Location

SLU Location

Arrival times and polarities were read from NetQuake and Transportable Array stations and located using the program elocate and the WUS velocity model listed below. The output of the program is elocate.txt. The azimuths and take-off angles from the program are used to plot the observed P-wave first motion data together with the waveform inversion nodal focal mechanism.

The SLU location is

2010/09/19 22:01:46.452 35.615 -97.247 5.87 km

NEIC Location

2010/09/19 22:01:47 35.595 -97.207 5.0 3.90 Oklahoma

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  2010/09/19 22:01:47:0  35.60  -97.21   5.0 3.9 Oklahoma
 
 Stations used:
   AG.HHAR TA.S35A TA.T30A TA.T34A TA.TUL1 TA.U33A TA.U34A 
   TA.V33A TA.V34A TA.V35A TA.W33A TA.W35A TA.W36A TA.W37A 
   TA.W38A TA.X34A TA.X35A TA.X36A TA.X37A TA.X38A TA.Y34A 
   TA.Y35A TA.Y37A TA.Y39A TA.Z35A 
 
 Filtering commands used:
   hp c 0.02 n 4
   lp c 0.08 n 4
 
 Best Fitting Double Couple
  Mo = 2.02e+21 dyne-cm
  Mw = 3.47 
  Z  = 3 km
  Plane   Strike  Dip  Rake
   NP1      195    90   160
   NP2      285    70     0
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.02e+21     14     148
    N   0.00e+00     70      15
    P  -2.02e+21     14     242

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     9.48e+20
       Mxy    -1.64e+21
       Mxz    -1.79e+20
       Myy    -9.48e+20
       Myz     6.67e+20
       Mzz     0.00e+00
                                                     
                                                     
                                                     
                                                     
                     ############--                  
                 ################------              
              ##################----------           
             ###################-----------          
           ####################--------------        
          #####################---------------       
         #####################-----------------      
        ####---------#########------------------     
        ---------------------#------------------     
       ----------------------#######-------------    
       ---------------------############---------    
       ---------------------###############------    
       --------------------##################----    
        -------------------####################-     
        ---   ------------######################     
         -- P ------------#####################      
          -   -----------#####################       
           --------------####################        
             -----------############   ####          
              ----------############ T ###           
                 ------#############                 
                     --############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  0.00e+00  -1.79e+20  -6.67e+20 
 -1.79e+20   9.48e+20   1.64e+21 
 -6.67e+20   1.64e+21  -9.48e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100919220147/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 = 70
     RAKE = 0
       MW = 3.47
       HS = 3.0

The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
SLUFM
 USGS/SLU Moment Tensor Solution
 ENS  2010/09/19 22:01:47:0  35.60  -97.21   5.0 3.9 Oklahoma
 
 Stations used:
   AG.HHAR TA.S35A TA.T30A TA.T34A TA.TUL1 TA.U33A TA.U34A 
   TA.V33A TA.V34A TA.V35A TA.W33A TA.W35A TA.W36A TA.W37A 
   TA.W38A TA.X34A TA.X35A TA.X36A TA.X37A TA.X38A TA.Y34A 
   TA.Y35A TA.Y37A TA.Y39A TA.Z35A 
 
 Filtering commands used:
   hp c 0.02 n 4
   lp c 0.08 n 4
 
 Best Fitting Double Couple
  Mo = 2.02e+21 dyne-cm
  Mw = 3.47 
  Z  = 3 km
  Plane   Strike  Dip  Rake
   NP1      195    90   160
   NP2      285    70     0
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.02e+21     14     148
    N   0.00e+00     70      15
    P  -2.02e+21     14     242

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     9.48e+20
       Mxy    -1.64e+21
       Mxz    -1.79e+20
       Myy    -9.48e+20
       Myz     6.67e+20
       Mzz     0.00e+00
                                                     
                                                     
                                                     
                                                     
                     ############--                  
                 ################------              
              ##################----------           
             ###################-----------          
           ####################--------------        
          #####################---------------       
         #####################-----------------      
        ####---------#########------------------     
        ---------------------#------------------     
       ----------------------#######-------------    
       ---------------------############---------    
       ---------------------###############------    
       --------------------##################----    
        -------------------####################-     
        ---   ------------######################     
         -- P ------------#####################      
          -   -----------#####################       
           --------------####################        
             -----------############   ####          
              ----------############ T ###           
                 ------#############                 
                     --############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  0.00e+00  -1.79e+20  -6.67e+20 
 -1.79e+20   9.48e+20   1.64e+21 
 -6.67e+20   1.64e+21  -9.48e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100919220147/index.html
	
First motions plot using the waveform inversion nodal planes and the elcoate
takeoff angles and azimuths. Symbols: o strong compression, + weak compression, Delta 
strong dilatation, - weak dilatation, X undetermined polarity.

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 4
lp c 0.08 n 4
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    50     5   3.42 0.3902
WVFGRD96    1.0   285    65     5   3.39 0.4050
WVFGRD96    2.0   285    55     0   3.47 0.4386
WVFGRD96    3.0   285    70     0   3.47 0.4502
WVFGRD96    4.0   285    70     0   3.49 0.4498
WVFGRD96    5.0   285    75    10   3.50 0.4441
WVFGRD96    6.0   285    80    15   3.52 0.4377
WVFGRD96    7.0   285    80    15   3.53 0.4315
WVFGRD96    8.0   285    80    20   3.56 0.4247
WVFGRD96    9.0   105    60     5   3.59 0.4185
WVFGRD96   10.0   105    65    10   3.59 0.4160
WVFGRD96   11.0   105    65    10   3.61 0.4136
WVFGRD96   12.0   105    65    10   3.62 0.4110
WVFGRD96   13.0   105    65    10   3.63 0.4075
WVFGRD96   14.0   105    65    10   3.64 0.4042
WVFGRD96   15.0   105    65     5   3.65 0.4001
WVFGRD96   16.0   105    65    10   3.66 0.3955
WVFGRD96   17.0   105    65    10   3.67 0.3903
WVFGRD96   18.0   105    65    10   3.68 0.3847
WVFGRD96   19.0   105    65    10   3.69 0.3787
WVFGRD96   20.0   105    70    10   3.69 0.3724
WVFGRD96   21.0   110    60    25   3.71 0.3669
WVFGRD96   22.0   110    60    25   3.72 0.3616
WVFGRD96   23.0   110    60    25   3.73 0.3561
WVFGRD96   24.0   110    60    25   3.74 0.3503
WVFGRD96   25.0   110    60    25   3.75 0.3441
WVFGRD96   26.0   110    60    25   3.76 0.3376
WVFGRD96   27.0   110    60    25   3.77 0.3306
WVFGRD96   28.0   110    60    30   3.78 0.3234
WVFGRD96   29.0   110    65    30   3.78 0.3163

The best solution is

WVFGRD96    3.0   285    70     0   3.47 0.4502

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 4
lp c 0.08 n 4
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.

Comparison of strong motion recordings

This earthquake was recorded by NetQuake accelerographs in the epicentral area. Although this earthquake was too small for the accelerometer signals to be low-pass filtered for use in the broadband moment tensor inversion, we were able to filter the traces near 1 Hz and comapre them to Grene's functions predicted using the moment tensor solution. This was done with care:

Both the predicted and observed ground velocities were filtered using the gsac command:

     hp c 0.25 n 3
     lp c 1.00 n 3

In addition, since the Green's functions have a triangular source pulse of duration 1 second, the following command is applied to the observed trace
     triangle w 1.0


Comparison of filtered observed (red) and predicted (blue) ground velocities in units of m/s. In general there is a agreement in the corresponding amplitudes. Even at these frequencies, the low frequency noise is significant. The difference in amplitude at short distance may be due to differences in distance and azimuth between the true location and the assumed NEIC location.

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 WUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

MODEL.01
Model after     8 iterations
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
     1.9000     3.4065     2.0089     2.2150  0.302E-02  0.679E-02   0.00       0.00       1.00       1.00    
     6.1000     5.5445     3.2953     2.6089  0.349E-02  0.784E-02   0.00       0.00       1.00       1.00    
    13.0000     6.2708     3.7396     2.7812  0.212E-02  0.476E-02   0.00       0.00       1.00       1.00    
    19.0000     6.4075     3.7680     2.8223  0.111E-02  0.249E-02   0.00       0.00       1.00       1.00    
     0.0000     7.9000     4.6200     3.2760  0.164E-10  0.370E-10   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=Mon Sep 20 20:04:28 CDT 2010

Last Changed 2010/09/19