Location

Location ANSS

2017/04/26 03:29:09 62.210 -149.372 41.4 4.1 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/04/26 03:29:09:0  62.21 -149.37  41.4 4.1 Alaska
 
 Stations used:
   AK.BPAW AK.BWN AK.CAPN AK.CAST AK.CUT AK.DIV AK.GHO AK.KLU 
   AK.KNK AK.KTH AK.MCK AK.NEA2 AK.PAX AK.PPLA AK.PWL AK.RC01 
   AK.RIDG AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AK.TRF AT.PMR 
   TA.M20K TA.M22K TA.M24K TA.N25K TA.O22K 
 
 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 = 2.26e+22 dyne-cm
  Mw = 4.17 
  Z  = 58 km
  Plane   Strike  Dip  Rake
   NP1       50    50   -75
   NP2      207    42   -107
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.26e+22      4     129
    N   0.00e+00     11     220
    P  -2.26e+22     78      21

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     8.22e+21
       Mxy    -1.14e+22
       Mxz    -5.33e+21
       Myy     1.33e+22
       Myz    -4.45e+20
       Mzz    -2.15e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ##############--------              
              #############---------------           
             ###########-------------------          
           ###########----------------------#        
          ###########-----------------------##       
         ##########-------------------------###      
        ##########--------------------------####     
        #########-----------   ------------#####     
       #########------------ P -----------#######    
       #########------------   -----------#######    
       ########-------------------------#########    
       ########------------------------##########    
        #######-----------------------##########     
        ######----------------------############     
         ######-------------------#############      
          #####----------------###########   #       
           ####-------------############## T         
             -##------####################           
              ---#########################           
                 -#####################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.15e+22  -5.33e+21   4.45e+20 
 -5.33e+21   8.22e+21   1.14e+22 
  4.45e+20   1.14e+22   1.33e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170426032909/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 = 50
     RAKE = -75
       MW = 4.17
       HS = 58.0

The NDK file is 20170426032909.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMT
 USGS/SLU Moment Tensor Solution
 ENS  2017/04/26 03:29:09:0  62.21 -149.37  41.4 4.1 Alaska
 
 Stations used:
   AK.BPAW AK.BWN AK.CAPN AK.CAST AK.CUT AK.DIV AK.GHO AK.KLU 
   AK.KNK AK.KTH AK.MCK AK.NEA2 AK.PAX AK.PPLA AK.PWL AK.RC01 
   AK.RIDG AK.RND AK.SAW AK.SCM AK.SKN AK.SSN AK.TRF AT.PMR 
   TA.M20K TA.M22K TA.M24K TA.N25K TA.O22K 
 
 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 = 2.26e+22 dyne-cm
  Mw = 4.17 
  Z  = 58 km
  Plane   Strike  Dip  Rake
   NP1       50    50   -75
   NP2      207    42   -107
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.26e+22      4     129
    N   0.00e+00     11     220
    P  -2.26e+22     78      21

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     8.22e+21
       Mxy    -1.14e+22
       Mxz    -5.33e+21
       Myy     1.33e+22
       Myz    -4.45e+20
       Mzz    -2.15e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ##############--------              
              #############---------------           
             ###########-------------------          
           ###########----------------------#        
          ###########-----------------------##       
         ##########-------------------------###      
        ##########--------------------------####     
        #########-----------   ------------#####     
       #########------------ P -----------#######    
       #########------------   -----------#######    
       ########-------------------------#########    
       ########------------------------##########    
        #######-----------------------##########     
        ######----------------------############     
         ######-------------------#############      
          #####----------------###########   #       
           ####-------------############## T         
             -##------####################           
              ---#########################           
                 -#####################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.15e+22  -5.33e+21   4.45e+20 
 -5.33e+21   8.22e+21   1.14e+22 
  4.45e+20   1.14e+22   1.33e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170426032909/index.html
	
