Arrival Times (from USGS)

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

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=     131.82
  DIP=      74.81
 RAKE=     138.24
  STK=     234.99
  DIP=      50.00
 RAKE=      20.00
DEPTH = 16.0 km
Mw = 4.52
Best Fit 0.8485 - 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
DUG	  352	  226	X
TPNV	  248	  360	i-
ELK	  321	  372	X
ISA	  244	  604	e+
PFO	  217	  622	e-

Broadband station distributiuon

The P-wave 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

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 valid response files:

Last Changed Fri Feb 13 08:30:25 CST 2004