Location

Location ANSS

2016/04/20 14:55:43 61.727 -148.194 2.5 3.5 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/04/20 14:55:43:0  61.73 -148.19   2.5 3.5 Alaska
 
 Stations used:
   AK.FID AK.GHO AK.GLB AK.KLU AK.KNK AK.MCK AK.PWL AK.RC01 
   AK.RND AK.SAW AK.SCM AT.PMR TA.M22K 
 
 Filtering commands used:
   cut o DIST/3.3 -20 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.33e+21 dyne-cm
  Mw = 3.35 
  Z  = 5 km
  Plane   Strike  Dip  Rake
   NP1       10    55   -45
   NP2      130    55   -135
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.33e+21      0      70
    N   0.00e+00     35     160
    P  -1.33e+21     55     340

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.37e+20
       Mxy     5.74e+20
       Mxz    -5.89e+20
       Myy     1.12e+21
       Myz     2.24e+20
       Mzz    -8.86e+20
                                                     
                                                     
                                                     
                                                     
                     ------------##                  
                 -----------------#####              
              ---------------------#######           
             -----------------------#######          
           #------------------------#########        
          ##-----------   -----------#########       
         ###----------- P -----------#########       
        #####----------   -----------######### T     
        ######-----------------------#########       
       #######-----------------------############    
       ########----------------------############    
       ##########--------------------############    
       ###########------------------#############    
        ############----------------############     
        ##############-------------#############     
         ###############-----------############      
          #################-------############       
           ####################--############        
             ###################-----------          
              ################------------           
                 ############----------              
                     #####---------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -8.86e+20  -5.89e+20  -2.24e+20 
 -5.89e+20  -2.37e+20  -5.74e+20 
 -2.24e+20  -5.74e+20   1.12e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160420145543/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 = 10
      DIP = 55
     RAKE = -45
       MW = 3.35
       HS = 5.0

The NDK file is 20160420145543.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  2016/04/20 14:55:43:0  61.73 -148.19   2.5 3.5 Alaska
 
 Stations used:
   AK.FID AK.GHO AK.GLB AK.KLU AK.KNK AK.MCK AK.PWL AK.RC01 
   AK.RND AK.SAW AK.SCM AT.PMR TA.M22K 
 
 Filtering commands used:
   cut o DIST/3.3 -20 o DIST/3.3 +50
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.33e+21 dyne-cm
  Mw = 3.35 
  Z  = 5 km
  Plane   Strike  Dip  Rake
   NP1       10    55   -45
   NP2      130    55   -135
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.33e+21      0      70
    N   0.00e+00     35     160
    P  -1.33e+21     55     340

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -2.37e+20
       Mxy     5.74e+20
       Mxz    -5.89e+20
       Myy     1.12e+21
       Myz     2.24e+20
       Mzz    -8.86e+20
                                                     
                                                     
                                                     
                                                     
                     ------------##                  
                 -----------------#####              
              ---------------------#######           
             -----------------------#######          
           #------------------------#########        
          ##-----------   -----------#########       
         ###----------- P -----------#########       
        #####----------   -----------######### T     
        ######-----------------------#########       
       #######-----------------------############    
       ########----------------------############    
       ##########--------------------############    
       ###########------------------#############    
        ############----------------############     
        ##############-------------#############     
         ###############-----------############      
          #################-------############       
           ####################--############        
             ###################-----------          
              ################------------           
                 ############----------              
                     #####---------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -8.86e+20  -5.89e+20  -2.24e+20 
 -5.89e+20  -2.37e+20  -5.74e+20 
 -2.24e+20  -5.74e+20   1.12e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160420145543/index.html
	
egional Moment Tensor (Mwr)
Moment	2.158e+14 N-m
Magnitude	3.5 Mwr
Depth	4.0 km
Percent DC	90 %
Half Duration	–
Catalog	AK
Data Source	US2
Contributor	US2
Nodal Planes
Plane	Strike	Dip	Rake
NP1	343	48	-81
NP2	150	43	-100
Principal Axes
Axis	Value	Plunge	Azimuth
T	2.214e+14 N-m	2	67
N	-0.116e+14 N-m	7	157
P	-2.098e+14 N-m	83	317

        

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

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 -20 o DIST/3.3 +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    1.0   210    60     5   2.99 0.2576
WVFGRD96    2.0    15    70   -40   3.17 0.3371
WVFGRD96    3.0     5    55   -55   3.29 0.4221
WVFGRD96    4.0     0    50   -60   3.35 0.4817
WVFGRD96    5.0    10    55   -45   3.35 0.4883
WVFGRD96    6.0    15    60   -35   3.36 0.4750
WVFGRD96    7.0    20    65   -25   3.37 0.4593
WVFGRD96    8.0    15    60   -35   3.43 0.4610
WVFGRD96    9.0    20    70   -20   3.42 0.4347
WVFGRD96   10.0    20    75   -15   3.42 0.4121
WVFGRD96   11.0   205    60    30   3.43 0.4084
WVFGRD96   12.0   205    60    30   3.45 0.4053
WVFGRD96   13.0   205    65    25   3.46 0.4015
WVFGRD96   14.0   205    65    20   3.47 0.3976
WVFGRD96   15.0   205    65    20   3.49 0.3947
WVFGRD96   16.0   205    65    20   3.50 0.3927
WVFGRD96   17.0   205    65    20   3.51 0.3898
WVFGRD96   18.0   200    70    20   3.51 0.3863
WVFGRD96   19.0   200    70    20   3.52 0.3830
WVFGRD96   20.0   205    70    15   3.53 0.3786
WVFGRD96   21.0   205    65    10   3.54 0.3747
WVFGRD96   22.0   205    65    10   3.55 0.3710
WVFGRD96   23.0   205    65    15   3.55 0.3666
WVFGRD96   24.0   205    65    10   3.56 0.3648
WVFGRD96   25.0   205    65    10   3.56 0.3630
WVFGRD96   26.0   205    65    10   3.57 0.3608
WVFGRD96   27.0   205    65    10   3.57 0.3589
WVFGRD96   28.0   210    65     0   3.60 0.3583
WVFGRD96   29.0   210    65    -5   3.61 0.3594

The best solution is

WVFGRD96    5.0    10    55   -45   3.35 0.4883

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 -20 o DIST/3.3 +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 Thu Apr 21 06:56:41 CDT 2016