Location

Location ANSS

2016/09/05 06:37:39 63.571 -150.754 10.7 3.6 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/09/05 06:37:39:0  63.57 -150.75  10.7 3.6 Alaska
 
 Stations used:
   AK.BWN AK.CAST AK.CCB AK.CHUM AK.COLD AK.CUT AK.DHY AK.DOT 
   AK.FID AK.GCSA AK.GHO AK.HDA AK.KLU AK.KNK AK.MCK AK.MDM 
   AK.MLY AK.NEA2 AK.PAX AK.PPD AK.PPLA AK.PWL AK.RIDG AK.RND 
   AK.SAW AK.SCM AK.SKN AK.SSN AK.TRF AK.WAT1 AK.WAT6 AK.WAT7 
   AK.WRH AT.PMR IU.COLA TA.F21K TA.G21K TA.G23K TA.H21K 
   TA.H22K TA.H23K TA.H24K TA.HARP TA.I21K TA.I26K TA.J20K 
   TA.J25K TA.J26L TA.K20K TA.K24K TA.L19K TA.L20K TA.L26K 
   TA.M19K TA.M20K TA.M22K TA.M23K TA.M26K TA.N20K TA.N25K 
   TA.POKR XV.F1TN XV.F6TP XV.F7TV XV.F8KN XV.FAPT XV.FPAP 
 
 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 = 2.75e+21 dyne-cm
  Mw = 3.56 
  Z  = 14 km
  Plane   Strike  Dip  Rake
   NP1       77    56   113
   NP2      220    40    60
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.75e+21     69      38
    N   0.00e+00     19     244
    P  -2.75e+21      9     151

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.84e+21
       Mxy     1.31e+21
       Mxz     1.07e+21
       Myy    -5.07e+20
       Myz     3.61e+20
       Mzz     2.35e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ----------------############           
             -------------#################          
           ------------######################        
          -----------#########################       
         ----------############################      
        ---------#############   ###############     
        --------############## T ###############     
       --------###############   ##############--    
       -------################################---    
       -------##############################-----    
       ------#############################-------    
        -----##########################---------     
        ##---#######################------------     
         ####--###############-----------------      
          ###---------------------------------       
           ##--------------------------------        
             #-----------------------------          
              #---------------------   ---           
                 ------------------- P               
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.35e+21   1.07e+21  -3.61e+20 
  1.07e+21  -1.84e+21  -1.31e+21 
 -3.61e+20  -1.31e+21  -5.07e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160905063739/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 = 220
      DIP = 40
     RAKE = 60
       MW = 3.56
       HS = 14.0

The NDK file is 20160905063739.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2016/09/05 06:37:39:0  63.57 -150.75  10.7 3.6 Alaska
 
 Stations used:
   AK.BWN AK.CAST AK.CCB AK.CHUM AK.COLD AK.CUT AK.DHY AK.DOT 
   AK.FID AK.GCSA AK.GHO AK.HDA AK.KLU AK.KNK AK.MCK AK.MDM 
   AK.MLY AK.NEA2 AK.PAX AK.PPD AK.PPLA AK.PWL AK.RIDG AK.RND 
   AK.SAW AK.SCM AK.SKN AK.SSN AK.TRF AK.WAT1 AK.WAT6 AK.WAT7 
   AK.WRH AT.PMR IU.COLA TA.F21K TA.G21K TA.G23K TA.H21K 
   TA.H22K TA.H23K TA.H24K TA.HARP TA.I21K TA.I26K TA.J20K 
   TA.J25K TA.J26L TA.K20K TA.K24K TA.L19K TA.L20K TA.L26K 
   TA.M19K TA.M20K TA.M22K TA.M23K TA.M26K TA.N20K TA.N25K 
   TA.POKR XV.F1TN XV.F6TP XV.F7TV XV.F8KN XV.FAPT XV.FPAP 
 
 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 = 2.75e+21 dyne-cm
  Mw = 3.56 
  Z  = 14 km
  Plane   Strike  Dip  Rake
   NP1       77    56   113
   NP2      220    40    60
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.75e+21     69      38
    N   0.00e+00     19     244
    P  -2.75e+21      9     151

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.84e+21
       Mxy     1.31e+21
       Mxz     1.07e+21
       Myy    -5.07e+20
       Myz     3.61e+20
       Mzz     2.35e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ----------------############           
             -------------#################          
           ------------######################        
          -----------#########################       
         ----------############################      
        ---------#############   ###############     
        --------############## T ###############     
       --------###############   ##############--    
       -------################################---    
       -------##############################-----    
       ------#############################-------    
        -----##########################---------     
        ##---#######################------------     
         ####--###############-----------------      
          ###---------------------------------       
           ##--------------------------------        
             #-----------------------------          
              #---------------------   ---           
                 ------------------- P               
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.35e+21   1.07e+21  -3.61e+20 
  1.07e+21  -1.84e+21  -1.31e+21 
 -3.61e+20  -1.31e+21  -5.07e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160905063739/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    90    45   -90   3.13 0.2401
WVFGRD96    2.0    90    45   -95   3.29 0.3188
WVFGRD96    3.0   355    45   -20   3.24 0.2772
WVFGRD96    4.0   195    25    20   3.32 0.3272
WVFGRD96    5.0   200    25    30   3.34 0.3837
WVFGRD96    6.0   200    30    30   3.35 0.4334
WVFGRD96    7.0   205    30    40   3.38 0.4747
WVFGRD96    8.0   210    30    45   3.46 0.5035
WVFGRD96    9.0   215    30    55   3.48 0.5398
WVFGRD96   10.0   215    35    55   3.50 0.5691
WVFGRD96   11.0   220    35    60   3.52 0.5918
WVFGRD96   12.0   220    35    60   3.53 0.6042
WVFGRD96   13.0   220    40    60   3.55 0.6126
WVFGRD96   14.0   220    40    60   3.56 0.6149
WVFGRD96   15.0   220    40    60   3.57 0.6100
WVFGRD96   16.0   220    40    60   3.58 0.5992
WVFGRD96   17.0   220    40    60   3.59 0.5836
WVFGRD96   18.0   215    45    55   3.60 0.5648
WVFGRD96   19.0   220    35    55   3.61 0.5459
WVFGRD96   20.0   220    35    55   3.61 0.5292
WVFGRD96   21.0   220    35    55   3.63 0.5113
WVFGRD96   22.0   215    35    50   3.63 0.4919
WVFGRD96   23.0   215    35    50   3.64 0.4716
WVFGRD96   24.0   215    35    50   3.64 0.4505
WVFGRD96   25.0   215    35    50   3.64 0.4296
WVFGRD96   26.0   215    35    50   3.65 0.4091
WVFGRD96   27.0   210    35    40   3.65 0.3907
WVFGRD96   28.0   215    35    45   3.65 0.3726
WVFGRD96   29.0   215    35    45   3.66 0.3544

The best solution is

WVFGRD96   14.0   220    40    60   3.56 0.6149

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 Mon Sep 5 17:31:38 CDT 2016