Location

2011/07/28 14:00:00 62.050 -151.290 81 5.30 Alask

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  2011/07/28 14:00:00:8  62.05 -151.29  81.0 5.3 Alask
 
 Stations used:
   AK.BAL AK.BMR AK.BRLK AK.CAST AK.CCB AK.CHUM AK.CNP AK.COLD 
   AK.CRQ AK.CTG AK.DHY AK.DIV AK.EYAK AK.FIB AK.FID AK.FYU 
   AK.GHO AK.HOM AK.KLU AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY 
   AK.PAX AK.PPLA AK.RAG AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN 
   AK.SWD AK.TGL AK.TRF AK.WRH AT.MENT AT.OHAK AT.PMR AT.SVW2 
   IU.COLA US.EGAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 8.51e+23 dyne-cm
  Mw = 5.22 
  Z  = 85 km
  Plane   Strike  Dip  Rake
   NP1      348    70   105
   NP2      130    25    55
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   8.51e+23     62     281
    N   0.00e+00     14     162
    P  -8.51e+23     23      66

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.10e+23
       Mxy    -2.99e+23
       Mxz    -5.89e+22
       Myy    -4.24e+23
       Myz    -6.27e+23
       Mzz     5.34e+23
                                                     
                                                     
                                                     
                                                     
                     ###-----------                  
                 #########-------------              
              #############---------------           
             ###############---------------          
           ##################----------------        
          -###################----------------       
         -#####################----------   ---      
        --######################--------- P ----     
        --######################---------   ----     
       ---##########   ##########----------------    
       ---########## T ##########----------------    
       ----#########   ##########----------------    
       ----#######################---------------    
        ----######################--------------     
        -----#####################--------------     
         -----####################-------------      
          ------##################------------       
           -------################-----------        
             -------##############---------          
              ----------##########------##           
                 ----------------######              
                     ------------##                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  5.34e+23  -5.89e+22   6.27e+23 
 -5.89e+22  -1.10e+23   2.99e+23 
  6.27e+23   2.99e+23  -4.24e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110728140000/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 = 130
      DIP = 25
     RAKE = 55
       MW = 5.22
       HS = 85.0

The NDK file is 20110728140000.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2011/07/28 14:00:00:8  62.05 -151.29  81.0 5.3 Alask
 
 Stations used:
   AK.BAL AK.BMR AK.BRLK AK.CAST AK.CCB AK.CHUM AK.CNP AK.COLD 
   AK.CRQ AK.CTG AK.DHY AK.DIV AK.EYAK AK.FIB AK.FID AK.FYU 
   AK.GHO AK.HOM AK.KLU AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY 
   AK.PAX AK.PPLA AK.RAG AK.RC01 AK.RND AK.SAW AK.SCM AK.SSN 
   AK.SWD AK.TGL AK.TRF AK.WRH AT.MENT AT.OHAK AT.PMR AT.SVW2 
   IU.COLA US.EGAK 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 8.51e+23 dyne-cm
  Mw = 5.22 
  Z  = 85 km
  Plane   Strike  Dip  Rake
   NP1      348    70   105
   NP2      130    25    55
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   8.51e+23     62     281
    N   0.00e+00     14     162
    P  -8.51e+23     23      66

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.10e+23
       Mxy    -2.99e+23
       Mxz    -5.89e+22
       Myy    -4.24e+23
       Myz    -6.27e+23
       Mzz     5.34e+23
                                                     
                                                     
                                                     
