2005/10/31 00:23:30 44.90N 113.45W 5. 4.5 Montana
USGS Felt map for this earthquake
USGS Felt reports page for Intermountain Western US
SLU Moment Tensor Solution 2005/10/31 00:23:30 44.90N 113.45W 5. 4.5 Montana Best Fitting Double Couple Mo = 5.50e+22 dyne-cm Mw = 4.46 Z = 14 km Plane Strike Dip Rake NP1 303 80 -113 NP2 190 25 -25 Principal Axes: Axis Value Plunge Azimuth T 5.50e+22 31 52 N 0.00e+00 23 307 P -5.50e+22 50 187 Moment Tensor: (dyne-cm) Component Value Mxx -6.66e+21 Mxy 1.67e+22 Mxz 4.19e+22 Myy 2.45e+22 Myz 2.25e+22 Mzz -1.78e+22 -----######### -----################# ------###################### -----######################### -----############################# ####-####################### ##### #####-----################### T ###### #####---------################ ####### #####-------------###################### #####-----------------#################### #####-------------------################## #####----------------------############### #####------------------------############# ####---------------------------######### #####----------------------------####### ####------------ ---------------#### ####----------- P -----------------# ####---------- ----------------- ###--------------------------- ####------------------------ ###------------------- #------------- Harvard Convention Moment Tensor: R T F -1.78e+22 4.19e+22 -2.25e+22 4.19e+22 -6.66e+21 -1.67e+22 -2.25e+22 -1.67e+22 2.45e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/NEW/20051031002330/index.html |
Harvard CMT Event name: 103105B Region name: WESTERN MONTANA Date (y/m/d): 2005/10/31 Information on data used in inversion Wave nsta nrec cutoff Body 0 0 0 Mantle 26 36 40 Timing and location information hr min sec lat lon depth mb Ms PDE 0 23 30.00 44.90 -113.45 5.0 4.5 4.5 CMT 0 23 34.90 44.83 -113.30 13.4 Error 0.50 0.03 0.05 3.0 Assumed half duration: 0.4 Mechanism information Exponent for moment tensor: 22 units: dyne-cm Mrr Mtt Mpp Mrt Mrp Mtp CMT -4.030 0.340 3.690 5.250 -3.300 -3.050 Error 0.790 0.400 0.530 1.510 1.100 0.320 Mw = 4.5 Scalar Moment = 7.91e+22 Fault plane: strike=176 dip=25 slip=-44 Fault plane: strike=307 dip=73 slip=-108 Eigenvector: eigenvalue: 8.17 plunge: 26 azimuth: 51 Eigenvector: eigenvalue: -0.53 plunge: 18 azimuth: 312 Eigenvector: eigenvalue: -7.65 plunge: 58 azimuth: 192 |
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 = 190 DIP = 25 RAKE = -25 MW = 4.46 HS = 14
Both the surface-wave spectral amplitude and the waveform inversion demand a near vertical dip-slip solution. The depth and moment estimates are considtent. The waveform inversion result is used to characterized this event.
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 0.014 3 lp c 0.06 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 315 50 -90 4.20 0.4428 WVFGRD96 1.0 115 50 -80 4.20 0.4089 WVFGRD96 2.0 135 50 -90 4.30 0.4446 WVFGRD96 3.0 325 30 -75 4.34 0.3774 WVFGRD96 4.0 180 20 -25 4.39 0.4059 WVFGRD96 5.0 180 15 -30 4.41 0.4678 WVFGRD96 6.0 185 15 -25 4.41 0.5185 WVFGRD96 7.0 185 20 -25 4.41 0.5598 WVFGRD96 8.0 185 15 -25 4.45 0.5917 WVFGRD96 9.0 180 20 -35 4.46 0.6183 WVFGRD96 10.0 185 20 -30 4.46 0.6397 WVFGRD96 11.0 185 20 -30 4.46 0.6538 WVFGRD96 12.0 180 20 -35 4.46 0.6638 WVFGRD96 13.0 185 20 -30 4.46 0.6686 WVFGRD96 14.0 190 25 -25 4.46 0.6699 WVFGRD96 15.0 195 25 -20 4.46 0.6694 WVFGRD96 16.0 195 25 -20 4.46 0.6662 WVFGRD96 17.0 195 25 -20 4.46 0.6602 WVFGRD96 18.0 200 25 -15 4.46 0.6532 WVFGRD96 19.0 200 25 -15 4.46 0.6448
The best solution is
WVFGRD96 14.0 190 25 -25 4.46 0.6699
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 0.014 3 lp c 0.06 3
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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= 319.99 DIP= 69.99 RAKE= -74.99 OR STK= 101.90 DIP= 24.82 RAKE= -125.42 DEPTH = 13.0 km Mw = 4.54 Best Fit 0.8947 - 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 HLID 208 168 iP_D MSO 350 218 iP_C LKWY 98 244 iP_C REDW 129 269 iP_D AHID 141 303 iP_D BW06 126 393 eP_X HWUT 157 397 iP_D HAWA 291 503 eP_- LAO 68 596 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) HLID 208 168 MSO 350 218 REDW 129 269 AHID 141 303 BW06 126 393 HWUT 157 397 WVOR 239 500 HAWA 291 503 LAO 68 596 LON 290 682 TTW 299 707 MVU 171 718 HUMO 255 807 MNV 210 818 TPH 204 821 ISCO 129 858 WDC 240 885 LLLB 318 895 EDM 0 926 DAC 202 1020 SDCO 137 1036 WUAZ 170 1057 CBB 307 1063 GSC 196 1104 ISA 204 1111 SAO 219 1127 EDB 303 1169 MWC 200 1250 GLA 186 1321 CBKS 116 1322 BBB 312 1349 BMBC 337 1383 BAR 193 1385 TUC 170 1417 KSU1 109 1533 MNTX 152 1625 WMOK 127 1684 FNBB 341 1688 EYMN 71 1716 DLBC 329 1884 LTX 150 1931 YKW3 358 1969 CCM 105 1997 JCT 139 1997 FCC 34 2026 SIT 320 2032 SLM 102 2044 MIAR 117 2047 FVM 104 2063 UALR 115 2118 SIUC 103 2171 NATX 125 2187 PVMO 107 2208 MPH 110 2271 USIN 101 2277 OXF 111 2350 WVT 105 2367 WCI 99 2371 KAPO 67 2383 ACSO 91 2540 LTL 121 2542 LRAL 111 2627 DAWY 332 2680 ERPA 85 2705 GOGA 106 2866 KGNO 78 2913 GAC 75 2953 CBN 91 3074 NCB 78 3096 NHSC 103 3137 PAL 84 3225 LSCT 82 3252 HRV 80 3351 DWPF 112 3386
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 0.014 3 lp c 0.06 3
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: