Revision 23 JUN 2016

A new model was created for the source region and the solution recomputed with better results. Details are given in TimpsonTX.pdf.

Location

2012/05/17 08:12:01 31.902 -94.332 5.0 4.30 Texas

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  2012/05/17 08:12:01:0  31.90  -94.33   5.0 4.3 Texas
 
 Stations used:
   AG.WLAR TA.140A TA.143A TA.240A TA.241A TA.242A TA.243A 
   TA.341A TA.441A TA.ABTX TA.TUL1 TA.W40A TA.X40A TA.X41A 
   TA.Y40A TA.Y41A TA.Z40A TA.Z41A TA.Z42A US.NATX 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +90
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.05 n 3 
 
 Best Fitting Double Couple
  Mo = 2.07e+23 dyne-cm
  Mw = 4.81 
  Z  = 5 km
  Plane   Strike  Dip  Rake
   NP1      240    90   -150
   NP2      150    60     0
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.07e+23     21      11
    N   0.00e+00     60     240
    P  -2.07e+23     21     109

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.55e+23
       Mxy     8.94e+22
       Mxz     8.94e+22
       Myy    -1.55e+23
       Myz    -5.16e+22
       Mzz     0.00e+00
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 #############   ######              
              ---############# T #########           
             ----#############   ##########          
           ------############################        
          -------#############################       
         --------#########################-----      
        ---------######################---------     
        ----------##################------------     
       -----------###############----------------    
       ------------###########-------------------    
       -------------######-----------------------    
       --------------##--------------------------    
        ------------##--------------------   ---     
        ---------######------------------- P ---     
         -----##########------------------   --      
          ################--------------------       
           #################-----------------        
             #################-------------          
              ###################---------           
                 ####################--              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  0.00e+00   8.94e+22   5.16e+22 
  8.94e+22   1.55e+23  -8.94e+22 
  5.16e+22  -8.94e+22  -1.55e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120517081201/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 = 150
      DIP = 60
     RAKE = 0
       MW = 4.81
       HS = 5.0

The NDK file is 20120517081201.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
 USGS/SLU Moment Tensor Solution
 ENS  2012/05/17 08:12:01:0  31.90  -94.33   5.0 4.3 Texas
 
 Stations used:
   AG.WLAR TA.140A TA.143A TA.240A TA.241A TA.242A TA.243A 
   TA.341A TA.441A TA.ABTX TA.TUL1 TA.W40A TA.X40A TA.X41A 
   TA.Y40A TA.Y41A TA.Z40A TA.Z41A TA.Z42A US.NATX 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +90
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.05 n 3 
 
 Best Fitting Double Couple
  Mo = 2.07e+23 dyne-cm
  Mw = 4.81 
  Z  = 5 km
  Plane   Strike  Dip  Rake
   NP1      240    90   -150
   NP2      150    60     0
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   2.07e+23     21      11
    N   0.00e+00     60     240
    P  -2.07e+23     21     109

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.55e+23
       Mxy     8.94e+22
       Mxz     8.94e+22
       Myy    -1.55e+23
       Myz    -5.16e+22
       Mzz     0.00e+00
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 #############   ######              
              ---############# T #########           
             ----#############   ##########          
           ------############################        
          -------#############################       
         --------#########################-----      
        ---------######################---------     
        ----------##################------------     
       -----------###############----------------    
       ------------###########-------------------    
       -------------######-----------------------    
       --------------##--------------------------    
        ------------##--------------------   ---     
        ---------######------------------- P ---     
         -----##########------------------   --      
          ################--------------------       
           #################-----------------        
             #################-------------          
              ###################---------           
                 ####################--              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  0.00e+00   8.94e+22   5.16e+22 
  8.94e+22   1.55e+23  -8.94e+22 
  5.16e+22  -8.94e+22  -1.55e+23 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120517081201/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 using wvfgrd96

