Location

SLU Location

First arrival times and polarities were selected and the earthquake located using elocate. The location results are given in the file elocate.txt. The takeoff angles are used to compare the selected first motions to the moment tensor soltuion in the plot below. The reason for this extra effort was to get a better idea of source depth. The ANSS Regional Moment Tensor solution ahd a depth of 19 km, while this moment tensor solution was 10 km. The elocate depth is 7 km using the same velocity model as for the moment tensor solution. This earthquake was in the same location at the 2017/09/09 04:15:29 M=3.1 earthquae at 38.425N 87.909W.

Location ANSS

2017/09/19 11:47:28 38.424 -87.910 11.7 3.8 Illinois

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/09/19 11:47:28:0  38.42  -87.91  11.7 3.8 Illinois
 
 Stations used:
   AG.LCAR ET.SWET IU.CCM IU.WCI IU.WVT N4.L42A N4.L46A 
   N4.N41A N4.N49A N4.O44A N4.O49A N4.P40B N4.P43A N4.P46A 
   N4.P48A N4.Q44B N4.R49A N4.R50A N4.S44A N4.S51A N4.T45B 
   N4.T47A N4.T50A N4.U49A N4.V51A N4.W50A NM.BLO NM.CGM3 
   NM.CLTN NM.FFIL NM.FVM NM.GLAT NM.HALT NM.HBAR NM.HICK 
   NM.LNXT NM.MGMO NM.OLIL NM.PARM NM.PBMO NM.PEBM NM.PLAL 
   NM.PVMO NM.SLM NM.USIN NM.UTMT NW.HQIL TA.SFIN US.HDIL 
   US.TZTN 
 
 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 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 6.10e+21 dyne-cm
  Mw = 3.79 
  Z  = 10 km
  Plane   Strike  Dip  Rake
   NP1      125    90   -10
   NP2      215    80   -180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.10e+21      7     170
    N   0.00e+00     80     305
    P  -6.10e+21      7      80

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     5.64e+21
       Mxy    -2.05e+21
       Mxz    -8.67e+20
       Myy    -5.64e+21
       Myz    -6.07e+20
       Mzz     9.25e+13
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ########################----           
             #######################-------          
           #######################-----------        
          ---####################-------------       
         -------###############----------------      
        -----------###########------------------     
        -------------#######------------------       
       -----------------###------------------- P     
       -------------------#-------------------       
       -----------------#####--------------------    
       ----------------#########-----------------    
        --------------#############-------------     
        -------------################-----------     
         -----------####################-------      
          ---------########################---       
           -------###########################        
             ----##########################          
              --##########################           
                 #############   ######              
                     ######### T ##                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  9.25e+13  -8.67e+20   6.07e+20 
 -8.67e+20   5.64e+21   2.05e+21 
  6.07e+20   2.05e+21  -5.64e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170919114728/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 = 125
      DIP = 90
     RAKE = -10
       MW = 3.79
       HS = 10.0

The NDK file is 20170919114728.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMWR
SLUFM
 USGS/SLU Moment Tensor Solution
 ENS  2017/09/19 11:47:28:0  38.42  -87.91  11.7 3.8 Illinois
 
 Stations used:
   AG.LCAR ET.SWET IU.CCM IU.WCI IU.WVT N4.L42A N4.L46A 
   N4.N41A N4.N49A N4.O44A N4.O49A N4.P40B N4.P43A N4.P46A 
   N4.P48A N4.Q44B N4.R49A N4.R50A N4.S44A N4.S51A N4.T45B 
   N4.T47A N4.T50A N4.U49A N4.V51A N4.W50A NM.BLO NM.CGM3 
   NM.CLTN NM.FFIL NM.FVM NM.GLAT NM.HALT NM.HBAR NM.HICK 
   NM.LNXT NM.MGMO NM.OLIL NM.PARM NM.PBMO NM.PEBM NM.PLAL 
   NM.PVMO NM.SLM NM.USIN NM.UTMT NW.HQIL TA.SFIN US.HDIL 
   US.TZTN 
 
