Location

2012/12/24 17:28:25 61.243 -150.775 57.1 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  2012/12/24 17:28:25:0  61.24 -150.77  57.1 4.0 Alaska
 
 Stations used:
   AK.GHO AK.KNK AK.PPLA AK.SAW AK.SCM AK.SKN AK.SSN AK.SWD 
   AT.PMR 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
 
 Best Fitting Double Couple
  Mo = 2.43e+22 dyne-cm
  Mw = 4.19 
  Z  = 70 km
  Plane   Strike  Dip  Rake
   NP1      188    82   -114
   NP2       80    25   -20
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.43e+22     33     298
    N   0.00e+00     23     192
    P  -2.43e+22     48      73

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.87e+21
       Mxy    -1.01e+22
       Mxz     1.67e+21
       Myy     3.49e+21
       Myz    -2.13e+22
       Mzz    -6.36e+21
                                                     
                                                     
                                                     
                                                     
                     #########-----                  
                 #############---------              
              ###############-------------           
             ################--------------          
           #################-----------------        
          ##################------------------       
         #####   ##########--------------------      
        ###### T ##########---------------------     
        ######   ##########---------   ---------     
       ###################---------- P ---------#    
       ###################----------   ---------#    
       ###################----------------------#    
       ##################----------------------##    
        #################---------------------##     
        -################--------------------###     
         -###############-------------------###      
          --#############-----------------####       
           ---###########--------------######        
             -----#######-----------#######          
              ----------#----#############           
                 --------##############              
                     ----##########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -6.36e+21   1.67e+21   2.13e+22 
  1.67e+21   2.87e+21   1.01e+22 
  2.13e+22   1.01e+22   3.49e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20121224172825/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 = 80
      DIP = 25
     RAKE = -20
       MW = 4.19
       HS = 70.0

The NDK file is 20121224172825.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2012/12/24 17:28:25:0  61.24 -150.77  57.1 4.0 Alaska
 
 Stations used:
   AK.GHO AK.KNK AK.PPLA AK.SAW AK.SCM AK.SKN AK.SSN AK.SWD 
   AT.PMR 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
 
 Best Fitting Double Couple
  Mo = 2.43e+22 dyne-cm
  Mw = 4.19 
  Z  = 70 km
  Plane   Strike  Dip  Rake
   NP1      188    82   -114
   NP2       80    25   -20
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.43e+22     33     298
    N   0.00e+00     23     192
    P  -2.43e+22     48      73

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.87e+21
       Mxy    -1.01e+22
       Mxz     1.67e+21
       Myy     3.49e+21
       Myz    -2.13e+22
       Mzz    -6.36e+21
                                                     
                                                     
                                                     
                                                     
                     #########-----                  
                 #############---------              
              ###############-------------           
             ################--------------          
           #################-----------------        
          ##################------------------       
         #####   ##########--------------------      
        ###### T ##########---------------------     
        ######   ##########---------   ---------     
       ###################---------- P ---------#    
       ###################----------   ---------#    
       ###################----------------------#    
       ##################----------------------##    
        #################---------------------##     
        -################--------------------###     
         -###############-------------------###      
          --#############-----------------####       
           ---###########--------------######        
             -----#######-----------#######          
              ----------#----#############           
                 --------##############              
                     ----##########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -6.36e+21   1.67e+21   2.13e+22 
  1.67e+21   2.87e+21   1.01e+22 
  2.13e+22   1.01e+22   3.49e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20121224172825/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
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    35    40   -90   3.23 0.2152
WVFGRD96    1.0    30    80    10   3.22 0.2190
WVFGRD96    2.0     5    75   -15   3.30 0.2817
WVFGRD96    3.0   205    80   -10   3.45 0.3142
WVFGRD96    4.0   205    85   -10   3.51 0.3144
WVFGRD96    5.0   190    60    20   3.46 0.3323
WVFGRD96    6.0   190    60    20   3.50 0.3575
WVFGRD96    7.0    10    60    15   3.54 0.3818
WVFGRD96    8.0    10    55    20   3.59 0.4057
WVFGRD96    9.0   190    80    30   3.62 0.4172
WVFGRD96   10.0   185    90    30   3.63 0.4307
WVFGRD96   11.0   185    90    25   3.66 0.4386
WVFGRD96   12.0   185    85    25   3.67 0.4419
WVFGRD96   13.0   185    85    25   3.69 0.4422
WVFGRD96   14.0   185    85    25   3.71 0.4416
WVFGRD96   15.0   185    70    20   3.71 0.4366
WVFGRD96   16.0   185    75    20   3.73 0.4323
WVFGRD96   17.0   185    75    20   3.74 0.4261
WVFGRD96   18.0   185    75    20   3.76 0.4176
WVFGRD96   19.0   190    70    20   3.78 0.4096
WVFGRD96   20.0   190    70    20   3.79 0.4001
WVFGRD96   21.0   190    70    20   3.80 0.3906
WVFGRD96   22.0   190    70    15   3.81 0.3800
WVFGRD96   23.0   190    70    15   3.82 0.3659
WVFGRD96   24.0   190    70    15   3.82 0.3550
WVFGRD96   25.0   190    65    15   3.83 0.3436
WVFGRD96   26.0    30    85    10   3.87 0.3460
WVFGRD96   27.0    35    85    20   3.87 0.3561
WVFGRD96   28.0    35    85    20   3.88 0.3651
WVFGRD96   29.0    35    85    20   3.89 0.3717
WVFGRD96   30.0    35    85    25   3.89 0.3796
WVFGRD96   31.0    45    70    25   3.90 0.3884
WVFGRD96   32.0    45    70    25   3.91 0.3989
WVFGRD96   33.0    45    70    25   3.92 0.4078
WVFGRD96   34.0    45    70    25   3.93 0.4181
WVFGRD96   35.0    45    70    25   3.94 0.4289
WVFGRD96   36.0    45    70    25   3.95 0.4389
WVFGRD96   37.0    45    85    40   3.96 0.4479
WVFGRD96   38.0    45    85    40   3.96 0.4585
WVFGRD96   39.0    50    80    40   3.97 0.4669
WVFGRD96   40.0    45    85    55   4.06 0.4679
WVFGRD96   41.0    45    85    55   4.07 0.4760
WVFGRD96   42.0    45    85    55   4.08 0.4787
WVFGRD96   43.0    45    80    50   4.08 0.4858
WVFGRD96   44.0    50    75    45   4.09 0.4905
WVFGRD96   45.0    50    75    45   4.10 0.4963
WVFGRD96   46.0    65    25   -30   4.09 0.5050
WVFGRD96   47.0    65    25   -30   4.09 0.5148
WVFGRD96   48.0    70    25   -30   4.10 0.5234
WVFGRD96   49.0    70    25   -30   4.11 0.5338
WVFGRD96   50.0    70    25   -30   4.12 0.5439
WVFGRD96   51.0    70    25   -30   4.12 0.5519
WVFGRD96   52.0    70    25   -30   4.13 0.5595
WVFGRD96   53.0    70    25   -30   4.13 0.5683
WVFGRD96   54.0    75    25   -25   4.13 0.5747
WVFGRD96   55.0    75    25   -25   4.14 0.5799
WVFGRD96   56.0    75    25   -25   4.14 0.5876
WVFGRD96   57.0    75    25   -25   4.15 0.5945
WVFGRD96   58.0    75    25   -25   4.15 0.5989
WVFGRD96   59.0    75    25   -25   4.15 0.6011
WVFGRD96   60.0    75    25   -25   4.16 0.6070
WVFGRD96   61.0    75    25   -25   4.16 0.6109
WVFGRD96   62.0    75    25   -25   4.16 0.6131
WVFGRD96   63.0    75    25   -25   4.17 0.6148
WVFGRD96   64.0    80    25   -20   4.17 0.6191
WVFGRD96   65.0    80    25   -20   4.17 0.6206
WVFGRD96   66.0    80    25   -20   4.17 0.6217
WVFGRD96   67.0    80    25   -20   4.18 0.6232
WVFGRD96   68.0    80    25   -20   4.18 0.6235
WVFGRD96   69.0    80    25   -20   4.18 0.6255
WVFGRD96   70.0    80    25   -20   4.19 0.6257
WVFGRD96   71.0    85    25   -15   4.18 0.6240
WVFGRD96   72.0    85    25   -15   4.19 0.6243
WVFGRD96   73.0    85    25   -15   4.19 0.6247
WVFGRD96   74.0    85    25   -15   4.19 0.6247
WVFGRD96   75.0    90    25   -15   4.20 0.6249
WVFGRD96   76.0    90    25   -15   4.20 0.6217
WVFGRD96   77.0    90    25   -15   4.21 0.6211
WVFGRD96   78.0    90    20   -15   4.20 0.6208
WVFGRD96   79.0    90    20   -15   4.21 0.6215
WVFGRD96   80.0    95    25   -10   4.21 0.6189
WVFGRD96   81.0    95    20   -10   4.21 0.6175
WVFGRD96   82.0    95    20   -10   4.21 0.6151
WVFGRD96   83.0    95    20   -10   4.22 0.6153
WVFGRD96   84.0    95    20   -10   4.22 0.6147
WVFGRD96   85.0    95    20   -10   4.22 0.6123
WVFGRD96   86.0   100    25    -5   4.22 0.6102
WVFGRD96   87.0   100    25    -5   4.23 0.6072
WVFGRD96   88.0   100    25    -5   4.23 0.6066
WVFGRD96   89.0   100    25    -5   4.23 0.6061
WVFGRD96   90.0   105    25    -5   4.24 0.6037
WVFGRD96   91.0   105    25    -5   4.24 0.6019
WVFGRD96   92.0   105    25    -5   4.25 0.5978
WVFGRD96   93.0   105    25    -5   4.25 0.5966
WVFGRD96   94.0   110    25     0   4.25 0.5954
WVFGRD96   95.0   110    25     0   4.25 0.5946
WVFGRD96   96.0   110    25     0   4.25 0.5928
WVFGRD96   97.0   110    25     0   4.26 0.5907
WVFGRD96   98.0   110    25     0   4.26 0.5874
WVFGRD96   99.0   110    25     0   4.26 0.5872

The best solution is

WVFGRD96   70.0    80    25   -20   4.19 0.6257

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
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 00:26:21 CST 2015