2006/02/05 03:25:52 44.74N 111.88W 5. 4.6 Montana
USGS Felt map for this earthquake
USGS Felt reports page for Intermountain Western US
SLU Moment Tensor Solution 2006/02/05 03:25:52 44.74N 111.88W 5. 4.6 Montana Best Fitting Double Couple Mo = 4.95e+22 dyne-cm Mw = 4.43 Z = 14 km Plane Strike Dip Rake NP1 264 66 -116 NP2 135 35 -45 Principal Axes: Axis Value Plunge Azimuth T 4.95e+22 17 13 N 0.00e+00 24 276 P -4.95e+22 60 135 Moment Tensor: (dyne-cm) Component Value Mxx 3.66e+22 Mxy 1.65e+22 Mxz 2.88e+22 Myy -3.63e+21 Myz -1.18e+22 Mzz -3.29e+22 ########## # ############## T ##### ################# ######## ############################## -################################# --################################## ---################################### ---##################################### ----########------------------------#### -----##----------------------------------# ----#------------------------------------- -#####------------------------------------ #######----------------- --------------- #######---------------- P -------------- ########--------------- -------------- ########------------------------------ #########--------------------------- ##########------------------------ ###########------------------- ###############-----------## ###################### ############## Harvard Convention Moment Tensor: R T F -3.29e+22 2.88e+22 1.18e+22 2.88e+22 3.66e+22 -1.65e+22 1.18e+22 -1.65e+22 -3.63e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/NEW/20060205032552/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.
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STK = 135 DIP = 35 RAKE = -45 MW = 4.43 HS = 14
The waveform inversion is preferred. The surface-wave solution agrees.
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 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 80 45 -90 4.06 0.4538 WVFGRD96 1.0 260 45 -85 4.10 0.4451 WVFGRD96 2.0 265 40 -80 4.21 0.5420 WVFGRD96 3.0 180 80 5 4.27 0.5284 WVFGRD96 4.0 175 70 -10 4.30 0.5114 WVFGRD96 5.0 160 35 -20 4.32 0.5827 WVFGRD96 6.0 160 35 -20 4.34 0.6651 WVFGRD96 7.0 155 35 -20 4.35 0.7250 WVFGRD96 8.0 150 30 -30 4.40 0.7668 WVFGRD96 9.0 145 30 -35 4.40 0.7978 WVFGRD96 10.0 135 30 -45 4.41 0.8198 WVFGRD96 11.0 130 30 -55 4.43 0.8343 WVFGRD96 12.0 135 35 -45 4.42 0.8432 WVFGRD96 13.0 135 35 -45 4.43 0.8471 WVFGRD96 14.0 135 35 -45 4.44 0.8460 WVFGRD96 15.0 135 40 -40 4.44 0.8430 WVFGRD96 16.0 140 40 -30 4.43 0.8359 WVFGRD96 17.0 140 40 -30 4.44 0.8255 WVFGRD96 18.0 140 40 -30 4.45 0.8111 WVFGRD96 19.0 145 45 -25 4.46 0.7940
The best solution is
WVFGRD96 13.0 135 35 -45 4.43 0.8471
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 0.10 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= 254.98 DIP= 59.99 RAKE= -120.00 OR STK= 124.08 DIP= 41.41 RAKE= -49.12 DEPTH = 0150 km Mw = 4.44 Best Fit 0.8193 - 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 LKWY 99 119 eP MOOW 140 143 iP_C LOHW 140 162 iP_C SNOW 147 168 iP_C REDW 151 174 iP_C AHID 164 228 iP_C HLID 238 241 eP_X MSO 326 282 eP_X BW06 139 288 eP_X HWUT 176 349 eP_X LAO 62 491 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) LKWY 99 119 MOOW 140 143 LOHW 140 162 SNOW 147 168 REDW 151 174 AHID 164 228 HLID 238 241 MSO 326 282 BW06 139 288 HWUT 176 349 LAO 62 491 RWWY 130 509 WVOR 247 603 PHWY 124 648 MVU 182 693 ISCO 135 754 LON 290 804 TPH 213 864 MNV 219 874 HUMO 259 924 SDCO 143 943 WDC 246 988 WUAZ 177 1025 DAC 209 1057 GSC 204 1129 ISA 211 1153 SAO 226 1197 CBKS 119 1204 MWC 206 1282 GLA 192 1322 TUC 176 1383 BAR 199 1402 KSU1 111 1411 MNTX 156 1554 WMOK 130 1576 EYMN 70 1605 JCT 142 1904 SLM 103 1919 FVM 105 1939 UALR 117 1999 SIUC 104 2046 SIT 318 2127 MPH 112 2149 USIN 102 2152 OXF 113 2228 WVT 107 2243 WCI 100 2246 PLAL 110 2296 LTL 123 2428 LRAL 113 2505 ERPA 85 2583
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 br c 0.12 0.2 n 8 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 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: