2007/05/08 15:46:49 45.39N 112.13W 13.5 4.7 Montana
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USGS Felt reports page for Intermountain Western US
SLU Moment Tensor Solution 2007/05/08 15:46:49 45.39N 112.13W 13.5 4.7 Montana Best Fitting Double Couple Mo = 4.37e+22 dyne-cm Mw = 4.36 Z = 10 km Plane Strike Dip Rake NP1 345 55 -85 NP2 156 35 -97 Principal Axes: Axis Value Plunge Azimuth T 4.37e+22 10 71 N 0.00e+00 4 162 P -4.37e+22 79 274 Moment Tensor: (dyne-cm) Component Value Mxx 4.30e+21 Mxy 1.29e+22 Mxz 1.74e+21 Myy 3.66e+22 Myz 1.49e+22 Mzz -4.09e+22 ---########### #---------############ ###------------############# ###---------------############ ####-----------------############# ####-------------------############# #####--------------------########## ######---------------------######### T # ######----------------------######## # #######--------- ----------############# #######--------- P -----------############ #######--------- -----------############ ########----------------------############ #######-----------------------########## ########----------------------########## ########---------------------######### ########-------------------######### #########-----------------######## #########---------------###### ##########------------###### ###########-------#### #############- Harvard Convention Moment Tensor: R T F -4.09e+22 1.74e+21 -1.49e+22 1.74e+21 4.30e+21 -1.29e+22 -1.49e+22 -1.29e+22 3.66e+22 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070508154649/index.html |
STK = 345 DIP = 55 RAKE = -85 MW = 4.36 HS = 10
The solutions from the two techniques are equivalent. Note that the waveform inversion down weighted the obervation at BOZ, the shortest distance, because subtle changes in location affect the separation into radial and vertical components and since the larger amplitudes here would dominate the solution. The additon of the TA data did not change the waveforms fit, except that the 0.02 - 0.05 band had to be sued instead of the 0.02 - 0.06 Hz band, perhaps because the current model coutl not fit both the PnL and the Surface wave. This requires more model studies for this area, which is slightly faster than the UTAH-Pechmann model. The surface wave amplitude data give about the soame solution.
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.
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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 0.05 n 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 180 40 -65 4.02 0.3526 WVFGRD96 1.0 195 50 -30 4.03 0.3395 WVFGRD96 2.0 10 65 -55 4.13 0.4057 WVFGRD96 3.0 5 70 -65 4.22 0.4505 WVFGRD96 4.0 0 65 -70 4.25 0.4991 WVFGRD96 5.0 360 65 -70 4.27 0.5402 WVFGRD96 6.0 355 60 -70 4.30 0.5697 WVFGRD96 7.0 350 60 -80 4.32 0.5967 WVFGRD96 8.0 350 60 -80 4.35 0.6293 WVFGRD96 9.0 345 55 -85 4.36 0.6319 WVFGRD96 10.0 350 55 -80 4.35 0.6205 WVFGRD96 11.0 -10 55 -80 4.34 0.5980 WVFGRD96 12.0 0 60 -65 4.32 0.5757 WVFGRD96 13.0 200 95 30 4.30 0.5672 WVFGRD96 14.0 200 95 30 4.30 0.5649 WVFGRD96 15.0 25 95 -25 4.30 0.5615 WVFGRD96 16.0 205 85 25 4.31 0.5580 WVFGRD96 17.0 25 95 -25 4.31 0.5526 WVFGRD96 18.0 205 85 25 4.32 0.5461 WVFGRD96 19.0 205 85 25 4.32 0.5392 WVFGRD96 20.0 205 85 20 4.33 0.5327 WVFGRD96 21.0 205 85 25 4.35 0.5212 WVFGRD96 22.0 205 85 25 4.35 0.5127 WVFGRD96 23.0 25 95 -25 4.36 0.5040 WVFGRD96 24.0 205 85 25 4.36 0.4949 WVFGRD96 25.0 25 95 -25 4.37 0.4858 WVFGRD96 26.0 205 85 25 4.37 0.4762 WVFGRD96 27.0 25 95 -25 4.37 0.4667 WVFGRD96 28.0 25 95 -25 4.38 0.4573 WVFGRD96 29.0 205 80 25 4.38 0.4483
The best solution is
WVFGRD96 9.0 345 55 -85 4.36 0.6319
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 0.05 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. |
The following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
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The surface-wave determined focal mechanism is shown here.
