Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Central and Southeastern US

Waveform inversion focal mechanism

This event was recorded on scale at short distances. The waveform inversion code was used to invert the PLAL and LRAL waveforms using the CUS model
Figure 1. Waveform inversion focal mechanism
Figure 2. Depth sensitivity for waveform mechanism
Figure 3. Mechanism sensitivity for waveform mechanism for depth of 2 km

The waveform inversion solution has the following parameters:

Depth  2.0
Strike 235
Dip     50
Rake   -60
Mw    3.62
The next figure compares the observed and predicted waveforms for these two stations. The traces are filtered using the SAC command
hp c 0.02 3
lp c 0.10 3
Figure 4. Waveform comparison for depth of 8 km

Focal Mechanism

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 5.
Figure 5. Location of broadband stations used to obtain focal mechanism


  STK=     221.50
  DIP=      60.50
 RAKE=     -84.28
  STK=      29.99
  DIP=      30.00
 RAKE=    -100.00
DEPTH = 4 km
Mw = 3.63
Best Fit 0.8793 - P-T axis plot gives solutions with FIT greater than FIT90

Focal Mechanism

First motion data

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

Sta Az(deg)    Dist(km)   First motion
LRLA      203      17      e-
PLAL	  332	  227      e+

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

Broadband station distributiuon

The distribution of broadband stations with azimuth and distance is

Sta Az(deg)    Dist(km)   
PLAL	  332	  227
OXF	  304	  273
MPH	  309	  351
UTMT	  334	  393
PVMO	  326	  440
UALR	  291	  531
USIN	  353	  535
WCI	    6	  563
FVM	  330	  620
NHSC	   89	  629
MIAR	  286	  634
BLO	    3	  666
SLM	  335	  675
DWPF	  135	  769
CBN	   54	 1028
WMOK	  282	 1109
JFWS	  346	 1119
CBKS	  302	 1312
ISCO	  299	 1824

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 0.16 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 Tue Aug 24 21:01:19 CDT 2004