Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Central and Southeastern US

Indiana University web page for this earthquake

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=      20.00
  DIP=      90.00
 RAKE=    -165.00
  STK=     290.00
  DIP=      75.00
 RAKE=      -0.01
DEPTH = 7.0 km
Mw = 4.15
Best Fit 0.8280 - P-T axis plot gives solutions with FIT greater than FIT90

Focal Mechanism

Lamont Solution (W. Y. Kim)

SLU Solution

This is the solution used in the source parameter tabulation

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, 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.
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
JFWS	  326	  187	e-
BLO	  141	  334	i+
SLM	  199	  340	e-
USIN	  164	  411	e+
FVM	  198	  414	X
CCM	  208	  433	i-

Broadband station distributiuon

The P-wave first motion data for focal mechanism studies are as follow:

Sta Az(deg)    Dist(km)   
PPMSH	   58	  102
JFWS	  326	  187
PPNVW	  145	  270
BLO	  141	  334
PPEGH	  145	  335
SLM	  199	  340
PPFAY	   86	  386
USIN	  164	  411
FVM	  198	  414
WCI	  147	  432
CCM	  208	  433
PPNAF	  143	  439
AAM	   77	  447
ACSO	  104	  523
PVMO	  187	  572
UTMT	  179	  577
MPH	  187	  717
PLAL	  174	  731
MCWV	  102	  798
UALR	  203	  807
SADO	   62	  874
LRAL	  169	  959
CBKS	  255	  966
KAPO	   28	 1013
CBN	  107	 1056
KGNO	   70	 1060
ULM	  333	 1106
WMOK	  232	 1144
NCB	   72	 1234
MUMO	  355	 1236
PAL	   88	 1260
RSSD	  288	 1264
HNH	   74	 1386
ISCO	  268	 1420
HKT	  208	 1427
HRV	   80	 1442
BRYW	   82	 1447
ANMO	  250	 1694
BW06	  282	 1704
FCC	  351	 1949
LMN	   68	 1996
SCHQ	   40	 2194
EDM	  314	 2236
DRLN	   60	 2585

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 Fri Jul 2 08:46:09 CDT 2004