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:

cut o DIST/3.3 -30 o DIST/3.3 +90
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.05 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    1.0   150    80     5   4.48 0.3494
WVFGRD96    2.0   330    90     0   4.62 0.4873
WVFGRD96    3.0   150    70     0   4.70 0.5492
WVFGRD96    4.0   150    60     0   4.76 0.5701
WVFGRD96    5.0   150    60     0   4.81 0.5719
WVFGRD96    6.0   150    65     0   4.83 0.5586
WVFGRD96    7.0   150    60     0   4.86 0.5378
WVFGRD96    8.0   150    55     0   4.89 0.5134
WVFGRD96    9.0   150    55     0   4.90 0.4877
WVFGRD96   10.0   150    60     0   4.91 0.4631
WVFGRD96   11.0   330    90    20   4.90 0.4412
WVFGRD96   12.0   150    90   -15   4.91 0.4235
WVFGRD96   13.0   330    90    15   4.92 0.4063

The best solution is

WVFGRD96    5.0   150    60     0   4.81 0.5719

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

cut o DIST/3.3 -30 o DIST/3.3 +90
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.05 n 3 
Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated.
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.

Surface-Wave Focal Mechanism

The following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
Location of broadband stations used to obtain focal mechanism from surface-wave spectral amplitudes

The surface-wave determined focal mechanism is shown here.


  NODAL PLANES 

  
  STK=      57.36
  DIP=      77.05
 RAKE=     149.13
  
             OR
  
  STK=     154.99
  DIP=      60.00
 RAKE=      15.00
 
 
DEPTH = 6.0 km
 
Mw = 4.94
Best Fit 0.7621 - P-T axis plot gives solutions with FIT greater than FIT90

First motion data

The P-wave first motion data for focal mechanism studies are as follow:

Sta Az    Dist   First motion

Surface-wave analysis

Surface wave analysis was performed using codes from Computer Programs in Seismology, specifically the multiple filter analysis program do_mft and the surface-wave radiation pattern search program srfgrd96.

Data preparation

Digital data were collected, instrument response removed and traces converted to Z, R an T components. Multiple filter analysis was applied to the Z and T traces to obtain the Rayleigh- and Love-wave spectral amplitudes, respectively. These were input to the search program which examined all depths between 1 and 25 km and all possible mechanisms.
Best mechanism fit as a function of depth. The preferred depth is given above. Lower hemisphere projection

Pressure-tension axis trends. Since the surface-wave spectra search does not distinguish between P and T axes and since there is a 180 ambiguity in strike, all possible P and T axes are plotted. First motion data and waveforms will be used to select the preferred mechanism. The purpose of this plot is to provide an idea of the possible range of solutions. The P and T-axes for all mechanisms with goodness of fit greater than 0.9 FITMAX (above) are plotted here.


Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the Love and Rayleigh wave radiation patterns. 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. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0-180 degrees are sampled.

Love-wave radiation patterns

Rayleigh-wave radiation patterns

Broadband station distribution

The distribution of broadband stations with azimuth and distance is
Listing of broadband stations used

Waveform comparison for this mechanism

Since the analysis of the surface-wave radiation patterns uses only spectral amplitudes and because the surfave-wave radiation patterns have a 180 degree symmetry, each surface-wave solution consists of four possible focal mechanisms corresponding to the interchange of the P- and T-axes and a roation of the mechanism by 180 degrees. To select one mechanism, P-wave first motion can be used. This was not possible in this case because all the P-wave first motions were emergent ( a feature of the P-wave wave takeoff angle, the station location and the mechanism). The other way to select among the mechanisms is to compute forward synthetics and compare the observed and predicted waveforms.

The fits to the waveforms with the given mechanism are show below:

This figure shows the fit to the three components of motion (Z - vertical, R-radial and T - transverse). For each station and component, the observed traces is shown in red and the model predicted trace in blue. The traces represent filtered ground velocity in units of meters/sec (the peak value is printed adjacent to each trace; each pair of traces to plotted to the same scale to emphasize the difference in levels). Both synthetic and observed traces have been filtered using the SAC commands:

hp c 0.01 n 4
lp c 0.025 n 4

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.

Appendix A


Spectra fit plots to each station

Velocity Model

The WUS model was sued for the suface wave study. The TTX model used for the waveform synthetic seismograms is as follows:

MODEL.01
Model after   100 iterations
ISOTROPIC
KGS
SPHERICAL 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    
     0.5000     2.9356     1.2494     2.3774   0.00       0.00       0.00       0.00       1.00       1.00    
     0.5000     3.4156     1.5736     2.2162   0.00       0.00       0.00       0.00       1.00       1.00    
     0.5000     3.9006     1.8839     2.3017   0.00       0.00       0.00       0.00       1.00       1.00    
     0.5000     4.3529     2.1458     2.3691   0.00       0.00       0.00       0.00       1.00       1.00    
     0.5000     4.7573     2.3986     2.4463   0.00       0.00       0.00       0.00       1.00       1.00    
     2.5000     5.0056     2.7330     2.5010  0.349E-02  0.784E-02   0.00       0.00       1.00       1.00    
     2.5000     5.3456     3.0909     2.5694  0.349E-02  0.784E-02   0.00       0.00       1.00       1.00    
     7.0000     5.8288     3.4764     2.6661  0.212E-02  0.476E-02   0.00       0.00       1.00       1.00    
     7.0000     6.0550     3.6108     2.7162  0.212E-02  0.476E-02   0.00       0.00       1.00       1.00    
     9.0000     6.6270     3.8965     2.8826  0.111E-02  0.249E-02   0.00       0.00       1.00       1.00    
    10.0000     6.6822     3.9286     2.8977  0.111E-02  0.249E-02   0.00       0.00       1.00       1.00    
    10.0000     8.2235     4.8094     3.3903  0.164E-10  0.370E-10   0.00       0.00       1.00       1.00    
    15.0000     8.0856     4.7281     3.3407  0.164E-10  0.370E-10   0.00       0.00       1.00       1.00    
    25.0000     7.9367     4.6420     3.2885  0.164E-10  0.370E-10   0.00       0.00       1.00       1.00    
    30.0000     7.9215     4.6319     3.2833  0.164E-10  0.370E-10   0.00       0.00       1.00       1.00    
    45.0000     8.1864     4.5458     3.3771  0.855E-02  0.131E-01   0.00       0.00       1.00       1.00    
    45.0000     8.3076     4.5524     3.4205  0.820E-02  0.128E-01   0.00       0.00       1.00       1.00    
    50.0000     8.3425     4.5380     3.4331  0.469E-02  0.741E-02   0.00       0.00       1.00       1.00    
    50.0000     8.3738     4.5444     3.4446  0.459E-02  0.725E-02   0.00       0.00       1.00       1.00    
    50.0000     8.4646     4.5820     3.4775  0.446E-02  0.709E-02   0.00       0.00       1.00       1.00    
    50.0000     8.6510     4.6719     3.5416  0.433E-02  0.690E-02   0.00       0.00       1.00       1.00    
    50.0000     9.2451     5.0263     3.7582  0.386E-02  0.610E-02   0.00       0.00       1.00       1.00    
    50.0000     9.5524     5.2059     3.8798  0.383E-02  0.602E-02   0.00       0.00       1.00       1.00    
    50.0000     9.7207     5.3121     3.9437  0.380E-02  0.595E-02   0.00       0.00       1.00       1.00    
    50.0000    10.0230     5.4925     4.0565  0.377E-02  0.588E-02   0.00       0.00       1.00       1.00    
    50.0000    10.2879     5.6509     4.1358  0.375E-02  0.581E-02   0.00       0.00       1.00       1.00    
     0.0000    11.0777     6.1473     4.4287  0.119E-02  0.183E-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 Thu Jun 23 08:54:52 CDT 2016