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= 36.66
DIP= 65.81
RAKE= -38.96
OR
STK= 144.99
DIP= 55.00
RAKE= -149.99
DEPTH = 10.0 km
Mw = 4.80
Best Fit 0.8851 - 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. The figure
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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 rpeferred 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. The Each solution is plotted as a vector at a given value of strike and dip witht he 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:
Sta Az(deg) Dist(km) First motion LKWY 359 109 e- BW06 143 113 e+ YMR 339 129 i- HWUT 204 239 e+ BOZ 337 250 ii- HVU 225 280 X CTU 200 340 X
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) BW06 143 113 YMR 339 129 HWUT 204 239 BOZ 337 250 HVU 225 280 SPUT 215 304 CTU 200 340 JLU 195 342 BGU 217 367 MPU 195 409 DUG 209 426 MSO 324 456 ELK 234 509 ISCO 135 578 BMN 241 665 WALA 337 666 WVOR 262 684 SDCO 146 768 TPH 226 840 MNV 231 867 TPNV 216 889 WUAZ 186 899 WCN 242 914 BEKR 247 925 PNT 315 952 ANMO 159 1016 YBH 263 1030 CBKS 117 1037 CMB 237 1043 WDC 256 1058 HLSB 310 1076 HRMB 309 1087 EDM 350 1094 ISA 221 1119 PIMB 308 1134 GOWB 305 1140 SILB 304 1140 COQB 308 1145 ENGB 306 1147 ANMB 308 1149 PGC 304 1154 SSIB 304 1155 KELB 304 1163 LLLB 316 1169 MGCB 303 1169 TSJB 303 1188 TWBB 302 1195 WSLR 312 1199 TWGB 302 1207 YOUB 304 1220 TUC 182 1251 KSU1 109 1254 ULM 51 1328 CBB 307 1346 PHC 308 1511 BBB 312 1628 CCM 104 1723 JCT 144 1729 SLM 101 1774 FVM 104 1791 UALR 116 1834 SIUC 103 1899 MOBC 312 1907 FNBB 337 1910 PVMO 107 1931 HKT 135 1988 MPH 111 1990 UTMT 106 2000 USIN 100 2010 FCC 28 2026 BLO 96 2048 OXF 112 2068 WCI 98 2107 YKW3 354 2130 DLBC 328 2136 IHLN 359 2195 KAPO 63 2222 LRAL 112 2345 SADO 76 2487 WHY 328 2505 KGNO 77 2705 GAC 73 2758 DAWY 331 2922 INK 342 3066 LMQ 68 3123 ICQ 63 3324 SCHQ 53 3346 FRB 35 3457 RES 8 3550 LMN 70 3571 DRLN 62 4030
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 osberved 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
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 osberved 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 0.50 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 Canadian Data were obntained from the excellent seismic network operated by the Geological Survey of Canada.
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 valide response file: CPCT, GLA, GLAT, HALT, PDFC, LON, YKW3
ELK had glitches on the horizontals: ELK
