Location

Location ANSS

2017/05/26 04:27:53 60.197 -146.218 9.1 3.8 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/05/26 04:27:53:0  60.20 -146.22   9.1 3.8 Alaska
 
 Stations used:
   AK.BARN AK.BRLK AK.CNP AK.CTG AK.CUT AK.DIV AK.DOT AK.EYAK 
   AK.FIRE AK.GHO AK.GLB AK.GLI AK.HIN AK.HMT AK.HOM AK.ISLE 
   AK.KLU AK.KNK AK.LOGN AK.MCAR AK.MCK AK.PAX AK.PPLA AK.PWL 
   AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SWD AK.TABL 
   AK.TGL AK.TRF AK.VRDI AK.WAX AT.MENT AT.PMR TA.L26K TA.L27K 
   TA.M22K TA.M24K TA.M26K TA.M27K TA.N25K TA.O22K TA.Q19K 
 
 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 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 9.23e+21 dyne-cm
  Mw = 3.91 
  Z  = 12 km
  Plane   Strike  Dip  Rake
   NP1      292    80   107
   NP2       50    20    30
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   9.23e+21     52     222
    N   0.00e+00     17     108
    P  -9.23e+21     33       7

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.43e+21
       Mxy     9.86e+20
       Mxz    -7.53e+21
       Myy     1.47e+21
       Myz    -3.48e+21
       Mzz     2.97e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              --------------   -----------           
             --------------- P ------------          
           -----------------   --------------        
          -----------------------------------#       
         -------------------------------------#      
        ########------------------------------##     
        ###############-----------------------##     
       #####################-------------------##    
       #########################--------------###    
       #############################----------###    
       #################################------###    
        ############   ####################--###     
        ############ T #####################--##     
         ###########   ####################----      
          ################################----       
           #############################-----        
             #########################-----          
              --###################-------           
                 ----########----------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.97e+21  -7.53e+21   3.48e+21 
 -7.53e+21  -4.43e+21  -9.86e+20 
  3.48e+21  -9.86e+20   1.47e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170526042753/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 = 50
      DIP = 20
     RAKE = 30
       MW = 3.91
       HS = 12.0

The NDK file is 20170526042753.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/05/26 04:27:53:0  60.20 -146.22   9.1 3.8 Alaska
 
 Stations used:
   AK.BARN AK.BRLK AK.CNP AK.CTG AK.CUT AK.DIV AK.DOT AK.EYAK 
   AK.FIRE AK.GHO AK.GLB AK.GLI AK.HIN AK.HMT AK.HOM AK.ISLE 
   AK.KLU AK.KNK AK.LOGN AK.MCAR AK.MCK AK.PAX AK.PPLA AK.PWL 
   AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SWD AK.TABL 
   AK.TGL AK.TRF AK.VRDI AK.WAX AT.MENT AT.PMR TA.L26K TA.L27K 
   TA.M22K TA.M24K TA.M26K TA.M27K TA.N25K TA.O22K TA.Q19K 
 
 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 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 9.23e+21 dyne-cm
  Mw = 3.91 
  Z  = 12 km
  Plane   Strike  Dip  Rake
   NP1      292    80   107
   NP2       50    20    30
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   9.23e+21     52     222
    N   0.00e+00     17     108
    P  -9.23e+21     33       7

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.43e+21
       Mxy     9.86e+20
       Mxz    -7.53e+21
       Myy     1.47e+21
       Myz    -3.48e+21
       Mzz     2.97e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              --------------   -----------           
             --------------- P ------------          
           -----------------   --------------        
          -----------------------------------#       
         -------------------------------------#      
        ########------------------------------##     
        ###############-----------------------##     
       #####################-------------------##    
       #########################--------------###    
       #############################----------###    
       #################################------###    
        ############   ####################--###     
        ############ T #####################--##     
         ###########   ####################----      
          ################################----       
           #############################-----        
             #########################-----          
              --###################-------           
                 ----########----------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.97e+21  -7.53e+21   3.48e+21 
 -7.53e+21  -4.43e+21  -9.86e+20 
  3.48e+21  -9.86e+20   1.47e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170526042753/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 0.06 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   105    50    90   3.63 0.3203
WVFGRD96    2.0   105    50    90   3.73 0.3846
WVFGRD96    3.0   105    55    90   3.79 0.3557
WVFGRD96    4.0   110    90   -80   3.84 0.3458
WVFGRD96    5.0    30    10    10   3.85 0.4163
WVFGRD96    6.0    40    10    20   3.84 0.4675
WVFGRD96    7.0    50    10    30   3.83 0.5016
WVFGRD96    8.0    50    10    30   3.91 0.5224
WVFGRD96    9.0    45    15    25   3.91 0.5407
WVFGRD96   10.0    55    15    35   3.91 0.5534
WVFGRD96   11.0    60    15    40   3.91 0.5603
WVFGRD96   12.0    50    20    30   3.91 0.5621
WVFGRD96   13.0    40    20    20   3.91 0.5602
WVFGRD96   14.0    40    20    20   3.91 0.5550
WVFGRD96   15.0    35    25    15   3.92 0.5474
WVFGRD96   16.0    35    25    15   3.93 0.5388
WVFGRD96   17.0    35    25    15   3.93 0.5285
WVFGRD96   18.0    35    25    15   3.93 0.5171
WVFGRD96   19.0    30    25    10   3.93 0.5054
WVFGRD96   20.0    25    25     5   3.94 0.4935
WVFGRD96   21.0    25    25     5   3.95 0.4821
WVFGRD96   22.0    25    25     5   3.96 0.4692
WVFGRD96   23.0    25    25     5   3.96 0.4570
WVFGRD96   24.0    25    25     5   3.97 0.4445
WVFGRD96   25.0    25    25     5   3.97 0.4318
WVFGRD96   26.0    25    25     5   3.98 0.4187
WVFGRD96   27.0    30    25    10   3.98 0.4058
WVFGRD96   28.0    30    25    10   3.98 0.3929
WVFGRD96   29.0    30    25    10   3.99 0.3797

The best solution is

WVFGRD96   12.0    50    20    30   3.91 0.5621

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 0.06 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 Fri May 26 04:14:04 CDT 2017