Location

Location ANSS

2020/01/03 02:57:36 58.469 -137.730 20.5 4.2 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2020/01/03 02:57:36:0  58.47 -137.73  20.5 4.2 Alaska
 
 Stations used:
   AK.BARN AK.BCP AK.BESE AK.CTG AK.LOGN AK.MCAR AK.PIN 
   AK.R32K AK.S31K AK.S32K AK.U33K AT.CRAG AT.SIT AT.SKAG 
   AT.YKU2 CN.HYT CN.WHY TA.N30M TA.N31M TA.N32M TA.O28M 
   TA.O29M TA.O30N TA.P29M TA.P30M TA.P32M TA.P33M TA.Q32M 
   TA.R33M TA.S34M US.WRAK 
 
 Filtering commands used:
   cut o DIST/3.3 -30 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 = 1.60e+22 dyne-cm
  Mw = 4.07 
  Z  = 28 km
  Plane   Strike  Dip  Rake
   NP1      280    70    35
   NP2      177    57   156
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.60e+22     39     142
    N   0.00e+00     50     306
    P  -1.60e+22      8      46

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.51e+21
       Mxy    -1.26e+22
       Mxz    -7.72e+21
       Myy    -4.40e+21
       Myz     3.20e+21
       Mzz     5.91e+21
                                                     
                                                     
                                                     
                                                     
                     ####----------                  
                 #######---------------              
              #########-----------------             
             #########------------------ P           
           ##########-------------------   --        
          ###########-------------------------       
         ###########---------------------------      
        ############----------------------------     
        ##----------###########-----------------     
       -------------##################-----------    
       -------------######################-------    
       -------------#########################----    
       -------------############################-    
        ------------############################     
        -------------###########################     
         ------------#############   ##########      
          ------------############ T #########       
           -----------############   ########        
             ----------####################          
              ----------##################           
                 ---------#############              
                     -------#######                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  5.91e+21  -7.72e+21  -3.20e+21 
 -7.72e+21  -1.51e+21   1.26e+22 
 -3.20e+21   1.26e+22  -4.40e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200103025736/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 = 280
      DIP = 70
     RAKE = 35
       MW = 4.07
       HS = 28.0

The NDK file is 20200103025736.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2020/01/03 02:57:36:0  58.47 -137.73  20.5 4.2 Alaska
 
 Stations used:
   AK.BARN AK.BCP AK.BESE AK.CTG AK.LOGN AK.MCAR AK.PIN 
   AK.R32K AK.S31K AK.S32K AK.U33K AT.CRAG AT.SIT AT.SKAG 
   AT.YKU2 CN.HYT CN.WHY TA.N30M TA.N31M TA.N32M TA.O28M 
   TA.O29M TA.O30N TA.P29M TA.P30M TA.P32M TA.P33M TA.Q32M 
   TA.R33M TA.S34M US.WRAK 
 
 Filtering commands used:
   cut o DIST/3.3 -30 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 = 1.60e+22 dyne-cm
  Mw = 4.07 
  Z  = 28 km
  Plane   Strike  Dip  Rake
   NP1      280    70    35
   NP2      177    57   156
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.60e+22     39     142
    N   0.00e+00     50     306
    P  -1.60e+22      8      46

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.51e+21
       Mxy    -1.26e+22
       Mxz    -7.72e+21
       Myy    -4.40e+21
       Myz     3.20e+21
       Mzz     5.91e+21
                                                     
                                                     
                                                     
                                                     
                     ####----------                  
                 #######---------------              
              #########-----------------             
             #########------------------ P           
           ##########-------------------   --        
          ###########-------------------------       
         ###########---------------------------      
        ############----------------------------     
        ##----------###########-----------------     
       -------------##################-----------    
       -------------######################-------    
       -------------#########################----    
       -------------############################-    
        ------------############################     
        -------------###########################     
         ------------#############   ##########      
          ------------############ T #########       
           -----------############   ########        
             ----------####################          
              ----------##################           
                 ---------#############              
                     -------#######                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  5.91e+21  -7.72e+21  -3.20e+21 
 -7.72e+21  -1.51e+21   1.26e+22 
 -3.20e+21   1.26e+22  -4.40e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200103025736/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 -30 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    45   -85   3.66 0.3380
WVFGRD96    2.0   135    45   -90   3.77 0.4313
WVFGRD96    3.0   145    45   -70   3.81 0.4092
WVFGRD96    4.0   160    40   -45   3.82 0.4089
WVFGRD96    5.0   165    35   -35   3.82 0.4182
WVFGRD96    6.0   165    35   -35   3.82 0.4263
WVFGRD96    7.0   170    40   -25   3.80 0.4319
WVFGRD96    8.0   165    35   -35   3.86 0.4462
WVFGRD96    9.0   170    35   -30   3.86 0.4520
WVFGRD96   10.0   175    40   -20   3.85 0.4585
WVFGRD96   11.0     0    40   -20   3.85 0.4632
WVFGRD96   12.0   110    70    50   3.87 0.4747
WVFGRD96   13.0   285    70    45   3.87 0.4941
WVFGRD96   14.0   285    70    45   3.89 0.5119
WVFGRD96   15.0   280    75    40   3.90 0.5288
WVFGRD96   16.0   280    75    40   3.91 0.5450
WVFGRD96   17.0   280    75    40   3.92 0.5600
WVFGRD96   18.0   280    70    35   3.94 0.5742
WVFGRD96   19.0   280    70    35   3.96 0.5883
WVFGRD96   20.0   280    70    35   3.97 0.6009
WVFGRD96   21.0   280    70    35   3.99 0.6128
WVFGRD96   22.0   280    70    35   4.00 0.6234
WVFGRD96   23.0   280    70    35   4.01 0.6324
WVFGRD96   24.0   280    70    35   4.03 0.6399
WVFGRD96   25.0   280    70    35   4.04 0.6458
WVFGRD96   26.0   280    70    35   4.05 0.6496
WVFGRD96   27.0   280    70    35   4.06 0.6519
WVFGRD96   28.0   280    70    35   4.07 0.6520
WVFGRD96   29.0   280    70    35   4.08 0.6498
WVFGRD96   30.0   280    70    35   4.09 0.6458
WVFGRD96   31.0   280    70    35   4.10 0.6401
WVFGRD96   32.0   280    70    35   4.11 0.6329
WVFGRD96   33.0   280    70    35   4.12 0.6240
WVFGRD96   34.0   280    70    35   4.13 0.6140
WVFGRD96   35.0   280    70    35   4.14 0.6033
WVFGRD96   36.0   280    70    35   4.15 0.5922
WVFGRD96   37.0   275    75    30   4.16 0.5812
WVFGRD96   38.0   275    75    30   4.17 0.5709
WVFGRD96   39.0   275    75    30   4.18 0.5606

The best solution is

WVFGRD96   28.0   280    70    35   4.07 0.6520

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 -30 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 Bureau of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Oklahoma Geological Survey, TexNet, the Iris stations, the Transportable Array of EarthScope and other networks.

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 Jan 3 06:08:19 CST 2020