Location

Location ANSS

2019/02/18 17:02:45 61.412 -149.947 39.7 4.4 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2019/02/18 17:02:45:0  61.41 -149.95  39.7 4.4 Alaska
 
 Stations used:
   AK.CAST AK.FIRE AK.GHO AK.KNK AK.PPLA AK.PWL AK.RC01 AK.RND 
   AK.SAW AK.SCM AK.SLK AK.SWD AT.PMR AV.ILS AV.ILSW AV.RDDF 
   AV.RDT AV.SPBG AV.SPCR AV.SPU AV.STLK GM.AD11 TA.M23K 
   TA.M24K TA.O22K 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 4.68e+22 dyne-cm
  Mw = 4.38 
  Z  = 49 km
  Plane   Strike  Dip  Rake
   NP1      205    70   -92
   NP2       30    20   -85
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.68e+22     25     296
    N   0.00e+00      2     205
    P  -4.68e+22     65     112

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     6.28e+21
       Mxy    -1.23e+22
       Mxz     1.45e+22
       Myy     2.37e+22
       Myz    -3.28e+22
       Mzz    -3.00e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ##################----              
              ###################--------#           
             ##################-----------#          
           ##################--------------##        
          ##################----------------##       
         ###   ############------------------##      
        #### T ###########-------------------###     
        ####   ##########--------------------###     
       #################---------------------####    
       ################----------------------####    
       ###############-----------   ---------####    
       ##############------------ P ---------####    
        #############------------   --------####     
        ############------------------------####     
         ###########-----------------------####      
          #########----------------------#####       
           ########---------------------#####        
             ######-------------------#####          
              #####-----------------######           
                 ##-------------#######              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.00e+22   1.45e+22   3.28e+22 
  1.45e+22   6.28e+21   1.23e+22 
  3.28e+22   1.23e+22   2.37e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190218170245/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 = 30
      DIP = 20
     RAKE = -85
       MW = 4.38
       HS = 49.0

The NDK file is 20190218170245.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2019/02/18 17:02:45:0  61.41 -149.95  39.7 4.4 Alaska
 
 Stations used:
   AK.CAST AK.FIRE AK.GHO AK.KNK AK.PPLA AK.PWL AK.RC01 AK.RND 
   AK.SAW AK.SCM AK.SLK AK.SWD AT.PMR AV.ILS AV.ILSW AV.RDDF 
   AV.RDT AV.SPBG AV.SPCR AV.SPU AV.STLK GM.AD11 TA.M23K 
   TA.M24K TA.O22K 
 
 Filtering commands used:
   cut o DIST/3.3 -40 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.08 n 3 
 
 Best Fitting Double Couple
  Mo = 4.68e+22 dyne-cm
  Mw = 4.38 
  Z  = 49 km
  Plane   Strike  Dip  Rake
   NP1      205    70   -92
   NP2       30    20   -85
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.68e+22     25     296
    N   0.00e+00      2     205
    P  -4.68e+22     65     112

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     6.28e+21
       Mxy    -1.23e+22
       Mxz     1.45e+22
       Myy     2.37e+22
       Myz    -3.28e+22
       Mzz    -3.00e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ##################----              
              ###################--------#           
             ##################-----------#          
           ##################--------------##        
          ##################----------------##       
         ###   ############------------------##      
        #### T ###########-------------------###     
        ####   ##########--------------------###     
       #################---------------------####    
       ################----------------------####    
       ###############-----------   ---------####    
       ##############------------ P ---------####    
        #############------------   --------####     
        ############------------------------####     
         ###########-----------------------####      
          #########----------------------#####       
           ########---------------------#####        
             ######-------------------#####          
              #####-----------------######           
                 ##-------------#######              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -3.00e+22   1.45e+22   3.28e+22 
  1.45e+22   6.28e+21   1.23e+22 
  3.28e+22   1.23e+22   2.37e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190218170245/index.html
	

Magnitudes

mLg Magnitude


(a) mLg computed using the IASPEI formula; (b) mLg residuals ; the values used for the trimmed mean are indicated.

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 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 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    1.0   135    85    15   3.47 0.1597
WVFGRD96    2.0    90    40   -90   3.75 0.2290
WVFGRD96    3.0   135    80    25   3.68 0.2424
WVFGRD96    4.0   135    75    25   3.72 0.2556
WVFGRD96    5.0   135    80    25   3.74 0.2608
WVFGRD96    6.0   310    90   -25   3.77 0.2630
WVFGRD96    7.0    45    60   -10   3.78 0.2772
WVFGRD96    8.0    50    55     5   3.82 0.2934
WVFGRD96    9.0    55    55    15   3.84 0.3075
WVFGRD96   10.0    55    55    15   3.86 0.3201
WVFGRD96   11.0    55    60    20   3.89 0.3321
WVFGRD96   12.0    60    60    30   3.91 0.3430
WVFGRD96   13.0    60    60    25   3.92 0.3516
WVFGRD96   14.0    60    60    25   3.93 0.3587
WVFGRD96   15.0    60    60    25   3.95 0.3642
WVFGRD96   16.0    80    60   -25   3.95 0.3723
WVFGRD96   17.0    75    55   -25   3.96 0.3814
WVFGRD96   18.0    75    55   -25   3.97 0.3897
WVFGRD96   19.0    75    55   -25   3.98 0.3967
WVFGRD96   20.0    75    55   -25   4.00 0.4029
WVFGRD96   21.0    75    50   -20   4.01 0.4087
WVFGRD96   22.0    80    50   -10   4.02 0.4157
WVFGRD96   23.0    80    50   -10   4.03 0.4222
WVFGRD96   24.0    80    50   -10   4.04 0.4289
WVFGRD96   25.0    80    45   -10   4.05 0.4344
WVFGRD96   26.0    75    45   -20   4.06 0.4418
WVFGRD96   27.0    75    45   -20   4.07 0.4480
WVFGRD96   28.0    75    45   -25   4.09 0.4559
WVFGRD96   29.0    75    45   -25   4.10 0.4632
WVFGRD96   30.0    70    40   -35   4.12 0.4712
WVFGRD96   31.0    70    40   -35   4.13 0.4799
WVFGRD96   32.0    65    35   -45   4.14 0.4883
WVFGRD96   33.0    65    35   -45   4.15 0.4965
WVFGRD96   34.0    60    30   -50   4.16 0.5040
WVFGRD96   35.0    60    30   -55   4.18 0.5123
WVFGRD96   36.0    60    30   -55   4.19 0.5190
WVFGRD96   37.0    50    25   -65   4.20 0.5245
WVFGRD96   38.0    50    25   -65   4.20 0.5303
WVFGRD96   39.0    50    25   -70   4.22 0.5367
WVFGRD96   40.0    40    20   -75   4.33 0.5316
WVFGRD96   41.0    35    20   -80   4.34 0.5348
WVFGRD96   42.0    35    20   -80   4.34 0.5378
WVFGRD96   43.0    30    20   -85   4.35 0.5405
WVFGRD96   44.0   205    70   -90   4.36 0.5427
WVFGRD96   45.0    30    20   -85   4.36 0.5445
WVFGRD96   46.0    30    20   -85   4.37 0.5463
WVFGRD96   47.0    25    20   -90   4.37 0.5465
WVFGRD96   48.0    25    20   -90   4.38 0.5465
WVFGRD96   49.0    30    20   -85   4.38 0.5465
WVFGRD96   50.0    30    20   -85   4.39 0.5459
WVFGRD96   51.0    30    20   -85   4.39 0.5445
WVFGRD96   52.0    30    20   -85   4.39 0.5431
WVFGRD96   53.0    25    20   -85   4.39 0.5407
WVFGRD96   54.0    25    20   -85   4.39 0.5387
WVFGRD96   55.0    25    20   -85   4.39 0.5363
WVFGRD96   56.0    25    20   -85   4.40 0.5327
WVFGRD96   57.0    25    20   -85   4.40 0.5297
WVFGRD96   58.0    25    20   -85   4.40 0.5262
WVFGRD96   59.0    25    20   -85   4.40 0.5219

The best solution is

WVFGRD96   49.0    30    20   -85   4.38 0.5465

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 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.08 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 Feb 18 12:47:18 CST 2019