Location

Location ANSS

2018/02/09 01:04:43 59.058 -153.082 68.5 4.3 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2018/02/09 01:04:43:0  59.06 -153.08  68.5 4.3 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.HOM AK.SWD AT.OHAK II.KDAK TA.N18K 
   TA.N19K TA.O18K TA.O19K TA.O22K TA.P18K TA.P19K TA.Q19K 
   TA.Q20K 
 
 Filtering commands used:
   cut o DIST/3.5 -30 o DIST/3.5 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 6.61e+22 dyne-cm
  Mw = 4.48 
  Z  = 76 km
  Plane   Strike  Dip  Rake
   NP1       85    75    30
   NP2      346    61   163
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.61e+22     32     309
    N   0.00e+00     57     109
    P  -6.61e+22      9     213

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.60e+22
       Mxy    -5.30e+22
       Mxz     2.72e+22
       Myy     9.47e+21
       Myz    -1.72e+22
       Mzz     1.65e+22
                                                     
                                                     
                                                     
                                                     
                     ###-----------                  
                 ##########------------              
              ##############--------------           
             #################-------------          
           ####################--------------        
          #####   ##############--------------       
         ###### T ###############--------------      
        #######   ################--------------     
        ##########################--------------     
       ############################--------------    
       ############################------------##    
       #############################-------######    
       ---#######################################    
        ----------------------------############     
        ----------------------------############     
         ---------------------------###########      
          --------------------------##########       
           -------------------------#########        
             ---   -----------------#######          
              -- P ----------------#######           
                   ---------------#####              
                     ------------##                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.65e+22   2.72e+22   1.72e+22 
  2.72e+22  -2.60e+22   5.30e+22 
  1.72e+22   5.30e+22   9.47e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180209010443/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 = 85
      DIP = 75
     RAKE = 30
       MW = 4.48
       HS = 76.0

The NDK file is 20180209010443.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMWR
 USGS/SLU Moment Tensor Solution
 ENS  2018/02/09 01:04:43:0  59.06 -153.08  68.5 4.3 Alaska
 
 Stations used:
   AK.BRLK AK.CNP AK.HOM AK.SWD AT.OHAK II.KDAK TA.N18K 
   TA.N19K TA.O18K TA.O19K TA.O22K TA.P18K TA.P19K TA.Q19K 
   TA.Q20K 
 
 Filtering commands used:
   cut o DIST/3.5 -30 o DIST/3.5 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 6.61e+22 dyne-cm
  Mw = 4.48 
  Z  = 76 km
  Plane   Strike  Dip  Rake
   NP1       85    75    30
   NP2      346    61   163
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.61e+22     32     309
    N   0.00e+00     57     109
    P  -6.61e+22      9     213

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.60e+22
       Mxy    -5.30e+22
       Mxz     2.72e+22
       Myy     9.47e+21
       Myz    -1.72e+22
       Mzz     1.65e+22
                                                     
                                                     
                                                     
                                                     
                     ###-----------                  
                 ##########------------              
              ##############--------------           
             #################-------------          
           ####################--------------        
          #####   ##############--------------       
         ###### T ###############--------------      
        #######   ################--------------     
        ##########################--------------     
       ############################--------------    
       ############################------------##    
       #############################-------######    
       ---#######################################    
        ----------------------------############     
        ----------------------------############     
         ---------------------------###########      
          --------------------------##########       
           -------------------------#########        
             ---   -----------------#######          
              -- P ----------------#######           
                   ---------------#####              
                     ------------##                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.65e+22   2.72e+22   1.72e+22 
  2.72e+22  -2.60e+22   5.30e+22 
  1.72e+22   5.30e+22   9.47e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180209010443/index.html
	
Regional Moment Tensor (Mwr)
Moment	6.994e+15 N-m
Magnitude	4.5 Mwr
Depth	74.0 km
Percent DC	96 %
Half Duration	–
Catalog	AK
Data Source	US3
Contributor	US3
Nodal Planes
Plane	Strike	Dip	Rake
NP1	345	59	163
NP2	84	76	32
Principal Axes
Axis	Value	Plunge	Azimuth
T	7.072e+15 N-m	33	308
N	-0.158e+15 N-m	55	105
P	-6.914e+15 N-m	11	211

        

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/3.5 -30 o DIST/3.5 +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   135    50   -60   3.61 0.2681
WVFGRD96    4.0   170    70    30   3.65 0.3268
WVFGRD96    6.0   165    75    20   3.71 0.3660
WVFGRD96    8.0   165    75    30   3.79 0.3920
WVFGRD96   10.0   165    75    25   3.84 0.4067
WVFGRD96   12.0   165    75    25   3.87 0.4048
WVFGRD96   14.0   340    80   -25   3.90 0.3900
WVFGRD96   16.0   250    65   -10   3.92 0.3708
WVFGRD96   18.0   250    65   -10   3.96 0.3857
WVFGRD96   20.0   250    65   -15   3.99 0.4025
WVFGRD96   22.0   250    65   -10   4.01 0.4197
WVFGRD96   24.0   245    65   -15   4.04 0.4393
WVFGRD96   26.0   245    65   -15   4.06 0.4574
WVFGRD96   28.0   245    65   -15   4.08 0.4689
WVFGRD96   30.0   245    65   -20   4.10 0.4787
WVFGRD96   32.0   245    65   -20   4.11 0.4819
WVFGRD96   34.0   240    65   -20   4.13 0.4839
WVFGRD96   36.0    75    85    20   4.16 0.4983
WVFGRD96   38.0    75    85    20   4.19 0.5160
WVFGRD96   40.0    80    75    30   4.27 0.5475
WVFGRD96   42.0    80    75    30   4.29 0.5589
WVFGRD96   44.0    80    75    30   4.31 0.5697
WVFGRD96   46.0    80    75    30   4.33 0.5825
WVFGRD96   48.0    80    75    30   4.35 0.5979
WVFGRD96   50.0    80    75    30   4.37 0.6150
WVFGRD96   52.0    80    75    25   4.38 0.6335
WVFGRD96   54.0    80    75    30   4.39 0.6536
WVFGRD96   56.0    80    75    30   4.41 0.6722
WVFGRD96   58.0    80    75    30   4.42 0.6882
WVFGRD96   60.0    80    75    30   4.42 0.7034
WVFGRD96   62.0    80    75    25   4.43 0.7153
WVFGRD96   64.0    80    75    25   4.44 0.7258
WVFGRD96   66.0    80    75    25   4.44 0.7325
WVFGRD96   68.0    80    75    25   4.45 0.7408
WVFGRD96   70.0    80    75    25   4.45 0.7445
WVFGRD96   72.0    80    75    25   4.46 0.7456
WVFGRD96   74.0    85    75    30   4.47 0.7496
WVFGRD96   76.0    85    75    30   4.48 0.7507
WVFGRD96   78.0    85    75    30   4.48 0.7502
WVFGRD96   80.0    85    75    30   4.48 0.7482
WVFGRD96   82.0    85    75    30   4.48 0.7469
WVFGRD96   84.0    85    75    30   4.49 0.7449
WVFGRD96   86.0    85    75    30   4.49 0.7410
WVFGRD96   88.0    85    75    30   4.49 0.7369
WVFGRD96   90.0    85    75    30   4.49 0.7335
WVFGRD96   92.0    85    75    30   4.49 0.7294
WVFGRD96   94.0    85    75    25   4.49 0.7250
WVFGRD96   96.0    85    75    25   4.49 0.7219
WVFGRD96   98.0    80    80    25   4.49 0.7181

The best solution is

WVFGRD96   76.0    85    75    30   4.48 0.7507

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/3.5 -30 o DIST/3.5 +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 Fri Feb 9 17:54:35 CST 2018