Location

2011/02/18 04:59:50 35.251 -92.409 5.0 4.00 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  2011/02/18 04:59:50:0  35.25  -92.41   5.0 4.0 Arkansas
 
 Stations used:
   AG.FCAR AG.HHAR AG.LCAR AG.WHAR AG.WLAR NM.MGMO NM.MPH 
   NM.OLIL NM.PBMO NM.PVMO NM.SIUC NM.SLM NM.UALR NM.USIN 
   NM.UTMT NM.X102 NM.X201 TA.137A TA.138A TA.139A TA.141A 
   TA.238A TA.240A TA.O38A TA.O39A TA.P40A TA.Q39A TA.Q40A 
   TA.R35A TA.R36A TA.R37A TA.R40A TA.S34A TA.S35A TA.S36A 
   TA.S38A TA.S39A TA.S40A TA.T34A TA.T35A TA.T36A TA.T37A 
   TA.T38A TA.T40A TA.TUL1 TA.U34A TA.U35A TA.U36A TA.U38A 
   TA.U39A TA.U40A TA.V33A TA.V34A TA.V35A TA.V36A TA.V37A 
   TA.V38A TA.V39A TA.W34A TA.W35A TA.W36A TA.W37B TA.W39A 
   TA.W40A TA.X34A TA.X36A TA.X37A TA.X38A TA.X39A TA.X40A 
   TA.Y36A TA.Y37A TA.Y38A TA.Y39A TA.Y40A TA.Z38A TA.Z39A 
   TA.Z40A 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 = 8.32e+21 dyne-cm
  Mw = 3.88 
  Z  = 8 km
  Plane   Strike  Dip  Rake
   NP1      201    81   155
   NP2      295    65    10
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   8.32e+21     24     155
    N   0.00e+00     63       2
    P  -8.32e+21     11     250

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.78e+21
       Mxy    -5.20e+21
       Mxz    -2.30e+21
       Myy    -5.88e+21
       Myz     2.75e+21
       Mzz     1.11e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 #################-----              
              ###################---------           
             ###################-----------          
           ####################--------------        
          ####################----------------       
         ---------------#####------------------      
        --------------------#-------------------     
        -------------------######---------------     
       --------------------#########-------------    
       -------------------#############----------    
       ------------------################--------    
       ------------------##################------    
        -   ------------####################----     
        - P -----------#######################--     
            -----------########################      
          ------------########################       
           -----------###########   #########        
             --------############ T #######          
              -------############   ######           
                 ----##################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.11e+21  -2.30e+21  -2.75e+21 
 -2.30e+21   4.78e+21   5.20e+21 
 -2.75e+21   5.20e+21  -5.88e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110218045950/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 = 65
     RAKE = 10
       MW = 3.88
       HS = 8.0

The NDK file is 20110218045950.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2011/02/18 04:59:50:0  35.25  -92.41   5.0 4.0 Arkansas
 
 Stations used:
   AG.FCAR AG.HHAR AG.LCAR AG.WHAR AG.WLAR NM.MGMO NM.MPH 
   NM.OLIL NM.PBMO NM.PVMO NM.SIUC NM.SLM NM.UALR NM.USIN 
   NM.UTMT NM.X102 NM.X201 TA.137A TA.138A TA.139A TA.141A 
   TA.238A TA.240A TA.O38A TA.O39A TA.P40A TA.Q39A TA.Q40A 
   TA.R35A TA.R36A TA.R37A TA.R40A TA.S34A TA.S35A TA.S36A 
   TA.S38A TA.S39A TA.S40A TA.T34A TA.T35A TA.T36A TA.T37A 
   TA.T38A TA.T40A TA.TUL1 TA.U34A TA.U35A TA.U36A TA.U38A 
   TA.U39A TA.U40A TA.V33A TA.V34A TA.V35A TA.V36A TA.V37A 
   TA.V38A TA.V39A TA.W34A TA.W35A TA.W36A TA.W37B TA.W39A 
   TA.W40A TA.X34A TA.X36A TA.X37A TA.X38A TA.X39A TA.X40A 
   TA.Y36A TA.Y37A TA.Y38A TA.Y39A TA.Y40A TA.Z38A TA.Z39A 
   TA.Z40A 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 = 8.32e+21 dyne-cm
  Mw = 3.88 
  Z  = 8 km
  Plane   Strike  Dip  Rake
   NP1      201    81   155
   NP2      295    65    10
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   8.32e+21     24     155
    N   0.00e+00     63       2
    P  -8.32e+21     11     250

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.78e+21
       Mxy    -5.20e+21
       Mxz    -2.30e+21
       Myy    -5.88e+21
       Myz     2.75e+21
       Mzz     1.11e+21
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 #################-----              
              ###################---------           
             ###################-----------          
           ####################--------------        
          ####################----------------       
         ---------------#####------------------      
        --------------------#-------------------     
        -------------------######---------------     
       --------------------#########-------------    
       -------------------#############----------    
       ------------------################--------    
       ------------------##################------    
        -   ------------####################----     
        - P -----------#######################--     
            -----------########################      
          ------------########################       
           -----------###########   #########        
             --------############ T #######          
              -------############   ######           
                 ----##################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  1.11e+21  -2.30e+21  -2.75e+21 
 -2.30e+21   4.78e+21   5.20e+21 
 -2.75e+21   5.20e+21  -5.88e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110218045950/index.html
	

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 0.02 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    80   -25   3.74 0.3794
WVFGRD96    1.0   290    85   -25   3.76 0.4016
WVFGRD96    2.0   110    75     0   3.78 0.4318
WVFGRD96    3.0   110    75     0   3.80 0.4446
WVFGRD96    4.0   110    75    -5   3.82 0.4497
WVFGRD96    5.0   110    70    -5   3.85 0.4487
WVFGRD96    6.0   295    65    10   3.86 0.4472
WVFGRD96    7.0   295    65    10   3.87 0.4510
WVFGRD96    8.0   295    65    10   3.88 0.4520
WVFGRD96    9.0   290    70    10   3.88 0.4512
WVFGRD96   10.0   295    65    10   3.90 0.4496
WVFGRD96   11.0   295    65    10   3.91 0.4464
WVFGRD96   12.0   290    70    10   3.91 0.4436
WVFGRD96   13.0   290    70    10   3.92 0.4404
WVFGRD96   14.0   290    70    10   3.93 0.4372
WVFGRD96   15.0   290    75    15   3.93 0.4339
WVFGRD96   16.0   290    80    15   3.94 0.4305
WVFGRD96   17.0   290    80    15   3.95 0.4267
WVFGRD96   18.0   290    80    15   3.95 0.4220
WVFGRD96   19.0   290    80    15   3.96 0.4168
WVFGRD96   20.0   295    75    20   3.98 0.4113
WVFGRD96   21.0   295    75    20   3.98 0.4053
WVFGRD96   22.0   295    70    20   3.99 0.3995
WVFGRD96   23.0   295    70    20   4.00 0.3932
WVFGRD96   24.0   295    70    20   4.01 0.3867
WVFGRD96   25.0   295    70    15   4.01 0.3801
WVFGRD96   26.0   295    65    15   4.02 0.3732
WVFGRD96   27.0   295    65    15   4.02 0.3661
WVFGRD96   28.0   295    65    15   4.03 0.3592
WVFGRD96   29.0   295    65    15   4.04 0.3520

The best solution is

WVFGRD96    8.0   295    65    10   3.88 0.4520

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

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 19:04:51 CST 2015