Location

Location ANSS

2017/05/04 03:13:47 59.887 -136.754 10.0 4.5 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/05/04 03:13:47:0  59.89 -136.75  10.0 4.5 Alaska
 
 Stations used:
   AK.BARN AK.BCP AK.BESE AK.CTG AK.GLB AK.JIS AK.LOGN AK.MCAR 
   AK.PIN AK.VRDI AT.SIT AT.YKU2 CN.DLBC CN.HYT NY.MAYO 
   TA.M27K TA.M30M TA.M31M TA.N31M TA.P29M TA.P33M 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 6.17e+22 dyne-cm
  Mw = 4.46 
  Z  = 8 km
  Plane   Strike  Dip  Rake
   NP1      270    75    20
   NP2      175    71   164
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.17e+22     25     133
    N   0.00e+00     65     305
    P  -6.17e+22      3      42

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.05e+22
       Mxy    -5.60e+22
       Mxz    -1.83e+22
       Myy    -1.33e+15
       Myz     1.50e+22
       Mzz     1.05e+22
                                                     
                                                     
                                                     
                                                     
                     ####----------                  
                 ########--------------              
              ##########---------------- P           
             ###########----------------             
           ############----------------------        
          #############-----------------------       
         ##############------------------------      
        ###############-------------------------     
        ###############-------------------------     
       #########-------#################---------    
       ##--------------########################--    
       ----------------##########################    
       ----------------##########################    
        ----------------########################     
        ----------------########################     
         ---------------##############   ######      
          ---------------############# T #####       
           --------------#############   ####        
             -------------#################          
              -------------###############           
                 -----------###########              
                     --------######                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.05e+22  -1.83e+22  -1.50e+22 
 -1.83e+22  -1.05e+22   5.60e+22 
 -1.50e+22   5.60e+22  -1.33e+15 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170504031347/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 = 270
      DIP = 75
     RAKE = 20
       MW = 4.46
       HS = 8.0

The NDK file is 20170504031347.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMT
 USGS/SLU Moment Tensor Solution
 ENS  2017/05/04 03:13:47:0  59.89 -136.75  10.0 4.5 Alaska
 
 Stations used:
   AK.BARN AK.BCP AK.BESE AK.CTG AK.GLB AK.JIS AK.LOGN AK.MCAR 
   AK.PIN AK.VRDI AT.SIT AT.YKU2 CN.DLBC CN.HYT NY.MAYO 
   TA.M27K TA.M30M TA.M31M TA.N31M TA.P29M TA.P33M 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 6.17e+22 dyne-cm
  Mw = 4.46 
  Z  = 8 km
  Plane   Strike  Dip  Rake
   NP1      270    75    20
   NP2      175    71   164
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.17e+22     25     133
    N   0.00e+00     65     305
    P  -6.17e+22      3      42

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -1.05e+22
       Mxy    -5.60e+22
       Mxz    -1.83e+22
       Myy    -1.33e+15
       Myz     1.50e+22
       Mzz     1.05e+22
                                                     
                                                     
                                                     
                                                     
                     ####----------                  
                 ########--------------              
              ##########---------------- P           
             ###########----------------             
           ############----------------------        
          #############-----------------------       
         ##############------------------------      
        ###############-------------------------     
        ###############-------------------------     
       #########-------#################---------    
       ##--------------########################--    
       ----------------##########################    
       ----------------##########################    
        ----------------########################     
        ----------------########################     
         ---------------##############   ######      
          ---------------############# T #####       
           --------------#############   ####        
             -------------#################          
              -------------###############           
                 -----------###########              
                     --------######                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.05e+22  -1.83e+22  -1.50e+22 
 -1.83e+22  -1.05e+22   5.60e+22 
 -1.50e+22   5.60e+22  -1.33e+15 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170504031347/index.html
	
Regional Moment Tensor (Mwr)
Moment	8.063e+15 N-m
Magnitude	4.5 Mwr
Depth	8.0 km
Percent DC	72 %
Half Duration	–
Catalog	US
Data Source	US3
Contributor	US3
Nodal Planes
Plane	Strike	Dip	Rake
NP1	345	52	131
NP2	110	53	50
Principal Axes
Axis	Value	Plunge	Azimuth
T	7.395e+15 N-m	59	318
N	1.202e+15 N-m	31	136
P	-8.597e+15 N-m	1	227

        

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/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 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    90    90   -15   4.14 0.2674
WVFGRD96    2.0   270    85    20   4.26 0.3406
WVFGRD96    3.0   270    85    20   4.31 0.3756
WVFGRD96    4.0   270    80    20   4.35 0.3996
WVFGRD96    5.0   270    80    20   4.38 0.4186
WVFGRD96    6.0   270    80    15   4.40 0.4322
WVFGRD96    7.0   270    80    15   4.43 0.4413
WVFGRD96    8.0   270    75    20   4.46 0.4487
WVFGRD96    9.0   270    80    15   4.48 0.4483
WVFGRD96   10.0   270    80    15   4.49 0.4437
WVFGRD96   11.0   265    70   -15   4.51 0.4417
WVFGRD96   12.0   265    70   -15   4.52 0.4368
WVFGRD96   13.0   265    70   -15   4.53 0.4309
WVFGRD96   14.0   265    70   -10   4.54 0.4243
WVFGRD96   15.0   265    70   -10   4.55 0.4161
WVFGRD96   16.0   265    70    -5   4.55 0.4063
WVFGRD96   17.0   265    70   -10   4.56 0.3956
WVFGRD96   18.0   265    70   -10   4.57 0.3839
WVFGRD96   19.0   265    70   -10   4.58 0.3716
WVFGRD96   20.0   265    70   -10   4.58 0.3588
WVFGRD96   21.0   265    70   -10   4.59 0.3466
WVFGRD96   22.0   265    70    -5   4.59 0.3345
WVFGRD96   23.0   265    70    -5   4.59 0.3227
WVFGRD96   24.0   265    70    -5   4.60 0.3115
WVFGRD96   25.0   265    70     0   4.60 0.3009
WVFGRD96   26.0   265    70     0   4.60 0.2905
WVFGRD96   27.0   265    70     0   4.61 0.2800
WVFGRD96   28.0   270    70     5   4.61 0.2708
WVFGRD96   29.0   270    70     5   4.62 0.2620

The best solution is

WVFGRD96    8.0   270    75    20   4.46 0.4487

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 +70
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 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 Thu May 4 05:58:08 CDT 2017