98/01/02 07:28:29 38.18N 112.46W 10.0 4.5Ml A UTAH
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.

NODAL PLANES STK= 131.82 DIP= 74.81 RAKE= 138.24 OR STK= 234.99 DIP= 50.00 RAKE= 20.00 DEPTH = 16.0 km Mw = 4.52 Best Fit 0.8485  PT axis plot gives solutions with FIT greater than FIT90
Surface wave analysis was performed using codes from Computer Programs in Seismology, specifically the multiple filter analysis program do_mft and the surfacewave 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 Lovewave 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

Pressuretension axis trends. Since the surfacewave 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 Taxes 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. 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 strikeslip fault striking at 75 or 165 degrees is preferred. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0180 degrees are sampled. 
The Pwave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion DUG 352 226 X TPNV 248 360 i ELK 321 372 X ISA 244 604 e+ PFO 217 622 e
The Pwave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) DUG 352 226 TPNV 248 360 ELK 321 372 HWUT 11 388 TPH 270 417 NEE 208 421 BMN 303 481 MNV 275 499 DAC 247 502 AHID 12 519 TIN 258 524 MLAC 265 564 BW06 25 567 ISA 244 604 SVD 224 617 ISCO 71 619 PFO 217 622 KNW 219 628 SND 217 638 WMC 218 639 FRD 217 642 CRY 218 643 RDM 219 643 ALQ 122 646 BZN 217 646 DGR 220 651 LVA2 216 653 PAS 230 682 CMB 271 695 USC 230 700 WVOR 314 706 TOV 234 727 RPV 229 729 SOL 218 736 SBC 240 773 PHL 249 785 SAO 262 810 SCZ 260 811 SACA 199 832 SAFE 195 839 OBTO 200 840 TELM 204 869 CBKS 82 1111 COR 312 1149 RAIO 319 1222 PMB 332 1605 MIAR 98 1740 CCM 84 1857 SLM 82 1937 WCI 82 2284 YKW3 358 2713 SSPA 74 2970 WIND 70 3253 VASR 71 3291 HRV 69 3474
Since the analysis of the surfacewave radiation patterns uses only spectral amplitudes and because the surfavewave radiation patterns have a 180 degree symmetry, each surfacewave solution consists of four possible focal mechanisms corresponding to the interchange of the P and Taxes and a roation of the mechanism by 180 degrees. To select one mechanism, Pwave first motion can be used. This was not possible in this case because all the Pwave first motions were emergent ( a feature of the Pwave 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:
This figure shows the fit to the three components of motion (Z  vertical, Rradial 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 0.05 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 100200 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 cmsec) 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: