Location

Location ANSS

2019/05/27 23:44:49 60.258 -152.520 97.2 4.0 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2019/05/27 23:44:49:0  60.26 -152.52  97.2 4.0 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CNP AK.CUT AK.GHO AK.HOM AK.KNK AK.PWL 
   AK.RC01 AK.SAW AK.SKN AK.SLK AK.SSN AK.SWD AT.PMR AV.ILSW 
   AV.STLK TA.L19K TA.M19K TA.M20K TA.M22K TA.N17K TA.N18K 
   TA.N19K TA.O18K TA.O19K TA.P18K TA.P19K TA.P23K TA.Q19K 
   TA.Q20K 
 
 Filtering commands used:
   cut o DIST/3.3 -50 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 3.43e+22 dyne-cm
  Mw = 4.29 
  Z  = 94 km
  Plane   Strike  Dip  Rake
   NP1       55    65    30
   NP2      311    63   152
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.43e+22     38     274
    N   0.00e+00     52      91
    P  -3.43e+22      1     183

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.41e+22
       Mxy    -3.03e+21
       Mxz     1.83e+21
       Myy     2.10e+22
       Myz    -1.66e+22
       Mzz     1.31e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ----------------------------           
             ------------------------------          
           ###########-----------------------        
          ###############--------------------#       
         ###################----------------###      
        #######################------------#####     
        #########################---------######     
       #######   ##################-----#########    
       ####### T ####################--##########    
       #######   ####################-###########    
       ############################-----#########    
        ########################---------#######     
        ######################------------######     
         ##################---------------#####      
          ############---------------------###       
           -##-----------------------------##        
             ------------------------------          
              ----------------------------           
                 ---------   ----------              
                     ----- P ------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.31e+22   1.83e+21   1.66e+22 
  1.83e+21  -3.41e+22   3.03e+21 
  1.66e+22   3.03e+21   2.10e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190527234449/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 = 55
      DIP = 65
     RAKE = 30
       MW = 4.29
       HS = 94.0

The NDK file is 20190527234449.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2019/05/27 23:44:49:0  60.26 -152.52  97.2 4.0 Alaska
 
 Stations used:
   AK.BRLK AK.CAPN AK.CNP AK.CUT AK.GHO AK.HOM AK.KNK AK.PWL 
   AK.RC01 AK.SAW AK.SKN AK.SLK AK.SSN AK.SWD AT.PMR AV.ILSW 
   AV.STLK TA.L19K TA.M19K TA.M20K TA.M22K TA.N17K TA.N18K 
   TA.N19K TA.O18K TA.O19K TA.P18K TA.P19K TA.P23K TA.Q19K 
   TA.Q20K 
 
 Filtering commands used:
   cut o DIST/3.3 -50 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 3.43e+22 dyne-cm
  Mw = 4.29 
  Z  = 94 km
  Plane   Strike  Dip  Rake
   NP1       55    65    30
   NP2      311    63   152
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.43e+22     38     274
    N   0.00e+00     52      91
    P  -3.43e+22      1     183

