Location

Location ANSS

2019/01/10 00:04:31 61.406 -149.922 18.3 4.1 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2019/01/10 00:04:31:0  61.41 -149.92  18.3 4.1 Alaska
 
 Stations used:
   AK.FID AK.FIRE AK.GHO AK.KLU AK.KNK AK.KTH AK.PPLA AK.PWL 
   AK.RC01 AK.RND AK.SAW AK.SKN AK.SLK AK.SWD AT.PMR AV.STLK 
   TA.M20K TA.M22K TA.N25K 
 
 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 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 1.35e+22 dyne-cm
  Mw = 4.02 
  Z  = 47 km
  Plane   Strike  Dip  Rake
   NP1      165    55   -70
   NP2      313    40   -116
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.35e+22      8     241
    N   0.00e+00     16     333
    P  -1.35e+22     72     126

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.69e+21
       Mxy     6.25e+21
       Mxz     1.43e+21
       Myy     9.22e+21
       Myz    -4.87e+21
       Mzz    -1.19e+22
                                                     
                                                     
                                                     
                                                     
                     -#############                  
                 ----##################              
              -------#####################           
             ######----------##############          
           ########--------------############        
          #########----------------###########       
         #########-------------------##########      
        ##########---------------------#########     
        ##########----------------------########     
       ############-----------------------#######    
       ############-----------------------#######    
       ############-----------   ----------######    
       #############---------- P -----------#####    
        ############----------   -----------####     
        #############-----------------------####     
         #   #########----------------------###      
           T ##########---------------------##       
             ###########--------------------#        
             ############------------------          
              #############---------------           
                 ############----------              
                     ###########---                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.19e+22   1.43e+21   4.87e+21 
  1.43e+21   2.69e+21  -6.25e+21 
  4.87e+21  -6.25e+21   9.22e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190110000431/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 = 165
      DIP = 55
     RAKE = -70
       MW = 4.02
       HS = 47.0

The NDK file is 20190110000431.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  2019/01/10 00:04:31:0  61.41 -149.92  18.3 4.1 Alaska
 
 Stations used:
   AK.FID AK.FIRE AK.GHO AK.KLU AK.KNK AK.KTH AK.PPLA AK.PWL 
   AK.RC01 AK.RND AK.SAW AK.SKN AK.SLK AK.SWD AT.PMR AV.STLK 
   TA.M20K TA.M22K TA.N25K 
 
 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 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 1.35e+22 dyne-cm
  Mw = 4.02 
  Z  = 47 km
  Plane   Strike  Dip  Rake
   NP1      165    55   -70
   NP2      313    40   -116
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.35e+22      8     241
    N   0.00e+00     16     333
    P  -1.35e+22     72     126

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.69e+21
       Mxy     6.25e+21
       Mxz     1.43e+21
       Myy     9.22e+21
       Myz    -4.87e+21
       Mzz    -1.19e+22
                                                     
                                                     
                                                     
                                                     
                     -#############                  
                 ----##################              
              -------#####################           
             ######----------##############          
           ########--------------############        
          #########----------------###########       
         #########-------------------##########      
        ##########---------------------#########     
        ##########----------------------########     
       ############-----------------------#######    
       ############-----------------------#######    
       ############-----------   ----------######    
       #############---------- P -----------#####    
        ############----------   -----------####     
        #############-----------------------####     
         #   #########----------------------###      
           T ##########---------------------##       
             ###########--------------------#        
             ############------------------          
              #############---------------           
                 ############----------              
                     ###########---                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.19e+22   1.43e+21   4.87e+21 
  1.43e+21   2.69e+21  -6.25e+21 
  4.87e+21  -6.25e+21   9.22e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190110000431/index.html
	
Regional Moment Tensor (Mwr)
Moment 8.070e+14 N-m
Magnitude 3.87 Mwr
Depth 38.0 km
Percent DC 90%
Half Duration -
Catalog US
Data Source US 3
Contributor US 3
Nodal Planes
Plane Strike Dip Rake
NP1 313 41 -129
NP2 180 60 -61
Principal Axes
Axis Value Plunge Azimuth
T 8.278e+14 N-m 10 250
N -0.435e+14 N-m 25 345
P -7.844e+14 N-m 63 139

        

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 
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    1.0   325    45    90   3.25 0.2519
WVFGRD96    2.0   145    45    95   3.42 0.3823
WVFGRD96    3.0   325    45    90   3.47 0.3683
WVFGRD96    4.0   140    50   -90   3.51 0.3438
WVFGRD96    5.0   345    40   -50   3.52 0.3515
WVFGRD96    6.0    15    65    50   3.50 0.3621
WVFGRD96    7.0    15    60    45   3.51 0.3753
WVFGRD96    8.0   345    35   -50   3.59 0.3773
WVFGRD96    9.0    25    35    25   3.58 0.3788
WVFGRD96   10.0    25    40    30   3.58 0.3905
WVFGRD96   11.0    25    40    30   3.59 0.3999
WVFGRD96   12.0    20    45    25   3.59 0.4088
WVFGRD96   13.0    25    45    30   3.60 0.4167
WVFGRD96   14.0    25    45    30   3.61 0.4228
WVFGRD96   15.0    25    45    25   3.62 0.4277
WVFGRD96   16.0    25    45    25   3.62 0.4322
WVFGRD96   17.0    20    50    25   3.62 0.4361
WVFGRD96   18.0    20    50    25   3.63 0.4391
WVFGRD96   19.0    25    45    20   3.65 0.4415
WVFGRD96   20.0   165    35   -45   3.68 0.4497
WVFGRD96   21.0   165    35   -45   3.70 0.4584
WVFGRD96   22.0   165    35   -45   3.71 0.4668
WVFGRD96   23.0   170    40   -40   3.71 0.4757
WVFGRD96   24.0   165    40   -50   3.72 0.4844
WVFGRD96   25.0   165    40   -50   3.73 0.4934
WVFGRD96   26.0   165    40   -50   3.74 0.5019
WVFGRD96   27.0   165    40   -50   3.75 0.5097
WVFGRD96   28.0   170    45   -45   3.76 0.5189
WVFGRD96   29.0   170    45   -45   3.77 0.5289
WVFGRD96   30.0   170    45   -45   3.78 0.5384
WVFGRD96   31.0   165    45   -50   3.78 0.5474
WVFGRD96   32.0   165    45   -50   3.79 0.5553
WVFGRD96   33.0   170    50   -50   3.80 0.5631
WVFGRD96   34.0   170    50   -50   3.81 0.5710
WVFGRD96   35.0   170    50   -55   3.82 0.5780
WVFGRD96   36.0   170    50   -55   3.84 0.5841
WVFGRD96   37.0   165    50   -60   3.85 0.5895
WVFGRD96   38.0   165    50   -60   3.86 0.5928
WVFGRD96   39.0   165    50   -60   3.88 0.5953
WVFGRD96   40.0   160    50   -65   3.96 0.5973
WVFGRD96   41.0   160    50   -65   3.97 0.6055
WVFGRD96   42.0   160    50   -65   3.98 0.6112
WVFGRD96   43.0   160    50   -65   3.99 0.6160
WVFGRD96   44.0   170    55   -65   4.00 0.6193
WVFGRD96   45.0   170    55   -65   4.01 0.6218
WVFGRD96   46.0   165    55   -70   4.02 0.6233
WVFGRD96   47.0   165    55   -70   4.02 0.6238
WVFGRD96   48.0   165    55   -70   4.03 0.6236
WVFGRD96   49.0   165    55   -70   4.04 0.6223
WVFGRD96   50.0   165    55   -70   4.04 0.6202
WVFGRD96   51.0   165    55   -70   4.05 0.6171
WVFGRD96   52.0   165    55   -70   4.05 0.6136
WVFGRD96   53.0   165    55   -70   4.05 0.6090
WVFGRD96   54.0   165    55   -70   4.06 0.6043
WVFGRD96   55.0   165    55   -70   4.06 0.5985
WVFGRD96   56.0   165    55   -70   4.06 0.5920
WVFGRD96   57.0   165    55   -65   4.06 0.5863
WVFGRD96   58.0   165    55   -65   4.07 0.5789
WVFGRD96   59.0   165    55   -65   4.07 0.5720

The best solution is

WVFGRD96   47.0   165    55   -70   4.02 0.6238

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 
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 Wed Jan 9 18:42:44 CST 2019