Location

2012/06/08 18:27:36 62.217 -147.879 39.4 4.30 Alaska

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports main page

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2012/06/08 18:27:36:0  62.22 -147.88  39.4 4.3 Alaska
 
 Stations used:
   AK.BAL AK.BMR AK.BWN AK.CCB AK.COLD AK.CRQ AK.CTG AK.DHY 
   AK.DIV AK.FYU AK.GHO AK.GLM AK.HDA AK.KLU AK.KNK AK.KTH 
   AK.MCK AK.MDM AK.MLY AK.PAX AK.PPD AK.PPLA AK.RAG AK.RIDG 
   AK.RND AK.SAW AK.SCM AK.SCRK AK.TGL AK.TRF AK.WRH AT.PMR 
   CN.DAWY IU.COLA US.EGAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 2.69e+22 dyne-cm
  Mw = 4.22 
  Z  = 51 km
  Plane   Strike  Dip  Rake
   NP1      255    60   -40
   NP2        8    56   -143
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.69e+22      2     312
    N   0.00e+00     42      44
    P  -2.69e+22     48     220

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     5.05e+21
       Mxy    -1.92e+22
       Mxz     1.10e+22
       Myy     9.93e+21
       Myz     7.72e+21
       Mzz    -1.50e+22
                                                     
                                                     
                                                     
                                                     
                     ###########---                  
                 ################------              
               ###################--------           
             T ####################--------          
           #   #####################---------        
          ##########################----------       
         ######################-----####-------      
        ###############--------------#########--     
        ###########------------------###########     
       #########---------------------############    
       ######------------------------############    
       ####-------------------------#############    
       ###--------------------------#############    
        ----------------------------############     
        ------------   ------------#############     
         ----------- P ------------############      
          ----------   -----------############       
           ----------------------############        
             -------------------###########          
              -----------------###########           
                 ------------##########              
                     ------########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.50e+22   1.10e+22  -7.72e+21 
  1.10e+22   5.05e+21   1.92e+22 
 -7.72e+21   1.92e+22   9.93e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120608182736/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 = 255
      DIP = 60
     RAKE = -40
       MW = 4.22
       HS = 51.0

The NDK file is 20120608182736.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2012/06/08 18:27:36:0  62.22 -147.88  39.4 4.3 Alaska
 
 Stations used:
   AK.BAL AK.BMR AK.BWN AK.CCB AK.COLD AK.CRQ AK.CTG AK.DHY 
   AK.DIV AK.FYU AK.GHO AK.GLM AK.HDA AK.KLU AK.KNK AK.KTH 
   AK.MCK AK.MDM AK.MLY AK.PAX AK.PPD AK.PPLA AK.RAG AK.RIDG 
   AK.RND AK.SAW AK.SCM AK.SCRK AK.TGL AK.TRF AK.WRH AT.PMR 
   CN.DAWY IU.COLA US.EGAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 2.69e+22 dyne-cm
  Mw = 4.22 
  Z  = 51 km
  Plane   Strike  Dip  Rake
   NP1      255    60   -40
   NP2        8    56   -143
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.69e+22      2     312
    N   0.00e+00     42      44
    P  -2.69e+22     48     220

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     5.05e+21
       Mxy    -1.92e+22
       Mxz     1.10e+22
       Myy     9.93e+21
       Myz     7.72e+21
       Mzz    -1.50e+22
                                                     
                                                     
                                                     
                                                     
                     ###########---                  
                 ################------              
               ###################--------           
             T ####################--------          
           #   #####################---------        
          ##########################----------       
         ######################-----####-------      
        ###############--------------#########--     
        ###########------------------###########     
       #########---------------------############    
       ######------------------------############    
       ####-------------------------#############    
       ###--------------------------#############    
        ----------------------------############     
        ------------   ------------#############     
         ----------- P ------------############      
          ----------   -----------############       
           ----------------------############        
             -------------------###########          
              -----------------###########           
                 ------------##########              
                     ------########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.50e+22   1.10e+22  -7.72e+21 
  1.10e+22   5.05e+21   1.92e+22 
 -7.72e+21   1.92e+22   9.93e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120608182736/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:

