Location

2010/10/16 09:43:19 35.278 -92.342 5.0 3.50 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  2010/10/16 09:43:19:0  35.28  -92.34   5.0 3.5 Arkansas
 
 Stations used:
   AG.HHAR AG.LCAR AG.WLAR IU.CCM NM.MGMO NM.SIUC NM.UALR 
   TA.Q36A TA.S36A TA.T36A TA.T37A TA.W37A TA.Y37A TA.Y39A 
   TA.Z38A US.MIAR 
 
 Filtering commands used:
   hp c 0.03 n 3
   lp c 0.10 n 3
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 2.02e+21 dyne-cm
  Mw = 3.47 
  Z  = 10 km
  Plane   Strike  Dip  Rake
   NP1      305    80    25
   NP2      210    65   169
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.02e+21     25     170
    N   0.00e+00     63     325
    P  -2.02e+21     10      76

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.50e+21
       Mxy    -7.53e+20
       Mxz    -8.39e+20
       Myy    -1.79e+21
       Myz    -2.00e+20
       Mzz     2.92e+20
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 #####################-              
              ####################--------           
             ##################------------          
           ##################----------------        
          ------############------------------       
         ----------#######---------------------      
        ---------------##--------------------        
        ---------------###------------------- P      
       ---------------######-----------------   -    
       --------------##########------------------    
       -------------##############---------------    
       -------------################-------------    
        -----------###################----------     
        ----------######################--------     
         ---------########################-----      
          --------##########################--       
           ------############################        
             ----############   ###########          
              ---############ T ##########           
                 ############   #######              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.92e+20  -8.39e+20   2.00e+20 
 -8.39e+20   1.50e+21   7.53e+20 
  2.00e+20   7.53e+20  -1.79e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20101016094319/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 = 305
      DIP = 80
     RAKE = 25
       MW = 3.47
       HS = 10.0

The depth control is not very sharp. Previous earthquakes here have a depth of about 5 km. The moment derived is OK.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2010/10/16 09:43:19:0  35.28  -92.34   5.0 3.5 Arkansas
 
 Stations used:
   AG.HHAR AG.LCAR AG.WLAR IU.CCM NM.MGMO NM.SIUC NM.UALR 
   TA.Q36A TA.S36A TA.T36A TA.T37A TA.W37A TA.Y37A TA.Y39A 
   TA.Z38A US.MIAR 
 
 Filtering commands used:
   hp c 0.03 n 3
   lp c 0.10 n 3
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 2.02e+21 dyne-cm
  Mw = 3.47 
  Z  = 10 km
  Plane   Strike  Dip  Rake
   NP1      305    80    25
   NP2      210    65   169
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.02e+21     25     170
    N   0.00e+00     63     325
    P  -2.02e+21     10      76

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.50e+21
       Mxy    -7.53e+20
       Mxz    -8.39e+20
       Myy    -1.79e+21
       Myz    -2.00e+20
       Mzz     2.92e+20
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 #####################-              
              ####################--------           
             ##################------------          
           ##################----------------        
          ------############------------------       
         ----------#######---------------------      
        ---------------##--------------------        
        ---------------###------------------- P      
       ---------------######-----------------   -    
       --------------##########------------------    
       -------------##############---------------    
       -------------################-------------    
        -----------###################----------     
        ----------######################--------     
         ---------########################-----      
          --------##########################--       
           ------############################        
             ----############   ###########          
              ---############ T ##########           
                 ############   #######              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  2.92e+20  -8.39e+20   2.00e+20 
 -8.39e+20   1.50e+21   7.53e+20 
  2.00e+20   7.53e+20  -1.79e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20101016094319/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 0.03 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   125    65    15   3.36 0.5974
WVFGRD96    1.0   125    75    15   3.35 0.6177
WVFGRD96    2.0   120    75   -10   3.36 0.6453
WVFGRD96    3.0   300    70   -15   3.39 0.6563
WVFGRD96    4.0   300    70   -15   3.40 0.6558
WVFGRD96    5.0   310    60    25   3.44 0.6593
WVFGRD96    6.0   310    65    25   3.43 0.6655
WVFGRD96    7.0   305    80    25   3.43 0.6706
WVFGRD96    8.0   305    80    25   3.44 0.6733
WVFGRD96    9.0   305    80    25   3.45 0.6751
WVFGRD96   10.0   305    80    25   3.47 0.6758
WVFGRD96   11.0   305    80    25   3.48 0.6751
WVFGRD96   12.0   305    80    25   3.49 0.6728
WVFGRD96   13.0   305    75    20   3.50 0.6688
WVFGRD96   14.0   305    75    20   3.51 0.6630
WVFGRD96   15.0   305    70    15   3.52 0.6555
WVFGRD96   16.0   305    70    15   3.53 0.6470
WVFGRD96   17.0   120    90   -20   3.54 0.6301
WVFGRD96   18.0   305    70    15   3.54 0.6228
WVFGRD96   19.0   305    60    10   3.56 0.6089
WVFGRD96   20.0   305    55    10   3.58 0.5948
WVFGRD96   21.0   305    50    10   3.60 0.5813
WVFGRD96   22.0   310    45    20   3.62 0.5679
WVFGRD96   23.0   315    35    30   3.67 0.5553
WVFGRD96   24.0   315    35    30   3.68 0.5457
WVFGRD96   25.0   315    30    30   3.71 0.5374
WVFGRD96   26.0   320    25    35   3.74 0.5316
WVFGRD96   27.0   320    25    35   3.75 0.5255
WVFGRD96   28.0   315    25    30   3.76 0.5193
WVFGRD96   29.0   320    20    35   3.79 0.5140

The best solution is

WVFGRD96   10.0   305    80    25   3.47 0.6758

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 0.03 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 Oct 16 09:05:54 CDT 2010

Last Changed 2010/10/16