Location

Location ANSS

2018/01/19 23:55:05 63.967 -148.994 4.6 128.0 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2018/01/19 23:55:05:0  63.97 -148.99   4.6 128.0 Alaska
 
 Stations used:
   AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV AK.GHO 
   AK.HDA AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PAX 
   AK.PPD AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN 
   AK.TRF AK.WRH AT.MENT AT.PMR IM.IL31 IU.COLA TA.H21K 
   TA.I20K TA.I21K TA.I23K TA.I26K TA.J20K TA.J25K TA.J26L 
   TA.L26K TA.L27K TA.POKR 
 
 Filtering commands used:
   cut o DIST/3.7 -30 o DIST/3.7 +60
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.00e+23 dyne-cm
  Mw = 4.60 
  Z  = 136 km
  Plane   Strike  Dip  Rake
   NP1       38    85   155
   NP2      130    65     5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.00e+23     21     351
    N   0.00e+00     65     208
    P  -1.00e+23     14      87

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     8.50e+22
       Mxy    -1.90e+22
       Mxz     3.14e+22
       Myy    -9.17e+22
       Myz    -2.87e+22
       Mzz     6.68e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 #######   ############              
              ########## T #############--           
             ###########   ############----          
           -##########################-------        
          ---########################---------       
         -----######################-----------      
        ------####################--------------     
        -------##################---------------     
       ----------###############-------------   -    
       -----------############--------------- P -    
       -------------#########----------------   -    
       ---------------#####----------------------    
        ----------------##----------------------     
        ----------------##----------------------     
         --------------######------------------      
          -----------###########--------------       
           --------#################---------        
             -----#########################          
              --##########################           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  6.68e+21   3.14e+22   2.87e+22 
  3.14e+22   8.50e+22   1.90e+22 
  2.87e+22   1.90e+22  -9.17e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180119235505/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 = 130
      DIP = 65
     RAKE = 5
       MW = 4.60
       HS = 136.0

The NDK file is 20180119235505.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2018/01/19 23:55:05:0  63.97 -148.99   4.6 128.0 Alaska
 
 Stations used:
   AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.DHY AK.DIV AK.GHO 
   AK.HDA AK.KNK AK.KTH AK.MCK AK.MDM AK.MLY AK.NEA2 AK.PAX 
   AK.PPD AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN 
   AK.TRF AK.WRH AT.MENT AT.PMR IM.IL31 IU.COLA TA.H21K 
   TA.I20K TA.I21K TA.I23K TA.I26K TA.J20K TA.J25K TA.J26L 
   TA.L26K TA.L27K TA.POKR 
 
 Filtering commands used:
   cut o DIST/3.7 -30 o DIST/3.7 +60
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.00e+23 dyne-cm
  Mw = 4.60 
  Z  = 136 km
  Plane   Strike  Dip  Rake
   NP1       38    85   155
   NP2      130    65     5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.00e+23     21     351
    N   0.00e+00     65     208
    P  -1.00e+23     14      87