                                                     
                     ###-----------                  
                 #########-------------              
              #############---------------           
             ###############---------------          
           ##################----------------        
          -###################----------------       
         -#####################----------   ---      
        --######################--------- P ----     
        --######################---------   ----     
       ---##########   ##########----------------    
       ---########## T ##########----------------    
       ----#########   ##########----------------    
       ----#######################---------------    
        ----######################--------------     
        -----#####################--------------     
         -----####################-------------      
          ------##################------------       
           -------################-----------        
             -------##############---------          
              ----------##########------##           
                 ----------------######              
                     ------------##                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  5.34e+23  -5.89e+22   6.27e+23 
 -5.89e+22  -1.10e+23   2.99e+23 
  6.27e+23   2.99e+23  -4.24e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110728140000/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    1.0    35    45   -75   4.35 0.1855
WVFGRD96    3.0    35    50   -70   4.52 0.2300
WVFGRD96    4.0    45    55   -55   4.53 0.2169
WVFGRD96    7.0    70    80    30   4.53 0.2295
WVFGRD96    9.0    70    80    35   4.58 0.2378
WVFGRD96   11.0    70    75    35   4.60 0.2434
WVFGRD96   13.0    70    75    30   4.62 0.2443
WVFGRD96   15.0   250    60    35   4.65 0.2465
WVFGRD96   17.0   250    60    35   4.66 0.2509
WVFGRD96   19.0   250    60    35   4.68 0.2525
WVFGRD96   21.0   250    65    35   4.69 0.2511
WVFGRD96   23.0   250    65    35   4.70 0.2504
WVFGRD96   25.0   250    65    35   4.72 0.2506
WVFGRD96   27.0   255    60    40   4.73 0.2483
WVFGRD96   29.0   255    60    40   4.75 0.2458
WVFGRD96   31.0    80    80    20   4.77 0.2480
WVFGRD96   33.0    80    80    20   4.79 0.2566
WVFGRD96   35.0    85    70    15   4.82 0.2651
WVFGRD96   37.0    85    70    15   4.84 0.2738
WVFGRD96   39.0    85    70    10   4.87 0.2819
WVFGRD96   41.0    80    40   -15   4.94 0.2843
WVFGRD96   43.0    80    40   -15   4.96 0.2900
WVFGRD96   45.0    85    40     0   4.99 0.3014
WVFGRD96   47.0    85    35     0   5.01 0.3137
WVFGRD96   49.0    90    35     5   5.02 0.3273
WVFGRD96   51.0    90    35     5   5.04 0.3400
WVFGRD96   53.0    90    35    10   5.06 0.3532
WVFGRD96   55.0    95    35    15   5.07 0.3646
WVFGRD96   57.0    95    35    20   5.09 0.3764
WVFGRD96   59.0   100    30    25   5.11 0.3914
WVFGRD96   61.0   100    30    25   5.12 0.4059
WVFGRD96   63.0   120    20    40   5.14 0.4222
WVFGRD96   65.0   125    20    50   5.16 0.4405
WVFGRD96   67.0   125    20    50   5.17 0.4573
WVFGRD96   69.0   125    20    50   5.18 0.4717
WVFGRD96   70.0   130    20    55   5.19 0.4784
WVFGRD96   71.0   130    20    55   5.19 0.4845
WVFGRD96   72.0   130    20    55   5.20 0.4897
WVFGRD96   73.0   130    20    55   5.20 0.4945
WVFGRD96   74.0   130    20    55   5.20 0.4989
WVFGRD96   75.0   130    20    55   5.21 0.5024
WVFGRD96   76.0   135    20    60   5.21 0.5062
WVFGRD96   77.0   135    20    60   5.21 0.5087
WVFGRD96   78.0   125    25    50   5.21 0.5114
WVFGRD96   79.0   125    25    50   5.21 0.5133
WVFGRD96   80.0   130    25    55   5.22 0.5157
WVFGRD96   81.0   130    25    55   5.22 0.5171
WVFGRD96   82.0   130    25    55   5.22 0.5183
WVFGRD96   83.0   130    25    55   5.22 0.5191
WVFGRD96   84.0   130    25    55   5.22 0.5195
WVFGRD96   85.0   130    25    55   5.22 0.5195
WVFGRD96   86.0   130    25    55   5.23 0.5190
WVFGRD96   87.0   130    25    55   5.23 0.5186
WVFGRD96   88.0   130    25    55   5.23 0.5179
WVFGRD96   89.0   130    25    55   5.23 0.5163
WVFGRD96   90.0   130    25    55   5.23 0.5152
WVFGRD96   91.0   130    25    55   5.23 0.5132
WVFGRD96   92.0   130    25    55   5.23 0.5115
WVFGRD96   93.0   130    25    55   5.23 0.5095
WVFGRD96   94.0   130    25    55   5.23 0.5067
WVFGRD96   95.0   130    25    55   5.23 0.5047
WVFGRD96   96.0   130    25    55   5.23 0.5016
WVFGRD96   97.0   130    25    55   5.23 0.4989
WVFGRD96   98.0   130    25    55   5.23 0.4959
WVFGRD96   99.0   130    25    55   5.23 0.4927
WVFGRD96  100.0   135    25    60   5.23 0.4895
WVFGRD96  101.0   135    25    60   5.23 0.4866
WVFGRD96  102.0   135    25    60   5.23 0.4828
WVFGRD96  103.0   135    25    60   5.23 0.4799
WVFGRD96  104.0   135    25    60   5.23 0.4762
WVFGRD96  105.0   135    25    60   5.23 0.4727
WVFGRD96  106.0   135    25    60   5.23 0.4690
WVFGRD96  107.0   135    25    60   5.23 0.4656
WVFGRD96  108.0   135    25    60   5.23 0.4615
WVFGRD96  109.0   135    25    60   5.23 0.4581
WVFGRD96  111.0   135    25    60   5.23 0.4509
WVFGRD96  113.0   135    25    60   5.23 0.4444
WVFGRD96  115.0   135    25    60   5.23 0.4372
WVFGRD96  117.0   130    30    60   5.23 0.4305
WVFGRD96  119.0   130    30    60   5.23 0.4237
WVFGRD96  121.0   130    30    60   5.23 0.4169
WVFGRD96  123.0   130    30    60   5.23 0.4106
WVFGRD96  125.0   130    30    60   5.23 0.4038
WVFGRD96  127.0   140    25    70   5.22 0.3974
WVFGRD96  129.0   140    25    70   5.22 0.3908

The best solution is

WVFGRD96   85.0   130    25    55   5.22 0.5195

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 20:52:03 CST 2015