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 usp000j7x7 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/usp000j7x7/executive.

2011/09/11 12:27:45 32.848 -100.769 5.0 4.3 Texas

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2011/09/11 12:27:45:0  32.85 -100.77   5.0 4.3 Texas
 
 Stations used:
   EP.KIDD IU.ANMO TA.136A TA.137A TA.233A TA.234A TA.236A 
   TA.333A TA.334A TA.335A TA.336A TA.433A TA.434A TA.435B 
   TA.436A TA.534A TA.535A TA.536A TA.633A TA.634A TA.635A 
   TA.733A TA.ABTX TA.MSTX TA.T25A TA.TUL1 TA.U32A TA.U35A 
   TA.V36A TA.W35A TA.W36A TA.W37B TA.WHTX TA.X35A TA.X36A 
   TA.X37A TA.Y35A TA.Y36A TA.Y37A TA.Y38A TA.Z33A TA.Z36A 
   US.AMTX US.JCT US.MNTX US.WMOK 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.05 n 3 
 
 Best Fitting Double Couple
  Mo = 4.68e+22 dyne-cm
  Mw = 4.38 
  Z  = 13 km
  Plane   Strike  Dip  Rake
   NP1      207    85   -160
   NP2      115    70    -5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.68e+22     11     339
    N   0.00e+00     69     219
    P  -4.68e+22     17      73

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     3.57e+22
       Mxy    -2.71e+22
       Mxz     3.90e+21
       Myy    -3.31e+22
       Myz    -1.58e+22
       Mzz    -2.62e+21
                                                     
                                                     
                                                     
                                                     
                       ############                  
                 ### T ##############--              
              ######   ############-------           
             #####################---------          
           ######################------------        
          ######################--------------       
         ######################----------------      
        ---###################-------------   --     
        ----#################-------------- P --     
       -------##############---------------   ---    
       ----------##########----------------------    
       ------------#######-----------------------    
       ---------------###------------------------    
        ----------------##----------------------     
        ----------------######------------------     
         --------------##############----------      
          ------------########################       
           ----------########################        
             -------#######################          
              ------######################           
                 --####################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.62e+21   3.90e+21   1.58e+22 
  3.90e+21   3.57e+22   2.71e+22 
  1.58e+22   2.71e+22  -3.31e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110911122745/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 = 115
      DIP = 70
     RAKE = -5
       MW = 4.38
       HS = 13.0

The NDK file is 20110911122745.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
GCMT
SLUFM
 USGS/SLU Moment Tensor Solution
 ENS  2011/09/11 12:27:45:0  32.85 -100.77   5.0 4.3 Texas
 
 Stations used:
   EP.KIDD IU.ANMO TA.136A TA.137A TA.233A TA.234A TA.236A 
   TA.333A TA.334A TA.335A TA.336A TA.433A TA.434A TA.435B 
   TA.436A TA.534A TA.535A TA.536A TA.633A TA.634A TA.635A 
   TA.733A TA.ABTX TA.MSTX TA.T25A TA.TUL1 TA.U32A TA.U35A 
   TA.V36A TA.W35A TA.W36A TA.W37B TA.WHTX TA.X35A TA.X36A 
   TA.X37A TA.Y35A TA.Y36A TA.Y37A TA.Y38A TA.Z33A TA.Z36A 
   US.AMTX US.JCT US.MNTX US.WMOK 
 
 Filtering commands used:
   cut o DIST/3.3 -30 o DIST/3.3 +70
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.05 n 3 
 
 Best Fitting Double Couple
  Mo = 4.68e+22 dyne-cm
  Mw = 4.38 
  Z  = 13 km
  Plane   Strike  Dip  Rake
   NP1      207    85   -160
   NP2      115    70    -5
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   4.68e+22     11     339
    N   0.00e+00     69     219
    P  -4.68e+22     17      73

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     3.57e+22
       Mxy    -2.71e+22
       Mxz     3.90e+21
       Myy    -3.31e+22
       Myz    -1.58e+22
       Mzz    -2.62e+21
                                                     
                                                     
                                                     
                                                     
                       ############                  
                 ### T ##############--              
              ######   ############-------           
             #####################---------          
           ######################------------        
          ######################--------------       
         ######################----------------      
        ---###################-------------   --     
        ----#################-------------- P --     
       -------##############---------------   ---    
       ----------##########----------------------    
       ------------#######-----------------------    
       ---------------###------------------------    
        ----------------##----------------------     
        ----------------######------------------     
         --------------##############----------      
          ------------########################       
           ----------########################        
             -------#######################          
              ------######################           
                 --####################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -2.62e+21   3.90e+21   1.58e+22 
  3.90e+21   3.57e+22   2.71e+22 
  1.58e+22   2.71e+22  -3.31e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110911122745/index.html
	
