USGS Felt map for this earthquake
USGS Felt reports page for Intermountain 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= 154.99
DIP= 64.99
RAKE= -120.00
OR
STK= 28.78
DIP= 38.29
RAKE= -43.01
DEPTH = 4.0 km
Mw = 4.28
Best Fit 0.8740 - 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.
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. 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 distribution of broadband stations with azimuth and distance is
Sta Az(deg) Dist(km) SDCO 332 108 ISCO 350 329 CBKS 63 503 WUAZ 257 600 BW06 330 764 HWUT 314 777 DUG 300 780 AHID 323 840 MIAR 101 1058 LAO 355 1094 HLID 316 1095 TPH 281 1096 BOZ 332 1123 DAC 270 1136 UALR 98 1159 MNV 282 1179 CCM 80 1215 ISA 268 1225 FVM 80 1287 SLM 77 1307 WVOR 302 1327 MSO 329 1333 PVMO 88 1360 MPH 94 1364 CMB 280 1373 SIUC 82 1394 JFWS 57 1419 OXF 96 1427 SAO 274 1473 USIN 80 1530 PLAL 93 1532 WDC 290 1583 HAWA 316 1607 NEW 326 1609 BLO 76 1632 ULM 23 1652 LRAL 99 1689 PNT 324 1821 MYNC 90 1882 AAM 65 1917 EDM 343 1931 LLLB 324 2038 OZB 315 2138 ALLY 68 2194 CBB 318 2195 MCWV 74 2208 ERPA 67 2213 KAPO 45 2281 NHSC 93 2290 SADO 60 2332 SSPA 72 2377 CBN 78 2430 BBB 320 2488 KGNO 62 2527 FCC 14 2558 PAL 71 2713 ACCN 65 2742 FNBB 338 2777 YKW4 350 2929 DLBC 331 3014 LMQ 56 3061 ICQ 53 3323 WHY 331 3384 LMN 61 3461 SCHQ 43 3503 DAWY 334 3799 FRB 28 3855 DRLN 55 4025
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 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
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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:
