2001/07/19 20:15:34 38.73N 111.52W 3.7 4.3M UTAH USGS 2001/07/19 20:15:33.79 38.7407 111.5562 0.06 4.25 ML 22
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= 189.99 DIP= 65.00 RAKE= 99.99 OR STK= 32.62 DIP= 26.81 RAKE= 69.59 DEPTH = 2.0 km Mw = 4.17 Best Fit 0.8497  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 MVU 247 65 iP_C SRU 63 96 iP_D MPU 356 142 iP_ DUG 326 196 iP_ CTU 354 218 iP_D NOQ 346 219 iP_+ HVU 342 355 iP_+ ELK 306 389 eP_X BW06 19 479 eP_X TPH 263 503 eP_X ISCO 75 523 eP_X BMN 292 524 eP_X MNV 268 578 eP_X
The Pwave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) MVU 248 65 DUG 326 197 ELK 306 389 TPNV 246 460 BW06 20 479 TPH 264 503 ISCO 75 523 BMN 293 525 DAN 218 570 MNV 269 579 HLID 337 589 DAC 245 602 GSC 233 605 MPM 242 606 ANMO 131 617 CWC 248 633 MLAC 262 652 LKWY 8 655 VTV 230 695 GLA 206 697 ISA 243 705 SVD 226 716 TUC 174 716 KNW 222 726 WVOR 307 729 SND 220 735 CRY 221 741 RDM 222 741 BOZ 359 769 MWC 232 770 JCS 218 776 CMB 267 778 PAS 232 783 MONP 216 784 OSI 237 789 MOD 298 825 TOV 234 828 RPV 230 830 SOL 220 833 CIA 228 857 RSSD 44 865 SBC 240 874 PHL 248 883 SAO 259 901 WDC 286 967 HAWA 325 1075 NEW 339 1152 LLLB 332 1552 SLM 84 1848 SIUC 86 1952
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 valide response file: HGU, CPCT, GLA, GLAT, HALT, JSU, KSU1, PDFC, LON, SPUT, USIN, YKW3
ELK had glitches on the horizontals: ELK