Location

2001/09/04 12:45:53 37.15N 104.65W 5 4.0 Colorado

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Central and Southeastern US

Focal Mechanism

 SLU Moment Tensor Solution
 2001/09/04 12:45:53 37.15N 104.65W 5 4.0 Colorado
 
 Best Fitting Double Couple
    Mo = 2.09e+22 dyne-cm
    Mw = 4.18 
    Z  = 3 km
     Plane   Strike  Dip  Rake
      NP1       23    61   -96
      NP2      215    30   -80
 Principal Axes:
   Axis    Value   Plunge  Azimuth
     T   2.09e+22     15     118
     N   0.00e+00      5      26
     P  -2.09e+22     74     279



 Moment Tensor: (dyne-cm)
    Component  Value
       Mxx     4.16e+21
       Mxy    -7.75e+21
       Mxz    -3.33e+21
       Myy     1.37e+22
       Myz     1.02e+22
       Mzz    -1.78e+22
                                                     
                                                     
                                                     
                                                     
                     ##############                  
                 ############--------#-              
              ##########-------------#####           
             ########-----------------#####          
           ########-------------------#######        
          #######---------------------########       
         #######----------------------#########      
        #######-----------------------##########     
        ######-----------------------###########     
       ######---------   ------------############    
       ######--------- P -----------#############    
       #####----------   -----------#############    
       #####-----------------------##############    
        ####----------------------##############     
        ####---------------------#########   ###     
         ###--------------------########## T ##      
          ###------------------###########   #       
           ##----------------################        
             #--------------###############          
              #-----------################           
                 ------################              
                     ##############                  
                                                     
                                                     
                                                     

 Harvard Convention
 Moment Tensor:
      R          T          F
 -1.78e+22  -3.33e+21  -1.02e+22 
 -3.33e+21   4.16e+21   7.75e+21 
 -1.02e+22   7.75e+21   1.37e+22 


Details of the solution is found at

http://www.eas.slu.edu/eqc/eqc_mt/NEW/20010904124553/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 = 215
      DIP = 30
     RAKE = -80
       MW = 4.18
       HS = 3

The waveform solution is preferred. It agrees with the surface-wave amplitude radiation pattern solution.

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 0.2 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   230    25   -45   4.16 0.5034
WVFGRD96    1.0    45    65   -70   4.12 0.5154
WVFGRD96    2.0    35    55   -80   4.13 0.5333
WVFGRD96    3.0   215    30   -80   4.18 0.5372
WVFGRD96    4.0   235    35   -70   4.19 0.5260
WVFGRD96    5.0   235    35   -70   4.20 0.5062
WVFGRD96    6.0    40    30    90   4.19 0.4926
WVFGRD96    7.0   260    65    45   4.10 0.4932
WVFGRD96    8.0   260    65    40   4.09 0.4991
WVFGRD96    9.0    95    60    30   4.10 0.5020
WVFGRD96   10.0    45    25   -75   4.17 0.5082
WVFGRD96   11.0    45    25   -75   4.16 0.5060
WVFGRD96   12.0    95    60    30   4.11 0.5048
WVFGRD96   13.0    95    60    30   4.12 0.5058
WVFGRD96   14.0    95    60    30   4.12 0.5049
WVFGRD96   15.0    95    60    25   4.12 0.5040
WVFGRD96   16.0    95    60    25   4.12 0.5045
WVFGRD96   17.0    95    60    25   4.13 0.5047
WVFGRD96   18.0    95    60    25   4.13 0.5045
WVFGRD96   19.0    95    65    25   4.14 0.5045

The best solution is

WVFGRD96    3.0   215    30   -80   4.18 0.5372

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 0.2 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=      20.00
  DIP=      65.00
 RAKE=    -104.99
  
             OR
  
  STK=     232.36
  DIP=      28.90
 RAKE=     -60.98
 
 
DEPTH = 4.0 km
 
Mw = 4.23
Best Fit 0.8346 - 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
ANMO      214  294 iP_D
ISCO      344  306 iP_D
WMOK      115  593 eP_X
WUAZ      255  630 eP_-
KNB       271  727 eP_X
PD31      328  751 eP_X
BW06      328  752 eP_X
RSSD        4  776 eP_X
TUC       228  777 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.

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

The distribution of broadband stations with azimuth and distance is

Sta Az(deg)    Dist(km)   
ANMO	  214	  294
ISCO	  344	  306
WMOK	  115	  593
WUAZ	  255	  630
MVU	  285	  682
KNB	  271	  727
BW06	  328	  752
HWUT	  312	  774
RSSD	    4	  776
TUC	  228	  777
DUG	  298	  786
AHID	  320	  832
LTX	  174	  872
FA20	   46	  982
FA17	   54	 1001
FA18	   52	 1002
DAN	  257	 1008
FA16	   56	 1011
FA15	   56	 1015
FA13	   60	 1028
GLA	  247	 1032
MIAR	  103	 1041
BOZ	  330	 1109
GSC	  263	 1111
SLA	  267	 1139
UALR	  100	 1140
SND	  253	 1157
FRD	  253	 1159
KNW	  254	 1159
WMC	  254	 1162
BZN	  253	 1165
CRY	  254	 1167
MONP	  250	 1172
RDM	  254	 1174
SVD	  257	 1176
JCS	  251	 1178
VTV	  260	 1180
CCM	   81	 1187
DGR	  255	 1187
PLM	  253	 1187
CWC	  270	 1200
BAR	  249	 1204
TIN	  274	 1206
SOL	  251	 1244
CHF	  260	 1248
ISA	  267	 1250
VCS	  260	 1251
MWC	  259	 1255
MLAC	  277	 1256
PAS	  259	 1268
SLM	   78	 1277
USC	  258	 1282
DJJ	  259	 1294
OSI	  262	 1300
RPV	  257	 1303
BAK	  266	 1313
CIA	  256	 1318
TOV	  260	 1324
FA08	   85	 1341
MPH	   95	 1342
LGU	  260	 1347
SIUC	   83	 1366
JFWS	   58	 1384
SBC	  262	 1393
OXF	   98	 1406
UTMT	   89	 1411
SNCC	  257	 1420
WVT	   89	 1507
PLAL	   94	 1510
WALA	  333	 1521
BLO	   76	 1602
ULM	   23	 1616
WCI	   80	 1621
FA07	   94	 1638
PNT	  323	 1812
FA04	   99	 1863
EDM	  342	 1910
LLLB	  323	 2029
KAPO	   46	 2244
SADO	   60	 2297
SSPA	   72	 2345
KGNO	   63	 2493
FCC	   14	 2524
MOBC	  319	 2759
HRV	   68	 2876
YKW2	  349	 2897
YKW1	  350	 2900
YKW4	  349	 2905
DLBC	  330	 3000

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 3
lp c 0.06 3
br c 0.12 0.2 n 4 p 2

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 Fri Feb 24 09:26:12 CST 2006