Location

Location ANSS

The ANSS event ID is usp000jafa and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000jafa/executive.

2011/11/06 15:07:05 35.484 -96.856 5.0 3.8 Oklahoma

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2011/11/06 15:07:05:0  35.48  -96.86   5.0 3.8 Oklahoma
 
 Stations used:
   AG.HHAR TA.Q34A TA.Q35A TA.Q36A TA.R34A TA.R35A TA.R36A 
   TA.R37A TA.R38A TA.S34A TA.S35A TA.S36A TA.S37A TA.S38A 
   TA.S39A TA.T34A TA.T35A TA.T36A TA.T38A TA.T39A TA.TUL1 
   TA.U32A TA.U35A TA.U36A TA.U37A TA.U38A TA.U40A TA.V35A 
   TA.V36A TA.V37A TA.V38A TA.V39A TA.V40A TA.W35A TA.W36A 
   TA.W37B TA.W38A TA.W39A TA.W40A TA.X35A TA.X36A TA.X38A 
   TA.X39A TA.Y35A TA.Y36A TA.Y37A TA.Y39A TA.Y40A TA.Z37A 
   US.KSU1 US.MIAR 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 5.13e+21 dyne-cm
  Mw = 3.74 
  Z  = 3 km
  Plane   Strike  Dip  Rake
   NP1      320    85   -15
   NP2       51    75   -175
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.13e+21      7       7
    N   0.00e+00     74     122
    P  -5.13e+21     14     275

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.96e+21
       Mxy     9.70e+20
       Mxz     5.10e+20
       Myy    -4.72e+21
       Myz     1.28e+21
       Mzz    -2.30e+20
                                                     
                                                     
                                                     
                                                     
                     ######## T ###                  
                 ############   #######              
              -###########################           
             ----##########################          
           --------#########################-        
          -----------######################---       
         -------------####################-----      
        ----------------################--------     
        ------------------#############---------     
       -   ----------------##########------------    
       - P ------------------######--------------    
       -   -------------------###----------------    
       ------------------------#-----------------    
        --------------------#####---------------     
        ------------------#########-------------     
         --------------#############-----------      
          ---------###################--------       
           ----########################------        
             ###########################---          
              ###########################-           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.30e+20   5.10e+20  -1.28e+21 
  5.10e+20   4.96e+21  -9.70e+20 
 -1.28e+21  -9.70e+20  -4.72e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20111106150705/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 = 320
      DIP = 85
     RAKE = -15
       MW = 3.74
       HS = 3.0

The NDK file is 20111106150705.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
USGSMT
 USGS/SLU Moment Tensor Solution
 ENS  2011/11/06 15:07:05:0  35.48  -96.86   5.0 3.8 Oklahoma
 
 Stations used:
   AG.HHAR TA.Q34A TA.Q35A TA.Q36A TA.R34A TA.R35A TA.R36A 
   TA.R37A TA.R38A TA.S34A TA.S35A TA.S36A TA.S37A TA.S38A 
   TA.S39A TA.T34A TA.T35A TA.T36A TA.T38A TA.T39A TA.TUL1 
   TA.U32A TA.U35A TA.U36A TA.U37A TA.U38A TA.U40A TA.V35A 
   TA.V36A TA.V37A TA.V38A TA.V39A TA.V40A TA.W35A TA.W36A 
   TA.W37B TA.W38A TA.W39A TA.W40A TA.X35A TA.X36A TA.X38A 
   TA.X39A TA.Y35A TA.Y36A TA.Y37A TA.Y39A TA.Y40A TA.Z37A 
   US.KSU1 US.MIAR 
 
 Filtering commands used:
   hp c 0.02 n 3
   lp c 0.06 n 3
 
 Best Fitting Double Couple
  Mo = 5.13e+21 dyne-cm
  Mw = 3.74 
  Z  = 3 km
  Plane   Strike  Dip  Rake
   NP1      320    85   -15
   NP2       51    75   -175
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   5.13e+21      7       7
    N   0.00e+00     74     122
    P  -5.13e+21     14     275

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     4.96e+21
       Mxy     9.70e+20
       Mxz     5.10e+20
       Myy    -4.72e+21
       Myz     1.28e+21
       Mzz    -2.30e+20
                                                     
                                                     
                                                     
                                                     
                     ######## T ###                  
                 ############   #######              
              -###########################           
             ----##########################          
           --------#########################-        
          -----------######################---       
         -------------####################-----      
        ----------------################--------     
        ------------------#############---------     
       -   ----------------##########------------    
       - P ------------------######--------------    
       -   -------------------###----------------    
       ------------------------#-----------------    
        --------------------#####---------------     
        ------------------#########-------------     
         --------------#############-----------      
          ---------###################--------       
           ----########################------        
             ###########################---          
              ###########################-           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.30e+20   5.10e+20  -1.28e+21 
  5.10e+20   4.96e+21  -9.70e+20 
 -1.28e+21  -9.70e+20  -4.72e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20111106150705/index.html
	
USGS/SLU Regional Moment Solution
OKLAHOMA

11/11/06 15:07:06.79

Epicenter:  35.550  -96.837
MW 3.8

USGS/SLU REGIONAL MOMENT TENSOR
Depth   5         No. of sta: 27
Moment Tensor;   Scale 10**14 Nm
  Mrr=-0.78       Mtt= 6.61
  Mpp=-5.83       Mrt=-1.30
  Mrp=-3.29       Mtp=-1.26
 Principal axes:
  T  Val=  6.87  Plg= 8  Azm=184
  N        0.79      62       78
  P       -7.66      27      278

Best Double Couple:Mo=7.3*10**14
 NP1:Strike= 53 Dip=78 Slip=-155
 NP2:       318     65       -14



< IMG SRC="b0006ku4_rmt_smt.gif">

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:

hp c 0.02 n 3
lp c 0.06 n 3
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   320    80   -25   3.59 0.3964
WVFGRD96    1.0   320    80   -20   3.62 0.4211
WVFGRD96    2.0   320    80   -20   3.71 0.4926
WVFGRD96    3.0   320    85   -15   3.74 0.5097
WVFGRD96    4.0   140    90    15   3.77 0.5068
WVFGRD96    5.0   320    90   -20   3.80 0.4985
WVFGRD96    6.0   320    90   -25   3.82 0.4935
WVFGRD96    7.0   320    90   -30   3.84 0.4932
WVFGRD96    8.0   145    85    35   3.88 0.4975
WVFGRD96    9.0   320    90   -35   3.89 0.4894
WVFGRD96   10.0   145    65    20   3.90 0.4871
WVFGRD96   11.0   145    65    20   3.91 0.4838
WVFGRD96   12.0   145    65    20   3.91 0.4793
WVFGRD96   13.0   145    70    20   3.92 0.4736
WVFGRD96   14.0   145    70    20   3.92 0.4672
WVFGRD96   15.0   145    70    20   3.93 0.4601
WVFGRD96   16.0   145    70    10   3.93 0.4532
WVFGRD96   17.0   140    65   -10   3.94 0.4486
WVFGRD96   18.0   140    65   -10   3.95 0.4434
WVFGRD96   19.0   140    65   -10   3.95 0.4374
WVFGRD96   20.0   140    65   -10   3.96 0.4310
WVFGRD96   21.0   140    75   -20   3.97 0.4255
WVFGRD96   22.0   140    75   -20   3.98 0.4211
WVFGRD96   23.0   140    80   -20   3.98 0.4169
WVFGRD96   24.0   140    80   -20   3.99 0.4124
WVFGRD96   25.0   140    80   -20   4.00 0.4070
WVFGRD96   26.0   140    80   -20   4.01 0.4012
WVFGRD96   27.0   140    80   -20   4.01 0.3949
WVFGRD96   28.0   140    85   -20   4.02 0.3892
WVFGRD96   29.0   140    85   -20   4.03 0.3836

The best solution is

WVFGRD96    3.0   320    85   -15   3.74 0.5097

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 0.02 n 3
lp c 0.06 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.

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 05:51:32 PM CDT 2024