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PAX AK.PPD 
   AK.PPLA AK.PWL AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK 
   AK.SKN AK.SSN AK.SWD AK.TRF AK.VRDI AK.WRH AT.MENT AT.PMR 
   AT.SVW2 AV.ILSW CN.DAWY IM.IL31 IU.COLA TA.H21K TA.H23K 
   TA.H24K TA.I23K TA.J20K TA.J25K TA.J26L TA.K20K TA.L19K 
   TA.L26K TA.L27K TA.M22K TA.M24K TA.M26K TA.M27K TA.N18K 
   TA.N19K TA.N25K TA.O19K TA.O22K TA.POKR TA.Q19K US.EGAK 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.40e+22 dyne-cm
  Mw = 4.03 
  Z  = 15 km
  Plane   Strike  Dip  Rake
   NP1       13    82   -114
   NP2      265    25   -20
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.40e+22     33     123
    N   0.00e+00     23      17
    P  -1.40e+22     48     258

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.67e+21
       Mxy    -5.78e+21
       Mxz    -2.02e+21
       Myy     9.91e+20
       Myz     1.21e+22
       Mzz    -3.66e+21
                                                     
                                                     
                                                     
                                                     
                     ############--                  
                 ################------              
              #############---###---------           
             ######--------------#####-----          
           #####----------------#########----        
          ####------------------###########---       
         ###--------------------#############--      
        ###---------------------##############--     
        #----------------------#################     
       ##----------------------#################-    
       #----------------------###################    
       ---------   -----------###################    
       --------- P -----------###################    
        --------   ----------###################     
        ---------------------#########   #######     
         -------------------########## T ######      
          -----------------###########   #####       
           ----------------##################        
             -------------#################          
              -----------#################           
                 --------##############              
                     ---###########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.66e+21  -2.02e+21  -1.21e+22 
 -2.02e+21   2.67e+21   5.78e+21 
 -1.21e+22   5.78e+21   9.91e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170502131354/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 = 265
      DIP = 25
     RAKE = -20
       MW = 4.03
       HS = 15.0

The NDK file is 20170502131354.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/05/02 13:13:54:0  63.15 -151.13  12.5 4.2 Alaska
 
 Stations used:
   AK.BARN AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.DIV AK.DOT 
   AK.EYAK AK.FIRE AK.FYU AK.GHO AK.GLB AK.HDA AK.KLU AK.KNK 
   AK.KTH AK.MCAR AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PAX AK.PPD 
   AK.PPLA AK.PWL AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK 
   AK.SKN AK.SSN AK.SWD AK.TRF AK.VRDI AK.WRH AT.MENT AT.PMR 
   AT.SVW2 AV.ILSW CN.DAWY IM.IL31 IU.COLA TA.H21K TA.H23K 
   TA.H24K TA.I23K TA.J20K TA.J25K TA.J26L TA.K20K TA.L19K 
   TA.L26K TA.L27K TA.M22K TA.M24K TA.M26K TA.M27K TA.N18K 
   TA.N19K TA.N25K TA.O19K TA.O22K TA.POKR TA.Q19K US.EGAK 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.40e+22 dyne-cm
  Mw = 4.03 
  Z  = 15 km
  Plane   Strike  Dip  Rake
   NP1       13    82   -114
   NP2      265    25   -20
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.40e+22     33     123
    N   0.00e+00     23      17
    P  -1.40e+22     48     258

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.67e+21
       Mxy    -5.78e+21
       Mxz    -2.02e+21
       Myy     9.91e+20
       Myz     1.21e+22
       Mzz    -3.66e+21
                                                     
                                                     
                                                     
                                                     
                     ############--                  
                 ################------              
              #############---###---------           
             ######--------------#####-----          
           #####----------------#########----        
          ####------------------###########---       
         ###--------------------#############--      
        ###---------------------##############--     
        #----------------------#################     
       ##----------------------#################-    
       #----------------------###################    
       ---------   -----------###################    
       --------- P -----------###################    
        --------   ----------###################     
        ---------------------#########   #######     
         -------------------########## T ######      
          -----------------###########   #####       
           ----------------##################        
             -------------#################          
              -----------#################           
                 --------##############              
                     ---###########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.66e+21  -2.02e+21  -1.21e+22 
 -2.02e+21   2.67e+21   5.78e+21 
 -1.21e+22   5.78e+21   9.91e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170502131354/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 +70
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   170    45    90   3.58 0.2454
WVFGRD96    2.0   170    45    90   3.73 0.3233
WVFGRD96    3.0    95    35     5   3.71 0.2514
WVFGRD96    4.0   105    25    15   3.77 0.3079
WVFGRD96    5.0   285    20    10   3.79 0.3742
WVFGRD96    6.0   280    20     5   3.81 0.4324
WVFGRD96    7.0   280    20     5   3.82 0.4777
WVFGRD96    8.0   280    15     5   3.91 0.5086
WVFGRD96    9.0   270    20   -10   3.93 0.5472
WVFGRD96   10.0   270    20   -10   3.95 0.5778
WVFGRD96   11.0   270    20   -10   3.96 0.6006
WVFGRD96   12.0   265    25   -20   3.98 0.6173
WVFGRD96   13.0   265    25   -20   4.00 0.6291
WVFGRD96   14.0   265    25   -20   4.01 0.6355
WVFGRD96   15.0   265    25   -20   4.03 0.6370
WVFGRD96   16.0   265    25   -20   4.04 0.6338
WVFGRD96   17.0   270    25   -15   4.05 0.6277
WVFGRD96   18.0   270    20   -15   4.06 0.6185
WVFGRD96   19.0   265    20   -20   4.07 0.6076
WVFGRD96   20.0   265    20   -20   4.08 0.5944
WVFGRD96   21.0   265    20   -20   4.10 0.5791
WVFGRD96   22.0   260    20   -25   4.11 0.5628
WVFGRD96   23.0   260    20   -25   4.11 0.5462
WVFGRD96   24.0   235    15   -45   4.12 0.5290
WVFGRD96   25.0   235    15   -45   4.13 0.5116
WVFGRD96   26.0   230    10   -50   4.14 0.4936
WVFGRD96   27.0   230    10   -50   4.15 0.4756
WVFGRD96   28.0   235    10   -45   4.15 0.4570
WVFGRD96   29.0   240    10   -35   4.15 0.4384

The best solution is

WVFGRD96   15.0   265    25   -20   4.03 0.6370

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 +70
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 May 2 09:04:11 CDT 2017