Location

2012/03/21 07:42:02 61.315 -150.063 39.7 3.8 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  2012/03/21 07:42:02:0  61.31 -150.06  39.7 3.8 Alaska
 
 Stations used:
   AK.BPAW AK.BRLK AK.GLI AK.RC01 AK.SKN AK.SSN AT.PMR 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 7.50e+21 dyne-cm
  Mw = 3.85 
  Z  = 51 km
  Plane   Strike  Dip  Rake
   NP1      185    70   -70
   NP2      318    28   -133
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.50e+21     22     260
    N   0.00e+00     19     358
    P  -7.50e+21     60     124

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.84e+20
       Mxy     1.98e+21
       Mxz     1.34e+21
       Myy     4.91e+21
       Myz    -5.30e+21
       Mzz    -4.53e+21
                                                     
                                                     
                                                     
                                                     
                     --------######                  
                 -----------###########              
              -############-----##########           
             #############---------########          
           ##############------------########        
          ###############--------------#######       
         ###############-----------------######      
        ################------------------######     
        ###############--------------------#####     
       ################---------------------#####    
       ################---------------------#####    
       ###   ##########----------------------####    
       ### T ##########----------   ---------####    
        ##   ##########---------- P ---------###     
        ###############----------   ---------###     
         ##############----------------------##      
          #############---------------------##       
           ############---------------------#        
             ###########-------------------          
              ##########------------------           
                 ########--------------              
                     #####---------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.53e+21   1.34e+21   5.30e+21 
  1.34e+21  -3.84e+20  -1.98e+21 
  5.30e+21  -1.98e+21   4.91e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120321074202/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 = 185
      DIP = 70
     RAKE = -70
       MW = 3.85
       HS = 51.0

The NDK file is 20120321074202.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2012/03/21 07:42:02:0  61.31 -150.06  39.7 3.8 Alaska
 
 Stations used:
   AK.BPAW AK.BRLK AK.GLI AK.RC01 AK.SKN AK.SSN AT.PMR 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 7.50e+21 dyne-cm
  Mw = 3.85 
  Z  = 51 km
  Plane   Strike  Dip  Rake
   NP1      185    70   -70
   NP2      318    28   -133
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.50e+21     22     260
    N   0.00e+00     19     358
    P  -7.50e+21     60     124

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.84e+20
       Mxy     1.98e+21
       Mxz     1.34e+21
       Myy     4.91e+21
       Myz    -5.30e+21
       Mzz    -4.53e+21
                                                     
                                                     
                                                     
