Location

Location ANSS

The ANSS event ID is usp000jkhb and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000jkhb/executive.

2012/05/17 08:12:01 31.926 -94.369 5.0 4.8 Texas

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2012/05/17 08:12:01:0  31.93  -94.37   5.0 4.8 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 = 1.93e+23 dyne-cm
  Mw = 4.79 
  Z  = 6 km
  Plane   Strike  Dip  Rake
   NP1      240    90   -165
   NP2      150    75     0
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   1.93e+23     11      14
    N   0.00e+00     75     240
    P  -1.93e+23     11     106

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     1.61e+23
       Mxy     9.31e+22
       Mxz     4.32e+22
       Myy    -1.61e+23
       Myz    -2.49e+22
       Mzz     0.00e+00
                                                     
                                                     
                                                     
                                                     
                     ##########   #                  
                 -############# T #####              
              ----#############   ########           
             -----#########################          
           --------##########################        
          ---------###########################       
         -----------######################-----      
        -------------##################---------     
        -------------###############------------     
       ---------------###########----------------    
       ----------------#######-------------------    
       -----------------##-----------------------    
       ----------------##--------------------   -    
        ------------######------------------- P      
        ---------###########-----------------        
         -----###############------------------      
          ####################----------------       
           #####################-------------        
             ####################----------          
              #####################-------           
                 #####################-              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
  0.00e+00   4.32e+22   2.49e+22 
  4.32e+22   1.61e+23  -9.31e+22 
  2.49e+22  -9.31e+22  -1.61e+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 or first motion observations is 

      STK = 150
      DIP = 75
     RAKE = 0
       MW = 4.79
       HS = 6.0

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

Magnitudes

Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.

mLg Magnitude


Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.

ML Magnitude


Left: ML computed using the IASPEI formula for Horizontal components. Center: 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. Right: Residuals from new relation as a function of distance and azimuth.


Left: ML computed using the IASPEI formula for Vertical components (research). Center: 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. Right: Residuals from new relation as a function of distance and azimuth.

Context

The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. 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). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.

Waveform Inversion using wvfgrd96

The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) 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's 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 are as follow: 

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   325    65    -5   4.44 0.2041
WVFGRD96    2.0   330    80     5   4.58 0.3133
WVFGRD96    3.0   330    75     5   4.66 0.3737
WVFGRD96    4.0   150    90    -5   4.71 0.4052
WVFGRD96    5.0   150    80     0   4.76 0.4250
WVFGRD96    6.0   150    75     0   4.79 0.4310
WVFGRD96    7.0   150    75     0   4.82 0.4298
WVFGRD96    8.0   150    65     0   4.86 0.4230
WVFGRD96    9.0   150    65     0   4.88 0.4123
WVFGRD96    0.0     0     8     0   0.00-2.0000
WVFGRD96    0.0     0     8     0   0.00-2.0000
WVFGRD96    0.0     0     8     0   0.00-2.0000
WVFGRD96    0.0     0     8     0   0.00-2.0000

The best solution is 

WVFGRD96    6.0   150    75     0   4.79 0.4310

The mechanism corresponding 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, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. 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. The time scale is relative to the first trace sample.

Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. 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

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