Location

2011/02/28 05:17:59 35.2430 -92.4280 5.0 3.80

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  2011/02/28 05:17:59:0  35.24  -92.43   5.0 3.8 
 
 Stations used:
   AG.WHAR NM.UALR NM.X102 NM.X201 TA.T40A TA.U38A TA.U40A 
   TA.V39A TA.W38A TA.W40A TA.X40A TA.Y39A TA.Z39A 
 
 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 = 3.05e+21 dyne-cm
  Mw = 3.59 
  Z  = 4 km
  Plane   Strike  Dip  Rake
   NP1       25    85   175
   NP2      115    85     5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.05e+21      7     340
    N   0.00e+00     83     160
    P  -3.05e+21      0      70

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.28e+21
       Mxy    -1.97e+21
       Mxz     3.50e+20
       Myy    -2.33e+21
       Myz    -1.30e+20
       Mzz     4.62e+19
                                                     
                                                     
                                                     
                                                     
                       ############                  
                 ### T #############---              
              ######   #############------           
             ######################--------          
           #######################-----------        
          ########################------------       
         -#######################-------------       
        -----###################-------------- P     
        --------###############---------------       
       ------------############------------------    
       ----------------#######-------------------    
       --------------------##--------------------    
       ---------------------###------------------    
        -------------------########-------------     
        ------------------##############--------     
         ----------------####################--      
          --------------######################       
           ------------######################        
             ---------#####################          
              -------#####################           
                 ---###################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.62e+19   3.50e+20   1.30e+20 
  3.50e+20   2.28e+21   1.97e+21 
  1.30e+20   1.97e+21  -2.33e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110228051759/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 = 115
      DIP = 85
     RAKE = 5
       MW = 3.59
       HS = 4.0

The NDK file is 20110228051759.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
SLUFM
 USGS/SLU Moment Tensor Solution
 ENS  2011/02/28 05:17:59:0  35.24  -92.43   5.0 3.8 
 
 Stations used:
   AG.WHAR NM.UALR NM.X102 NM.X201 TA.T40A TA.U38A TA.U40A 
   TA.V39A TA.W38A TA.W40A TA.X40A TA.Y39A TA.Z39A 
 
 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 = 3.05e+21 dyne-cm
  Mw = 3.59 
  Z  = 4 km
  Plane   Strike  Dip  Rake
   NP1       25    85   175
   NP2      115    85     5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.05e+21      7     340
    N   0.00e+00     83     160
    P  -3.05e+21      0      70

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.28e+21
       Mxy    -1.97e+21
       Mxz     3.50e+20
       Myy    -2.33e+21
       Myz    -1.30e+20
       Mzz     4.62e+19
                                                     
                                                     
                                                     
                                                     
                       ############                  
                 ### T #############---              
              ######   #############------           
             ######################--------          
           #######################-----------        
          ########################------------       
         -#######################-------------       
        -----###################-------------- P     
        --------###############---------------       
       ------------############------------------    
       ----------------#######-------------------    
       --------------------##--------------------    
       ---------------------###------------------    
        -------------------########-------------     
        ------------------##############--------     
         ----------------####################--      
          --------------######################       
           ------------######################        
             ---------#####################          
              -------#####################           
                 ---###################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.62e+19   3.50e+20   1.30e+20 
  3.50e+20   2.28e+21   1.97e+21 
  1.30e+20   1.97e+21  -2.33e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110228051759/index.html
 USGS/SLU Moment Tensor Solution
 ENS  2011/02/28 05:18:00:0  35.25  -92.36   3.8 3.8 Arkansas
 
 Stations used:
   AG.WHAR NM.UALR NM.X102 NM.X201 TA.T40A TA.U38A TA.U40A 
   TA.V39A TA.W38A TA.W40A TA.X40A TA.Y39A TA.Z39A 
 
 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 = 3.05e+21 dyne-cm
  Mw = 3.59 
  Z  = 4 km
  Plane   Strike  Dip  Rake
   NP1       25    85   175
   NP2      115    85     5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.05e+21      7     340
    N   0.00e+00     83     160
    P  -3.05e+21      0      70

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.28e+21
       Mxy    -1.97e+21
       Mxz     3.50e+20
       Myy    -2.33e+21
       Myz    -1.30e+20
       Mzz     4.62e+19
                                                     
                                                     
                                                     
                                                     
                       ############                  
                 ### T #############---              
              ######   #############------           
             ######################--------          
           #######################-----------        
          ########################------------       
         -#######################-------------       
        -----###################-------------- P     
        --------###############---------------       
       ------------############------------------    
       ----------------#######-------------------    
       --------------------##--------------------    
       ---------------------###------------------    
        -------------------########-------------     
        ------------------##############--------     
         ----------------####################--      
          --------------######################       
           ------------######################        
             ---------#####################          
              -------#####################           
                 ---###################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  4.62e+19   3.50e+20   1.30e+20 
  3.50e+20   2.28e+21   1.97e+21 
  1.30e+20   1.97e+21  -2.33e+21 


Details of the solution is found at

http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20110228051800/index.html
	


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

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   295    35    15   3.53 0.5152
WVFGRD96    1.0   290    45    10   3.51 0.5316
WVFGRD96    2.0   280    55   -30   3.56 0.5736
WVFGRD96    3.0   295    90   -10   3.53 0.5962
WVFGRD96    4.0   115    85     5   3.59 0.6069
WVFGRD96    5.0   115    90     5   3.62 0.5991
WVFGRD96    6.0   295    85     0   3.65 0.5848
WVFGRD96    7.0   295    70     0   3.66 0.5629
WVFGRD96    8.0   295    65     0   3.68 0.5422
WVFGRD96    9.0   295    65     0   3.71 0.5197
WVFGRD96   10.0   295    55     5   3.74 0.4991
WVFGRD96   11.0   295    55     5   3.76 0.4742
WVFGRD96   12.0   295    55     0   3.76 0.4514
WVFGRD96   13.0   290    55     0   3.77 0.4297
WVFGRD96   14.0   290    55     0   3.78 0.4124
WVFGRD96   15.0   290    55     0   3.79 0.3965
WVFGRD96   16.0   290    55    -5   3.78 0.3827
WVFGRD96   17.0   290    55    -5   3.79 0.3721
WVFGRD96   18.0   290    55    -5   3.80 0.3628
WVFGRD96   19.0   290    55    -5   3.81 0.3543
WVFGRD96   20.0   290    50    -5   3.83 0.3462
WVFGRD96   21.0   290    50    -5   3.84 0.3389
WVFGRD96   22.0   290    50    -5   3.84 0.3335
WVFGRD96   23.0   290    50    -5   3.85 0.3286
WVFGRD96   24.0   290    50    -5   3.86 0.3241
WVFGRD96   25.0   285    45    -5   3.86 0.3205
WVFGRD96   26.0   285    45    -5   3.87 0.3171
WVFGRD96   27.0   285    45    -5   3.88 0.3140
WVFGRD96   28.0   285    45   -10   3.88 0.3120
WVFGRD96   29.0   285    45   -10   3.89 0.3101

The best solution is

WVFGRD96    4.0   115    85     5   3.59 0.6069

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

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 Dec 6 19:26:25 CST 2015