                                                     
                     --------######                  
                 -----------###########              
              -############-----##########           
             #############---------########          
           ##############------------########        
          ###############--------------#######       
         ###############-----------------######      
        ################------------------######     
        ###############--------------------#####     
       ################---------------------#####    
       ################---------------------#####    
       ###   ##########----------------------####    
       ### T ##########----------   ---------####    
        ##   ##########---------- P ---------###     
        ###############----------   ---------###     
         ##############----------------------##      
          #############---------------------##       
           ############---------------------#        
             ###########-------------------          
              ##########------------------           
                 ########--------------              
                     #####---------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.53e+21   1.34e+21   5.30e+21 
  1.34e+21  -3.84e+20  -1.98e+21 
  5.30e+21  -1.98e+21   4.91e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120321074202/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 0.10 n 3
br c 0.12 0.25 n 4 p 2
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    40    45    95   3.00 0.1939
WVFGRD96    1.0   175    80    15   2.95 0.1439
WVFGRD96    2.0     5    40    90   3.17 0.2262
WVFGRD96    3.0   170    85   -25   3.20 0.2406
WVFGRD96    4.0   170    85   -30   3.26 0.2881
WVFGRD96    5.0   170    85   -35   3.30 0.3364
WVFGRD96    6.0   170    80   -35   3.33 0.3748
WVFGRD96    7.0   165    70   -40   3.35 0.4084
WVFGRD96    8.0   145    30    35   3.40 0.4284
WVFGRD96    9.0   145    30    35   3.40 0.4465
WVFGRD96   10.0   140    35    25   3.39 0.4578
WVFGRD96   11.0   140    35    25   3.39 0.4679
WVFGRD96   12.0   140    40    20   3.40 0.4766
WVFGRD96   13.0   140    40    20   3.41 0.4861
WVFGRD96   14.0   135    40    15   3.42 0.4941
WVFGRD96   15.0   135    40    15   3.43 0.5012
WVFGRD96   16.0   135    45    15   3.44 0.5072
WVFGRD96   17.0   135    45    10   3.45 0.5135
WVFGRD96   18.0   135    45    10   3.46 0.5180
WVFGRD96   19.0   135    45    10   3.47 0.5226
WVFGRD96   20.0   135    45    10   3.49 0.5260
WVFGRD96   21.0   135    45    10   3.50 0.5264
WVFGRD96   22.0   135    40    10   3.51 0.5279
WVFGRD96   23.0   160    40   -15   3.56 0.5340
WVFGRD96   24.0   160    40   -15   3.57 0.5389
WVFGRD96   25.0   160    40   -15   3.58 0.5430
WVFGRD96   26.0   160    40   -15   3.59 0.5463
WVFGRD96   27.0   165    45   -10   3.60 0.5484
WVFGRD96   28.0   160    40   -15   3.61 0.5514
WVFGRD96   29.0   160    45   -15   3.62 0.5561
WVFGRD96   30.0   205    90   -65   3.57 0.5593
WVFGRD96   31.0    30    85    70   3.59 0.5682
WVFGRD96   32.0    30    85    70   3.59 0.5701
WVFGRD96   33.0   205    90   -65   3.60 0.5729
WVFGRD96   34.0    25    90    65   3.60 0.5751
WVFGRD96   35.0    25    90    65   3.61 0.5762
WVFGRD96   36.0    25    90    65   3.62 0.5770
WVFGRD96   37.0   200    85   -65   3.62 0.5802
WVFGRD96   38.0   195    80   -60   3.63 0.5792
WVFGRD96   39.0   195    80   -60   3.63 0.5788
WVFGRD96   40.0   195    80   -70   3.76 0.5685
WVFGRD96   41.0   195    80   -70   3.77 0.5733
WVFGRD96   42.0   195    80   -70   3.78 0.5784
WVFGRD96   43.0    55    15   -45   3.79 0.5826
WVFGRD96   44.0    65    20   -40   3.80 0.5867
WVFGRD96   45.0   185    70   -70   3.81 0.5941
WVFGRD96   46.0   180    65   -70   3.82 0.5980
WVFGRD96   47.0   185    70   -70   3.82 0.6024
WVFGRD96   48.0   180    65   -70   3.83 0.6046
WVFGRD96   49.0   185    70   -70   3.84 0.6074
WVFGRD96   50.0   185    70   -70   3.84 0.6074
WVFGRD96   51.0   185    70   -70   3.85 0.6077
WVFGRD96   52.0   185    70   -70   3.86 0.6065
WVFGRD96   53.0   185    70   -70   3.86 0.6041
WVFGRD96   54.0   180    70   -70   3.88 0.6015
WVFGRD96   55.0    20    15   -65   3.90 0.5996
WVFGRD96   56.0     0    15   -80   3.91 0.6013
WVFGRD96   57.0    -5    15   -85   3.92 0.5996
WVFGRD96   58.0     0    15   -80   3.92 0.5983
WVFGRD96   59.0    10    20   -70   3.92 0.5978
WVFGRD96   60.0    10    20   -70   3.93 0.5953
WVFGRD96   61.0    10    20   -70   3.93 0.5937
WVFGRD96   62.0    10    20   -70   3.94 0.5906
WVFGRD96   63.0    10    20   -70   3.94 0.5862
WVFGRD96   64.0    10    20   -70   3.94 0.5817
WVFGRD96   65.0    10    20   -70   3.95 0.5765
WVFGRD96   66.0    15    20   -65   3.95 0.5695
WVFGRD96   67.0    15    20   -65   3.95 0.5637
WVFGRD96   68.0    75    35   -60   3.93 0.5677
WVFGRD96   69.0    75    35   -60   3.93 0.5631
WVFGRD96   70.0    70    35   -60   3.93 0.5591
WVFGRD96   71.0    70    35   -60   3.93 0.5549
WVFGRD96   72.0    65    35   -65   3.92 0.5501
WVFGRD96   73.0    65    35   -65   3.92 0.5450
WVFGRD96   74.0    65    35   -65   3.93 0.5400
WVFGRD96   75.0   180    90   -70   4.01 0.5347
WVFGRD96   76.0   180    90   -70   4.01 0.5335
WVFGRD96   77.0   180    90   -70   4.01 0.5322
WVFGRD96   78.0   180    90   -70   4.02 0.5300
WVFGRD96   79.0   180    90   -70   4.02 0.5278
WVFGRD96   80.0   180    90   -75   4.01 0.5262
WVFGRD96   81.0   180    90   -75   4.02 0.5237
WVFGRD96   82.0     0    85    75   4.01 0.5285
WVFGRD96   83.0   180    90   -75   4.02 0.5190
WVFGRD96   84.0   180    90   -75   4.02 0.5159
WVFGRD96   85.0   180    90   -75   4.02 0.5127
WVFGRD96   86.0   180    90   -75   4.02 0.5100
WVFGRD96   87.0   180    90   -75   4.02 0.5069
WVFGRD96   88.0   145    40   -10   4.09 0.5161
WVFGRD96   89.0   145    40   -10   4.09 0.5138
WVFGRD96   90.0   145    45   -10   4.11 0.5117
WVFGRD96   91.0   145    45   -10   4.11 0.5110
WVFGRD96   92.0   145    45   -10   4.11 0.5096
WVFGRD96   93.0   145    45   -10   4.11 0.5077
WVFGRD96   94.0   145    45   -10   4.11 0.5052
WVFGRD96   95.0   145    45   -10   4.11 0.5026
WVFGRD96   96.0   145    45   -10   4.11 0.5009
WVFGRD96   97.0   145    45   -10   4.11 0.4989
WVFGRD96   98.0   145    45   -10   4.11 0.4961
WVFGRD96   99.0   145    45   -10   4.11 0.4930

The best solution is

WVFGRD96   51.0   185    70   -70   3.85 0.6077

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 0.10 n 3
br c 0.12 0.25 n 4 p 2
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 00:24:31 CST 2015