Location

Location ANSS

2017/06/11 12:40:25 36.377 -92.990 1.8 3.7 Arkansas

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2017/06/11 12:40:25:0  36.38  -92.99   1.8 3.7 Arkansas
 
 Stations used:
   AG.FCAR AG.LCAR AG.WHAR GS.OK048 IU.CCM N4.P40B N4.R40B 
   N4.S39B N4.T35B N4.T42B N4.U38B NM.CGM3 NM.FVM NM.SLM 
   US.MIAR 
 
 Filtering commands used:
   cut o DIST/3.3 -20 o DIST/3.3 +30
   rtr
   taper w 0.1
   hp c 0.04 n 3 
   lp c 0.15 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 3.16e+21 dyne-cm
  Mw = 3.60 
  Z  = 5 km
  Plane   Strike  Dip  Rake
   NP1       25    90   -175
   NP2      295    85     0
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.16e+21      4     160
    N   0.00e+00     85      25
    P  -3.16e+21      4     250

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.41e+21
       Mxy    -2.02e+21
       Mxz    -1.16e+20
       Myy    -2.41e+21
       Myz     2.50e+20
       Mzz     0.00e+00
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ###################---              
              #####################-------           
             #####################---------          
           #######################-----------        
          #######################-------------       
         --#####################---------------      
        --------###############-----------------     
        -------------#########------------------     
       ------------------####--------------------    
       ---------------------#--------------------    
       ---------------------#####----------------    
       --------------------##########------------    
           ---------------##############--------     
         P --------------##################-----     
           -------------######################-      
          -------------#######################       
           -----------#######################        
             ---------#####################          
              -------#####################           
                 ---#############   ###              
                     ############ T                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  0.00e+00  -1.16e+20  -2.50e+20 
 -1.16e+20   2.41e+21   2.02e+21 
 -2.50e+20   2.02e+21  -2.41e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170611124025/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 = 295
      DIP = 85
     RAKE = 0
       MW = 3.60
       HS = 5.0

The NDK file is 20170611124025.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  2017/06/11 12:40:25:0  36.38  -92.99   1.8 3.7 Arkansas
 
 Stations used:
   AG.FCAR AG.LCAR AG.WHAR GS.OK048 IU.CCM N4.P40B N4.R40B 
   N4.S39B N4.T35B N4.T42B N4.U38B NM.CGM3 NM.FVM NM.SLM 
   US.MIAR 
 
 Filtering commands used:
   cut o DIST/3.3 -20 o DIST/3.3 +30
   rtr
   taper w 0.1
   hp c 0.04 n 3 
   lp c 0.15 n 3 
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 3.16e+21 dyne-cm
  Mw = 3.60 
  Z  = 5 km
  Plane   Strike  Dip  Rake
   NP1       25    90   -175
   NP2      295    85     0
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.16e+21      4     160
    N   0.00e+00     85      25
    P  -3.16e+21      4     250

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.41e+21
       Mxy    -2.02e+21
       Mxz    -1.16e+20
       Myy    -2.41e+21
       Myz     2.50e+20
       Mzz     0.00e+00
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ###################---              
              #####################-------           
             #####################---------          
           #######################-----------        
          #######################-------------       
         --#####################---------------      
        --------###############-----------------     
        -------------#########------------------     
       ------------------####--------------------    
       ---------------------#--------------------    
       ---------------------#####----------------    
       --------------------##########------------    
           ---------------##############--------     
         P --------------##################-----     
           -------------######################-      
          -------------#######################       
           -----------#######################        
             ---------#####################          
              -------#####################           
                 ---#############   ###              
                     ############ T                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  0.00e+00  -1.16e+20  -2.50e+20 
 -1.16e+20   2.41e+21   2.02e+21 
 -2.50e+20   2.02e+21  -2.41e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170611124025/index.html
	
Regional Moment Tensor (Mwr)
Moment	3.932e+14 N-m
Magnitude	3.7 Mwr
Depth	6.0 km
Percent DC	83 %
Half Duration	–
Catalog	US
Data Source	US2
Contributor	US2
Nodal Planes
Plane	Strike	Dip	Rake
NP1	296	84	9
NP2	205	81	174
Principal Axes
Axis	Value	Plunge	Azimuth
T	4.091e+14 N-m	10	161
N	-0.340e+14 N-m	80	330
P	-3.751e+14 N-m	2	70

        

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 -20 o DIST/3.3 +30
rtr
taper w 0.1
hp c 0.04 n 3 
lp c 0.15 n 3 
br c 0.12 0.25 n 4 p 2
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   295    90   -10   3.46 0.6246
WVFGRD96    2.0   115    90   -25   3.53 0.6809
WVFGRD96    3.0   295    85    10   3.54 0.7311
WVFGRD96    4.0   295    90     5   3.57 0.7623
WVFGRD96    5.0   295    85     0   3.60 0.7743
WVFGRD96    6.0   295    85    -5   3.61 0.7705
WVFGRD96    7.0   115    90    10   3.63 0.7540
WVFGRD96    8.0   115    90    10   3.64 0.7312
WVFGRD96    9.0   295    90   -10   3.65 0.7000
WVFGRD96   10.0   295    85    10   3.65 0.6656
WVFGRD96   11.0   115    90   -10   3.66 0.6412
WVFGRD96   12.0   115    90   -10   3.67 0.6212
WVFGRD96   13.0   115    90   -10   3.67 0.6024
WVFGRD96   14.0   115    90    -5   3.68 0.5927
WVFGRD96   15.0   295    90     5   3.69 0.5836
WVFGRD96   16.0   115    90    -5   3.69 0.5752
WVFGRD96   17.0   295    90     0   3.70 0.5701
WVFGRD96   18.0   115    90     5   3.71 0.5644
WVFGRD96   19.0   115    90     5   3.72 0.5598
WVFGRD96   20.0   115    90     5   3.73 0.5509
WVFGRD96   21.0   110    90     0   3.73 0.5371
WVFGRD96   22.0   110    90   -10   3.74 0.5255
WVFGRD96   23.0   110    90   -10   3.74 0.5171
WVFGRD96   24.0   290    85    10   3.75 0.5087
WVFGRD96   25.0   290    90     5   3.75 0.5006
WVFGRD96   26.0   110    90    -5   3.76 0.4919
WVFGRD96   27.0   110    90     0   3.76 0.4828
WVFGRD96   28.0   110    90    -5   3.76 0.4796
WVFGRD96   29.0   290    85    15   3.78 0.4915

The best solution is

WVFGRD96    5.0   295    85     0   3.60 0.7743

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 +30
rtr
taper w 0.1
hp c 0.04 n 3 
lp c 0.15 n 3 
br c 0.12 0.25 n 4 p 2
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 Sun Jun 11 08:50:03 CDT 2017