Location

2010/10/15 10:20:19 35.276 -92.322 5.0 4.40 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  2010/10/15 10:20:19:0  35.28  -92.32   5.0 4.4 Arkansas
 
 Stations used:
   AG.FCAR AG.LCAR AG.WLAR IU.CCM IU.WVT NM.MGMO NM.MPH 
   NM.OLIL NM.SIUC NM.SLM NM.UALR NM.USIN NM.UTMT TA.139A 
   TA.O36A TA.P35A TA.P36A TA.Q34A TA.Q36A TA.Q37A TA.R35A 
   TA.R36A TA.S35A TA.S36A TA.T34A TA.T35A TA.T36A TA.T37A 
   TA.TUL1 TA.V34A TA.V35A TA.W34A TA.W35A TA.W36A TA.W37A 
   TA.W38A TA.X37A TA.X38A TA.Y37A TA.Y38A TA.Y39A TA.Z37A 
   TA.Z38A TA.Z39A US.KSU1 US.MIAR US.OXF 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
 
 Best Fitting Double Couple
  Mo = 7.00e+21 dyne-cm
  Mw = 3.83 
  Z  = 5 km
  Plane   Strike  Dip  Rake
   NP1      211    85   -170
   NP2      120    80    -5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.00e+21      4     345
    N   0.00e+00     79     237
    P  -7.00e+21     11      76

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     6.10e+21
       Mxy    -3.34e+21
       Mxz     1.09e+20
       Myy    -5.89e+21
       Myz    -1.34e+21
       Mzz    -2.09e+20
                                                     
                                                     
                                                     
                                                     
                      T ###########                  
                 ####   ##############-              
              #######################-----           
             #######################-------          
           #######################-----------        
          #######################-------------       
         ---####################---------------      
        ------#################--------------        
        --------#############---------------- P      
       ------------#########-----------------   -    
       ---------------#####----------------------    
       ------------------#-----------------------    
       ------------------###---------------------    
        ----------------#######-----------------     
        ---------------############-------------     
         -------------#################--------      
          -----------########################-       
           ---------#########################        
             ------########################          
              ----########################           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.09e+20   1.09e+20   1.34e+21 
  1.09e+20   6.10e+21   3.34e+21 
  1.34e+21   3.34e+21  -5.89e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20101015102019/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 = 120
      DIP = 80
     RAKE = -5
       MW = 3.83
       HS = 5.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
USGSMT
 USGS/SLU Moment Tensor Solution
 ENS  2010/10/15 10:20:19:0  35.28  -92.32   5.0 4.4 Arkansas
 
 Stations used:
   AG.FCAR AG.LCAR AG.WLAR IU.CCM IU.WVT NM.MGMO NM.MPH 
   NM.OLIL NM.SIUC NM.SLM NM.UALR NM.USIN NM.UTMT TA.139A 
   TA.O36A TA.P35A TA.P36A TA.Q34A TA.Q36A TA.Q37A TA.R35A 
   TA.R36A TA.S35A TA.S36A TA.T34A TA.T35A TA.T36A TA.T37A 
   TA.TUL1 TA.V34A TA.V35A TA.W34A TA.W35A TA.W36A TA.W37A 
   TA.W38A TA.X37A TA.X38A TA.Y37A TA.Y38A TA.Y39A TA.Z37A 
   TA.Z38A TA.Z39A US.KSU1 US.MIAR US.OXF 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.10 n 3
 
 Best Fitting Double Couple
  Mo = 7.00e+21 dyne-cm
  Mw = 3.83 
  Z  = 5 km
  Plane   Strike  Dip  Rake
   NP1      211    85   -170
   NP2      120    80    -5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.00e+21      4     345
    N   0.00e+00     79     237
    P  -7.00e+21     11      76

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     6.10e+21
       Mxy    -3.34e+21
       Mxz     1.09e+20
       Myy    -5.89e+21
       Myz    -1.34e+21
       Mzz    -2.09e+20
                                                     
                                                     
                                                     
                                                     
                      T ###########                  
                 ####   ##############-              
              #######################-----           
             #######################-------          
           #######################-----------        
          #######################-------------       
         ---####################---------------      
        ------#################--------------        
        --------#############---------------- P      
       ------------#########-----------------   -    
       ---------------#####----------------------    
       ------------------#-----------------------    
       ------------------###---------------------    
        ----------------#######-----------------     
        ---------------############-------------     
         -------------#################--------      
          -----------########################-       
           ---------#########################        
             ------########################          
              ----########################           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.09e+20   1.09e+20   1.34e+21 
  1.09e+20   6.10e+21   3.34e+21 
  1.34e+21   3.34e+21  -5.89e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20101015102019/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
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   120    85     0   3.69 0.3380
WVFGRD96    1.0   300    85   -10   3.72 0.3634
WVFGRD96    2.0   120    70   -10   3.78 0.4168
WVFGRD96    3.0   120    70   -10   3.81 0.4409
WVFGRD96    4.0   120    70   -10   3.83 0.4486
WVFGRD96    5.0   120    80    -5   3.83 0.4591
WVFGRD96    6.0   120    80    -5   3.84 0.4535
WVFGRD96    7.0   120    85    -5   3.84 0.4518
WVFGRD96    8.0   120    85    -5   3.85 0.4436
WVFGRD96    9.0   300    90     5   3.86 0.4402
WVFGRD96   10.0   300    90     5   3.88 0.4376
WVFGRD96   11.0   305    75    10   3.89 0.4351
WVFGRD96   12.0   305    75    10   3.90 0.4326
WVFGRD96   13.0   305    75    10   3.91 0.4288
WVFGRD96   14.0   305    75    10   3.92 0.4243
WVFGRD96   15.0   305    75    10   3.93 0.4186
WVFGRD96   16.0   305    75    10   3.94 0.4120
WVFGRD96   17.0   305    75    10   3.94 0.4043
WVFGRD96   18.0   305    75    10   3.95 0.3960
WVFGRD96   19.0   305    75    10   3.96 0.3879
WVFGRD96   20.0   305    70    10   3.97 0.3796
WVFGRD96   21.0   305    70    10   3.98 0.3705
WVFGRD96   22.0   305    70    10   3.98 0.3610
WVFGRD96   23.0   305    70    10   3.99 0.3522
WVFGRD96   24.0   305    70    10   3.99 0.3426
WVFGRD96   25.0   305    70    10   3.99 0.3340
WVFGRD96   26.0   305    70    10   4.00 0.3265
WVFGRD96   27.0   305    70    10   4.00 0.3187
WVFGRD96   28.0   305    70    10   4.00 0.3114
WVFGRD96   29.0   305    60     5   4.02 0.3069

The best solution is

WVFGRD96    5.0   120    80    -5   3.83 0.4591

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
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

A quick QC of data quality is given at the link QC/index.html.

The following stations did not have a valid response files:

DATE=Sat Oct 16 09:07:09 CDT 2010

Last Changed 2010/10/15