Location

2011/06/18 20:40:29 62.086 -148.260 23 4.00 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  2011/06/18 20:40:29:0  62.09 -148.26  23.0 4.0 Alaska
 
 Stations used:
   AK.BAL AK.BMR AK.CRQ AK.DIV AK.EYAK AK.KLU AK.MCK AK.MDM 
   AK.MLY AK.PAX AK.PPLA AK.RAG AK.SAW AK.SCM AK.SWD AK.TRF 
   AT.PMR AT.SVW2 IU.COLA 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 1.72e+22 dyne-cm
  Mw = 4.09 
  Z  = 46 km
  Plane   Strike  Dip  Rake
   NP1       70    50   -60
   NP2      208    48   -121
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.72e+22      1     139
    N   0.00e+00     23     230
    P  -1.72e+22     67      47

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     8.71e+21
       Mxy    -9.75e+21
       Mxz    -4.32e+21
       Myy     5.94e+21
       Myz    -4.30e+21
       Mzz    -1.47e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ################------              
              ###############-------------           
             #############-----------------          
           #############---------------------        
          ############------------------------       
         ############--------------------------      
        ############-----------   -------------#     
        ###########------------ P -------------#     
       ###########-------------   ------------###    
       ##########----------------------------####    
       #########----------------------------#####    
       #########--------------------------#######    
        ########------------------------########     
        #######----------------------###########     
         -#####-------------------#############      
          ----##-------------#################       
           -----#############################        
             ---########################             
              ---####################### T           
                 -#####################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.47e+22  -4.32e+21   4.30e+21 
 -4.32e+21   8.71e+21   9.75e+21 
  4.30e+21   9.75e+21   5.94e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110618204029/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 = 70
      DIP = 50
     RAKE = -60
       MW = 4.09
       HS = 46.0

The NDK file is 20110618204029.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2011/06/18 20:40:29:0  62.09 -148.26  23.0 4.0 Alaska
 
 Stations used:
   AK.BAL AK.BMR AK.CRQ AK.DIV AK.EYAK AK.KLU AK.MCK AK.MDM 
   AK.MLY AK.PAX AK.PPLA AK.RAG AK.SAW AK.SCM AK.SWD AK.TRF 
   AT.PMR AT.SVW2 IU.COLA 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 1.72e+22 dyne-cm
  Mw = 4.09 
  Z  = 46 km
  Plane   Strike  Dip  Rake
   NP1       70    50   -60
   NP2      208    48   -121
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.72e+22      1     139
    N   0.00e+00     23     230
    P  -1.72e+22     67      47

