Location

SLU Location

2002/06/18 17:37:17 37.983 -87.795 17.0 4.9 Indiana

The program elocate was used to locate this event. The source depth was fixed at 17 km, which is the depth determined from the waveform inversion. Waveforms were downloaded using the IRIS FetchData mechanism on 11/27/2013. The following are noted: a) none of the PP stations in Indiana were used because of calibration and timing differences; b) I flipped the BLO waveform because of original problems with polarity and wiring.

The phase picks and location are given in the file elocate.txt. The computed take-off angles are used in the comparison of the P-wave first motions to the waveform focal mechanism.

NEIC Location and Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports main page

Focal Mechanism

 USGS/SLU Moment Tensor Solution
 ENS  2002/06/18 17:37:17:0  37.98  -87.79  17.0 4.9 Indiana
 
 Stations used:
   IU.CCM IU.WCI IU.WVT NM.BLO NM.MPH NM.PLAL NM.SIUC NM.SLM 
   NM.UALR NM.UTMT SP.DWDAN US.ACSO US.BLA US.GOGA US.JFWS 
   US.MIAR US.MYNC US.OXF 
 
 Filtering commands used:
   cut a -30 a 180
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 6.84e+22 dyne-cm
  Mw = 4.49 
  Z  = 17 km
  Plane   Strike  Dip  Rake
   NP1      125    90    10
   NP2       35    80   180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.84e+22      7     350
    N   0.00e+00     80     125
    P  -6.84e+22      7     260

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     6.33e+22
       Mxy    -2.30e+22
       Mxz     9.73e+21
       Myy    -6.33e+22
       Myz     6.81e+21
       Mzz    -1.04e+15
                                                     
                                                     
                                                     
                                                     
                     ## T #########                  
                 ######   #############              
              ##########################--           
             ##########################----          
           ###########################-------        
          ---########################---------       
         -------####################-----------      
        -----------################-------------     
        -------------#############--------------     
       -----------------#########----------------    
       --------------------#####-----------------    
          -------------------#-------------------    
        P -------------------###-----------------    
          ------------------#######-------------     
        ------------------###########-----------     
         ----------------###############-------      
          -------------####################---       
           -----------#######################        
             -------#######################          
              ----########################           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.04e+15   9.73e+21  -6.81e+21 
  9.73e+21   6.33e+22   2.30e+22 
 -6.81e+21   2.30e+22  -6.33e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20020618173717/index.html
        

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is

      STK = 125
      DIP = 90
     RAKE = 10
       MW = 4.49
       HS = 17.0

The NDK file is 20020618173717.ndk The waveform inversion is preferred.

Moment Tensor Comparison

The following compares this source inversion to others
SLU
SLUFM
 USGS/SLU Moment Tensor Solution
 ENS  2002/06/18 17:37:17:0  37.98  -87.79  17.0 4.9 Indiana
 
 Stations used:
   IU.CCM IU.WCI IU.WVT NM.BLO NM.MPH NM.PLAL NM.SIUC NM.SLM 
   NM.UALR NM.UTMT SP.DWDAN US.ACSO US.BLA US.GOGA US.JFWS 
   US.MIAR US.MYNC US.OXF 
 
 Filtering commands used:
   cut a -30 a 180
   rtr
   taper w 0.1
   hp c 0.02 n 3 
   lp c 0.10 n 3 
 
 Best Fitting Double Couple
  Mo = 6.84e+22 dyne-cm
  Mw = 4.49 
  Z  = 17 km
  Plane   Strike  Dip  Rake
   NP1      125    90    10
   NP2       35    80   180
  Principal Axes:
   Axis    Value   Plunge  Azimuth
    T   6.84e+22      7     350
    N   0.00e+00     80     125
    P  -6.84e+22      7     260

 Moment Tensor: (dyne-cm)
    Component   Value
       Mxx     6.33e+22
       Mxy    -2.30e+22
       Mxz     9.73e+21
       Myy    -6.33e+22
       Myz     6.81e+21
       Mzz    -1.04e+15
                                                     
                                                     
                                                     
                                                     
                     ## T #########                  
                 ######   #############              
              ##########################--           
             ##########################----          
           ###########################-------        
          ---########################---------       
         -------####################-----------      
        -----------################-------------     
        -------------#############--------------     
       -----------------#########----------------    
       --------------------#####-----------------    
          -------------------#-------------------    
        P -------------------###-----------------    
          ------------------#######-------------     
        ------------------###########-----------     
         ----------------###############-------      
          -------------####################---       
           -----------#######################        
             -------#######################          
              ----########################           
                 ######################              
                     ##############                  
                                                     
                                                     
                                                     
 Global CMT Convention Moment Tensor:
      R          T          P
 -1.04e+15   9.73e+21  -6.81e+21 
  9.73e+21   6.33e+22   2.30e+22 
 -6.81e+21   2.30e+22  -6.33e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20020618173717/index.html
	


First motions and takeoff angles from an elocate run.

