Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Intermountain 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=     154.99
  DIP=      64.99
 RAKE=    -120.00
  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

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.

Data preparation

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.
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 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.

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.

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
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

Broadband station distributiuon

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

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 observed and predicted waveforms.

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 0.10 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 Mon Aug 2 15:49:04 CDT 2004