 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 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 6.10e+21 dyne-cm
  Mw = 3.79 
  Z  = 10 km
  Plane   Strike  Dip  Rake
   NP1      125    90   -10
   NP2      215    80   -180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.10e+21      7     170
    N   0.00e+00     80     305
    P  -6.10e+21      7      80

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     5.64e+21
       Mxy    -2.05e+21
       Mxz    -8.67e+20
       Myy    -5.64e+21
       Myz    -6.07e+20
       Mzz     9.25e+13
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ########################----           
             #######################-------          
           #######################-----------        
          ---####################-------------       
         -------###############----------------      
        -----------###########------------------     
        -------------#######------------------       
       -----------------###------------------- P     
       -------------------#-------------------       
       -----------------#####--------------------    
       ----------------#########-----------------    
        --------------#############-------------     
        -------------################-----------     
         -----------####################-------      
          ---------########################---       
           -------###########################        
             ----##########################          
              --##########################           
                 #############   ######              
                     ######### T ##                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  9.25e+13  -8.67e+20   6.07e+20 
 -8.67e+20   5.64e+21   2.05e+21 
  6.07e+20   2.05e+21  -5.64e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170919114728/index.html
	
Regional Moment Tensor (Mwr)
Moment	7.302e+14 N-m
Magnitude	3.8 Mwr
Depth	19.0 km
Percent DC	87 %
Half Duration	–
Catalog	US
Data Source	US2
Contributor	US2
Nodal Planes
Plane	Strike	Dip	Rake
NP1	126	86	-10
NP2	217	80	-176
Principal Axes
Axis	Value	Plunge	Azimuth
T	7.053e+14 N-m	4	172
N	0.474e+14 N-m	79	284
P	-7.527e+14 N-m	10	81

        


First motions and takeoff angles from an elocate run.

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 +40
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   310    75    25   3.65 0.4646
WVFGRD96    2.0   310    80    30   3.70 0.5139
WVFGRD96    3.0   305    90    25   3.71 0.5569
WVFGRD96    4.0   125    85   -20   3.72 0.5920
WVFGRD96    5.0   125    85   -20   3.73 0.6184
WVFGRD96    6.0   125    85   -15   3.74 0.6378
WVFGRD96    7.0   125    85   -15   3.75 0.6509
WVFGRD96    8.0   125    85   -15   3.76 0.6591
WVFGRD96    9.0   125    90   -10   3.78 0.6646
WVFGRD96   10.0   125    90   -10   3.79 0.6666
WVFGRD96   11.0   125    90   -10   3.80 0.6660
WVFGRD96   12.0   125    90   -10   3.81 0.6624
WVFGRD96   13.0   125    90   -10   3.82 0.6558
WVFGRD96   14.0   125    90   -10   3.82 0.6463
WVFGRD96   15.0   125    90   -10   3.83 0.6346
WVFGRD96   16.0   305    90    10   3.84 0.6211
WVFGRD96   17.0   125    90   -10   3.85 0.6085
WVFGRD96   18.0   125    90    -5   3.85 0.5947
WVFGRD96   19.0   125    90   -10   3.86 0.5804
WVFGRD96   20.0   125    90   -10   3.87 0.5663
WVFGRD96   21.0   125    90   -10   3.88 0.5513
WVFGRD96   22.0   125    90   -10   3.88 0.5374
WVFGRD96   23.0   125    90   -10   3.89 0.5236
WVFGRD96   24.0   305    90    15   3.89 0.5115
WVFGRD96   25.0   125    90   -15   3.90 0.5003
WVFGRD96   26.0   125    90   -15   3.90 0.4900
WVFGRD96   27.0   305    90    15   3.91 0.4806
WVFGRD96   28.0   125    90   -15   3.91 0.4722
WVFGRD96   29.0   125    90   -15   3.92 0.4644

The best solution is

WVFGRD96   10.0   125    90   -10   3.79 0.6666

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 0.10 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 Sep 19 09:39:39 CDT 2017