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.41e+22
       Mxy    -3.03e+21
       Mxz     1.83e+21
       Myy     2.10e+22
       Myz    -1.66e+22
       Mzz     1.31e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ----------------------------           
             ------------------------------          
           ###########-----------------------        
          ###############--------------------#       
         ###################----------------###      
        #######################------------#####     
        #########################---------######     
       #######   ##################-----#########    
       ####### T ####################--##########    
       #######   ####################-###########    
       ############################-----#########    
        ########################---------#######     
        ######################------------######     
         ##################---------------#####      
          ############---------------------###       
           -##-----------------------------##        
             ------------------------------          
              ----------------------------           
                 ---------   ----------              
                     ----- P ------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.31e+22   1.83e+21   1.66e+22 
  1.83e+21  -3.41e+22   3.03e+21 
  1.66e+22   3.03e+21   2.10e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190527234449/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/3.3 -50 o DIST/3.3 +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   115    55   -60   3.45 0.2192
WVFGRD96    4.0   325    65    15   3.46 0.2525
WVFGRD96    6.0   145    65    20   3.54 0.2852
WVFGRD96    8.0   145    65    20   3.62 0.3039
WVFGRD96   10.0   145    70    20   3.66 0.3055
WVFGRD96   12.0   145    70    15   3.69 0.2980
WVFGRD96   14.0   145    70    15   3.72 0.2838
WVFGRD96   16.0   145    70    15   3.74 0.2648
WVFGRD96   18.0   225    70   -25   3.77 0.2595
WVFGRD96   20.0   225    70   -20   3.79 0.2618
WVFGRD96   22.0   225    70   -20   3.82 0.2691
WVFGRD96   24.0   230    70    15   3.83 0.2798
WVFGRD96   26.0   230    70    15   3.86 0.2973
WVFGRD96   28.0   225    70   -10   3.89 0.3156
WVFGRD96   30.0    50    80    20   3.91 0.3332
WVFGRD96   32.0    50    80    20   3.93 0.3533
WVFGRD96   34.0    50    80    20   3.95 0.3707
WVFGRD96   36.0    50    80    20   3.98 0.3853
WVFGRD96   38.0    50    80    20   4.01 0.3984
WVFGRD96   40.0    55    70    30   4.08 0.4211
WVFGRD96   42.0    55    70    30   4.10 0.4239
WVFGRD96   44.0    55    75    30   4.12 0.4308
WVFGRD96   46.0    55    75    30   4.14 0.4417
WVFGRD96   48.0    55    75    30   4.16 0.4550
WVFGRD96   50.0    55    75    30   4.17 0.4672
WVFGRD96   52.0    55    70    30   4.19 0.4812
WVFGRD96   54.0    55    70    30   4.20 0.4921
WVFGRD96   56.0    55    70    30   4.21 0.5082
WVFGRD96   58.0    55    70    30   4.21 0.5208
WVFGRD96   60.0    55    70    30   4.22 0.5354
WVFGRD96   62.0    55    70    35   4.23 0.5476
WVFGRD96   64.0    55    70    35   4.24 0.5593
WVFGRD96   66.0    55    70    30   4.24 0.5693
WVFGRD96   68.0    55    70    30   4.25 0.5766
WVFGRD96   70.0    55    70    30   4.25 0.5851
WVFGRD96   72.0    55    70    30   4.26 0.5935
WVFGRD96   74.0    55    70    30   4.26 0.5997
WVFGRD96   76.0    55    65    30   4.26 0.6050
WVFGRD96   78.0    55    65    30   4.27 0.6097
WVFGRD96   80.0    55    65    30   4.27 0.6137
WVFGRD96   82.0    55    65    30   4.27 0.6156
WVFGRD96   84.0    55    65    30   4.28 0.6172
WVFGRD96   86.0    55    65    30   4.28 0.6188
WVFGRD96   88.0    55    65    30   4.28 0.6211
WVFGRD96   90.0    55    65    30   4.29 0.6234
WVFGRD96   92.0    55    65    30   4.29 0.6246
WVFGRD96   94.0    55    65    30   4.29 0.6252
WVFGRD96   96.0    55    65    30   4.30 0.6242
WVFGRD96   98.0    55    65    30   4.30 0.6234
WVFGRD96  100.0    55    65    30   4.30 0.6225
WVFGRD96  102.0    55    65    30   4.31 0.6222
WVFGRD96  104.0    55    65    30   4.31 0.6210
WVFGRD96  106.0    55    60    30   4.31 0.6197
WVFGRD96  108.0    55    60    30   4.31 0.6189
WVFGRD96  110.0    55    60    30   4.31 0.6168
WVFGRD96  112.0    55    60    30   4.32 0.6152
WVFGRD96  114.0    55    60    30   4.32 0.6143
WVFGRD96  116.0    55    60    30   4.32 0.6123
WVFGRD96  118.0    55    60    30   4.32 0.6075
WVFGRD96  120.0    55    60    30   4.33 0.6067
WVFGRD96  122.0    55    60    30   4.33 0.6049
WVFGRD96  124.0    55    60    30   4.33 0.6012
WVFGRD96  126.0    50    60    25   4.34 0.5995
WVFGRD96  128.0    50    60    25   4.34 0.5956
WVFGRD96  130.0    50    60    25   4.34 0.5939
WVFGRD96  132.0    50    60    25   4.35 0.5913
WVFGRD96  134.0    50    60    25   4.35 0.5888
WVFGRD96  136.0    50    60    25   4.35 0.5849
WVFGRD96  138.0    50    60    25   4.35 0.5823
WVFGRD96  140.0    50    60    25   4.36 0.5807
WVFGRD96  142.0    50    60    25   4.36 0.5752
WVFGRD96  144.0    50    60    25   4.36 0.5742
WVFGRD96  146.0    50    60    25   4.36 0.5699
WVFGRD96  148.0    50    60    25   4.36 0.5685

The best solution is

WVFGRD96   94.0    55    65    30   4.29 0.6252

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/3.3 -50 o DIST/3.3 +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 Tue May 28 00:53:06 CDT 2019