Location

2010/11/20 19:06:34 35.316 -92.317 2.8 4.20 Arkansas

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports main page

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2010/11/20 19:06:34:8  35.32  -92.32   2.8 4.2 Arkansas
 
 Stations used:
   AG.FCAR AG.HHAR AG.LCAR AG.WLAR IU.CCM IU.WVT NM.MGMO 
   NM.MPH NM.PVMO NM.SIUC NM.SLM NM.UALR NM.USIN NM.UTMT 
   TA.139A TA.238A TA.O36A TA.P35A TA.P36A TA.Q34A TA.Q35A 
   TA.Q36A TA.Q37A TA.R34A TA.R35A TA.R36A TA.R37A TA.S33A 
   TA.S34A TA.S35A TA.S36A TA.S37A TA.T33A TA.T34A TA.T35A 
   TA.T36A TA.T37A TA.TUL1 TA.U33A TA.U34A TA.U35A TA.U36A 
   TA.U37A TA.U38A TA.V33A TA.V34A TA.V35A TA.V36A TA.V37A 
   TA.V38A TA.W33A TA.W34A TA.W35A TA.W36A TA.W37A TA.W38A 
   TA.X36A TA.X37A TA.X38A TA.Y37A TA.Y38A TA.Y39A TA.Z35A 
   TA.Z36A TA.Z37A TA.Z38A TA.Z39A US.KSU1 US.LRAL US.MIAR 
   US.OXF 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 8.04e+21 dyne-cm
  Mw = 3.87 
  Z  = 5 km
  Plane   Strike  Dip  Rake
   NP1      195    90   170
   NP2      285    80     0
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   8.04e+21      7     150
    N   0.00e+00     80      15
    P  -8.04e+21      7     240

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     3.96e+21
       Mxy    -6.85e+21
       Mxz    -3.61e+20
       Myy    -3.96e+21
       Myz     1.35e+21
       Mzz     0.00e+00
                                                     
                                                     
                                                     
                                                     
                     ############--                  
                 ################------              
              ##################----------           
             ###################-----------          
           ####################--------------        
          #####################---------------       
         #####################-----------------      
        ######################------------------     
        -----------##########-------------------     
       ---------------------#--------------------    
       ---------------------######---------------    
       ---------------------###########----------    
       --------------------################------    
        -------------------####################-     
        ------------------######################     
         -   -------------#####################      
           P ------------#####################       
             ------------####################        
             -----------###################          
              ----------############   ###           
                 ------############# T               
                     --############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  0.00e+00  -3.61e+20  -1.35e+21 
 -3.61e+20   3.96e+21   6.85e+21 
 -1.35e+21   6.85e+21  -3.96e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20101120190634/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 = 80
     RAKE = 0
       MW = 3.87
       HS = 5.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2010/11/20 19:06:34:8  35.32  -92.32   2.8 4.2 Arkansas
 
 Stations used:
   AG.FCAR AG.HHAR AG.LCAR AG.WLAR IU.CCM IU.WVT NM.MGMO 
   NM.MPH NM.PVMO NM.SIUC NM.SLM NM.UALR NM.USIN NM.UTMT 
   TA.139A TA.238A TA.O36A TA.P35A TA.P36A TA.Q34A TA.Q35A 
   TA.Q36A TA.Q37A TA.R34A TA.R35A TA.R36A TA.R37A TA.S33A 
   TA.S34A TA.S35A TA.S36A TA.S37A TA.T33A TA.T34A TA.T35A 
   TA.T36A TA.T37A TA.TUL1 TA.U33A TA.U34A TA.U35A TA.U36A 
   TA.U37A TA.U38A TA.V33A TA.V34A TA.V35A TA.V36A TA.V37A 
   TA.V38A TA.W33A TA.W34A TA.W35A TA.W36A TA.W37A TA.W38A 
   TA.X36A TA.X37A TA.X38A TA.Y37A TA.Y38A TA.Y39A TA.Z35A 
   TA.Z36A TA.Z37A TA.Z38A TA.Z39A US.KSU1 US.LRAL US.MIAR 
   US.OXF 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 8.04e+21 dyne-cm
  Mw = 3.87 
  Z  = 5 km
  Plane   Strike  Dip  Rake
   NP1      195    90   170
   NP2      285    80     0
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   8.04e+21      7     150
    N   0.00e+00     80      15
    P  -8.04e+21      7     240