NODAL PLANES STK= 4.87 DIP= 57.20 RAKE= -57.27 OR STK= 134.99 DIP= 45.00 RAKE= -129.99 DEPTH = 10.0 km Mw = 4.40 Best Fit 0.8673 - 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 G15A 228 38 -12345 BOZ 45 55 -12345 F15A 330 57 -12345 G14A 261 106 -12345 F14A 296 107 -12345 E15A 341 121 -12345 E14A 318 155 -12345 G13A 259 168 -12345 F13A 285 177 -12345 D15A 351 186 -12345 H13A 242 191 -12345 E13A 307 198 -12345 D14A 331 216 -12345 MSO 321 220 -12345 TPAW 155 222 -12345 LOHW 147 223 -12345 RLMT 94 226 -12345 I13A 224 228 -12345 H12A 247 235 -12345 F12A 280 247 -12345 D13A 317 261 -12345 HLID 224 265 -12345 J13A 217 275 -12345 C14A 335 292 -12345 D12A 308 309 -12345 H11A 257 315 -12345 C13A 324 316 -12345 F11A 281 319 -12345 G11A 272 324 -12345 K14A 195 327 -12345 E11A 290 334 -12345 J12A 226 336 -12345 K13A 208 343 -12345 I11A 243 345 -12345 BW06 143 349 -12345 EGMT 30 353 -12345 J11A 235 368 -12345 D11A 301 373 -12345 H10A 258 375 -12345 B13A 333 376 -12345 K12A 217 379 -12345 G10A 270 391 -12345 L13A 202 395 -12345 I10A 250 398 -12345 BMO 265 411 -12345 WALA 343 439 -12345 LAO 69 483 -12345 NEW 313 504 -12345
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.
Search over depth. The best fit corresponds to the largest value in the last column.
SRFGRD96 1.0 125. 35. 15. 0.876 0.893 4.25 4.25 0.7818 SRFGRD96 2.0 130. 45. 25. 0.860 0.906 4.23 4.23 0.7782 SRFGRD96 3.0 125. 25. 25. 0.896 0.911 4.36 4.36 0.8079 SRFGRD96 4.0 125. 30. 20. 0.897 0.924 4.33 4.33 0.8280 SRFGRD96 5.0 120. 35. 15. 0.898 0.934 4.32 4.32 0.8371 SRFGRD96 6.0 130. 40. 40. 0.904 0.943 4.34 4.33 0.8492 SRFGRD96 7.0 30. 75. 50. 0.896 0.945 4.32 4.32 0.8412 SRFGRD96 8.0 135. 45. 50. 0.904 0.952 4.37 4.37 0.8532 SRFGRD96 9.0 145. 45. 65. 0.911 0.953 4.41 4.41 0.8618 SRFGRD96 10.0 135. 45. 50. 0.915 0.953 4.40 4.40 0.8673 SRFGRD96 11.0 125. 45. 35. 0.908 0.948 4.38 4.38 0.8607 SRFGRD96 12.0 30. 75. 45. 0.891 0.948 4.37 4.36 0.8344 SRFGRD96 13.0 125. 50. 35. 0.884 0.945 4.38 4.38 0.8351 SRFGRD96 14.0 40. 60. 40. 0.878 0.943 4.38 4.38 0.8191 SRFGRD96 15.0 30. 70. 40. 0.871 0.937 4.38 4.38 0.8133 SRFGRD96 16.0 35. 65. 35. 0.858 0.937 4.38 4.38 0.7859 SRFGRD96 17.0 30. 70. 35. 0.846 0.930 4.39 4.38 0.7707 SRFGRD96 18.0 30. 65. 30. 0.829 0.925 4.39 4.38 0.7606 SRFGRD96 19.0 30. 70. 30. 0.820 0.921 4.40 4.39 0.7299 SRFGRD96 20.0 35. 70. 30. 0.805 0.912 4.39 4.40 0.7264 SRFGRD96 21.0 30. 75. 30. 0.791 0.908 4.40 4.40 0.7129 SRFGRD96 22.0 30. 70. 35. 0.778 0.900 4.43 4.43 0.6988 SRFGRD96 23.0 35. 65. 30. 0.761 0.894 4.43 4.43 0.6787 SRFGRD96 24.0 25. 80. 35. 0.737 0.884 4.43 4.43 0.6458 SRFGRD96 25.0 30. 70. 30. 0.737 0.882 4.44 4.44 0.6492
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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) E15A 341 121 E14A 318 155 G13A 259 168 F13A 285 177 D15A 351 186 H13A 242 191 E13A 307 198 MSO 315 205 D14A 331 216 RLMT 101 223 MOOW 152 224 I13A 224 228 H12A 247 235 LOHW 151 243 TPAW 159 244 F12A 280 247 SNOW 156 253 D13A 317 261 J13A 217 275 HLID 221 289 C14A 335 292 D12A 308 309 H11A 257 315 C13A 324 316 F11A 281 319 G11A 272 324 EGMT 32 327 K14A 195 327 E11A 290 334 J12A 226 336 K13A 208 343 I11A 243 345 BW06 146 367 J11A 235 368 D11A 301 373 H10A 258 375 B13A 333 376 K12A 217 379 G10A 270 391 L13A 202 395 I10A 250 398 F10A 281 402 E10A 289 405 WALA 341 416 BMO 261 420 HVU 188 422 K11A 228 427 J10A 241 429 L12A 214 429 A13A 337 430 B12A 324 431 M15A 184 437 D10A 297 438 G09A 270 443 H09A 261 443 M14A 193 443 F09A 276 453 L11A 220 462 B11A 318 467 LAO 72 468 I09A 252 472 A12A 327 476 K10A 234 478 M13A 201 478 C10A 306 479 E09A 287 483 NEW 310 491 M12A 208 496 N15A 184 501 J09A 245 503 B10A 312 505 L10A 225 508 A11A 322 510 D09A 294 511 N14A 190 512 F08A 277 520 H08A 262 525 M11A 215 531 N13A 199 532 G08A 271 535 I08A 254 537 K09A 238 539 C09A 302 541 E08A 285 551 J08A 248 554 D08A 292 555 N12A 206 557 M10A 221 559 B09A 308 569 A10A 316 573 L09A 232 583 HAWA 282 587 N11A 211 587 K08A 242 589 G07A 271 591 C08A 299 593 H07A 264 593 DUG 186 597 I07A 259 602 O13A 195 604 O12A 202 608 E07A 285 612 J07A 251 614 L08A 237 614 MPU 177 615 M09A 226 616 A09A 312 630 N10A 216 630 WVOR 239 631 D07A 291 636 B08A 304 640 K07A 245 644 H06A 266 650 O11A 208 652 C07A 296 659 I06A 259 660 G06A 272 667 N09A 223 668 M08A 231 669 O10A 214 670 F06A 277 676 P13A 194 678 DGMT 59 680 J06A 252 682 L07A 240 691 P12A 200 696 B07A 302 697 E06A 284 697 D06A 290 700 N08A 226 708 PNT 309 708 K06A 249 711 O09A 217 713 P11A 206 714 G05A 272 720 H05A 267 721 M07A 234 723 C06A 296 727 F05A 278 729 Q13A 193 732 P10A 210 733 I05A 262 736 Q12A 198 740 A07A 306 745 O08A 223 751 N07B 230 756 E05A 283 757 K05A 250 762 J05A 255 763 P09A 214 768 C05A 293 776 Q11A 203 783 D05A 288 785 B06A 299 792 R14A 186 792 L05A 245 793 H04A 268 796 P08A 220 799 O07A 226 801 F04A 278 804 Q10A 207 810 G04A 272 811 R12A 196 811 N06A 233 812 A06A 304 818 M06C 238 821 B05A 296 824 ISCO 138 827 K04A 252 829 E04A 283 831 R11A 201 833 I04A 261 835 Q09A 212 840 D04A 287 844 S14A 186 852 A05A 302 853 P07A 223 855 M05C 242 856 O06A 230 856 R10A 205 862 L04A 249 863 Q08A 215 869 C04A 291 870 G03A 273 873 ELFS 236 878 M04C 246 880 S13A 190 880 H03A 269 882 A04A 298 884 J03A 259 891 R09A 209 894 S12A 196 894 E03A 282 895 I03A 264 897 P06A 228 899 Q07A 220 904 S10A 206 909 S11A 200 912 HATC 239 913 B04A 293 917 D03A 286 917 LLLB 311 923 O04C 236 923 R08A 214 926 T14A 185 929 M03C 244 930 NLWA 287 930 T15A 181 930 T16A 177 935 H02A 269 938 I02A 265 939 J02A 260 939 O05C 233 939 T11A 197 942 T13A 190 942 S09A 208 951 K02A 256 955 P05C 229 971 R06C 221 974 M02C 247 975 T12A 194 987 L02A 253 990 R05C 224 991 R07C 218 994 SHB 301 996 U14A 185 1001 O03C 237 1008 U13A 189 1009 S08C 212 1011 U12A 192 1016 TPNV 202 1017 LAVA 227 1024 K01A 258 1028 U11A 196 1033 N02C 245 1040 M01C 252 1044 O02C 240 1046 SUTB 233 1049 Q04C 230 1055 V13A 189 1071 R04C 226 1077 CMB 223 1083 OZB 295 1085 V14A 185 1087 V11A 196 1097 V12A 193 1098 Q03C 231 1109 CBB 301 1114 WUAZ 177 1114 O01C 243 1119 S05C 220 1125 T06C 217 1126 HELL 213 1127 P01C 238 1131 W14A 184 1133 W15A 181 1134 W12A 193 1144 W13A 188 1153 S04C 224 1164 X13A 188 1208 U05C 216 1209 X15A 180 1211 X14A 183 1215 X16A 177 1220 EDB 298 1226 U04C 219 1238 ECSD 94 1240 BNLO 226 1243 V05C 214 1244 CBKS 122 1260 AGMN 70 1268 Y15A 181 1270 Y14A 184 1274 HAST 222 1275 PHC 302 1278 Y16A 177 1280 V03C 220 1292 Y19A 168 1293 Y13A 187 1294 Y12C 190 1309 V04C 217 1311 ULM 61 1314 Z14A 183 1338 Z16A 177 1340 BMBC 332 1365 Z18A 172 1378 BBB 308 1386 115A 180 1409 119A 169 1423 116A 178 1425 117A 175 1428 118A 172 1428 109C 199 1453 KSU1 114 1456 AMTX 140 1474 216A 178 1488 218A 172 1501 219A 170 1508 318A 172 1560 SCIA 98 1566 319A 170 1576 EYMN 73 1588 MNTX 157 1641 WMOK 132 1644 FNBB 338 1659 MOBC 308 1666 JFWS 92 1761 COWI 80 1776 WRAK 318 1857 DLBC 326 1881 YKW2 356 1887 YKW1 356 1892 YKW3 356 1902 FCC 33 1905 HDIL 99 1927 SLM 105 1953 FVM 108 1976 JCT 143 1983 MIAR 122 1986 UALR 119 2052 SIUC 107 2082 PVMO 111 2126 BLO 100 2208 WHY 326 2250 KAPO 68 2253 OXF 115 2276 AAM 89 2296 ACSO 94 2432 SADO 80 2571 TZTN 103 2579 ERPA 87 2589 BLA 98 2782 GOGA 109 2782 EGAK 330 2783 LONY 78 2925 NCB 80 2972 NHSC 106 3048 LBNH 78 3140
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 n 3 lp c 0.05 n 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 digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface
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: