Location

2002/03/31 18:35:01 43.16 -110.73 3.5 Wyoming

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Intermountain Western US

Focal Mechanism

 SLU Moment Tensor Solution
 2002/03/31 18:35:01 43.16 -110.73  3.5 Wyoming 
 
 Best Fitting Double Couple
    Mo = 3.98e+21 dyne-cm
    Mw = 3.70 
    Z  = 9 km
     Plane   Strike  Dip  Rake
      NP1      196    66   -64
      NP2      325    35   -135
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   3.98e+21     17     267
     N   0.00e+00     24       4
     P  -3.98e+21     60     145



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -6.47e+20
       Mxy     6.91e+20
       Mxz     1.34e+21
       Myy     3.29e+21
       Myz    -2.11e+21
       Mzz    -2.65e+21
                                                     
                                                     
                                                     
                                                     
                     ------------##                  
                 --####--------########              
              ###############-############           
             ###############-----##########          
           ###############---------##########        
          ###############------------#########       
         ################--------------########      
        ################----------------########     
        ###############------------------#######     
       ###############--------------------#######    
       ##   ##########--------------------#######    
       ## T #########----------------------######    
       ##   #########----------------------######    
        #############----------   ---------#####     
        #############---------- P ---------#####     
         ###########-----------   ---------####      
          ##########-----------------------###       
           #########----------------------###        
             ########---------------------#          
              #######--------------------#           
                 ####------------------              
                     #-------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -2.65e+21   1.34e+21   2.11e+21 
  1.34e+21  -6.47e+20  -6.91e+20 
  2.11e+21  -6.91e+20   3.29e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/NEW/20020331183501/index.html
        

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
Figure 1. Location of broadband stations used to obtain focal mechanism

Preferred Solution

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

      STK = 325
      DIP = 35
     RAKE = -135
       MW = 3.70
       HS = 9

The surface-wave is preferred. The waveform solution is not very sensitive to the mechanism paramters. This solution is marginal at best.

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:

hp c 0.02 3
lp c 0.06 3
br c 0.12 0.25 n 4 p 2
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   180    60    15   3.46 0.5368
WVFGRD96    1.0   175    75    10   3.48 0.5542
WVFGRD96    2.0   360    75    25   3.54 0.6109
WVFGRD96    3.0   355    85    10   3.56 0.6149
WVFGRD96    4.0    25    85    70   3.70 0.6621
WVFGRD96    5.0   175    10     5   3.84 0.7129
WVFGRD96    6.0   175    10     5   3.83 0.7275
WVFGRD96    7.0    40    75    65   3.66 0.7304
WVFGRD96    8.0   255    15    90   3.80 0.7320
WVFGRD96    9.0   250    15    85   3.80 0.7329
WVFGRD96   10.0   250    20    90   3.77 0.7299
WVFGRD96   11.0    65    65    80   3.74 0.7260
WVFGRD96   12.0    70    65    85   3.75 0.7199
WVFGRD96   13.0    70    65    85   3.74 0.7127
WVFGRD96   14.0    60    60    75   3.71 0.7035
WVFGRD96   15.0   190    30    25   3.79 0.6967
WVFGRD96   16.0   190    30    25   3.79 0.6895
WVFGRD96   17.0   185    35    20   3.79 0.6830
WVFGRD96   18.0   185    35    20   3.80 0.6761
WVFGRD96   19.0   185    35    20   3.80 0.6681

The best solution is

WVFGRD96    9.0   250    15    85   3.80 0.7329

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 componnet is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. The number in black at the rightr of each predicted traces 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 bandpass filter used in the processing and for the display was

hp c 0.02 3
lp c 0.06 3
br c 0.12 0.25 n 4 p 2
Figure 3. Waveform comparison for depth of 8 km
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.

Surface-Wave Focal Mechanism


  NODAL PLANES 

  
  STK=      15.66
  DIP=      66.07
 RAKE=     -63.66
  
             OR
  
  STK=     145.00
  DIP=      35.00
 RAKE=    -134.99
 
 
DEPTH = 9.0 km
 
Mw = 3.70
Best Fit 0.8986 - 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(deg)    Dist(km)   First motion
BW06      114  105 iP_C
HWUT      202  186 iP_D
HLID      280  302 eP_+
DUG       208  372 eP_-
ELK       236  460 eP_X

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.

The velocity model used for the search is a modified Utah model .

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 distributiuon

Sta Az(deg)    Dist(km)   
AHID	  215	   53
BW06	  114	  105
LKWY	   10	  158
HWUT	  202	  186
BOZ	  346	  286
HLID	  280	  302
DUG	  208	  372
ELK	  236	  460
RSSD	   76	  551
ISCO	  129	  567
NEW	  321	  754

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 velocity model used for the waveform fit is a modified Utah model .

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 3
lp c 0.06 3

Discussion

The Future

Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.

Acknowledgements

Dr. Harley Benz, USGS, provided the USGS USNSN digital data.

Appendix A

The figures below show the observed spectral amplitudes (units of cm-sec) at each station and the theoretical predictions as a function of period for the mechanism given above. The modified Utah model earth model was used to define the Green's functions. For each station, the Love and Rayleigh wave spectrail amplitudes are plotted with the same scaling so that one can get a sense fo the effects of the effects of the focal mechanism and depth on the excitation of each.

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 Sat Feb 25 20:38:44 CST 2006