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     8.50e+22
       Mxy    -1.90e+22
       Mxz     3.14e+22
       Myy    -9.17e+22
       Myz    -2.87e+22
       Mzz     6.68e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 #######   ############              
              ########## T #############--           
             ###########   ############----          
           -##########################-------        
          ---########################---------       
         -----######################-----------      
        ------####################--------------     
        -------##################---------------     
       ----------###############-------------   -    
       -----------############--------------- P -    
       -------------#########----------------   -    
       ---------------#####----------------------    
        ----------------##----------------------     
        ----------------##----------------------     
         --------------######------------------      
          -----------###########--------------       
           --------#################---------        
             -----#########################          
              --##########################           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  6.68e+21   3.14e+22   2.87e+22 
  3.14e+22   8.50e+22   1.90e+22 
  2.87e+22   1.90e+22  -9.17e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180119235505/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/3.7 -30 o DIST/3.7 +60
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    45    65   -20   3.53 0.1902
WVFGRD96    4.0    45    60   -15   3.62 0.2194
WVFGRD96    6.0    50    65   -10   3.69 0.2465
WVFGRD96    8.0    50    65   -15   3.78 0.2676
WVFGRD96   10.0    55    70    15   3.82 0.2713
WVFGRD96   12.0    50    70    10   3.85 0.2664
WVFGRD96   14.0    50    75    10   3.88 0.2543
WVFGRD96   16.0   140    75    20   3.90 0.2440
WVFGRD96   18.0   140    75    20   3.93 0.2451
WVFGRD96   20.0   140    75    20   3.96 0.2465
WVFGRD96   22.0   140    80    20   3.98 0.2491
WVFGRD96   24.0   140    80    20   4.00 0.2512
WVFGRD96   26.0   140    80    20   4.02 0.2514
WVFGRD96   28.0   140    80    20   4.03 0.2482
WVFGRD96   30.0   140    80    25   4.04 0.2422
WVFGRD96   32.0   140    80    25   4.04 0.2337
WVFGRD96   34.0   140    75    20   4.05 0.2256
WVFGRD96   36.0   140    75    20   4.06 0.2201
WVFGRD96   38.0   140    75    20   4.09 0.2202
WVFGRD96   40.0   145    55    20   4.17 0.2312
WVFGRD96   42.0   140    65    20   4.19 0.2354
WVFGRD96   44.0   140    70    20   4.20 0.2408
WVFGRD96   46.0   140    70    15   4.22 0.2475
WVFGRD96   48.0   140    70    20   4.25 0.2561
WVFGRD96   50.0   140    70    20   4.26 0.2663
WVFGRD96   52.0   140    70    15   4.28 0.2777
WVFGRD96   54.0   140    70    15   4.29 0.2914
WVFGRD96   56.0   140    65    15   4.32 0.3099
WVFGRD96   58.0   140    65    15   4.33 0.3317
WVFGRD96   60.0   140    60    15   4.36 0.3561
WVFGRD96   62.0   140    60    15   4.38 0.3828
WVFGRD96   64.0   140    60    15   4.39 0.4103
WVFGRD96   66.0   140    60    15   4.41 0.4356
WVFGRD96   68.0   140    60    15   4.42 0.4582
WVFGRD96   70.0   140    55    15   4.44 0.4741
WVFGRD96   72.0   140    55    15   4.45 0.4874
WVFGRD96   74.0   135    55    10   4.46 0.5066
WVFGRD96   76.0   135    55    10   4.47 0.5301
WVFGRD96   78.0   135    55    10   4.48 0.5550
WVFGRD96   80.0   135    55    10   4.49 0.5744
WVFGRD96   82.0   135    55    10   4.50 0.5865
WVFGRD96   84.0   135    55    10   4.50 0.5939
WVFGRD96   86.0   135    55    10   4.51 0.5999
WVFGRD96   88.0   135    55    10   4.51 0.6052
WVFGRD96   90.0   135    55    10   4.52 0.6105
WVFGRD96   92.0   135    55    10   4.52 0.6167
WVFGRD96   94.0   135    55    10   4.53 0.6222
WVFGRD96   96.0   135    55    10   4.53 0.6266
WVFGRD96   98.0   135    55    10   4.54 0.6304
WVFGRD96  100.0   135    55    10   4.54 0.6334
WVFGRD96  102.0   135    60    10   4.54 0.6356
WVFGRD96  104.0   135    60    10   4.54 0.6400
WVFGRD96  106.0   130    60     5   4.55 0.6430
WVFGRD96  108.0   130    60     5   4.55 0.6454
WVFGRD96  110.0   130    60     5   4.55 0.6488
WVFGRD96  112.0   130    60     5   4.56 0.6528
WVFGRD96  114.0   130    60     5   4.56 0.6550
WVFGRD96  116.0   130    60     5   4.57 0.6560
WVFGRD96  118.0   130    60     5   4.57 0.6588
WVFGRD96  120.0   130    60     5   4.58 0.6602
WVFGRD96  122.0   130    60     5   4.58 0.6607
WVFGRD96  124.0   130    60     5   4.58 0.6625
WVFGRD96  126.0   130    60     5   4.59 0.6613
WVFGRD96  128.0   130    60     5   4.59 0.6616
WVFGRD96  130.0   130    65     5   4.59 0.6627
WVFGRD96  132.0   130    65     5   4.59 0.6619
WVFGRD96  134.0   130    65     5   4.59 0.6625
WVFGRD96  136.0   130    65     5   4.60 0.6633
WVFGRD96  138.0   130    65     0   4.60 0.6626
WVFGRD96  140.0   130    65     0   4.60 0.6623
WVFGRD96  142.0   130    65     0   4.61 0.6602
WVFGRD96  144.0   130    65     5   4.61 0.6604
WVFGRD96  146.0   130    65     0   4.61 0.6577
WVFGRD96  148.0   130    65     0   4.62 0.6568

The best solution is

WVFGRD96  136.0   130    65     5   4.60 0.6633

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/3.7 -30 o DIST/3.7 +60
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 Fri Jan 19 21:06:54 CST 2018