Location

2006/05/18 10:16:21 44.17N 110.34W 0. 3.9 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
 2006/05/18 10:16:21 44.17N 110.34W   0. 3.9 Wyoming
 
 Best Fitting Double Couple
    Mo = 6.46e+21 dyne-cm
    Mw = 3.84 
    Z  = 8 km
     Plane   Strike  Dip  Rake
      NP1      314    71   -126
      NP2      200    40   -30
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   6.46e+21     18      70
     N   0.00e+00     34     327
     P  -6.46e+21     50     183



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx    -1.94e+21
       Mxy     1.73e+21
       Mxz     3.83e+21
       Myy     5.12e+21
       Myz     1.99e+21
       Mzz    -3.18e+21
                                                     
                                                     
                                                     
                                                     
                     -------------#                  
                 -----------###########              
              -----------#################           
             ##--------####################          
           #########-########################        
          #########----#######################       
         #########--------#################   #      
        #########-----------############### T ##     
        ########--------------#############   ##     
       #########----------------#################    
       ########-------------------###############    
       ########--------------------##############    
       ########----------------------############    
        #######-----------------------##########     
        #######-------------------------########     
         ######-----------   ------------######      
          #####----------- P -------------####       
           #####----------   -------------###        
             ####--------------------------          
              ####------------------------           
                 ##--------------------              
                     --------------                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -3.18e+21   3.83e+21  -1.99e+21 
  3.83e+21  -1.94e+21  -1.73e+21 
 -1.99e+21  -1.73e+21   5.12e+21 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/NEW/20060518101621/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 = 200
      DIP = 40
     RAKE = -30
       MW = 3.84
       HS = 8

The waveform inversion is preferred. The surface-wave solution is compatible

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 0.10 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    95    30    90   3.54 0.4141
WVFGRD96    1.0    85    75   -10   3.58 0.3911
WVFGRD96    2.0    70    30    45   3.69 0.4435
WVFGRD96    3.0    65    30    55   3.72 0.5010
WVFGRD96    4.0   205    35   -10   3.72 0.5750
WVFGRD96    5.0   205    35   -15   3.75 0.6324
WVFGRD96    6.0   200    40   -25   3.78 0.6661
WVFGRD96    7.0   200    40   -30   3.80 0.6787
WVFGRD96    8.0   200    40   -30   3.84 0.6814
WVFGRD96    9.0   200    40   -30   3.85 0.6702
WVFGRD96   10.0   200    45   -25   3.86 0.6524
WVFGRD96   11.0   205    45   -15   3.86 0.6288
WVFGRD96   12.0   205    45   -15   3.87 0.6019
WVFGRD96   13.0   210    45   -15   3.88 0.5736
WVFGRD96   14.0   290    75   -45   3.90 0.5477
WVFGRD96   15.0   290    75   -40   3.91 0.5272
WVFGRD96   16.0   290    75   -40   3.92 0.5070
WVFGRD96   17.0   285    65   -40   3.94 0.4871
WVFGRD96   18.0   285    65   -40   3.94 0.4697
WVFGRD96   19.0   285    65   -40   3.95 0.4535

The best solution is

WVFGRD96    8.0   200    40   -30   3.84 0.6814

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 0.10 3
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=     349.98
  DIP=      65.00
 RAKE=    -104.99
  
             OR
  
  STK=     202.34
  DIP=      28.90
 RAKE=     -60.98
 
 
DEPTH = 9.0 km
 
Mw = 3.92
Best Fit 0.8952 - 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
LKWY      353   44 ePg
MOOW      215   57 ePg
REDW      205   99 ePg
AHID      202  168 ePn
BW06      158  169 ePn
HWUT      200  302 ePn
HLID      260  335 ePn
RWWY      136  376 ePn
MSO       318  408 ePn
LAO        48  427 ePn

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)   
LKWY	  353	   44
IMW	  238	   57
MOOW	  215	   57
REDW	  205	   99
AHID	  202	  168
BW06	  158	  169
HWUT	  200	  302
HLID	  260	  335
RWWY	  136	  376
MSO	  318	  408
LAO	   48	  427
PHWY	  127	  511
ISCO	  140	  624
WVOR	  257	  701
HAWA	  292	  762
TPH	  223	  890
MNV	  228	  913
LON	  292	  941
HUMO	  265	 1037
CBKS	  120	 1066
WDC	  252	 1080
GSC	  212	 1130
ISA	  219	 1172
SAO	  233	 1249
KSU1	  112	 1273
MWC	  214	 1289
GLA	  199	 1295
AMTX	  141	 1317
EYMN	   67	 1514
CCM	  106	 1737
JCT	  145	 1780
MIAR	  121	 1792
UALR	  118	 1861
SIUC	  105	 1911
NATX	  130	 1942
MPH	  113	 2012
UTMT	  108	 2016
USIN	  102	 2019
OXF	  114	 2090
WCI	  100	 2114
PLAL	  111	 2158

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 0.10 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. The digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface

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 Thu May 18 08:32:41 CDT 2006