USGS/SLU Regional Moment Solution
WESTERN TEXAS

11/09/11 12:27:45.27

Epicenter:  32.855 -100.899
MW 4.3

USGS/SLU REGIONAL MOMENT TENSOR
Depth   6         No. of sta: 17
Moment Tensor;   Scale 10**15 Nm
  Mrr=-0.55       Mtt= 3.29
  Mpp=-2.74       Mrt=-0.71
  Mrp=-0.66       Mtp= 1.78
 Principal axes:
  T  Val=  3.94  Plg=11  Azm=164
  N       -0.64      75       26
  P       -3.30      10      256

Best Double Couple:Mo=3.7*10**15
 NP1:Strike=210 Dip=89 Slip= 165
 NP2:       300     75         1


eptember 11, 2011, WESTERN TEXAS, MW=4.5

Meredith Nettles

CENTROID-MOMENT-TENSOR  SOLUTION
GCMT EVENT:     S201109111227A
DATA: IU II LD G  CU TA US BK
SURFACE WAVES:  80S, 107C, T= 50
TIMESTAMP:      Q-20110911220257
CENTROID LOCATION:
ORIGIN TIME:      12:27:45.8 0.5
LAT:32.83N 0.03;LON:100.80W 0.02
DEP: 12.0  FIX;TRIANG HDUR:  0.4
MOMENT TENSOR: SCALE 10**22 D-CM
RR=-1.600 0.242; TT= 5.360 0.252
PP=-3.760 0.237; RT= 1.570 0.873
RP= 2.710 0.786; TP= 3.420 0.214
PRINCIPAL AXES:
1.(T) VAL=  7.156;PLG=16;AZM=339
2.(N)      -1.166;    58;    223
3.(P)      -5.990;    28;     77
BEST DBLE.COUPLE:M0= 6.57*10**22
NP1: STRIKE=115;DIP=59;SLIP=  -9
NP2: STRIKE=210;DIP=82;SLIP=-148

             #########
       ### T ###########--
     #####   ##########-----
   ##################---------
  ##################-----------
 -#################-------------
 -###############----------   --
 ----############----------- P ---
 -----##########------------   ---
 -------#######-------------------
 ---------####--------------------
 -------------------------------
 ----------#####----------------
  --------#############----####
   -------####################
     ----###################
       -##################
           ###########
        


First motions and takeoff angles from an 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.

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 0.02 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   300    85     0   4.09 0.4294
WVFGRD96    2.0   300    80     0   4.17 0.5166
WVFGRD96    3.0   120    80     5   4.20 0.5509
WVFGRD96    4.0   115    70   -10   4.24 0.5749
WVFGRD96    5.0   115    70   -10   4.27 0.5947
WVFGRD96    6.0   115    70   -10   4.29 0.6118
WVFGRD96    7.0   115    70   -10   4.31 0.6275
WVFGRD96    8.0   115    65   -10   4.34 0.6431
WVFGRD96    9.0   115    65    -5   4.35 0.6493
WVFGRD96   10.0   115    65    -5   4.36 0.6534
WVFGRD96   11.0   115    65    -5   4.37 0.6557
WVFGRD96   12.0   115    70    -5   4.38 0.6570
WVFGRD96   13.0   115    70    -5   4.38 0.6571
WVFGRD96   14.0   115    70    -5   4.39 0.6558
WVFGRD96   15.0   115    70    -5   4.40 0.6531
WVFGRD96   16.0   115    70    -5   4.41 0.6491
WVFGRD96   17.0   115    70    -5   4.41 0.6440
WVFGRD96   18.0   120    80    25   4.44 0.6388
WVFGRD96   19.0   120    80    25   4.44 0.6349
WVFGRD96   20.0   120    80    25   4.45 0.6299
WVFGRD96   21.0   120    80    25   4.46 0.6236
WVFGRD96   22.0   120    85    25   4.47 0.6169
WVFGRD96   23.0   120    85    25   4.47 0.6107
WVFGRD96   24.0   120    85    25   4.48 0.6037
WVFGRD96   25.0   120    85    25   4.48 0.5959
WVFGRD96   26.0   300    75    -5   4.48 0.5877
WVFGRD96   27.0   300    75    -5   4.48 0.5793
WVFGRD96   28.0   300    75    -5   4.49 0.5703
WVFGRD96   29.0   300    75    -5   4.50 0.5610

The best solution is

WVFGRD96   13.0   115    70    -5   4.38 0.6571

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 0.02 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.

Velocity Model

The CUS.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
CUS Model with Q from simple gamma values
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
  1.0000  5.0000  2.8900  2.5000 0.172E-02 0.387E-02 0.00  0.00  1.00  1.00 
  9.0000  6.1000  3.5200  2.7300 0.160E-02 0.363E-02 0.00  0.00  1.00  1.00 
 10.0000  6.4000  3.7000  2.8200 0.149E-02 0.336E-02 0.00  0.00  1.00  1.00 
 20.0000  6.7000  3.8700  2.9020 0.000E-04 0.000E-04 0.00  0.00  1.00  1.00 
  0.0000  8.1500  4.7000  3.3640 0.194E-02 0.431E-02 0.00  0.00  1.00  1.00 
Last Changed Sat Apr 27 04:16:32 PM CDT 2024