Location

2013/08/04 07:57:54 61.410 -149.886 18.3 3.9 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  2013/08/04 07:57:54:0  61.41 -149.89  18.3 3.9 Alaska
 
 Stations used:
   AK.FID AK.GHO AK.GLI AK.RC01 AK.SAW AK.SCM AK.SKN AK.SSN 
   AK.SWD AT.PMR 
 
 Filtering commands used:
   cut a -30 a 120
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 1.02e+22 dyne-cm
  Mw = 3.94 
  Z  = 48 km
  Plane   Strike  Dip  Rake
   NP1      220    70   -60
   NP2      341    36   -144
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.02e+22     19     288
    N   0.00e+00     28      29
    P  -1.02e+22     55     168

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.38e+21
       Mxy    -1.97e+21
       Mxz     5.70e+21
       Myy     8.08e+21
       Myz    -4.08e+21
       Mzz    -5.70e+21
                                                     
                                                     
                                                     
                                                     
                     ##------------                  
                 ############----------              
              ##################---------#           
             ######################-#######          
           ######################---#########        
          #####################-------########       
         ####################----------########      
        ##   ##############-------------########     
        ## T ############----------------#######     
       ###   ###########-----------------########    
       ###############--------------------#######    
       ##############---------------------#######    
       #############----------------------#######    
        ###########-----------------------######     
        ##########----------   -----------######     
         ########----------- P -----------#####      
          ######------------   ----------#####       
           ####-------------------------#####        
             ##-------------------------###          
              #-----------------------####           
                 --------------------##              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -5.70e+21   5.70e+21   4.08e+21 
  5.70e+21  -2.38e+21   1.97e+21 
  4.08e+21   1.97e+21   8.08e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130804075754/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 = 220
      DIP = 70
     RAKE = -60
       MW = 3.94
       HS = 48.0

The NDK file is 20130804075754.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2013/08/04 07:57:54:0  61.41 -149.89  18.3 3.9 Alaska
 
 Stations used:
   AK.FID AK.GHO AK.GLI AK.RC01 AK.SAW AK.SCM AK.SKN AK.SSN 
   AK.SWD AT.PMR 
 
 Filtering commands used:
   cut a -30 a 120
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 1.02e+22 dyne-cm
  Mw = 3.94 
  Z  = 48 km
  Plane   Strike  Dip  Rake
   NP1      220    70   -60
   NP2      341    36   -144
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.02e+22     19     288
    N   0.00e+00     28      29
    P  -1.02e+22     55     168

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.38e+21
       Mxy    -1.97e+21
       Mxz     5.70e+21
       Myy     8.08e+21
       Myz    -4.08e+21
       Mzz    -5.70e+21
                                                     
                                                     
                                                     
                                                     
                     ##------------                  
                 ############----------              
              ##################---------#           
             ######################-#######          
           ######################---#########        
          #####################-------########       
         ####################----------########      
        ##   ##############-------------########     
        ## T ############----------------#######     
       ###   ###########-----------------########    
       ###############--------------------#######    
       ##############---------------------#######    
       #############----------------------#######    
        ###########-----------------------######     
        ##########----------   -----------######     
         ########----------- P -----------#####      
          ######------------   ----------#####       
           ####-------------------------#####        
             ##-------------------------###          
              #-----------------------####           
                 --------------------##              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -5.70e+21   5.70e+21   4.08e+21 
  5.70e+21  -2.38e+21   1.97e+21 
  4.08e+21   1.97e+21   8.08e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130804075754/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:

cut a -30 a 120
rtr
taper w 0.1
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   195    45    85   3.21 0.2647
WVFGRD96    1.0   195    45    90   3.24 0.2639
WVFGRD96    2.0   195    45    85   3.37 0.3454
WVFGRD96    3.0   190    50    80   3.42 0.3495
WVFGRD96    4.0    15    45   -90   3.46 0.3489
WVFGRD96    5.0   225    80   -10   3.42 0.3568
WVFGRD96    6.0   230    80    -5   3.43 0.3697
WVFGRD96    7.0   240    70    -5   3.45 0.3840
WVFGRD96    8.0   240    70    -5   3.48 0.3994
WVFGRD96    9.0    70    60    30   3.52 0.4079
WVFGRD96   10.0    70    65    30   3.54 0.4210
WVFGRD96   11.0    70    65    30   3.55 0.4353
WVFGRD96   12.0    70    65    30   3.56 0.4481
WVFGRD96   13.0    70    65    30   3.57 0.4591
WVFGRD96   14.0    70    65    30   3.59 0.4688
WVFGRD96   15.0    75    65    25   3.60 0.4779
WVFGRD96   16.0    70    70    30   3.61 0.4863
WVFGRD96   17.0    70    70    25   3.62 0.4941
WVFGRD96   18.0    70    70    25   3.63 0.5016
WVFGRD96   19.0    70    70    25   3.64 0.5087
WVFGRD96   20.0    70    70    25   3.65 0.5148
WVFGRD96   21.0   230    70   -35   3.67 0.5247
WVFGRD96   22.0   230    70   -40   3.68 0.5380
WVFGRD96   23.0   230    70   -40   3.69 0.5510
WVFGRD96   24.0   230    70   -40   3.69 0.5625
WVFGRD96   25.0   230    70   -40   3.70 0.5734
WVFGRD96   26.0   230    70   -40   3.71 0.5840
WVFGRD96   27.0   230    70   -40   3.72 0.5926
WVFGRD96   28.0   230    70   -40   3.72 0.6010
WVFGRD96   29.0   230    70   -40   3.73 0.6076
WVFGRD96   30.0   230    75   -45   3.74 0.6138
WVFGRD96   31.0   230    75   -45   3.75 0.6220
WVFGRD96   32.0   230    75   -45   3.75 0.6281
WVFGRD96   33.0   225    70   -45   3.76 0.6349
WVFGRD96   34.0   225    70   -45   3.76 0.6399
WVFGRD96   35.0   225    70   -45   3.77 0.6464
WVFGRD96   36.0   225    70   -45   3.78 0.6518
WVFGRD96   37.0   225    70   -45   3.79 0.6570
WVFGRD96   38.0   225    70   -45   3.79 0.6607
WVFGRD96   39.0   220    70   -45   3.81 0.6653
WVFGRD96   40.0   225    70   -60   3.90 0.6621
WVFGRD96   41.0   225    70   -55   3.90 0.6679
WVFGRD96   42.0   225    70   -55   3.91 0.6722
WVFGRD96   43.0   220    70   -55   3.91 0.6767
WVFGRD96   44.0   220    70   -55   3.92 0.6794
WVFGRD96   45.0   220    70   -55   3.93 0.6820
WVFGRD96   46.0   220    70   -55   3.93 0.6831
WVFGRD96   47.0   220    70   -60   3.94 0.6837
WVFGRD96   48.0   220    70   -60   3.94 0.6839
WVFGRD96   49.0   220    70   -60   3.95 0.6832
WVFGRD96   50.0   220    70   -60   3.95 0.6822
WVFGRD96   51.0   220    70   -60   3.95 0.6801
WVFGRD96   52.0   220    70   -60   3.96 0.6781
WVFGRD96   53.0   220    70   -60   3.96 0.6752
WVFGRD96   54.0   220    70   -60   3.96 0.6725
WVFGRD96   55.0   220    70   -60   3.96 0.6692
WVFGRD96   56.0   220    70   -60   3.97 0.6650
WVFGRD96   57.0   220    70   -60   3.97 0.6615
WVFGRD96   58.0   220    70   -60   3.97 0.6562
WVFGRD96   59.0   215    70   -60   3.97 0.6533
WVFGRD96   60.0   215    70   -60   3.98 0.6490
WVFGRD96   61.0   215    70   -60   3.98 0.6451
WVFGRD96   62.0   215    70   -60   3.98 0.6406
WVFGRD96   63.0   215    70   -60   3.98 0.6363
WVFGRD96   64.0   215    70   -60   3.98 0.6328
WVFGRD96   65.0   215    70   -60   3.98 0.6281
WVFGRD96   66.0   215    70   -60   3.98 0.6238
WVFGRD96   67.0   215    70   -60   3.99 0.6204
WVFGRD96   68.0   215    70   -60   3.99 0.6152
WVFGRD96   69.0   215    70   -60   3.99 0.6114
WVFGRD96   70.0   215    70   -60   3.99 0.6073
WVFGRD96   71.0   215    70   -60   3.99 0.6021
WVFGRD96   72.0   215    70   -60   3.99 0.5988
WVFGRD96   73.0   215    70   -60   3.99 0.5943
WVFGRD96   74.0   215    70   -60   3.99 0.5894
WVFGRD96   75.0   215    70   -60   3.99 0.5860
WVFGRD96   76.0   215    75   -60   4.00 0.5817
WVFGRD96   77.0   215    75   -60   4.00 0.5786
WVFGRD96   78.0   215    75   -60   4.00 0.5754
WVFGRD96   79.0   215    75   -60   4.00 0.5729
WVFGRD96   80.0   215    75   -65   4.00 0.5696
WVFGRD96   81.0   215    75   -65   4.01 0.5664
WVFGRD96   82.0   215    75   -65   4.01 0.5643
WVFGRD96   83.0   215    75   -65   4.01 0.5611
WVFGRD96   84.0   215    75   -65   4.01 0.5579
WVFGRD96   85.0   215    75   -65   4.01 0.5554
WVFGRD96   86.0   215    75   -65   4.01 0.5531
WVFGRD96   87.0   215    75   -70   4.01 0.5494
WVFGRD96   88.0   215    80   -70   4.02 0.5475
WVFGRD96   89.0   215    80   -70   4.02 0.5457
WVFGRD96   90.0   215    80   -70   4.03 0.5442
WVFGRD96   91.0   215    80   -75   4.03 0.5414
WVFGRD96   92.0   215    80   -75   4.03 0.5408
WVFGRD96   93.0   215    80   -80   4.04 0.5388
WVFGRD96   94.0   215    80   -85   4.05 0.5373
WVFGRD96   95.0   215    80   -85   4.05 0.5356
WVFGRD96   96.0   215    80   -85   4.05 0.5345
WVFGRD96   97.0   215    80   -85   4.05 0.5329
WVFGRD96   98.0   215    80   -85   4.05 0.5307
WVFGRD96   99.0   215    80   -85   4.05 0.5291
WVFGRD96  100.0   215    80   -85   4.05 0.5273
WVFGRD96  101.0   215    80   -85   4.06 0.5255
WVFGRD96  102.0    15    10  -110   4.06 0.5228
WVFGRD96  103.0   215    80   -85   4.06 0.5211
WVFGRD96  104.0   215    80   -90   4.07 0.5189
WVFGRD96  105.0   215    80   -90   4.07 0.5168
WVFGRD96  106.0   215    80   -90   4.07 0.5144
WVFGRD96  107.0   215    80   -90   4.07 0.5117
WVFGRD96  108.0    10    10  -110   4.07 0.5103
WVFGRD96  109.0    10    10  -110   4.07 0.5075
WVFGRD96  110.0    50    10   -75   4.08 0.5049
WVFGRD96  111.0    50    10   -70   4.08 0.5024
WVFGRD96  112.0    60    10   -60   4.09 0.5002
WVFGRD96  113.0    60    10   -60   4.09 0.4982
WVFGRD96  114.0    60    10   -60   4.09 0.4954
WVFGRD96  115.0    65    10   -55   4.10 0.4926
WVFGRD96  116.0    65    10   -55   4.10 0.4907
WVFGRD96  117.0    65    10   -55   4.10 0.4879
WVFGRD96  118.0    65    10   -55   4.10 0.4858
WVFGRD96  119.0    70    10   -50   4.10 0.4828
WVFGRD96  120.0    70    10   -50   4.10 0.4799
WVFGRD96  121.0    70    10   -50   4.10 0.4780
WVFGRD96  122.0    50     5   -70   4.10 0.4753
WVFGRD96  123.0    50     5   -70   4.10 0.4726
WVFGRD96  124.0    50     5   -70   4.10 0.4697
WVFGRD96  125.0    60     5   -60   4.10 0.4668
WVFGRD96  126.0    60     5   -60   4.10 0.4647
WVFGRD96  127.0    60     5   -60   4.10 0.4622
WVFGRD96  128.0    60     5   -60   4.11 0.4590
WVFGRD96  129.0    60     5   -60   4.11 0.4565

The best solution is

WVFGRD96   48.0   220    70   -60   3.94 0.6839

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

cut a -30 a 120
rtr
taper w 0.1
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 00:22:12 CST 2015