2007/02/25 03:52:21 42.47N 110.67W 1 3.7 Wyoming
USGS Felt map for this earthquake
USGS Felt reports page for Intermountain Western US
SLU Moment Tensor Solution 2007/02/25 03:52:21 42.47N 110.67W 1 3.7 Wyoming Best Fitting Double Couple Mo = 9.77e+21 dyne-cm Mw = 3.96 Z = 11 km Plane Strike Dip Rake NP1 325 70 -75 NP2 107 25 -125 Principal Axes: Axis Value Plunge Azimuth T 9.77e+21 24 43 N 0.00e+00 14 140 P -9.77e+21 62 258 Moment Tensor: (dyne-cm) Component Value Mxx 4.23e+21 Mxy 3.66e+21 Mxz 3.44e+21 Myy 1.84e+21 Myz 6.42e+21 Mzz -6.07e+21 ############## ###################### -----####################### ---------################ ## -------------############## T #### ----------------############ ##### -------------------################### ---------------------################### -----------------------################# #------------------------################# #------------ -----------############### ##----------- P ------------############## ###---------- -------------############# ##---------------------------########### ####-------------------------########### ####-------------------------########- #####------------------------######- ######----------------------####-- #######----------------------- ############--------######-- ###################### ############## Harvard Convention Moment Tensor: R T F -6.07e+21 3.44e+21 -6.42e+21 3.44e+21 4.23e+21 -3.66e+21 -6.42e+21 -3.66e+21 1.84e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070225035221/index.html |
STK = 325 DIP = 70 RAKE = -75 MW = 3.96 HS = 11
Both the surface-wave and waveform inversion give the same solution.
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.
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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 n 3 lp c 0.10 n 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 0.5 140 40 -90 3.60 0.1913 WVFGRD96 1.0 185 90 5 3.48 0.1430 WVFGRD96 2.0 140 40 -90 3.73 0.1944 WVFGRD96 3.0 360 70 -30 3.68 0.1660 WVFGRD96 4.0 185 30 -10 3.73 0.1699 WVFGRD96 5.0 180 20 -25 3.81 0.2136 WVFGRD96 6.0 165 15 -45 3.86 0.2525 WVFGRD96 7.0 155 20 -60 3.89 0.2823 WVFGRD96 8.0 320 70 -80 3.96 0.3002 WVFGRD96 9.0 320 70 -80 3.96 0.3161 WVFGRD96 10.0 320 70 -80 3.97 0.3226 WVFGRD96 11.0 325 70 -75 3.96 0.3230 WVFGRD96 12.0 325 70 -75 3.96 0.3213 WVFGRD96 13.0 325 70 -75 3.96 0.3173 WVFGRD96 14.0 325 70 -70 3.96 0.3129 WVFGRD96 15.0 325 70 -70 3.96 0.3073 WVFGRD96 16.0 120 65 60 3.96 0.3022 WVFGRD96 17.0 120 65 60 3.96 0.3006 WVFGRD96 18.0 120 65 60 3.97 0.2975 WVFGRD96 19.0 120 65 60 3.98 0.2932 WVFGRD96 20.0 115 70 55 3.98 0.2878 WVFGRD96 21.0 115 65 55 4.04 0.2837 WVFGRD96 22.0 115 65 55 4.05 0.2799 WVFGRD96 23.0 115 65 55 4.06 0.2750 WVFGRD96 24.0 115 65 55 4.07 0.2695 WVFGRD96 25.0 115 65 55 4.08 0.2633 WVFGRD96 26.0 115 65 50 4.08 0.2565 WVFGRD96 27.0 115 65 50 4.09 0.2508 WVFGRD96 28.0 115 60 50 4.10 0.2444 WVFGRD96 29.0 110 60 45 4.11 0.2386
The best solution is
WVFGRD96 11.0 325 70 -75 3.96 0.3230
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 n 3 lp c 0.10 n 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. |
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.
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The surface-wave determined focal mechanism is shown here.
NODAL PLANES STK= 309.98 DIP= 64.99 RAKE= -105.00 OR STK= 162.35 DIP= 28.91 RAKE= -60.97 DEPTH = 10.0 km Mw = 4.05 Best Fit 0.7607 - P-T axis plot gives solutions with FIT greater than FIT90
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion AHID 313 48 eP_- BW06 70 97 iP_C SPU 229 196 eP_- CTU 205 217 eP_X JLU 198 217 iP_- YFT 357 221 eP_+ NOQ 211 235 eP_X YMR 355 246 eP_X YNR 360 250 iP_+ BGU 230 261 eP_X MPU 197 284 eP_X NLU 203 303 eP_X DUG 216 309 eP_X RLMT 20 316 eP_X HLID 293 328 eP_X TMU 187 355 eP_X BOZ 348 362 eP_X SRU 178 373 eP_X ELK 245 426 eP_X ISCO 123 518 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.
The velocity model used for the search is a modified Utah model .
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. |
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. |
Sta Az(deg) Dist(km) AHID 314 48 BW06 70 97 SPU 230 196 CTU 205 217 JLU 198 217 YFT 357 221 LKWY 5 234 NOQ 211 235 YUF 3 250 BGU 230 261 MPU 197 284 NLU 203 303 DUG 216 309 RLMT 20 316 HLID 293 328 TMU 188 355 BOZ 348 362 SRU 178 373 ELK 245 426 HMU 181 503 ISCO 123 518 MSO 333 550 LAO 36 587 CCUT 204 592 BMO 299 597 MVCO 162 613 EGMT 6 622 WVOR 272 655 SDCO 138 685 WUAZ 185 774 TPNV 220 777 NEW 324 818 HAWA 305 829 DGMT 35 837
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 n 3 lp c 0.10 n 3 br c 0.12 0.2 n 4 p 2
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 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
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.
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files: