2005/07/26 04:08:36 45.40N 112.55W 14 5.44 Montana
USGS Felt map for this earthquake
USGS Felt reports page for Intermountain Western US
05/07/26 04:08:36.09 WESTERN MONTANA Epicenter: 45.399 -112.550 MW 5.5 USGS MOMENT TENSOR SOLUTION Depth 9 No. of sta: 17 Moment Tensor; Scale 10**17 Nm Mrr=-1.66 Mtt= 0.91 Mff= 0.75 Mrt=-0.61 Mrf=-0.39 Mtf=-1.17 Principal axes: T Val= 2.01 Plg= 3 Azm=222 N -0.04 23 131 P -1.97 67 319 Best Double Couple:Mo=2.0*10**17 NP1:Strike=335 Dip=47 Slip= -57 NP2: 112 52 -120 ####### ----############# ----------########### --------------########### ------------------########### ---------------------########## ----------- --------######### ##---------- P ---------######### ###--------- ----------######## ####---------------------######## ######--------------------####### ########------------------####### ###########--------------###### ###############----------####-- ########################----- ##################---- T ##################-- ################- ####### |
July 26, 2005, WESTERN MONTANA, MW=5.6 Natasha Maternovskaya CENTROID, MOMENT TENSOR SOLUTION HARVARD EVENT-FILE NAME C072605A DATA USED: GSN L.P. BODY WAVES: 50S, 98C, T= 40 SURFACE WAVES: 69S,155C, T= 50 CENTROID LOCATION: ORIGIN TIME 04:08:39.4 0.1 LAT 45.34N 0.01;LON 112.50W 0.01 DEP 13.3 0.5;HALF-DURATION 1.5 MOMENT TENSOR; SCALE 10**24 D-CM MRR=-1.95 0.04; MTT=-0.04 0.03 MPP= 1.99 0.04; MRT=-1.13 0.09 MRP= 0.13 0.08; MTP=-1.46 0.03 PRINCIPAL AXES: 1.(T) VAL= 2.84;PLG= 8;AZM=240 2.(N) -0.31; 28; 146 3.(P) -2.54; 61; 345 BEST DOUBLE COUPLE:M0=2.7*10**24 NP1:STRIKE=359;DIP=44;SLIP= -48 NP2:STRIKE=127;DIP=59;SLIP=-123 -------#### -------------###### -----------------###### --------------------####### ##--------- --------####### ###--------- P ---------####### ####-------- ---------####### ######-------------------######## #######-------------------####### ########------------------####### ##########---------------######## ###########-------------####### # ##########----------####### T #############------####### ##################-#####- #################------ ##############----- #######---- |
SLU Moment Tensor Solution 2005/07/26 04:08:36 45.40N 112.55W 14 5.44 Montana Best Fitting Double Couple Mo = 1.80e+24 dyne-cm Mw = 5.47 Z = 10 km Plane Strike Dip Rake NP1 359 58 -42 NP2 115 55 -140 Principal Axes: Axis Value Plunge Azimuth T 1.80e+24 2 58 N 0.00e+00 39 149 P -1.80e+24 51 325 Moment Tensor: (dyne-cm) Component Value Mxx 2.78e+22 Mxy 1.14e+24 Mxz -6.92e+23 Myy 1.06e+24 Myz 5.49e+23 Mzz -1.09e+24 --------###### --------------######## ------------------########## --------------------########## -----------------------########## ----------- ----------########## T ------------ P -----------######### ##----------- -----------############# ###-------------------------############ #####------------------------############# ######-----------------------############# #######----------------------############# #########--------------------############# ###########-----------------############ #############---------------############ ################----------############ ####################-----#########-- #######################----------- ####################---------- ##################---------- ##############-------- #########----- Harvard Convention Moment Tensor: R T F -1.09e+24 -6.92e+23 -5.49e+23 -6.92e+23 2.78e+22 -1.14e+24 -5.49e+23 -1.14e+24 1.06e+24 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/NEW/20050726040836/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 = 115 DIP = 55 RAKE = -140 MW = 5.47 HS = 10
The preferred solution is based on the surface-wave amplitude spectrum fit because sprectral holes int he Rayleigh mechanism are a direct indicator of depth and mechanism.
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 0.01 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 0.5 200 60 20 5.07 0.3415 WVFGRD96 1.0 200 65 25 5.10 0.3552 WVFGRD96 2.0 195 70 15 5.17 0.3942 WVFGRD96 3.0 195 70 10 5.22 0.4003 WVFGRD96 4.0 190 65 -5 5.27 0.3968 WVFGRD96 5.0 10 75 -40 5.32 0.4063 WVFGRD96 6.0 360 60 -45 5.37 0.4345 WVFGRD96 7.0 360 60 -45 5.38 0.4591 WVFGRD96 8.0 360 60 -50 5.42 0.4839 WVFGRD96 9.0 360 55 -45 5.43 0.5033 WVFGRD96 10.0 360 55 -45 5.44 0.5175 WVFGRD96 11.0 360 55 -45 5.44 0.5252 WVFGRD96 12.0 10 65 -35 5.42 0.5316 WVFGRD96 13.0 10 65 -35 5.43 0.5361 WVFGRD96 14.0 10 65 -30 5.44 0.5379 WVFGRD96 15.0 10 65 -30 5.45 0.5377 WVFGRD96 16.0 10 65 -30 5.45 0.5358 WVFGRD96 17.0 10 65 -25 5.46 0.5309 WVFGRD96 18.0 10 65 -25 5.47 0.5258 WVFGRD96 19.0 15 75 -25 5.47 0.5200
The best solution is
WVFGRD96 14.0 10 65 -30 5.44 0.5379
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 0.01 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= 114.99 DIP= 59.99 RAKE= -135.00 OR STK= 358.42 DIP= 52.24 RAKE= -39.24 DEPTH = 10.0 km Mw = 5.47 Best Fit 0.8617 - 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 MSO 326 192 eP_X LKWY 118 193 iP_C REDW 149 264 eP_- AHID 158 315 eP_+ BW06 140 378 iP_C WALA 346 420 iP_- HWUT 169 429 eP_+ WVOR 238 590 iP_C
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 326 192 LKWY 118 193 REDW 148 264 AHID 158 315 BW06 140 378 WALA 346 420 HWUT 169 429 LAO 71 510 WVOR 238 590 PNT 312 688 TTW 293 746 SLEB 329 763 GNW 291 825 EDM 356 872 TPH 207 901 PGC 298 904 LLLB 313 905 SHB 303 972 WDC 240 974 SDCO 143 1033 OZB 297 1057 CBB 303 1090 DAC 204 1098 GSC 199 1178 ISA 207 1191 EDB 300 1201 SAO 221 1214 PHC 303 1254 MWC 203 1327 ULM 61 1356 BBB 309 1367 GLA 189 1385 BAR 196 1456 TUC 173 1461 KSU1 112 1487 MNTX 155 1643 MOBC 309 1646 FNBB 338 1660 DLBC 327 1874 YKW3 357 1917 LTX 153 1946 SLM 104 1987 FVM 106 2010 KNDN 5 2019 SNPN 2 2022 MGTN 4 2046 BOXN 4 2064 LON 285 2076 MLON 4 2076 UALR 118 2079 MCKN 3 2100 ILKN 356 2104 GALN 354 2107 LGSN 3 2115 SIUC 106 2116 CTLN 355 2132 DVKN 3 2134 LDGN 3 2141 EKTN 2 2154 PVMO 110 2158 GLWN 4 2163 NATX 128 2164 YMBN 1 2171 ACKN 2 2186 COWN 2 2216 USIN 103 2220 UTMT 109 2225 MPH 113 2226 WHY 327 2244 LUPN 2 2268 KAPO 68 2297 OXF 114 2306 WCI 101 2311 YRTN 27 2334 PLAL 111 2370 LRAL 113 2582 SADO 80 2614 ALLY 88 2636 MCWV 92 2743 QILN 24 2832 SDMD 90 2989 NHSC 105 3082 PAL 85 3149 HRV 81 3272 DWPF 114 3343
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 0.01 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: