2006/06/18 00:05:33 45.74N 111.80W 5. 4.3 Montana
DATE ORIGIN LAT N LONG W DEPTH MAG NO DM GAP M RMS ERH ERZ Q 2060618 000532.40 45.60 111.90 11.53 4.03 51 21 44 1 0.14 0.3 0.4 B Michael Stickney, Director Earthquake Studies Office Montana Bureau of Mines and Geology Montana Tech of the University of Montana 1300 W Park St Butte, MT 59701
USGS Felt map for this earthquake
USGS Felt reports page for Intermountain Western US
SLU Moment Tensor Solution 2006/06/18 00:05:32 45.60N 111.90W 11 4.0 Montana Best Fitting Double Couple Mo = 2.02e+22 dyne-cm Mw = 4.17 Z = 10 km Plane Strike Dip Rake NP1 305 60 -120 NP2 174 41 -49 Principal Axes: Axis Value Plunge Azimuth T 2.02e+22 10 56 N 0.00e+00 26 321 P -2.02e+22 62 166 Moment Tensor: (dyne-cm) Component Value Mxx 1.94e+21 Mxy 1.01e+22 Mxz 1.01e+22 Myy 1.32e+22 Myz 8.80e+20 Mzz -1.51e+22 ---########### -----################# ------###################### ------######################## #######-####################### #######--------################# T # ########------------############# ## ########----------------################ ########------------------############## #########---------------------############ #########----------------------########### #########------------------------######### #########-------------------------######## #########----------- ------------##### #########----------- P ------------##### #########---------- -------------### #########--------------------------# #########------------------------- ########---------------------- ########-------------------- #######--------------- ######-------- Harvard Convention Moment Tensor: R T F -1.51e+22 1.01e+22 -8.80e+20 1.01e+22 1.94e+21 -1.01e+22 -8.80e+20 -1.01e+22 1.32e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/NEW/20060618000533/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 = 305 DIP = 60 RAKE = -120 MW = 4.17 HS = 10
Both techniques give the same depth and moment. The waveforms inversionis more strike-slip like. However, To use the data at the closest station, BOZ, we had to use the Montana Bureau of Mines and Geology location. The surface-wave spectral amplitude solution is preferred.
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion BOZ 129 17 eP_X BOZ 77 22 iP_C LKWY 139 171 eP_+ MSO 307 205 eP_X IMW 161 216 iP_D MOOW 159 237 iP_D REDW 164 275 eP_D EGMT 31 298 iP_C HLID 222 319 eP_- AHID 170 336 eP_X BW06 151 376 ePn LAO 74 443 ePn HWUT 178 460 ePn
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 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 105 75 -20 3.63 0.3196 WVFGRD96 1.0 105 80 -10 3.66 0.3310 WVFGRD96 2.0 105 75 -15 3.77 0.3634 WVFGRD96 3.0 290 85 35 3.84 0.3395 WVFGRD96 4.0 290 80 45 3.89 0.3591 WVFGRD96 5.0 290 80 45 3.92 0.3796 WVFGRD96 6.0 360 50 -35 3.93 0.3997 WVFGRD96 7.0 360 50 -35 3.95 0.4174 WVFGRD96 8.0 360 50 -35 4.00 0.4397 WVFGRD96 9.0 10 65 -25 4.02 0.4484 WVFGRD96 10.0 190 60 -20 4.04 0.4628 WVFGRD96 11.0 190 60 -15 4.06 0.4754 WVFGRD96 12.0 190 60 -15 4.08 0.4840 WVFGRD96 13.0 190 65 -15 4.10 0.4887 WVFGRD96 14.0 190 65 -15 4.11 0.4897 WVFGRD96 15.0 190 65 -15 4.12 0.4858 WVFGRD96 16.0 190 70 -15 4.14 0.4800 WVFGRD96 17.0 190 70 -15 4.15 0.4708 WVFGRD96 18.0 195 75 -5 4.18 0.4596 WVFGRD96 19.0 190 75 -10 4.18 0.4463 WVFGRD96 20.0 190 75 -10 4.19 0.4348 WVFGRD96 21.0 190 70 -15 4.22 0.4248 WVFGRD96 22.0 190 70 -10 4.24 0.4164 WVFGRD96 23.0 195 70 0 4.26 0.4085 WVFGRD96 24.0 195 70 0 4.27 0.3998 WVFGRD96 25.0 195 75 0 4.28 0.3922 WVFGRD96 26.0 195 75 0 4.28 0.3846 WVFGRD96 27.0 195 75 0 4.29 0.3762 WVFGRD96 28.0 195 75 0 4.30 0.3666 WVFGRD96 29.0 195 55 -15 4.29 0.3585
The best solution is
WVFGRD96 14.0 190 65 -15 4.11 0.4897
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
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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= 304.98 DIP= 59.99 RAKE= -120.00 OR STK= 174.08 DIP= 41.41 RAKE= -49.12 DEPTH = 10.0 km Mw = 4.17 Best Fit 0.8864 - P-T axis plot gives solutions with FIT greater than FIT90
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) MSO 307 205 IMW 161 216 MOOW 159 237 REDW 164 275 EGMT 31 298 HLID 222 319 AHID 170 336 BW06 151 376 LAO 74 443 HWUT 178 460 RWWY 139 583 HAWA 280 603 WVOR 238 660 PHWY 132 711 LON 282 780 MVU 183 805 ISCO 140 832 HUMO 253 958 TPH 210 963 MNV 215 967 SDCO 147 1031 WDC 240 1044 DAC 207 1158 GSC 202 1234 ISA 209 1253 SAO 222 1282 MWC 205 1385 GLA 192 1433 TUC 176 1494 BAR 198 1510 AMTX 141 1525 EYMN 74 1564 MNTX 158 1655 CCM 109 1900 SLM 106 1941 LTX 156 1956 FVM 108 1965 JCT 144 1990 UALR 120 2046 SIUC 108 2071 PVMO 112 2117 NATX 130 2143 UTMT 110 2182 MPH 115 2188 WVT 110 2272 PLAL 112 2330 LTL 126 2486 ERPA 88 2570 MCWV 93 2685 SSPA 90 2794 CBN 94 2949 NCB 80 2951 NHSC 106 3036 PAL 86 3087 HRV 82 3208 DWPF 116 3306
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 4 p 2
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files: