Location

2013/07/26 20:37:19 58.024 -151.745 40.0 4.3 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  2013/07/26 20:37:19:0  58.02 -151.74  40.0 4.3 Alaska
 
 Stations used:
   AK.BRLK AK.CHI AK.CNP AK.FID AK.GHO AK.GLI AK.HIN AK.HOM 
   AK.KNK AK.RC01 AK.SII AK.SKN AK.SSN AK.SWD AT.CHGN AT.OHAK 
   AT.PMR AT.SVW2 II.KDAK 
 
 Filtering commands used:
   cut a -30 a 180
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 7.59e+22 dyne-cm
  Mw = 4.52 
  Z  = 40 km
  Plane   Strike  Dip  Rake
   NP1      220    50   -90
   NP2       40    40   -90
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.59e+22      5     310
    N   0.00e+00     -0     220
    P  -7.59e+22     85     130

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     3.09e+22
       Mxy    -3.68e+22
       Mxz     8.47e+21
       Myy     4.38e+22
       Myz    -1.01e+22
       Mzz    -7.47e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              #####################------#           
              ################------------#          
            T ############-----------------##        
          #   ##########-------------------###       
         #############---------------------####      
        ############-----------------------#####     
        ###########------------------------#####     
       ###########-------------------------######    
       ##########-----------   -----------#######    
       #########------------ P -----------#######    
       ########-------------   ----------########    
        ######--------------------------########     
        ######-------------------------#########     
         #####-----------------------##########      
          ####---------------------###########       
           ###-------------------############        
             #----------------#############          
              #-----------################           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -7.47e+22   8.47e+21   1.01e+22 
  8.47e+21   3.09e+22   3.68e+22 
  1.01e+22   3.68e+22   4.38e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130726203719/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 = 40
      DIP = 40
     RAKE = -90
       MW = 4.52
       HS = 40.0

The NDK file is 20130726203719.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMT
 USGS/SLU Moment Tensor Solution
 ENS  2013/07/26 20:37:19:0  58.02 -151.74  40.0 4.3 Alaska
 
 Stations used:
   AK.BRLK AK.CHI AK.CNP AK.FID AK.GHO AK.GLI AK.HIN AK.HOM 
   AK.KNK AK.RC01 AK.SII AK.SKN AK.SSN AK.SWD AT.CHGN AT.OHAK 
   AT.PMR AT.SVW2 II.KDAK 
 
 Filtering commands used:
   cut a -30 a 180
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.06 n 3 
 
 Best Fitting Double Couple
  Mo = 7.59e+22 dyne-cm
  Mw = 4.52 
  Z  = 40 km
  Plane   Strike  Dip  Rake
   NP1      220    50   -90
   NP2       40    40   -90
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.59e+22      5     310
    N   0.00e+00     -0     220
    P  -7.59e+22     85     130

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     3.09e+22
       Mxy    -3.68e+22
       Mxz     8.47e+21
       Myy     4.38e+22
       Myz    -1.01e+22
       Mzz    -7.47e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              #####################------#           
              ################------------#          
            T ############-----------------##        
          #   ##########-------------------###       
         #############---------------------####      
        ############-----------------------#####     
        ###########------------------------#####     
       ###########-------------------------######    
       ##########-----------   -----------#######    
       #########------------ P -----------#######    
       ########-------------   ----------########    
        ######--------------------------########     
        ######-------------------------#########     
         #####-----------------------##########      
          ####---------------------###########       
           ###-------------------############        
             #----------------#############          
              #-----------################           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -7.47e+22   8.47e+21   1.01e+22 
  8.47e+21   3.09e+22   3.68e+22 
  1.01e+22   3.68e+22   4.38e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130726203719/index.html
	
USGS/SLU Regional Moment Solution

13/07/26 20:37:19.00

Epicenter:  58.024 -151.745
MW 4.6

USGS/SLU REGIONAL MOMENT TENSOR
Depth  40         No. of sta: 30
Moment Tensor;   Scale 10**15 Nm
  Mrr=-7.74       Mtt= 2.11
  Mpp= 5.63       Mrt=-1.46
  Mrp=-0.62       Mtp= 4.58
 Principal axes:
  T  Val=  8.89  Plg= 5  Azm=125
  N       -0.94       7      215
  P       -7.96      82        1

Best Double Couple:Mo=8.5*10**15
 NP1:Strike= 41 Dip=50 Slip= -81
 NP2:       207     41      -101


        

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 180
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    1.0   115    45    85   3.75 0.2858
WVFGRD96    2.0    75    45    85   3.95 0.3658
WVFGRD96    3.0    80    45    90   4.02 0.3894
WVFGRD96    4.0   215    85     5   3.96 0.3738
WVFGRD96    5.0   215    85     0   3.99 0.3818
WVFGRD96    6.0    35    90     0   4.01 0.3818
WVFGRD96    7.0    30    75   -20   4.04 0.3907
WVFGRD96    8.0    30    70   -25   4.08 0.4041
WVFGRD96    9.0    30    70   -25   4.09 0.4077
WVFGRD96   10.0    30    70   -20   4.09 0.4083
WVFGRD96   11.0    30    65   -20   4.10 0.4092
WVFGRD96   12.0    75    75   -40   4.06 0.4212
WVFGRD96   13.0    70    70   -40   4.08 0.4392
WVFGRD96   14.0    70    65   -40   4.09 0.4557
WVFGRD96   15.0    70    65   -40   4.10 0.4715
WVFGRD96   16.0    70    65   -40   4.11 0.4867
WVFGRD96   17.0    70    65   -40   4.12 0.5005
WVFGRD96   18.0    70    65   -40   4.13 0.5164
WVFGRD96   19.0    70    65   -40   4.14 0.5319
WVFGRD96   20.0    35    30   -90   4.22 0.5533
WVFGRD96   21.0   215    60   -90   4.24 0.5731
WVFGRD96   22.0    40    35   -85   4.24 0.5921
WVFGRD96   23.0    40    35   -85   4.25 0.6105
WVFGRD96   24.0    40    35   -85   4.25 0.6274
WVFGRD96   25.0    40    35   -85   4.26 0.6430
WVFGRD96   26.0    40    35   -85   4.27 0.6569
WVFGRD96   27.0    40    35   -85   4.28 0.6693
WVFGRD96   28.0    40    35   -85   4.29 0.6801
WVFGRD96   29.0    45    40   -80   4.29 0.6905
WVFGRD96   30.0    50    40   -75   4.29 0.6999
WVFGRD96   31.0    50    40   -75   4.30 0.7090
WVFGRD96   32.0    50    40   -75   4.31 0.7158
WVFGRD96   33.0    45    40   -80   4.32 0.7198
WVFGRD96   34.0    45    40   -80   4.33 0.7223
WVFGRD96   35.0    45    40   -80   4.34 0.7227
WVFGRD96   36.0    45    40   -80   4.35 0.7204
WVFGRD96   37.0    45    40   -80   4.36 0.7158
WVFGRD96   38.0    50    45   -80   4.37 0.7112
WVFGRD96   39.0    45    45   -80   4.38 0.7064
WVFGRD96   40.0    40    40   -90   4.52 0.7421
WVFGRD96   41.0    40    40   -90   4.52 0.7395
WVFGRD96   42.0    45    45   -85   4.52 0.7337
WVFGRD96   43.0   220    45   -95   4.53 0.7271
WVFGRD96   44.0    45    45   -90   4.54 0.7185
WVFGRD96   45.0    45    45   -90   4.54 0.7086
WVFGRD96   46.0   220    45   -90   4.55 0.6978
WVFGRD96   47.0    40    45   -90   4.55 0.6859
WVFGRD96   48.0    45    50   -90   4.55 0.6733
WVFGRD96   49.0    45    50   -90   4.56 0.6607
WVFGRD96   50.0    45    50   -90   4.56 0.6483
WVFGRD96   51.0    45    50   -90   4.56 0.6352
WVFGRD96   52.0    45    50   -90   4.56 0.6212
WVFGRD96   53.0    45    50   -90   4.56 0.6074
WVFGRD96   54.0   225    40   -90   4.56 0.5929
WVFGRD96   55.0    45    50   -90   4.56 0.5791
WVFGRD96   56.0   225    40   -90   4.56 0.5649
WVFGRD96   57.0    45    50   -90   4.56 0.5516
WVFGRD96   58.0    45    50   -90   4.55 0.5380
WVFGRD96   59.0    45    50   -90   4.55 0.5253

The best solution is

WVFGRD96   40.0    40    40   -90   4.52 0.7421

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 180
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 00:22:07 CST 2015