Location

Location SLU

The purpose of the SLU location was to check the first motions against the prediction of the regional moment tensor inversion. The results of the run are in elocate.txt. The WUS velocity model was used. The fist motions are consistent with the RMT solution.

Location ANSS

2017/07/21 05:24:15 59.850 -136.720 1.4 3.7 Alaska

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/07/21 05:24:15:0  59.85 -136.72   1.4 3.7 Alaska
 
 Stations used:
   AK.BARN AK.BCP AK.BESE AK.CTG AK.GLB AK.GRNC AK.JIS AK.LOGN 
   AK.MESA AK.PIN AK.TABL AK.WAX AT.SIT AT.SKAG AT.YKU2 
   CN.DLBC CN.HYT NY.MAYO NY.MMPY TA.M26K TA.M30M TA.M31M 
   TA.N31M TA.P29M TA.P33M 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 7.50e+21 dyne-cm
  Mw = 3.85 
  Z  = 17 km
  Plane   Strike  Dip  Rake
   NP1      137    73   100
   NP2      285    20    60
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.50e+21     61      62
    N   0.00e+00     10     313
    P  -7.50e+21     27     218

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.25e+21
       Mxy    -2.15e+21
       Mxz     3.89e+21
       Myy    -9.21e+20
       Myz     4.69e+21
       Mzz     4.17e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              -----###############--------           
             #--######################-----          
           ##--##########################----        
          #----############################---       
         #------############################---      
        #--------############################---     
        -----------#############   ###########--     
       -------------############ T ############--    
       --------------###########   #############-    
       ----------------#########################-    
       ------------------#######################-    
        ------------------######################     
        --------------------####################     
         ---------------------#################      
          -------   -------------#############       
           ------ P ----------------#########        
             ----   -------------------####          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.17e+21   3.89e+21  -4.69e+21 
  3.89e+21  -3.25e+21   2.15e+21 
 -4.69e+21   2.15e+21  -9.21e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170721052415/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 = 285
      DIP = 20
     RAKE = 60
       MW = 3.85
       HS = 17.0

The NDK file is 20170721052415.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
SLUFM
 USGS/SLU Moment Tensor Solution
 ENS  2017/07/21 05:24:15:0  59.85 -136.72   1.4 3.7 Alaska
 
 Stations used:
   AK.BARN AK.BCP AK.BESE AK.CTG AK.GLB AK.GRNC AK.JIS AK.LOGN 
   AK.MESA AK.PIN AK.TABL AK.WAX AT.SIT AT.SKAG AT.YKU2 
   CN.DLBC CN.HYT NY.MAYO NY.MMPY TA.M26K TA.M30M TA.M31M 
   TA.N31M TA.P29M TA.P33M 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +40
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 7.50e+21 dyne-cm
  Mw = 3.85 
  Z  = 17 km
  Plane   Strike  Dip  Rake
   NP1      137    73   100
   NP2      285    20    60
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.50e+21     61      62
    N   0.00e+00     10     313
    P  -7.50e+21     27     218

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx    -3.25e+21
       Mxy    -2.15e+21
       Mxz     3.89e+21
       Myy    -9.21e+20
       Myz     4.69e+21
       Mzz     4.17e+21
                                                     
                                                     
                                                     
                                                     
                     --------------                  
                 ----------------------              
              -----###############--------           
             #--######################-----          
           ##--##########################----        
          #----############################---       
         #------############################---      
        #--------############################---     
        -----------#############   ###########--     
       -------------############ T ############--    
       --------------###########   #############-    
       ----------------#########################-    
       ------------------#######################-    
        ------------------######################     
        --------------------####################     
         ---------------------#################      
          -------   -------------#############       
           ------ P ----------------#########        
             ----   -------------------####          
              ----------------------------           
                 ----------------------              
                     --------------                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.17e+21   3.89e+21  -4.69e+21 
  3.89e+21  -3.25e+21   2.15e+21 
 -4.69e+21   2.15e+21  -9.21e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170721052415/index.html
	


First motions and takeoff angles from an elocate run.

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 +40
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   150    45   -90   3.55 0.4333
WVFGRD96    2.0   150    45   -90   3.65 0.5078
WVFGRD96    3.0    20    45   -35   3.68 0.4168
WVFGRD96    4.0    30    35   -15   3.70 0.4114
WVFGRD96    5.0    30    30   -15   3.72 0.4491
WVFGRD96    6.0    30    25   -20   3.71 0.4891
WVFGRD96    7.0    35    30   -10   3.72 0.5204
WVFGRD96    8.0    30    25   -20   3.79 0.5447
WVFGRD96    9.0   270    15    45   3.78 0.5725
WVFGRD96   10.0   275    15    50   3.79 0.6156
WVFGRD96   11.0   280    15    55   3.80 0.6498
WVFGRD96   12.0   285    15    60   3.81 0.6768
WVFGRD96   13.0   280    20    55   3.82 0.6974
WVFGRD96   14.0   285    20    60   3.83 0.7124
WVFGRD96   15.0   285    20    60   3.84 0.7225
WVFGRD96   16.0   285    20    60   3.84 0.7281
WVFGRD96   17.0   285    20    60   3.85 0.7299
WVFGRD96   18.0   295    20    70   3.86 0.7292
WVFGRD96   19.0   295    20    70   3.87 0.7258
WVFGRD96   20.0   285    20    55   3.87 0.7204
WVFGRD96   21.0   285    20    55   3.88 0.7144
WVFGRD96   22.0   285    20    55   3.89 0.7059
WVFGRD96   23.0   285    20    55   3.90 0.6959
WVFGRD96   24.0   280    20    50   3.90 0.6848
WVFGRD96   25.0   280    20    50   3.91 0.6726
WVFGRD96   26.0   285    15    55   3.92 0.6598
WVFGRD96   27.0   280    15    50   3.92 0.6472
WVFGRD96   28.0   270    15    40   3.93 0.6344
WVFGRD96   29.0   270    15    40   3.93 0.6218

The best solution is

WVFGRD96   17.0   285    20    60   3.85 0.7299

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 +40
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 Fri Jul 21 13:06:23 CDT 2017