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

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Intermountain Western US

Focal Mechanism

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
Figure 1. Location of broadband stations used to obtain focal mechanism


  STK=     189.99
  DIP=      65.00
 RAKE=     -99.99
  STK=      32.62
  DIP=      26.81
 RAKE=     -69.59
DEPTH = 2.0 km
Mw = 4.17
Best Fit 0.8497 - P-T axis plot gives solutions with FIT greater than FIT90

Focal Mechanism

Surface-wave analysis

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 .

Data preparation

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
Best mechanism fit as a function of depth. The preferred depth is given above. Lower hemisphere projection

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.

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.

Love-wave radiation patterns

Rayleigh-wave radiation patterns

First motion data

The P-wave 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

Broadband station distributiuon

The P-wave 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

Waveform comparison for this mechanism

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:

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 0.05 3


The Future

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.

Appendix A

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.

Quality Control

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

Last Changed Tue Oct 5 17:24:32 CDT 2004