Location

Location ANSS

2016/07/28 01:42:17 61.230 -139.237 7.5 3.7 Yukon, Canada

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2016/07/28 01:42:17:0  61.23 -139.24   7.5 3.7  Yukon, Canada
 
 Stations used:
   AK.BARN AK.BCP AK.BERG AK.BESE AK.BMR AK.CRQ AK.CTG AK.DIV 
   AK.DOT AK.FID AK.GRNC AK.ISLE AK.JIS AK.KLU AK.LOGN AK.MCAR 
   AK.MESA AK.PAX AK.PIN AK.RIDG AK.SCRK AK.SSP AK.SUCK 
   AK.TABL AK.TGL AK.VRDI AK.WAX AK.YAH AT.MENT AT.SKAG 
   CN.DAWY CN.HYT CN.WHY NY.FARO NY.MAYO TA.J26L TA.J29M 
   TA.K27K TA.L26K TA.L27K TA.M26K TA.M27K TA.M30M TA.M31M 
   TA.N25K TA.N31M 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 = 5.31e+21 dyne-cm
  Mw = 3.75 
  Z  = 17 km
  Plane   Strike  Dip  Rake
   NP1      100    60    85
   NP2      290    30    99
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.31e+21     74     357
    N   0.00e+00      4     102
    P  -5.31e+21     15     194

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.30e+21
       Mxy    -1.16e+21
       Mxz     2.64e+21
       Myy    -2.75e+20
       Myz     2.31e+20
       Mzz     4.58e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ----------------------------           
             -----###############----------          
           ---#######################--------        
          -############################-------       
         ################################------      
        ##################   ##############-----     
        ################## T ###############----     
       -##################   ################----    
       ---####################################---    
       -----###################################--    
       -------#################################--    
        ---------##############################-     
        --------------#####################-----     
         --------------------------------------      
          ------------------------------------       
           ----------------------------------        
             ------------------------------          
              --------   -----------------           
                 ----- P --------------              
                     -   ----------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.58e+21   2.64e+21  -2.31e+20 
  2.64e+21  -4.30e+21   1.16e+21 
 -2.31e+20   1.16e+21  -2.75e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160728014217/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 = 100
      DIP = 60
     RAKE = 85
       MW = 3.75
       HS = 17.0

The NDK file is 20160728014217.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  2016/07/28 01:42:17:0  61.23 -139.24   7.5 3.7  Yukon, Canada
 
 Stations used:
   AK.BARN AK.BCP AK.BERG AK.BESE AK.BMR AK.CRQ AK.CTG AK.DIV 
   AK.DOT AK.FID AK.GRNC AK.ISLE AK.JIS AK.KLU AK.LOGN AK.MCAR 
   AK.MESA AK.PAX AK.PIN AK.RIDG AK.SCRK AK.SSP AK.SUCK 
   AK.TABL AK.TGL AK.VRDI AK.WAX AK.YAH AT.MENT AT.SKAG 
   CN.DAWY CN.HYT CN.WHY NY.FARO NY.MAYO TA.J26L TA.J29M 
   TA.K27K TA.L26K TA.L27K TA.M26K TA.M27K TA.M30M TA.M31M 
   TA.N25K TA.N31M 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 = 5.31e+21 dyne-cm
  Mw = 3.75 
  Z  = 17 km
  Plane   Strike  Dip  Rake
   NP1      100    60    85
   NP2      290    30    99
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.31e+21     74     357
    N   0.00e+00      4     102
    P  -5.31e+21     15     194

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.30e+21
       Mxy    -1.16e+21
       Mxz     2.64e+21
       Myy    -2.75e+20
       Myz     2.31e+20
       Mzz     4.58e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              ----------------------------           
             -----###############----------          
           ---#######################--------        
          -############################-------       
         ################################------      
        ##################   ##############-----     
        ################## T ###############----     
       -##################   ################----    
       ---####################################---    
       -----###################################--    
       -------#################################--    
        ---------##############################-     
        --------------#####################-----     
         --------------------------------------      
          ------------------------------------       
           ----------------------------------        
             ------------------------------          
              --------   -----------------           
                 ----- P --------------              
                     -   ----------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.58e+21   2.64e+21  -2.31e+20 
  2.64e+21  -4.30e+21   1.16e+21 
 -2.31e+20   1.16e+21  -2.75e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160728014217/index.html
	
Regional Moment Tensor (Mwr)
Moment	5.260e+14 N-m
Magnitude	3.7 Mwr
Depth	16.0 km
Percent DC	99 %
Half Duration	–
Catalog	AK
Data Source	US2
Contributor	US2
Nodal Planes
Plane	Strike	Dip	Rake
NP1	269	28	83
NP2	97	62	94
Principal Axes
Axis	Value	Plunge	Azimuth
T	5.252e+14 N-m	73	16
N	0.016e+14 N-m	3	275
P	-5.268e+14 N-m	17	185

        

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   135    45   -95   3.39 0.3441
WVFGRD96    2.0   120    45   -90   3.51 0.4127
WVFGRD96    3.0    10    75     0   3.53 0.3698
WVFGRD96    4.0    10    50     0   3.58 0.3513
WVFGRD96    5.0    15    30     5   3.61 0.3737
WVFGRD96    6.0    15    20     5   3.61 0.4164
WVFGRD96    7.0   295    10   -75   3.60 0.4523
WVFGRD96    8.0   285    10   -85   3.69 0.4773
WVFGRD96    9.0   280    15   -90   3.69 0.5085
WVFGRD96   10.0   100    75   -90   3.69 0.5291
WVFGRD96   11.0   100    70   -90   3.70 0.5424
WVFGRD96   12.0   285    25    95   3.71 0.5624
WVFGRD96   13.0   100    60    85   3.73 0.5862
WVFGRD96   14.0   100    60    85   3.73 0.6043
WVFGRD96   15.0   100    60    85   3.74 0.6161
WVFGRD96   16.0   100    60    85   3.74 0.6227
WVFGRD96   17.0   100    60    85   3.75 0.6252
WVFGRD96   18.0   100    60    85   3.76 0.6241
WVFGRD96   19.0   100    65    80   3.76 0.6209
WVFGRD96   20.0   100    65    80   3.77 0.6155
WVFGRD96   21.0   100    65    80   3.78 0.6096
WVFGRD96   22.0   100    65    80   3.79 0.6015
WVFGRD96   23.0   100    65    80   3.79 0.5921
WVFGRD96   24.0   100    65    80   3.80 0.5815
WVFGRD96   25.0   100    70    80   3.81 0.5703
WVFGRD96   26.0   100    70    80   3.81 0.5591
WVFGRD96   27.0   100    70    75   3.82 0.5471
WVFGRD96   28.0   100    70    75   3.83 0.5344
WVFGRD96   29.0   100    70    80   3.83 0.5211

The best solution is

WVFGRD96   17.0   100    60    85   3.75 0.6252

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 Jul 28 10:59:03 CDT 2016