Regional Moment Tensor (Mwr)
Moment	2.351e+15 N-m
Magnitude	4.2 Mwr
Depth	59.0 km
Percent DC	89 %
Half Duration	–
Catalog	AK
Data Source	US3
Contributor	US3
Nodal Planes
Plane	Strike	Dip	Rake
NP1	206	48	-110
NP2	54	46	-69
Principal Axes
Axis	Value	Plunge	Azimuth
T	2.283e+15 N-m	1	310
N	0.130e+15 N-m	15	219
P	-2.413e+15 N-m	75	44

        

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    2.0   225    50    95   3.45 0.2112
WVFGRD96    4.0   190    25    40   3.49 0.1979
WVFGRD96    6.0   185    30    35   3.50 0.2275
WVFGRD96    8.0   205    65    80   3.62 0.2519
WVFGRD96   10.0   200    65    70   3.64 0.2647
WVFGRD96   12.0    45    50   -70   3.67 0.2675
WVFGRD96   14.0    50    50   -65   3.69 0.2659
WVFGRD96   16.0    55    50   -60   3.72 0.2624
WVFGRD96   18.0    55    50   -60   3.74 0.2586
WVFGRD96   20.0    60    55   -55   3.76 0.2513
WVFGRD96   22.0    70    50   -40   3.79 0.2450
WVFGRD96   24.0    70    50   -40   3.80 0.2425
WVFGRD96   26.0   265    70    50   3.82 0.2440
WVFGRD96   28.0   265    75    50   3.84 0.2439
WVFGRD96   30.0    70    85   -50   3.86 0.2471
WVFGRD96   32.0    70    80   -50   3.88 0.2583
WVFGRD96   34.0    70    65   -50   3.90 0.2855
WVFGRD96   36.0    70    65   -50   3.92 0.3186
WVFGRD96   38.0    65    60   -55   3.95 0.3513
WVFGRD96   40.0    60    60   -65   4.06 0.3908
WVFGRD96   42.0    60    60   -65   4.09 0.4076
WVFGRD96   44.0    60    60   -65   4.11 0.4250
WVFGRD96   46.0    55    55   -70   4.13 0.4405
WVFGRD96   48.0    55    55   -70   4.14 0.4521
WVFGRD96   50.0    55    55   -70   4.15 0.4590
WVFGRD96   52.0    55    55   -70   4.16 0.4643
WVFGRD96   54.0    55    55   -70   4.16 0.4659
WVFGRD96   56.0    50    50   -75   4.17 0.4682
WVFGRD96   58.0    50    50   -75   4.17 0.4686
WVFGRD96   60.0    50    50   -75   4.18 0.4682
WVFGRD96   62.0    50    50   -75   4.18 0.4668
WVFGRD96   64.0    55    50   -70   4.18 0.4643
WVFGRD96   66.0    55    50   -70   4.18 0.4609
WVFGRD96   68.0    50    45   -75   4.18 0.4568
WVFGRD96   70.0    50    45   -75   4.18 0.4535
WVFGRD96   72.0    50    45   -75   4.18 0.4500
WVFGRD96   74.0    55    45   -70   4.18 0.4458
WVFGRD96   76.0    55    45   -70   4.18 0.4407
WVFGRD96   78.0    55    45   -70   4.18 0.4364
WVFGRD96   80.0    55    45   -70   4.18 0.4325
WVFGRD96   82.0    55    45   -70   4.18 0.4279
WVFGRD96   84.0    55    45   -70   4.18 0.4239
WVFGRD96   86.0    60    45   -65   4.19 0.4200
WVFGRD96   88.0    60    45   -65   4.19 0.4149
WVFGRD96   90.0    60    45   -65   4.19 0.4110
WVFGRD96   92.0    60    45   -65   4.19 0.4073
WVFGRD96   94.0    60    45   -65   4.19 0.4031
WVFGRD96   96.0    60    45   -65   4.19 0.3983
WVFGRD96   98.0    60    45   -65   4.19 0.3940

The best solution is

WVFGRD96   58.0    50    50   -75   4.17 0.4686

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 Wed Apr 26 05:53:36 CDT 2017