Location

Location ANSS

2017/07/24 21:26:40 59.619 -152.543 101.5 4.0 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/07/24 21:26:40:0  59.62 -152.54 101.5 4.0 Alaska
 
 Stations used:
   AK.CNP AK.FIRE AK.HOM AK.RC01 AV.ILSW II.KDAK TA.N19K 
   TA.O18K TA.P18K TA.P19K TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.5 -30 o DIST/3.5 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 7.50e+21 dyne-cm
  Mw = 3.85 
  Z  = 100 km
  Plane   Strike  Dip  Rake
   NP1      215    90   -35
   NP2      305    55   -180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.50e+21     24     266
    N   0.00e+00     55      35
    P  -7.50e+21     24     164

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -5.77e+21
       Mxy     2.10e+21
       Mxz     2.47e+21
       Myy     5.77e+21
       Myz    -3.52e+21
       Mzz     3.76e+14
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              -------------------------###           
             -------------------------#####          
           ############-------------#########        
          ##################-------###########       
         ######################---#############      
        #########################-##############     
        #######################-----############     
       #######################--------###########    
       ####   ###############-----------#########    
       #### T #############--------------########    
       ####   ############----------------#######    
        #################------------------#####     
        ###############---------------------####     
         #############-----------------------##      
          ###########------------------------#       
           #########-------------------------        
             #####-------------   ---------          
              ###-------------- P --------           
                 --------------   -----              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  3.76e+14   2.47e+21   3.52e+21 
  2.47e+21  -5.77e+21  -2.10e+21 
  3.52e+21  -2.10e+21   5.77e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170724212640/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 = 215
      DIP = 90
     RAKE = -35
       MW = 3.85
       HS = 100.0

The NDK file is 20170724212640.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/07/24 21:26:40:0  59.62 -152.54 101.5 4.0 Alaska
 
 Stations used:
   AK.CNP AK.FIRE AK.HOM AK.RC01 AV.ILSW II.KDAK TA.N19K 
   TA.O18K TA.P18K TA.P19K TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.5 -30 o DIST/3.5 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 7.50e+21 dyne-cm
  Mw = 3.85 
  Z  = 100 km
  Plane   Strike  Dip  Rake
   NP1      215    90   -35
   NP2      305    55   -180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.50e+21     24     266
    N   0.00e+00     55      35
    P  -7.50e+21     24     164

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -5.77e+21
       Mxy     2.10e+21
       Mxz     2.47e+21
       Myy     5.77e+21
       Myz    -3.52e+21
       Mzz     3.76e+14
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              -------------------------###           
             -------------------------#####          
           ############-------------#########        
          ##################-------###########       
         ######################---#############      
        #########################-##############     
        #######################-----############     
       #######################--------###########    
       ####   ###############-----------#########    
       #### T #############--------------########    
       ####   ############----------------#######    
        #################------------------#####     
        ###############---------------------####     
         #############-----------------------##      
          ###########------------------------#       
           #########-------------------------        
             #####-------------   ---------          
              ###-------------- P --------           
                 --------------   -----              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  3.76e+14   2.47e+21   3.52e+21 
  2.47e+21  -5.77e+21  -2.10e+21 
  3.52e+21  -2.10e+21   5.77e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170724212640/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 using wvfgrd96

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 o DIST/3.5 -30 o DIST/3.5 +50
rtr
taper w 0.1
hp c 0.03 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    2.0   155    30    75   2.93 0.1627
WVFGRD96    4.0   135    30    45   3.00 0.1930
WVFGRD96    6.0   285    40   -30   3.02 0.2108
WVFGRD96    8.0   275    35   -50   3.15 0.2236
WVFGRD96   10.0   260    30   -80   3.23 0.2233
WVFGRD96   12.0    85    55   -80   3.26 0.2192
WVFGRD96   14.0    85    55   -80   3.28 0.2103
WVFGRD96   16.0    85    55   -80   3.30 0.1972
WVFGRD96   18.0    85    55   -80   3.31 0.1805
WVFGRD96   20.0    30    75   -40   3.24 0.1668
WVFGRD96   22.0   250    65    80   3.40 0.1712
WVFGRD96   24.0   245    65    80   3.42 0.1805
WVFGRD96   26.0   250    65    80   3.46 0.1902
WVFGRD96   28.0   250    65    80   3.49 0.1977
WVFGRD96   30.0   245    65    80   3.50 0.2008
WVFGRD96   32.0   245    65    80   3.52 0.1968
WVFGRD96   34.0   245    70    75   3.53 0.1989
WVFGRD96   36.0   245    70    75   3.53 0.2024
WVFGRD96   38.0   240    70    70   3.52 0.2001
WVFGRD96   40.0    45    90   -60   3.52 0.1986
WVFGRD96   42.0   180    40   -70   3.51 0.2261
WVFGRD96   44.0   180    45   -70   3.56 0.2646
WVFGRD96   46.0   180    45   -65   3.59 0.2984
WVFGRD96   48.0   185    55   -65   3.64 0.3283
WVFGRD96   50.0   185    55   -65   3.66 0.3445
WVFGRD96   52.0   190    60   -60   3.67 0.3508
WVFGRD96   54.0   190    60   -60   3.68 0.3568
WVFGRD96   56.0   195    60   -55   3.67 0.3618
WVFGRD96   58.0   195    60   -55   3.68 0.3669
WVFGRD96   60.0   195    60   -55   3.68 0.3741
WVFGRD96   62.0   200    65   -50   3.69 0.3835
WVFGRD96   64.0   200    70   -50   3.72 0.3920
WVFGRD96   66.0   205    75   -45   3.72 0.4016
WVFGRD96   68.0   205    75   -45   3.73 0.4112
WVFGRD96   70.0   205    75   -45   3.74 0.4204
WVFGRD96   72.0   205    75   -45   3.75 0.4289
WVFGRD96   74.0   210    80   -40   3.75 0.4357
WVFGRD96   76.0   210    80   -40   3.76 0.4452
WVFGRD96   78.0   210    85   -40   3.78 0.4512
WVFGRD96   80.0   210    85   -40   3.79 0.4570
WVFGRD96   82.0   210    85   -40   3.80 0.4639
WVFGRD96   84.0   215    90   -40   3.82 0.4684
WVFGRD96   86.0   215    90   -40   3.82 0.4726
WVFGRD96   88.0   215    90   -40   3.83 0.4774
WVFGRD96   90.0   215    90   -35   3.82 0.4795
WVFGRD96   92.0   215    90   -35   3.83 0.4843
WVFGRD96   94.0   215    90   -35   3.83 0.4852
WVFGRD96   96.0   215    90   -35   3.84 0.4881
WVFGRD96   98.0   215    90   -35   3.84 0.4883
WVFGRD96  100.0   215    90   -35   3.85 0.4906
WVFGRD96  102.0   215    90   -35   3.85 0.4889
WVFGRD96  104.0   215    90   -35   3.85 0.4894
WVFGRD96  106.0   215    90   -35   3.86 0.4873
WVFGRD96  108.0   210    85   -30   3.84 0.4871
WVFGRD96  110.0   210    85   -30   3.85 0.4857
WVFGRD96  112.0   210    85   -30   3.85 0.4849
WVFGRD96  114.0   210    85   -30   3.85 0.4833
WVFGRD96  116.0   210    85   -30   3.86 0.4821
WVFGRD96  118.0   210    85   -30   3.86 0.4800

The best solution is

WVFGRD96  100.0   215    90   -35   3.85 0.4906

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 o DIST/3.5 -30 o DIST/3.5 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 
Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated.
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 Jul 24 19:17:17 CDT 2017