2001/09/04 12:45:53 37.15N 104.65W 5 4.0 Colorado
USGS Felt map for this earthquake
USGS Felt reports page for Central and Southeastern US
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/Earthquake_Center/NEW/20010904124553/index.html
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The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
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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.
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 2The 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
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The best fit as a function of depth is given in the following figure:
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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
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| 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. |
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
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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 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.
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.
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| 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. |
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| 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. |
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
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:
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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
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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.
Dr. Harley Benz, USGS, provided the USGS USNSN digital data.
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.
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Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
