Location

SLU Location

To check the ANSS location or to compare the observed P-wave first motions to the moment tensor solution, P- and S-wave first arrival times were manually read together with the P-wave first motions. The subsequent output of the program elocate is given in the file elocate.txt. The first motion plot is shown below.

Location ANSS

The ANSS event ID is usp000h78h and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000h78h/executive.

2010/02/13 05:30:55 35.530 -97.272 8.1 3.2 Oklahoma

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2010/02/13 05:30:55:0  35.53  -97.27   8.1 3.2 Oklahoma
 
 Stations used:
   GS.OK001 GS.OK002 GS.OK003 GS.OK004 GS.OK005 GS.OK006 
 
 Filtering commands used:
   hp c 0.50 n 2
   lp c 1.00 n 2
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 7.16e+20 dyne-cm
  Mw = 3.17 
  Z  = 5 km
  Plane   Strike  Dip  Rake
   NP1       57    80   -170
   NP2      325    80   -10
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.16e+20      0     191
    N   0.00e+00     76     100
    P  -7.16e+20     14     281

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     6.67e+20
       Mxy     2.58e+20
       Mxz    -3.33e+19
       Myy    -6.24e+20
       Myz     1.66e+20
       Mzz    -4.25e+19
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ----########################           
             -------#######################          
           -----------#######################        
          -------------#####################--       
         ----------------##################----      
        ------------------###############-------     
        -   ----------------###########---------     
       -- P -----------------#######-------------    
       --   ------------------####---------------    
       ------------------------#-----------------    
       ---------------------#####----------------    
        -----------------#########--------------     
        --------------#############-------------     
         ----------#################-----------      
          ----#######################---------       
           ###########################-------        
             ##########################----          
              ##########################--           
                 ######   #############              
                     ## T #########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.25e+19  -3.33e+19  -1.66e+20 
 -3.33e+19   6.67e+20  -2.58e+20 
 -1.66e+20  -2.58e+20  -6.24e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100213053055/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 = 325
      DIP = 80
     RAKE = -10
       MW = 3.17
       HS = 5.0

The NDK file is 20100213053055.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
SLU
SLUFM
 USGS/SLU Moment Tensor Solution
 ENS  2010/02/13 05:30:55:0  35.53  -97.27   8.1 3.2 Oklahoma
 
 Stations used:
   GS.OK001 GS.OK002 GS.OK003 GS.OK004 GS.OK005 GS.OK006 
 
 Filtering commands used:
   hp c 0.50 n 2
   lp c 1.00 n 2
   br c 0.12 0.25 n 4 p 2
 
 Best Fitting Double Couple
  Mo = 7.16e+20 dyne-cm
  Mw = 3.17 
  Z  = 5 km
  Plane   Strike  Dip  Rake
   NP1       57    80   -170
   NP2      325    80   -10
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   7.16e+20      0     191
    N   0.00e+00     76     100
    P  -7.16e+20     14     281

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     6.67e+20
       Mxy     2.58e+20
       Mxz    -3.33e+19
       Myy    -6.24e+20
       Myz     1.66e+20
       Mzz    -4.25e+19
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ######################              
              ----########################           
             -------#######################          
           -----------#######################        
          -------------#####################--       
         ----------------##################----      
        ------------------###############-------     
        -   ----------------###########---------     
       -- P -----------------#######-------------    
       --   ------------------####---------------    
       ------------------------#-----------------    
       ---------------------#####----------------    
        -----------------#########--------------     
        --------------#############-------------     
         ----------#################-----------      
          ----#######################---------       
           ###########################-------        
             ##########################----          
              ##########################--           
                 ######   #############              
                     ## T #########                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -4.25e+19  -3.33e+19  -1.66e+20 
 -3.33e+19   6.67e+20  -2.58e+20 
 -1.66e+20  -2.58e+20  -6.24e+20 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20100213053055/index.html
	
First motions plotted with waveform inversion 
nodal planes. Take-off angles are from WUS model and
elocate run.

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.

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:

hp c 0.50 n 2
lp c 1.00 n 2
br c 0.12 0.25 n 4 p 2
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   150    90     0   2.42 0.2947
WVFGRD96    1.0    20    80     0   2.47 0.3078
WVFGRD96    2.0   290    55   -25   2.73 0.1387
WVFGRD96    3.0   325    80   -20   3.03 0.2812
WVFGRD96    4.0   325    80   -15   3.13 0.4226
WVFGRD96    5.0   325    80   -10   3.17 0.4562
WVFGRD96    6.0   325    75    -5   3.18 0.4549
WVFGRD96    7.0   325    65     0   3.19 0.4430
WVFGRD96    8.0   330    60     0   3.23 0.4255
WVFGRD96    9.0   330    65     0   3.29 0.4370
WVFGRD96   10.0   330    65     0   3.32 0.4253
WVFGRD96   11.0   330    65     0   3.34 0.4072
WVFGRD96   12.0   330    70     5   3.36 0.3641
WVFGRD96   13.0   335    70     5   3.36 0.3139
WVFGRD96   14.0   140    90   -30   3.38 0.2995
WVFGRD96   15.0   340    55    15   3.23 0.1142
WVFGRD96   16.0   340    25    20   3.12 0.0631
WVFGRD96   17.0   345    25    25   3.12 0.0608
WVFGRD96   18.0   335    30     5   3.10 0.0457
WVFGRD96   19.0   110    75     0   3.22 0.0370

The best solution is

WVFGRD96    5.0   325    80   -10   3.17 0.4562

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

hp c 0.50 n 2
lp c 1.00 n 2
br c 0.12 0.25 n 4 p 2
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.

Velocity Model

The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).

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    
Last Changed Sat Apr 27 11:23:29 AM CDT 2024