Location

Location ANSS

The ANSS event ID is us20003jvi and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us20003jvi/executive.

2015/09/16 02:30:02 35.978 -96.795 4.0 3.7 Oklahoma

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2015/09/16 02:30:02:0  35.98  -96.79   4.0 3.7 Oklahoma
 
 Stations used:
   AG.HHAR GS.KAN14 GS.KAN17 GS.OK025 GS.OK029 GS.OK032 
   N4.T35B N4.U38B N4.Z35B N4.Z38B OK.CHOK OK.CROK OK.FNO 
   OK.OKCFA OK.X37A TA.W39A US.KSU1 US.MIAR US.WMOK 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.03 n 3 
   lp c 0.07 n 3 
 
 Best Fitting Double Couple
  Mo = 3.89e+21 dyne-cm
  Mw = 3.66 
  Z  = 3 km
  Plane   Strike  Dip  Rake
   NP1      245    90   180
   NP2      335    90    -0
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   3.89e+21     -0      20
    N   0.00e+00     90     245
    P  -3.89e+21     -0     110

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     2.98e+21
       Mxy     2.50e+21
       Mxz    -1.54e+14
       Myy    -2.98e+21
       Myz     7.19e+13
       Mzz    -0.00e+00
                                                     
                                                     
                                                     
                                                     
                     ############ T                  
                 ---#############   ###              
              -------#####################           
             ---------#####################          
           -----------#######################        
          -------------#######################       
         ---------------######################-      
        -----------------#################------     
        ------------------############----------     
       --------------------#######---------------    
       ---------------------##-------------------    
       -------------------##---------------------    
       ---------------#######--------------------    
        ----------############----------------       
        ------#################--------------- P     
         -######################--------------       
          #######################-------------       
           #######################-----------        
             #####################---------          
              #####################-------           
                 ###################---              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -0.00e+00  -1.54e+14  -7.19e+13 
 -1.54e+14   2.98e+21  -2.50e+21 
 -7.19e+13  -2.50e+21  -2.98e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20150916023002/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 = 155
      DIP = 90
     RAKE = 0
       MW = 3.66
       HS = 3.0

The NDK file is 20150916023002.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 +70
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 n 3 
The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   150    90     0   3.49 0.4118
WVFGRD96    2.0   150    90     0   3.59 0.4798
WVFGRD96    3.0   155    90     0   3.66 0.4999
WVFGRD96    4.0   335    90     0   3.69 0.4933
WVFGRD96    5.0   155    80     5   3.72 0.4718
WVFGRD96    6.0   155    75     5   3.74 0.4493
WVFGRD96    7.0   155    75     5   3.75 0.4295
WVFGRD96    8.0   335    70    10   3.78 0.4153
WVFGRD96    9.0   155    70     5   3.79 0.3927
WVFGRD96   10.0   155    70     5   3.80 0.3768
WVFGRD96   11.0   155    70     5   3.81 0.3620
WVFGRD96   12.0   335    70    10   3.82 0.3523
WVFGRD96   13.0   335    70    10   3.82 0.3399
WVFGRD96   14.0   335    70    10   3.83 0.3287
WVFGRD96   15.0   335    70    10   3.83 0.3185
WVFGRD96   16.0   335    70    10   3.84 0.3100
WVFGRD96   17.0   335    70    10   3.84 0.3026
WVFGRD96   18.0   335    70    10   3.85 0.2961
WVFGRD96   19.0   335    70    10   3.85 0.2904
WVFGRD96   20.0   335    70    10   3.86 0.2853
WVFGRD96   21.0   335    70    10   3.87 0.2809
WVFGRD96   22.0   335    70    10   3.87 0.2767
WVFGRD96   23.0   335    70    10   3.88 0.2729
WVFGRD96   24.0   335    70    10   3.88 0.2695
WVFGRD96   25.0   335    70    10   3.89 0.2663
WVFGRD96   26.0   335    70    10   3.90 0.2635
WVFGRD96   27.0   335    70    10   3.90 0.2609
WVFGRD96   28.0   335    70    10   3.91 0.2585
WVFGRD96   29.0   335    70    10   3.92 0.2560

The best solution is

WVFGRD96    3.0   155    90     0   3.66 0.4999

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 +70
rtr
taper w 0.1
hp c 0.03 n 3 
lp c 0.07 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 Fri Apr 26 11:04:20 PM CDT 2024