Location

Location ANSS

2018/05/15 20:31:53 61.184 -140.468 0.0 4.1 Yukon

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2018/05/15 20:31:53:0  61.18 -140.47   0.0 4.1 Yukon
 
 Stations used:
   AK.BARN AK.BCP AK.BESE AK.BMR AK.CTG AK.DIV AK.EYAK AK.FID 
   AK.GLB AK.GLI AK.HIN AK.HMT AK.ISLE AK.KLU AK.KNK AK.LOGN 
   AK.MCAR AK.MESA AK.RAG AK.SAW AK.SCM AK.SCRK AK.VRDI AK.WAX 
   CN.DAWY CN.HYT CN.WHY NY.MAYO TA.J26L TA.J29N TA.J30M 
   TA.K29M TA.L27K TA.L29M TA.M26K TA.M27K TA.M29M TA.M30M 
   TA.M31M TA.N25K TA.N30M TA.N31M TA.N32M TA.O30N TA.P29M 
   TA.P32M TA.P33M TA.S31K US.EGAK 
 
 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 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.02e+22 dyne-cm
  Mw = 3.94 
  Z  = 10 km
  Plane   Strike  Dip  Rake
   NP1      344    76   -164
   NP2      250    75   -15
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.02e+22      0     117
    N   0.00e+00     69      26
    P  -1.02e+22     21     207

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.97e+21
       Mxy    -7.74e+21
       Mxz     3.03e+21
       Myy     6.29e+21
       Myz     1.62e+21
       Mzz    -1.32e+21
                                                     
                                                     
                                                     
                                                     
                     ##------------                  
                 #######---------------              
              ###########-----------------           
             #############-----------------          
           ################------------------        
          ##################------------------       
         ####################------------------      
        ######################-------###########     
        ########################################     
       ##################------##################    
       #############------------#################    
       ##########---------------#################    
       #######-------------------################    
        ###----------------------###############     
        #------------------------############        
         -------------------------########### T      
          ------------------------###########        
           --------   ------------###########        
             ------ P ------------#########          
              -----   ------------########           
                 -----------------#####              
                     -------------#                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.32e+21   3.03e+21  -1.62e+21 
  3.03e+21  -4.97e+21   7.74e+21 
 -1.62e+21   7.74e+21   6.29e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180515203153/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 = 250
      DIP = 75
     RAKE = -15
       MW = 3.94
       HS = 10.0

The NDK file is 20180515203153.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  2018/05/15 20:31:53:0  61.18 -140.47   0.0 4.1 Yukon
 
 Stations used:
   AK.BARN AK.BCP AK.BESE AK.BMR AK.CTG AK.DIV AK.EYAK AK.FID 
   AK.GLB AK.GLI AK.HIN AK.HMT AK.ISLE AK.KLU AK.KNK AK.LOGN 
   AK.MCAR AK.MESA AK.RAG AK.SAW AK.SCM AK.SCRK AK.VRDI AK.WAX 
   CN.DAWY CN.HYT CN.WHY NY.MAYO TA.J26L TA.J29N TA.J30M 
   TA.K29M TA.L27K TA.L29M TA.M26K TA.M27K TA.M29M TA.M30M 
   TA.M31M TA.N25K TA.N30M TA.N31M TA.N32M TA.O30N TA.P29M 
   TA.P32M TA.P33M TA.S31K US.EGAK 
 
 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 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 1.02e+22 dyne-cm
  Mw = 3.94 
  Z  = 10 km
  Plane   Strike  Dip  Rake
   NP1      344    76   -164
   NP2      250    75   -15
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.02e+22      0     117
    N   0.00e+00     69      26
    P  -1.02e+22     21     207

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -4.97e+21
       Mxy    -7.74e+21
       Mxz     3.03e+21
       Myy     6.29e+21
       Myz     1.62e+21
       Mzz    -1.32e+21
                                                     
                                                     
                                                     
