Location

Location ANSS

2016/07/11 20:05:57 63.804 -149.238 112.6 4.2 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/07/11 20:05:57:0  63.80 -149.24 112.6 4.2 Alaska
 
 Stations used:
   AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.GHO AK.GLI 
   AK.HDA AK.KNK AK.KTH AK.MCK AK.MDM AK.NEA2 AK.PAX AK.RC01 
   AK.RND AK.SAW AK.SCM AK.SCRK AK.TRF AK.WRH AT.PMR IM.IL31 
   IU.COLA TA.H23K TA.H24K TA.I23K TA.J20K TA.J25K TA.J26L 
   TA.K20K TA.L19K TA.L26K TA.M22K TA.M26K TA.POKR 
 
 Filtering commands used:
   cut o DIST/4.5 -30 o DIST/4.5 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.04e+22 dyne-cm
  Mw = 4.14 
  Z  = 140 km
  Plane   Strike  Dip  Rake
   NP1      339    66   129
   NP2       95    45    35
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.04e+22     52     295
    N   0.00e+00     35     140
    P  -2.04e+22     12      41

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.57e+21
       Mxy    -1.27e+22
       Mxz     1.03e+21
       Myy    -2.14e+21
       Myz    -1.18e+22
       Mzz     1.17e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ######----------------              
              ###########--------------              
             ##############------------ P -          
           #################-----------   ---        
          ####################----------------       
         ######################----------------      
        ##########   ###########----------------     
        ########## T ############---------------     
       ###########   ############----------------    
       -##########################---------------    
       --##########################--------------    
       ---#########################-------------#    
        ----########################-----------#     
        ------######################--------####     
         --------###################------#####      
          ------------########################       
           --------------------------########        
             ------------------------######          
              ----------------------######           
                 -------------------###              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.17e+22   1.03e+21   1.18e+22 
  1.03e+21  -9.57e+21   1.27e+22 
  1.18e+22   1.27e+22  -2.14e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160711200557/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 = 95
      DIP = 45
     RAKE = 35
       MW = 4.14
       HS = 140.0

The NDK file is 20160711200557.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2016/07/11 20:05:57:0  63.80 -149.24 112.6 4.2 Alaska
 
 Stations used:
   AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.GHO AK.GLI 
   AK.HDA AK.KNK AK.KTH AK.MCK AK.MDM AK.NEA2 AK.PAX AK.RC01 
   AK.RND AK.SAW AK.SCM AK.SCRK AK.TRF AK.WRH AT.PMR IM.IL31 
   IU.COLA TA.H23K TA.H24K TA.I23K TA.J20K TA.J25K TA.J26L 
   TA.K20K TA.L19K TA.L26K TA.M22K TA.M26K TA.POKR 
 
 Filtering commands used:
   cut o DIST/4.5 -30 o DIST/4.5 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 2.04e+22 dyne-cm
  Mw = 4.14 
  Z  = 140 km
  Plane   Strike  Dip  Rake
   NP1      339    66   129
   NP2       95    45    35
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.04e+22     52     295
    N   0.00e+00     35     140
    P  -2.04e+22     12      41

