Location

Location ANSS

2019/12/17 02:15:27 60.594 -152.264 106.4 3.9 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2019/12/17 02:15:27:0  60.59 -152.26 106.4 3.9 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.CUT AK.HOM AK.KNK AK.L19K AK.O18K AK.O19K 
   AK.PPLA AK.Q19K AK.RC01 AK.SAW AK.SKN AK.SSN AV.ILSW 
   AV.STLK TA.M22K TA.P19K 
 
 Filtering commands used:
   cut o DIST/3.6 -20 o DIST/3.6 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 3.31e+22 dyne-cm
  Mw = 4.28 
  Z  = 130 km
  Plane   Strike  Dip  Rake
   NP1      332    69   148
   NP2       75    60    25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.31e+22     38     291
    N   0.00e+00     52     122
    P  -3.31e+22      5      25

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.43e+22
       Mxy    -1.95e+22
       Mxz     2.88e+21
       Myy     1.22e+22
       Myz    -1.63e+22
       Mzz     1.21e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ###--------------- P -              
              ########-------------   ----           
             ############------------------          
           ###############-------------------        
          ##################------------------       
         ####################------------------      
        ######   ##############-----------------     
        ###### T ###############----------------     
       #######   ################-------------###    
       ###########################-----------####    
       ############################--------######    
       ############################-----#########    
        ############################-###########     
        --#######################----###########     
         -------#########------------##########      
          ----------------------------########       
           ---------------------------#######        
             -------------------------#####          
              ------------------------####           
                 ---------------------#              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.21e+22   2.88e+21   1.63e+22 
  2.88e+21  -2.43e+22   1.95e+22 
  1.63e+22   1.95e+22   1.22e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191217021527/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 = 75
      DIP = 60
     RAKE = 25
       MW = 4.28
       HS = 130.0

The NDK file is 20191217021527.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2019/12/17 02:15:27:0  60.59 -152.26 106.4 3.9 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.CUT AK.HOM AK.KNK AK.L19K AK.O18K AK.O19K 
   AK.PPLA AK.Q19K AK.RC01 AK.SAW AK.SKN AK.SSN AV.ILSW 
   AV.STLK TA.M22K TA.P19K 
 
 Filtering commands used:
   cut o DIST/3.6 -20 o DIST/3.6 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 3.31e+22 dyne-cm
  Mw = 4.28 
  Z  = 130 km
  Plane   Strike  Dip  Rake
   NP1      332    69   148
   NP2       75    60    25
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.31e+22     38     291
    N   0.00e+00     52     122
    P  -3.31e+22      5      25

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.43e+22
       Mxy    -1.95e+22
       Mxz     2.88e+21
       Myy     1.22e+22
       Myz    -1.63e+22
       Mzz     1.21e+22
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ###--------------- P -              
              ########-------------   ----           
             ############------------------          
           ###############-------------------        
          ##################------------------       
         ####################------------------      
        ######   ##############-----------------     
        ###### T ###############----------------     
       #######   ################-------------###    
       ###########################-----------####    
       ############################--------######    
       ############################-----#########    
        ############################-###########     
        --#######################----###########     
         -------#########------------##########      
          ----------------------------########       
           ---------------------------#######        
             -------------------------#####          
              ------------------------####           
                 ---------------------#              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.21e+22   2.88e+21   1.63e+22 
  2.88e+21  -2.43e+22   1.95e+22 
  1.63e+22   1.95e+22   1.22e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191217021527/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/3.6 -20 o DIST/3.6 +40
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 
br c 0.12 0.25 n 4 p 2
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   325    60   -25   3.48 0.4108
WVFGRD96    4.0   325    85    25   3.56 0.4581
WVFGRD96    6.0   340    65    35   3.63 0.4988
WVFGRD96    8.0   345    60    40   3.70 0.5158
WVFGRD96   10.0   330    65    30   3.70 0.5205
WVFGRD96   12.0   330    70    30   3.71 0.5192
WVFGRD96   14.0   325    75    25   3.73 0.5135
WVFGRD96   16.0   140    90   -25   3.75 0.5059
WVFGRD96   18.0   250    75    25   3.78 0.5103
WVFGRD96   20.0   250    75    25   3.81 0.5263
WVFGRD96   22.0   250    75    20   3.84 0.5449
WVFGRD96   24.0   245    75    20   3.86 0.5616
WVFGRD96   26.0   245    75    20   3.88 0.5727
WVFGRD96   28.0   245    80    20   3.90 0.5776
WVFGRD96   30.0   245    80    20   3.92 0.5798
WVFGRD96   32.0   245    80    20   3.93 0.5777
WVFGRD96   34.0   245    80    20   3.95 0.5764
WVFGRD96   36.0   245    80    20   3.98 0.5755
WVFGRD96   38.0   240    70    15   4.00 0.5744
WVFGRD96   40.0   240    65    20   4.06 0.5747
WVFGRD96   42.0   240    65    20   4.09 0.5659
WVFGRD96   44.0   235    70    15   4.10 0.5553
WVFGRD96   46.0   235    65    10   4.11 0.5491
WVFGRD96   48.0   230    70   -10   4.13 0.5479
WVFGRD96   50.0   230    70   -15   4.14 0.5507
WVFGRD96   52.0   230    70   -15   4.15 0.5546
WVFGRD96   54.0   230    70   -15   4.17 0.5606
WVFGRD96   56.0   230    70   -15   4.18 0.5669
WVFGRD96   58.0   230    70   -20   4.19 0.5723
WVFGRD96   60.0    65    70    35   4.19 0.5835
WVFGRD96   62.0    65    70    35   4.20 0.5954
WVFGRD96   64.0    65    70    35   4.20 0.6036
WVFGRD96   66.0    65    65    35   4.20 0.6138
WVFGRD96   68.0    65    65    30   4.20 0.6225
WVFGRD96   70.0    65    65    30   4.20 0.6310
WVFGRD96   72.0    65    65    30   4.21 0.6383
WVFGRD96   74.0    65    65    30   4.21 0.6444
WVFGRD96   76.0    70    65    35   4.22 0.6493
WVFGRD96   78.0    70    65    35   4.22 0.6562
WVFGRD96   80.0    70    65    35   4.22 0.6602
WVFGRD96   82.0    70    65    35   4.23 0.6628
WVFGRD96   84.0    70    65    35   4.23 0.6668
WVFGRD96   86.0    70    65    30   4.22 0.6711
WVFGRD96   88.0    70    65    30   4.23 0.6740
WVFGRD96   90.0    70    65    30   4.23 0.6770
WVFGRD96   92.0    70    65    30   4.23 0.6793
WVFGRD96   94.0    70    65    30   4.23 0.6811
WVFGRD96   96.0    70    65    30   4.24 0.6830
WVFGRD96   98.0    70    65    30   4.24 0.6846
WVFGRD96  100.0    70    65    30   4.24 0.6856
WVFGRD96  102.0    70    65    30   4.24 0.6864
WVFGRD96  104.0    70    65    30   4.25 0.6869
WVFGRD96  106.0    70    65    30   4.25 0.6870
WVFGRD96  108.0    70    65    30   4.25 0.6869
WVFGRD96  110.0    70    65    30   4.25 0.6864
WVFGRD96  112.0    70    65    30   4.25 0.6853
WVFGRD96  114.0    75    60    30   4.26 0.6849
WVFGRD96  116.0    75    60    30   4.26 0.6853
WVFGRD96  118.0    75    60    25   4.26 0.6858
WVFGRD96  120.0    75    60    25   4.26 0.6857
WVFGRD96  122.0    75    60    25   4.27 0.6852
WVFGRD96  124.0    75    60    25   4.27 0.6852
WVFGRD96  126.0    75    60    25   4.27 0.6869
WVFGRD96  128.0    75    60    25   4.28 0.6881
WVFGRD96  130.0    75    60    25   4.28 0.6883
WVFGRD96  132.0    75    55    25   4.27 0.6874
WVFGRD96  134.0    75    55    25   4.28 0.6873
WVFGRD96  136.0    75    55    20   4.28 0.6869
WVFGRD96  138.0    75    55    20   4.28 0.6873

The best solution is

WVFGRD96  130.0    75    60    25   4.28 0.6883

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/3.6 -20 o DIST/3.6 +40
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.10 n 3 
br c 0.12 0.25 n 4 p 2
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 Dec 16 21:19:41 CST 2019