Location

Location ANSS

2016/04/21 10:14:00 63.038 -151.599 11.3 4.2 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/04/21 10:14:00:0  63.04 -151.60  11.3 4.2 Alaska
 
 Stations used:
   AK.BMR AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV 
   AK.DOT AK.EYAK AK.FID AK.GHO AK.GLB AK.GLI AK.KLU AK.KNK 
   AK.KTH AK.MCK AK.MDM AK.NEA2 AK.PAX AK.PPLA AK.PWL AK.RC01 
   AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SWD AK.TRF AK.WRH 
   AT.MENT AT.PMR AT.SVW2 AT.TTA AV.ILSW IM.IL31 IU.COLA 
   TA.H21K TA.H23K TA.H24K TA.I21K TA.I23K TA.J20K TA.J26L 
   TA.K20K TA.L19K TA.L26K TA.M22K TA.M24K TA.N18K TA.N19K 
   TA.N25K TA.O18K TA.O19K TA.P19K TA.POKR TA.TCOL 
 
 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 = 1.22e+22 dyne-cm
  Mw = 3.99 
  Z  = 15 km
  Plane   Strike  Dip  Rake
   NP1       68    62   139
   NP2      180    55    35
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.22e+22     48      31
    N   0.00e+00     42     219
    P  -1.22e+22      4     125

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.43e+15
       Mxy     8.16e+21
       Mxz     5.71e+21
       Myy    -6.56e+21
       Myz     2.39e+21
       Mzz     6.56e+21
                                                     
                                                     
                                                     
                                                     
                     -----#########                  
                 --------##############              
              ---------###################           
             ---------#####################          
           ----------########################        
          -----------###########   ###########       
         -----------############ T ############      
        ------------############   ############-     
        -----------###########################--     
       ------------#########################-----    
       ------------########################------    
       ------------#####################---------    
       ------------###################-----------    
        -----------################-------------     
        ------------###########-----------------     
         #----------#####----------------------      
          ###########----------------------          
           ##########---------------------- P        
             #########---------------------          
              #########-------------------           
                 ########--------------              
                     #####---------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  6.56e+21   5.71e+21  -2.39e+21 
  5.71e+21  -1.43e+15  -8.16e+21 
 -2.39e+21  -8.16e+21  -6.56e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160421101400/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 = 180
      DIP = 55
     RAKE = 35
       MW = 3.99
       HS = 15.0

The NDK file is 20160421101400.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2016/04/21 10:14:00:0  63.04 -151.60  11.3 4.2 Alaska
 
 Stations used:
   AK.BMR AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV 
   AK.DOT AK.EYAK AK.FID AK.GHO AK.GLB AK.GLI AK.KLU AK.KNK 
   AK.KTH AK.MCK AK.MDM AK.NEA2 AK.PAX AK.PPLA AK.PWL AK.RC01 
   AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SWD AK.TRF AK.WRH 
   AT.MENT AT.PMR AT.SVW2 AT.TTA AV.ILSW IM.IL31 IU.COLA 
   TA.H21K TA.H23K TA.H24K TA.I21K TA.I23K TA.J20K TA.J26L 
   TA.K20K TA.L19K TA.L26K TA.M22K TA.M24K TA.N18K TA.N19K 
   TA.N25K TA.O18K TA.O19K TA.P19K TA.POKR TA.TCOL 
 
 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 = 1.22e+22 dyne-cm
  Mw = 3.99 
  Z  = 15 km
  Plane   Strike  Dip  Rake
   NP1       68    62   139
   NP2      180    55    35
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.22e+22     48      31
    N   0.00e+00     42     219
    P  -1.22e+22      4     125

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.43e+15
       Mxy     8.16e+21
       Mxz     5.71e+21
       Myy    -6.56e+21
       Myz     2.39e+21
       Mzz     6.56e+21
                                                     
                                                     
                                                     
                                                     
                     -----#########                  
                 --------##############              
              ---------###################           
             ---------#####################          
           ----------########################        
          -----------###########   ###########       
         -----------############ T ############      
        ------------############   ############-     
        -----------###########################--     
       ------------#########################-----    
       ------------########################------    
       ------------#####################---------    
       ------------###################-----------    
        -----------################-------------     
        ------------###########-----------------     
         #----------#####----------------------      
          ###########----------------------          
           ##########---------------------- P        
             #########---------------------          
              #########-------------------           
                 ########--------------              
                     #####---------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  6.56e+21   5.71e+21  -2.39e+21 
  5.71e+21  -1.43e+15  -8.16e+21 
 -2.39e+21  -8.16e+21  -6.56e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160421101400/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

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   355    85     5   3.46 0.2798
WVFGRD96    2.0   175    90    -5   3.59 0.3328
WVFGRD96    3.0   170    75   -25   3.66 0.3234
WVFGRD96    4.0   170    35     0   3.71 0.3434
WVFGRD96    5.0   170    35     0   3.73 0.3927
WVFGRD96    6.0   170    35     0   3.75 0.4345
WVFGRD96    7.0   170    40     0   3.77 0.4698
WVFGRD96    8.0   175    35    10   3.85 0.4990
WVFGRD96    9.0   180    40    25   3.87 0.5347
WVFGRD96   10.0   180    45    30   3.90 0.5691
WVFGRD96   11.0   185    50    45   3.93 0.5987
WVFGRD96   12.0   185    50    40   3.94 0.6215
WVFGRD96   13.0   185    50    40   3.96 0.6353
WVFGRD96   14.0   180    55    35   3.98 0.6424
WVFGRD96   15.0   180    55    35   3.99 0.6435
WVFGRD96   16.0   180    55    35   4.00 0.6391
WVFGRD96   17.0   180    55    35   4.01 0.6299
WVFGRD96   18.0   180    55    35   4.02 0.6168
WVFGRD96   19.0   180    55    35   4.03 0.6012
WVFGRD96   20.0   180    55    35   4.04 0.5831
WVFGRD96   21.0   180    55    35   4.05 0.5618
WVFGRD96   22.0   180    55    35   4.06 0.5407
WVFGRD96   23.0   180    55    35   4.06 0.5190
WVFGRD96   24.0   180    50    35   4.06 0.4968
WVFGRD96   25.0   180    50    35   4.07 0.4752
WVFGRD96   26.0   180    50    35   4.07 0.4547
WVFGRD96   27.0   180    50    35   4.08 0.4346
WVFGRD96   28.0   180    45    40   4.07 0.4155
WVFGRD96   29.0   180    45    40   4.08 0.3977

The best solution is

WVFGRD96   15.0   180    55    35   3.99 0.6435

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 Thu Apr 21 07:18:48 CDT 2016