hp c 0.02 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    0.5    45    45    85   3.40 0.2287
WVFGRD96    1.0    50    45    90   3.45 0.2387
WVFGRD96    2.0   230    45    95   3.56 0.3000
WVFGRD96    3.0   225    45    85   3.63 0.3088
WVFGRD96    4.0     0    75    15   3.58 0.2967
WVFGRD96    5.0     0    80    15   3.60 0.2869
WVFGRD96    6.0    90    80   -10   3.62 0.2891
WVFGRD96    7.0   275    70    30   3.66 0.3029
WVFGRD96    8.0   275    70    35   3.71 0.3181
WVFGRD96    9.0   275    70    35   3.72 0.3260
WVFGRD96   10.0   275    70    35   3.73 0.3301
WVFGRD96   11.0   275    70    35   3.74 0.3322
WVFGRD96   12.0   275    70    35   3.75 0.3330
WVFGRD96   13.0   280    65    35   3.76 0.3341
WVFGRD96   14.0   180    85   -55   3.78 0.3402
WVFGRD96   15.0   105    60    50   3.75 0.3486
WVFGRD96   16.0    80    75   -45   3.75 0.3600
WVFGRD96   17.0    80    75   -45   3.76 0.3748
WVFGRD96   18.0    80    75   -45   3.77 0.3878
WVFGRD96   19.0    80    75   -45   3.78 0.3998
WVFGRD96   20.0    80    70   -45   3.79 0.4105
WVFGRD96   21.0    80    70   -45   3.81 0.4224
WVFGRD96   22.0    80    70   -40   3.83 0.4330
WVFGRD96   23.0    80    70   -40   3.84 0.4429
WVFGRD96   24.0    80    70   -40   3.85 0.4516
WVFGRD96   25.0   260    45   -30   3.88 0.4634
WVFGRD96   26.0   260    50   -30   3.89 0.4744
WVFGRD96   27.0   260    50   -30   3.90 0.4854
WVFGRD96   28.0   260    50   -30   3.92 0.4953
WVFGRD96   29.0   260    50   -30   3.93 0.5039
WVFGRD96   30.0   260    50   -30   3.94 0.5115
WVFGRD96   31.0   260    55   -30   3.95 0.5193
WVFGRD96   32.0   260    55   -25   3.97 0.5274
WVFGRD96   33.0   260    60   -30   3.97 0.5380
WVFGRD96   34.0   260    60   -30   3.98 0.5481
WVFGRD96   35.0   260    60   -30   3.99 0.5574
WVFGRD96   36.0   260    60   -30   4.00 0.5656
WVFGRD96   37.0   260    60   -30   4.02 0.5724
WVFGRD96   38.0   260    60   -30   4.03 0.5776
WVFGRD96   39.0   260    60   -30   4.04 0.5792
WVFGRD96   40.0   255    55   -35   4.14 0.6028
WVFGRD96   41.0   255    55   -35   4.15 0.6130
WVFGRD96   42.0   255    60   -35   4.15 0.6220
WVFGRD96   43.0   255    60   -40   4.16 0.6304
WVFGRD96   44.0   255    60   -40   4.16 0.6377
WVFGRD96   45.0   255    60   -40   4.17 0.6433
WVFGRD96   46.0   255    60   -40   4.18 0.6483
WVFGRD96   47.0   255    60   -40   4.19 0.6515
WVFGRD96   48.0   255    60   -40   4.20 0.6539
WVFGRD96   49.0   255    60   -40   4.20 0.6549
WVFGRD96   50.0   255    60   -40   4.21 0.6551
WVFGRD96   51.0   255    60   -40   4.22 0.6554
WVFGRD96   52.0   255    60   -40   4.22 0.6540
WVFGRD96   53.0   255    60   -40   4.23 0.6520
WVFGRD96   54.0   255    60   -35   4.24 0.6487
WVFGRD96   55.0   255    65   -35   4.24 0.6451
WVFGRD96   56.0   255    65   -35   4.24 0.6419
WVFGRD96   57.0   255    65   -35   4.25 0.6383
WVFGRD96   58.0   255    65   -35   4.25 0.6335
WVFGRD96   59.0   255    65   -35   4.26 0.6282
WVFGRD96   60.0   255    65   -35   4.26 0.6221
WVFGRD96   61.0   255    65   -35   4.26 0.6156
WVFGRD96   62.0   255    65   -35   4.26 0.6077
WVFGRD96   63.0   255    65   -35   4.27 0.6009
WVFGRD96   64.0   255    65   -35   4.27 0.5925
WVFGRD96   65.0   255    65   -35   4.27 0.5843
WVFGRD96   66.0   260    70   -30   4.26 0.5772
WVFGRD96   67.0   260    70   -30   4.27 0.5699
WVFGRD96   68.0   260    70   -30   4.27 0.5624
WVFGRD96   69.0   260    70   -30   4.27 0.5548

The best solution is

WVFGRD96   51.0   255    60   -40   4.22 0.6554

The mechanism corresponding 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

hp c 0.02 n 3
lp c 0.06 n 3
Figure 3. Waveform comparison for selected depth
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 Dec 7 00:24:54 CST 2015