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     3.96e+21
       Mxy    -6.85e+21
       Mxz    -3.61e+20
       Myy    -3.96e+21
       Myz     1.35e+21
       Mzz     0.00e+00
                                                     
                                                     
                                                     
                                                     
                     ############--                  
                 ################------              
              ##################----------           
             ###################-----------          
           ####################--------------        
          #####################---------------       
         #####################-----------------      
        ######################------------------     
        -----------##########-------------------     
       ---------------------#--------------------    
       ---------------------######---------------    
       ---------------------###########----------    
       --------------------################------    
        -------------------####################-     
        ------------------######################     
         -   -------------#####################      
           P ------------#####################       
             ------------####################        
             -----------###################          
              ----------############   ###           
                 ------############# T               
                     --############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  0.00e+00  -3.61e+20  -1.35e+21 
 -3.61e+20   3.96e+21   6.85e+21 
 -1.35e+21   6.85e+21  -3.96e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20101120190634/index.html
	

Waveform Inversion

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:

hp c 0.02 n 3
lp c 0.10 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    0.5   285    75   -15   3.78 0.4500
WVFGRD96    1.0   285    75   -15   3.81 0.4727
WVFGRD96    2.0   285    80    -5   3.82 0.5029
WVFGRD96    3.0   105    90     0   3.84 0.5204
WVFGRD96    4.0   285    85     0   3.86 0.5286
WVFGRD96    5.0   285    80     0   3.87 0.5303
WVFGRD96    6.0   285    75     0   3.88 0.5299
WVFGRD96    7.0   285    75     5   3.89 0.5285
WVFGRD96    8.0   285    75     5   3.90 0.5266
WVFGRD96    9.0   285    75     5   3.91 0.5250
WVFGRD96   10.0   285    90    20   3.92 0.5230
WVFGRD96   11.0   285    90    15   3.93 0.5212
WVFGRD96   12.0   285    90    15   3.93 0.5191
WVFGRD96   13.0   285    90    15   3.94 0.5168
WVFGRD96   14.0   285    90    15   3.95 0.5139
WVFGRD96   15.0   285    90    15   3.96 0.5098
WVFGRD96   16.0   285    90    15   3.96 0.5047
WVFGRD96   17.0   105    85   -15   3.97 0.4988
WVFGRD96   18.0   105    85   -15   3.98 0.4916
WVFGRD96   19.0   105    85   -15   3.99 0.4838
WVFGRD96   20.0   105    85   -15   4.00 0.4756
WVFGRD96   21.0   105    80   -15   4.00 0.4668
WVFGRD96   22.0   105    80   -15   4.01 0.4574
WVFGRD96   23.0   105    80   -15   4.02 0.4473
WVFGRD96   24.0   105    80   -15   4.02 0.4367
WVFGRD96   25.0   105    80   -15   4.03 0.4262
WVFGRD96   26.0   105    75   -15   4.03 0.4152
WVFGRD96   27.0   105    75   -15   4.04 0.4045
WVFGRD96   28.0   105    75   -15   4.04 0.3938
WVFGRD96   29.0   105    75   -15   4.05 0.3841

The best solution is

WVFGRD96    5.0   285    80     0   3.87 0.5303

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

hp c 0.02 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2
Figure 3. Waveform comparison for selected depth
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

The Future

Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.

Acknowledgements

Dr. Harley Benz, USGS, provided the USGS USNSN digital data. The digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface.

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint L ouis University, Universityof Memphis, Lamont Doehrty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.

Velocity Model

The CUS 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:

DATE=Sat Nov 20 15:09:42 CST 2010

Last Changed 2010/11/20