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -9.57e+21
       Mxy    -1.27e+22
       Mxz     1.03e+21
       Myy    -2.14e+21
       Myz    -1.18e+22
       Mzz     1.17e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ######----------------              
              ###########--------------              
             ##############------------ P -          
           #################-----------   ---        
          ####################----------------       
         ######################----------------      
        ##########   ###########----------------     
        ########## T ############---------------     
       ###########   ############----------------    
       -##########################---------------    
       --##########################--------------    
       ---#########################-------------#    
        ----########################-----------#     
        ------######################--------####     
         --------###################------#####      
          ------------########################       
           --------------------------########        
             ------------------------######          
              ----------------------######           
                 -------------------###              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.17e+22   1.03e+21   1.18e+22 
  1.03e+21  -9.57e+21   1.27e+22 
  1.18e+22   1.27e+22  -2.14e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160711200557/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/4.5 -30 o DIST/4.5 +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   145    50   -80   3.26 0.2355
WVFGRD96    4.0   205    20   -20   3.25 0.1769
WVFGRD96    6.0   310    90   -65   3.27 0.2349
WVFGRD96    8.0   215    25   -10   3.37 0.2668
WVFGRD96   10.0   140    75    60   3.41 0.2977
WVFGRD96   12.0   145    70    65   3.45 0.3168
WVFGRD96   14.0   240    40    35   3.49 0.3221
WVFGRD96   16.0   245    40    35   3.52 0.3249
WVFGRD96   18.0   240    45    35   3.56 0.3210
WVFGRD96   20.0   240    45    30   3.58 0.3120
WVFGRD96   22.0   240    45    25   3.61 0.2987
WVFGRD96   24.0   240    45    25   3.62 0.2853
WVFGRD96   26.0   290    65    45   3.63 0.2737
WVFGRD96   28.0   290    65    45   3.65 0.2646
WVFGRD96   30.0   285    65    40   3.67 0.2544
WVFGRD96   32.0   285    65    40   3.67 0.2404
WVFGRD96   34.0   175    55   -40   3.69 0.2527
WVFGRD96   36.0   175    55   -40   3.70 0.2662
WVFGRD96   38.0   175    50   -45   3.71 0.2763
WVFGRD96   40.0   165    50   -55   3.82 0.3048
WVFGRD96   42.0   150    40   -90   3.84 0.3093
WVFGRD96   44.0   150    40   -90   3.86 0.3084
WVFGRD96   46.0   335    50   -85   3.88 0.3042
WVFGRD96   48.0   335    50   -85   3.89 0.2991
WVFGRD96   50.0   335    50   -85   3.90 0.2932
WVFGRD96   52.0   340    50   -80   3.91 0.2872
WVFGRD96   54.0   340    50   -80   3.91 0.2807
WVFGRD96   56.0   335    50   -75   3.91 0.2747
WVFGRD96   58.0   340    50   -70   3.92 0.2697
WVFGRD96   60.0   250    55    25   3.98 0.2807
WVFGRD96   62.0   275    60    40   3.96 0.3031
WVFGRD96   64.0   275    60    40   3.98 0.3384
WVFGRD96   66.0   275    60    40   3.99 0.3720
WVFGRD96   68.0   275    60    40   4.01 0.4007
WVFGRD96   70.0   100    45    65   3.99 0.4277
WVFGRD96   72.0   100    45    65   4.00 0.4555
WVFGRD96   74.0   100    45    60   4.01 0.4780
WVFGRD96   76.0   100    45    60   4.01 0.4980
WVFGRD96   78.0   100    45    55   4.02 0.5172
WVFGRD96   80.0   100    45    55   4.03 0.5358
WVFGRD96   82.0   100    45    55   4.03 0.5517
WVFGRD96   84.0   100    45    55   4.03 0.5656
WVFGRD96   86.0   100    45    50   4.04 0.5793
WVFGRD96   88.0   100    45    50   4.05 0.5923
WVFGRD96   90.0   100    45    50   4.05 0.6039
WVFGRD96   92.0   100    45    50   4.05 0.6144
WVFGRD96   94.0    95    45    45   4.06 0.6256
WVFGRD96   96.0    95    45    45   4.06 0.6355
WVFGRD96   98.0    95    45    45   4.06 0.6452
WVFGRD96  100.0    95    45    45   4.07 0.6552
WVFGRD96  102.0    95    45    45   4.07 0.6635
WVFGRD96  104.0    95    45    45   4.07 0.6713
WVFGRD96  106.0    95    45    40   4.08 0.6786
WVFGRD96  108.0    95    45    40   4.09 0.6843
WVFGRD96  110.0    95    45    40   4.09 0.6924
WVFGRD96  112.0    95    45    40   4.09 0.6979
WVFGRD96  114.0    95    45    40   4.10 0.7028
WVFGRD96  116.0    95    45    40   4.10 0.7076
WVFGRD96  118.0    95    45    40   4.10 0.7117
WVFGRD96  120.0    95    45    40   4.11 0.7159
WVFGRD96  122.0    95    45    40   4.11 0.7189
WVFGRD96  124.0    95    45    40   4.11 0.7224
WVFGRD96  126.0    95    45    40   4.11 0.7244
WVFGRD96  128.0    95    45    40   4.12 0.7270
WVFGRD96  130.0    95    45    40   4.12 0.7281
WVFGRD96  132.0    95    45    40   4.12 0.7293
WVFGRD96  134.0    95    45    40   4.12 0.7313
WVFGRD96  136.0    95    45    40   4.13 0.7317
WVFGRD96  138.0    95    45    40   4.13 0.7326
WVFGRD96  140.0    95    45    35   4.14 0.7335
WVFGRD96  142.0    95    45    40   4.13 0.7323
WVFGRD96  144.0    95    45    35   4.14 0.7332
WVFGRD96  146.0    95    45    35   4.15 0.7315
WVFGRD96  148.0    95    45    40   4.14 0.7315

The best solution is

WVFGRD96  140.0    95    45    35   4.14 0.7335

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/4.5 -30 o DIST/4.5 +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 11 16:54:19 CDT 2016