Waveform Inversion

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for 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 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 a -30 a 180
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.10 n 3 
The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   290    65   -25   4.18 0.3630
WVFGRD96    1.0   290    70   -25   4.19 0.3842
WVFGRD96    2.0   115    90     5   4.19 0.4167
WVFGRD96    3.0   300    85     5   4.23 0.4253
WVFGRD96    4.0   295    75   -15   4.24 0.4267
WVFGRD96    5.0   295    70   -15   4.26 0.4424
WVFGRD96    6.0   295    70   -15   4.27 0.4665
WVFGRD96    7.0   295    75   -15   4.29 0.4931
WVFGRD96    8.0   295    75   -15   4.31 0.5195
WVFGRD96    9.0   300    75   -10   4.35 0.5505
WVFGRD96   10.0   305    80   -10   4.39 0.5825
WVFGRD96   11.0   305    80   -10   4.41 0.6105
WVFGRD96   12.0   305    80   -10   4.42 0.6324
WVFGRD96   13.0   305    85   -10   4.44 0.6502
WVFGRD96   14.0   305    85   -10   4.46 0.6642
WVFGRD96   15.0   305    85   -10   4.47 0.6718
WVFGRD96   16.0   125    90    10   4.48 0.6737
WVFGRD96   17.0   305    85   -10   4.50 0.6754
WVFGRD96   18.0   305    85   -10   4.51 0.6716
WVFGRD96   19.0   125    90    10   4.52 0.6644
WVFGRD96   20.0   305    85   -10   4.54 0.6581
WVFGRD96   21.0   305    85   -10   4.55 0.6456
WVFGRD96   22.0   305    85   -10   4.55 0.6338
WVFGRD96   23.0   305    85   -10   4.56 0.6182
WVFGRD96   24.0   305    85   -10   4.57 0.5996
WVFGRD96   25.0   305    85   -10   4.57 0.5817
WVFGRD96   26.0   300    80    -5   4.57 0.5624
WVFGRD96   27.0   300    80    -5   4.57 0.5442
WVFGRD96   28.0   300    80    -5   4.58 0.5261
WVFGRD96   29.0   300    80    -5   4.58 0.5073

The best solution is

WVFGRD96   17.0   305    85   -10   4.50 0.6754

The mechanism correspond 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 and because the velocity model used in the predictions may not be perfect. 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 a -30 a 180
rtr
taper w 0.1
hp c 0.02 n 3 
lp c 0.10 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.
Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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=      30.00
  DIP=      84.99
 RAKE=    -170.00
  
             OR
  
  STK=     299.12
  DIP=      80.04
 RAKE=      -5.08
 
 
DEPTH = 19.0 km
 
Mw = 4.58
Best Fit 0.8834 - P-T axis plot gives solutions with FIT greater than FIT90

First motion data

The P-wave first motion data for focal mechanism studies are as follow:

Sta Az    Dist   First motion

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

Broadband station distribution

The distribution of broadband stations with azimuth and distance is
Listing of broadband stations used

Waveform comparison for this mechanism

Since the analysis of the surface-wave radiation patterns uses only spectral amplitudes and because the surfave-wave radiation patterns have a 180 degree symmetry, each surface-wave solution consists of four possible focal mechanisms corresponding to the interchange of the P- and T-axes and a roation of the mechanism by 180 degrees. To select one mechanism, P-wave first motion can be used. This was not possible in this case because all the P-wave first motions were emergent ( a feature of the P-wave wave takeoff angle, the station location and the mechanism). The other way to select among the mechanisms is to compute forward synthetics and compare the observed and predicted waveforms.

The fits to the waveforms with the given mechanism are show below:

This figure shows the fit to the three components of motion (Z - vertical, R-radial and T - transverse). For each station and component, the observed traces is shown in red and the model predicted trace in blue. The traces represent filtered ground velocity in units of meters/sec (the peak value is printed adjacent to each trace; each pair of traces to plotted to the same scale to emphasize the difference in levels). Both synthetic and observed traces have been filtered using the SAC commands:

hp c 0.02 n 3
lp c 0.10 n 3

Discussion

Acknowledgements

Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Iris stations and the Transportable Array of EarthScope.

Appendix A


Spectra fit plots to each station

Velocity Model

The CUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:

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 

Quality Control

Here we tabulate the reasons for not using certain digital data sets

The following stations did not have a valid response files:

Last Changed Wed Nov 27 14:40:19 CST 2013