                                                     
                     ##------------                  
                 #######---------------              
              ###########-----------------           
             #############-----------------          
           ################------------------        
          ##################------------------       
         ####################------------------      
        ######################-------###########     
        ########################################     
       ##################------##################    
       #############------------#################    
       ##########---------------#################    
       #######-------------------################    
        ###----------------------###############     
        #------------------------############        
         -------------------------########### T      
          ------------------------###########        
           --------   ------------###########        
             ------ P ------------#########          
              -----   ------------########           
                 -----------------#####              
                     -------------#                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.32e+21   3.03e+21  -1.62e+21 
  3.03e+21  -4.97e+21   7.74e+21 
 -1.62e+21   7.74e+21   6.29e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180515203153/index.html
	
Regional Moment Tensor (Mwr)
Moment	8.655e+14 N-m
Magnitude	3.9 Mwr
Depth	14.0 km
Percent DC	85 %
Half Duration	–
Catalog	US
Data Source	US2
Contributor	US2
Nodal Planes
Plane	Strike	Dip	Rake
NP1	346	75	-167
NP2	252	78	-15
Principal Axes
Axis	Value	Plunge	Azimuth
T	8.292e+14 N-m	2	299
N	0.685e+14 N-m	70	34
P	-8.978e+14 N-m	20	209

        

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 0.10 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   255    70     0   3.80 0.5813
WVFGRD96    2.0   255    70     0   3.83 0.6120
WVFGRD96    3.0   250    70   -20   3.86 0.6296
WVFGRD96    4.0   250    70   -20   3.88 0.6446
WVFGRD96    5.0   250    75   -20   3.89 0.6570
WVFGRD96    6.0   250    75   -20   3.90 0.6663
WVFGRD96    7.0   250    75   -15   3.90 0.6722
WVFGRD96    8.0   250    75   -15   3.91 0.6761
WVFGRD96    9.0   250    75   -15   3.93 0.6780
WVFGRD96   10.0   250    75   -15   3.94 0.6791
WVFGRD96   11.0   250    75   -15   3.95 0.6770
WVFGRD96   12.0   250    75   -15   3.96 0.6726
WVFGRD96   13.0   250    75   -15   3.97 0.6658
WVFGRD96   14.0   250    75   -15   3.98 0.6570
WVFGRD96   15.0   250    75   -15   3.99 0.6476
WVFGRD96   16.0   250    75   -15   4.00 0.6380
WVFGRD96   17.0   255    80   -10   4.01 0.6285
WVFGRD96   18.0   255    80   -10   4.02 0.6179
WVFGRD96   19.0   255    80   -10   4.02 0.6066
WVFGRD96   20.0   250    75   -15   4.04 0.5968
WVFGRD96   21.0   250    75   -15   4.04 0.5867
WVFGRD96   22.0   250    75   -15   4.05 0.5773
WVFGRD96   23.0   250    75   -15   4.06 0.5674
WVFGRD96   24.0   255    75    -5   4.06 0.5587
WVFGRD96   25.0   255    75    -5   4.06 0.5500
WVFGRD96   26.0   255    75    -5   4.07 0.5413
WVFGRD96   27.0   255    75   -10   4.08 0.5340
WVFGRD96   28.0   255    75    -5   4.08 0.5269
WVFGRD96   29.0   255    75    -5   4.08 0.5188

The best solution is

WVFGRD96   10.0   250    75   -15   3.94 0.6791

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 0.10 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 CUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

MODEL.01
CUS Model with Q from simple gamma values
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
  1.0000  5.0000  2.8900  2.5000 0.172E-02 0.387E-02 0.00  0.00  1.00  1.00 
  9.0000  6.1000  3.5200  2.7300 0.160E-02 0.363E-02 0.00  0.00  1.00  1.00 
 10.0000  6.4000  3.7000  2.8200 0.149E-02 0.336E-02 0.00  0.00  1.00  1.00 
 20.0000  6.7000  3.8700  2.9020 0.000E-04 0.000E-04 0.00  0.00  1.00  1.00 
  0.0000  8.1500  4.7000  3.3640 0.194E-02 0.431E-02 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 Tue May 15 16:04:11 CDT 2018