2006/06/17 16:22:14 45.72N 111.84W 5. 3.9 Montana
DATE ORIGIN LAT N LONG W DEPTH MAG NO DM GAP M RMS ERH ERZ Q 2060617 162213.54 45.60 111.91 11.24 3.84 48 22 44 1 0.16 0.3 0.5 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
2006/06/17 16:22:13 45.60N 111.91W 11 3.84 Montana
Best Fitting Double Couple
Mo = 6.24e+21 dyne-cm
Mw = 3.83
Z = 10 km
Plane Strike Dip Rake
NP1 308 51 -124
NP2 175 50 -55
Principal Axes:
Axis Value Plunge Azimuth
T 6.24e+21 1 61
N 0.00e+00 26 331
P -6.24e+21 64 152
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.14e+20
Mxy 3.14e+21
Mxz 2.21e+21
Myy 4.52e+21
Myz -1.08e+21
Mzz -5.03e+21
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#########------##################
#########-----------############## T
##########--------------###########
##########------------------############
##########-------------------###########
###########---------------------##########
###########----------------------#########
###########------------------------#######
###########-------------------------######
###########----------- ----------#####
###########----------- P -----------####
###########---------- -----------###
##########-------------------------#
##########------------------------
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##########------------------
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Harvard Convention
Moment Tensor:
R T F
-5.03e+21 2.21e+21 1.08e+21
2.21e+21 5.14e+20 -3.14e+21
1.08e+21 -3.14e+21 4.52e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/NEW/20060617162214/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 = 175
DIP = 50
RAKE = -55
MW = 3.83
HS = 10
There is good agreement between the waveform inversion and surface-wave spectral amplitude focal mechanisms. Care was taken in the use of the BOZ station, which is only about 19 km away to downweight its influence on the waveform grid search. The Montana Bureau of Mines and Geology solution is required to fit the BOZ waveforms. The Montana local magnitude agrees with the determined Mw.
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion BOZ 76 22 iP_C LKWY 138 171 ePn IMW 160 214 iP_D MOOW 158 235 iP_D REDW 163 273 ePn HLID 221 315 ePn AHID 170 333 eP_- LAO 74 447 ePb HWUT 177 457 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 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 90 45 -80 3.39 0.2948
WVFGRD96 1.0 110 90 0 3.46 0.3087
WVFGRD96 2.0 110 90 0 3.55 0.3419
WVFGRD96 3.0 110 90 0 3.61 0.3362
WVFGRD96 4.0 190 55 -25 3.68 0.3260
WVFGRD96 5.0 300 80 50 3.71 0.3595
WVFGRD96 6.0 300 80 50 3.73 0.3911
WVFGRD96 7.0 305 75 50 3.74 0.4061
WVFGRD96 8.0 175 45 -55 3.83 0.4181
WVFGRD96 9.0 170 45 -60 3.83 0.4294
WVFGRD96 10.0 175 50 -55 3.83 0.4332
WVFGRD96 11.0 175 50 -50 3.81 0.4312
WVFGRD96 12.0 180 55 -45 3.81 0.4286
WVFGRD96 13.0 185 55 -35 3.81 0.4238
WVFGRD96 14.0 185 60 -35 3.82 0.4189
WVFGRD96 15.0 185 60 -35 3.82 0.4141
WVFGRD96 16.0 190 65 -25 3.84 0.4090
WVFGRD96 17.0 190 65 -25 3.85 0.4039
WVFGRD96 18.0 190 65 -25 3.85 0.3980
WVFGRD96 19.0 190 65 -20 3.86 0.3917
WVFGRD96 20.0 190 65 -20 3.86 0.3862
WVFGRD96 21.0 190 65 -25 3.92 0.3790
WVFGRD96 22.0 190 65 -25 3.93 0.3701
WVFGRD96 23.0 190 65 -25 3.94 0.3606
WVFGRD96 24.0 25 90 50 3.92 0.3515
WVFGRD96 25.0 205 90 -50 3.93 0.3448
WVFGRD96 26.0 25 90 50 3.94 0.3373
WVFGRD96 27.0 205 90 -50 3.94 0.3279
WVFGRD96 28.0 25 90 45 3.96 0.3192
WVFGRD96 29.0 30 90 45 3.95 0.3101
The best solution is
WVFGRD96 10.0 175 50 -55 3.83 0.4332
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= 309.98
DIP= 59.99
RAKE= -120.00
OR
STK= 179.08
DIP= 41.41
RAKE= -49.12
DEPTH = 10.0 km
Mw = 3.91
Best Fit 0.8870 - P-T axis plot gives solutions with FIT greater than FIT90
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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. |
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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. |
Sta Az(deg) Dist(km) IMW 160 214 MOOW 158 235 REDW 163 273 HLID 221 315 AHID 170 333 LAO 74 447 HWUT 177 457 RWWY 139 583 HAWA 280 600 WVOR 239 656 PHWY 131 712 LON 282 778 ISCO 140 831 HUMO 253 954 TPH 210 959 MNV 215 963 WDC 240 1040 GSC 202 1230 ISA 209 1249 SAO 222 1278 MWC 205 1381 GLA 191 1430 TUC 176 1491 EYMN 74 1568 MNTX 158 1653 CCM 109 1902 FVM 108 1967 MIAR 122 1983 JCT 144 1989 UALR 120 2047 WCI 102 2263 PLAL 112 2332 LRAL 115 2546 ERPA 88 2573 SSPA 90 2797 NCB 80 2954 HRV 82 3212 DWPF 116 3308
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:
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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 0.10 3 br c 0.12 0.2 n 4 p 2
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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:
