2000/11/24 04:20:05 44.82N 110.59W 10.0 4.6Mb B YELLOWSTONE REGION
2000 11 24 420 6.12 44 44.96 110 41.60 4.89 4.76 28 72 4.0 0.16 0.4 0.9
USGS Felt map for this earthquake
USGS Felt reports page for Intermountain Western US
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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NODAL PLANES
STK= 109.99
DIP= 64.99
RAKE= -110.00
OR
STK= 330.72
DIP= 31.61
RAKE= -53.75
DEPTH = 6.0 km
Mw = 4.46
Best Fit 0.8229 - 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, intreument 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. A nearly vertical strike-slip fault striking at 75 or 165 degrees is preferred. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0-180 degrees are sampled. |
The P-wave first motion data for focal mechanism studies are as follow (1 = strong compression 2 = weak compression, -1 = strong dilatation, -2 = weak dilataion, 0 = indeterminate polarity. Take off angle is measured with respect to downward vertical)
Sta Az(deg) Takeoff Angle First motion 137 46 -2 'Y35 ' 142 63 -2 'Y32 ' 352 46 -2 'Y61 ' 104 46 0 'Y64 ' 110 63 0 'Y43 ' 157 46 0 'BW06 ' 188 46 0 'AHID ' 191 45 0 'HWUT ' 247 46 0 'HLID ' 289 63 0 'Y13 ' 297 46 0 'Y12 ' 312 63 0 'Y24 ' 314 46 0 'Y23 ' 323 63 0 'BOZ ' 327 63 0 'Y37 ' 335 63 0 'Y39 ' 101 78 1 'Y100 ' 146 63 1 'Y101 ' 176 63 1 'Y19 ' 183 46 1 'Y07 ' 193 63 1 'Y18 ' 202 63 1 'Y05 ' 206 78 1 'Y103 ' 208 63 1 'Y17 ' 215 63 1 'Y04 ' 231 63 1 'Y03 ' 241 63 1 'Y16 ' 244 63 1 'Y02 ' 259 63 1 'Y01 ' 260 63 1 'Y15 ' 356 78 1 'Y105 ' 120 63 2 'Y44 ' 138 46 2 'Y34 ' 140 46 2 'Y33 ' 162 63 2 'Y20 ' 172 64 2 'Y102 ' 177 63 2 'Y106 ' 19 63 2 'Y47 ' 281 63 2 'Y14 ' 355 63 2 'Y40 ' 71 63 2 'Y50 ' 95 46 2 'Y63 '
Sta Az(deg) Dist(km) Y40 348 61 Y16 240 93 Y17 211 93 Y15 258 120 BOZ 319 123 Y18 197 124 Y19 180 134 Y44 125 137 Y47 18 137 Y03 231 178 Y13 287 183 Y37 324 188 Y05 204 198 Y61 350 200 Y24 310 205 Y01 258 210 Y63 98 214 Y12 294 216 AHID 190 232 BW06 160 242 Y23 312 244 Y07 186 252 Y64 107 260 HLID 247 336 HWUT 193 366 CTU 192 468 RSSD 96 527 WALA 333 535 MPU 190 541 DUG 200 545 L03 134 588 L02 134 590 ELK 222 591 L01 134 592 NEW 310 630 SRU 180 634 WVOR 251 701 MVU 191 715 HAWA 287 719 BMN 230 730 UT54 180 822 UT53 178 831 UT52 177 844 PNT 309 848 MOD 251 851 UT51 176 862 AZ50 176 873 AZ49 175 884 LON 288 898 AZ48 174 900 AZ47 173 915 AZ46 172 926 TPH 219 931 NM44 171 943 MNV 224 948 EDM 349 956 NM43 171 963 BEK 239 974 WCN 234 975 NM42 170 976 TPNV 210 996 NM40 169 1003 OMM 105 1003 COR 273 1007 NM39 168 1022 WUAZ 184 1035 MLAC 224 1055 TIN 219 1075 WDC 248 1086 CMB 231 1112 CBKS 123 1120 CWC 217 1125 NM32 164 1129 MPM 213 1135 NM31 164 1147 ANMO 161 1152 GSC 209 1182 ISA 216 1217 NM26 161 1227 MB04B 164 1234 ULM 56 1259 VTV 209 1277 SCZ 229 1290 SVD 207 1314 OSI 214 1329 MB01 161 1334 MWC 211 1339 KNW 205 1341 PFO 204 1343 PHL 222 1344 PAS 211 1351 RDM 206 1354 SND 204 1354 WMC 205 1354 CRY 205 1357 DGR 206 1357 GLA 197 1357 FRD 204 1360 BZN 205 1362 DJJ 212 1367 USC 211 1369 LVA2 204 1373 TOV 214 1378 PLM 205 1383 TUC 181 1389 SBC 217 1392 NM15 158 1400 JCS 204 1402 MONP 203 1417 CIA 211 1436 BAR 203 1447 SOL 206 1449 NM10 156 1476 NM08 156 1514 TX06 155 1547 TX05 155 1560 TX01 154 1624 MOBC 309 1806 SLM 105 1822 FCC 30 1913 YKW1 354 1982
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 0.06 3
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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.
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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:
