Location

Location ANSS

2017/07/10 04:31:08 59.738 -153.142 105.5 4.3 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/07/10 04:31:08:0  59.74 -153.14 105.5 4.3 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CNP AK.FIRE AK.GHO AK.HOM AK.PWL AK.RC01 
   AK.SSN AK.SWD AT.OHAK AT.PMR AT.SVW2 AV.ILSW II.KDAK 
   TA.L19K TA.M19K TA.M20K TA.M22K TA.N18K TA.N19K TA.O18K 
   TA.O19K TA.O22K TA.P18K TA.P19K TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.4 -40 o DIST/3.4 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 7.59e+22 dyne-cm
  Mw = 4.52 
  Z  = 112 km
  Plane   Strike  Dip  Rake
   NP1       45    80    65
   NP2      295    27   157
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.59e+22     49     288
    N   0.00e+00     25      50
    P  -7.59e+22     31     155

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.33e+22
       Mxy     1.18e+22
       Mxz     4.17e+22
       Myy     1.98e+22
       Myz    -4.96e+22
       Mzz     2.35e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              -------########-------------           
             ---###################-------#          
           --#########################---####        
          ####################################       
         ##############################---#####      
        #############################------#####     
        #########   ################---------###     
       ########## T ##############------------###    
       ##########   ############--------------###    
       ########################----------------##    
       ######################------------------##    
        ###################--------------------#     
        #################----------------------#     
         ##############------------------------      
          ##########--------------------------       
           #######---------------   ---------        
             ##------------------ P -------          
              -------------------   ------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.35e+22   4.17e+22   4.96e+22 
  4.17e+22  -4.33e+22  -1.18e+22 
  4.96e+22  -1.18e+22   1.98e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170710043108/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 = 45
      DIP = 80
     RAKE = 65
       MW = 4.52
       HS = 112.0

The NDK file is 20170710043108.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2017/07/10 04:31:08:0  59.74 -153.14 105.5 4.3 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CNP AK.FIRE AK.GHO AK.HOM AK.PWL AK.RC01 
   AK.SSN AK.SWD AT.OHAK AT.PMR AT.SVW2 AV.ILSW II.KDAK 
   TA.L19K TA.M19K TA.M20K TA.M22K TA.N18K TA.N19K TA.O18K 
   TA.O19K TA.O22K TA.P18K TA.P19K TA.Q19K 
 
 Filtering commands used:
   cut o DIST/3.4 -40 o DIST/3.4 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 7.59e+22 dyne-cm
  Mw = 4.52 
  Z  = 112 km
  Plane   Strike  Dip  Rake
   NP1       45    80    65
   NP2      295    27   157
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.59e+22     49     288
    N   0.00e+00     25      50
    P  -7.59e+22     31     155

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.33e+22
       Mxy     1.18e+22
       Mxz     4.17e+22
       Myy     1.98e+22
       Myz    -4.96e+22
       Mzz     2.35e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              -------########-------------           
             ---###################-------#          
           --#########################---####        
          ####################################       
         ##############################---#####      
        #############################------#####     
        #########   ################---------###     
       ########## T ##############------------###    
       ##########   ############--------------###    
       ########################----------------##    
       ######################------------------##    
        ###################--------------------#     
        #################----------------------#     
         ##############------------------------      
          ##########--------------------------       
           #######---------------   ---------        
             ##------------------ P -------          
              -------------------   ------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.35e+22   4.17e+22   4.96e+22 
  4.17e+22  -4.33e+22  -1.18e+22 
  4.96e+22  -1.18e+22   1.98e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170710043108/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/3.4 -40 o DIST/3.4 +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   215    75   -40   3.44 0.0972
WVFGRD96    4.0   215    80   -40   3.54 0.1225
WVFGRD96    6.0   215    80   -35   3.60 0.1476
WVFGRD96    8.0   215    80   -40   3.69 0.1611
WVFGRD96   10.0   310    55    15   3.73 0.1712
WVFGRD96   12.0   310    55    15   3.77 0.1801
WVFGRD96   14.0   305    60   -10   3.80 0.1826
WVFGRD96   16.0   305    60   -10   3.83 0.1817
WVFGRD96   18.0   300    65   -20   3.86 0.1795
WVFGRD96   20.0   220    75   -20   3.90 0.1764
WVFGRD96   22.0   225    80   -20   3.93 0.1812
WVFGRD96   24.0   230    85   -15   3.95 0.1831
WVFGRD96   26.0   230    85   -15   3.96 0.1796
WVFGRD96   28.0    55    85    10   3.98 0.1784
WVFGRD96   30.0    55    80     5   3.99 0.1779
WVFGRD96   32.0    55    80     0   4.00 0.1761
WVFGRD96   34.0    50    85    -5   4.03 0.1809
WVFGRD96   36.0    50    85   -10   4.05 0.1884
WVFGRD96   38.0   230    90     5   4.09 0.1940
WVFGRD96   40.0    50    85   -15   4.15 0.2072
WVFGRD96   42.0    50    85   -15   4.18 0.2136
WVFGRD96   44.0    50    85   -10   4.21 0.2165
WVFGRD96   46.0   230    90    10   4.23 0.2159
WVFGRD96   48.0    50    85    -5   4.25 0.2163
WVFGRD96   50.0    55    90     0   4.27 0.2190
WVFGRD96   52.0   235    90     0   4.29 0.2230
WVFGRD96   54.0    50    75    20   4.30 0.2434
WVFGRD96   56.0    50    75    25   4.32 0.2712
WVFGRD96   58.0    50    75    25   4.34 0.2947
WVFGRD96   60.0    50    75    25   4.36 0.3146
WVFGRD96   62.0    50    75    30   4.37 0.3318
WVFGRD96   64.0    50    75    30   4.39 0.3476
WVFGRD96   66.0    50    75    30   4.40 0.3628
WVFGRD96   68.0    50    75    30   4.41 0.3755
WVFGRD96   70.0    50    75    30   4.42 0.3887
WVFGRD96   72.0    50    75    30   4.43 0.3999
WVFGRD96   74.0    50    75    30   4.44 0.4099
WVFGRD96   76.0    70    60    30   4.44 0.4235
WVFGRD96   78.0    70    55    25   4.45 0.4359
WVFGRD96   80.0    70    55    25   4.45 0.4489
WVFGRD96   82.0    70    55    25   4.46 0.4601
WVFGRD96   84.0    70    55    25   4.47 0.4713
WVFGRD96   86.0    70    55    25   4.47 0.4795
WVFGRD96   88.0    70    55    25   4.47 0.4885
WVFGRD96   90.0    70    50    20   4.49 0.4959
WVFGRD96   92.0    75    50    30   4.47 0.5028
WVFGRD96   94.0    50    80    60   4.49 0.5112
WVFGRD96   96.0    50    80    65   4.49 0.5215
WVFGRD96   98.0    50    80    65   4.50 0.5303
WVFGRD96  100.0    50    80    65   4.50 0.5396
WVFGRD96  102.0    50    80    65   4.51 0.5464
WVFGRD96  104.0    50    80    65   4.51 0.5515
WVFGRD96  106.0    50    80    65   4.51 0.5545
WVFGRD96  108.0    50    80    65   4.52 0.5574
WVFGRD96  110.0    50    80    65   4.52 0.5601
WVFGRD96  112.0    45    80    65   4.52 0.5607
WVFGRD96  114.0    45    80    65   4.52 0.5598
WVFGRD96  116.0    45    80    65   4.53 0.5585
WVFGRD96  118.0    45    80    65   4.53 0.5563

The best solution is

WVFGRD96  112.0    45    80    65   4.52 0.5607

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/3.4 -40 o DIST/3.4 +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 10 05:36:46 CDT 2017