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     8.71e+21
       Mxy    -9.75e+21
       Mxz    -4.32e+21
       Myy     5.94e+21
       Myz    -4.30e+21
       Mzz    -1.47e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ################------              
              ###############-------------           
             #############-----------------          
           #############---------------------        
          ############------------------------       
         ############--------------------------      
        ############-----------   -------------#     
        ###########------------ P -------------#     
       ###########-------------   ------------###    
       ##########----------------------------####    
       #########----------------------------#####    
       #########--------------------------#######    
        ########------------------------########     
        #######----------------------###########     
         -#####-------------------#############      
          ----##-------------#################       
           -----#############################        
             ---########################             
              ---####################### T           
                 -#####################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.47e+22  -4.32e+21   4.30e+21 
 -4.32e+21   8.71e+21   9.75e+21 
  4.30e+21   9.75e+21   5.94e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110618204029/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   225    45    90   3.37 0.2263
WVFGRD96    1.0    45    45    90   3.41 0.2297
WVFGRD96    2.0    45    45    90   3.52 0.2910
WVFGRD96    3.0   210    45    80   3.61 0.3028
WVFGRD96    4.0   165    85     5   3.60 0.3076
WVFGRD96    5.0   165    85     0   3.63 0.3034
WVFGRD96    6.0   165    80    -5   3.66 0.2951
WVFGRD96    7.0   165    80    -5   3.68 0.2893
WVFGRD96    8.0   165    75   -10   3.71 0.2862
WVFGRD96    9.0    80    85   -35   3.70 0.2862
WVFGRD96   10.0    90    90   -45   3.68 0.2880
WVFGRD96   11.0   275    80    50   3.70 0.3016
WVFGRD96   12.0   275    80    50   3.71 0.3167
WVFGRD96   13.0   275    80    50   3.72 0.3302
WVFGRD96   14.0    90    90   -50   3.71 0.3402
WVFGRD96   15.0    90    90   -50   3.73 0.3563
WVFGRD96   16.0    90    90   -50   3.74 0.3708
WVFGRD96   17.0    90    90   -50   3.75 0.3841
WVFGRD96   18.0    85    80   -55   3.74 0.3965
WVFGRD96   19.0    85    80   -50   3.76 0.4105
WVFGRD96   20.0    85    80   -50   3.77 0.4246
WVFGRD96   21.0    85    80   -50   3.79 0.4361
WVFGRD96   22.0    85    75   -55   3.79 0.4491
WVFGRD96   23.0    85    75   -50   3.80 0.4613
WVFGRD96   24.0    80    70   -55   3.80 0.4744
WVFGRD96   25.0    80    70   -55   3.81 0.4867
WVFGRD96   26.0    80    70   -50   3.83 0.4985
WVFGRD96   27.0    80    65   -55   3.83 0.5105
WVFGRD96   28.0    80    65   -55   3.84 0.5224
WVFGRD96   29.0    80    65   -55   3.85 0.5336
WVFGRD96   30.0    80    65   -50   3.86 0.5438
WVFGRD96   31.0    80    65   -50   3.87 0.5532
WVFGRD96   32.0    80    60   -55   3.87 0.5618
WVFGRD96   33.0    80    60   -55   3.88 0.5700
WVFGRD96   34.0    80    60   -55   3.89 0.5775
WVFGRD96   35.0    80    60   -50   3.90 0.5839
WVFGRD96   36.0    75    55   -55   3.91 0.5895
WVFGRD96   37.0    75    55   -55   3.92 0.5955
WVFGRD96   38.0    75    55   -55   3.93 0.5988
WVFGRD96   39.0    70    50   -60   3.95 0.5998
WVFGRD96   40.0    70    55   -60   4.05 0.5932
WVFGRD96   41.0    70    50   -60   4.06 0.5994
WVFGRD96   42.0    70    50   -60   4.06 0.6043
WVFGRD96   43.0    70    50   -60   4.07 0.6088
WVFGRD96   44.0    70    50   -60   4.08 0.6110
WVFGRD96   45.0    70    50   -60   4.09 0.6122
WVFGRD96   46.0    70    50   -60   4.09 0.6123
WVFGRD96   47.0    70    50   -60   4.10 0.6110
WVFGRD96   48.0    70    50   -60   4.10 0.6088
WVFGRD96   49.0    65    45   -65   4.11 0.6063
WVFGRD96   50.0    65    45   -65   4.12 0.6027
WVFGRD96   51.0    65    45   -65   4.12 0.5989
WVFGRD96   52.0    70    45   -60   4.12 0.5939
WVFGRD96   53.0    70    45   -60   4.12 0.5887
WVFGRD96   54.0    70    45   -60   4.13 0.5834
WVFGRD96   55.0    70    45   -60   4.13 0.5768
WVFGRD96   56.0    70    45   -60   4.13 0.5693
WVFGRD96   57.0    70    45   -60   4.14 0.5618
WVFGRD96   58.0    70    45   -60   4.14 0.5533
WVFGRD96   59.0    70    45   -60   4.14 0.5444

The best solution is

WVFGRD96   46.0    70    50   -60   4.09 0.6123

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

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 Sun Dec 6 20:46:59 CST 2015