2005/06/02 11:35:10 36.14N 89.46W 16 4.0 Tennessee

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Central and Southeastern 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

Waveform Inversion

The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.

The observed and predicted traces are filtered using the following gsac commands:

hp c 0.07 3
lp c 0.10 3
The results of this grid search from 0.5 to 19 km depth are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    0.5   325    50    60   3.65 0.4750
WVFGRD96    1.0   325    50    65   3.70 0.4936
WVFGRD96    2.0   340    50    85   3.78 0.4543
WVFGRD96    3.0   155    80   -70   3.74 0.4146
WVFGRD96    4.0   150    80   -70   3.73 0.4561
WVFGRD96    5.0   150    80   -70   3.73 0.4930
WVFGRD96    6.0   150    80   -70   3.73 0.5206
WVFGRD96    7.0   150    80   -70   3.73 0.5436
WVFGRD96    8.0   155    85   -70   3.73 0.5591
WVFGRD96    9.0   155    85   -70   3.74 0.5717
WVFGRD96   10.0   155    85   -75   3.78 0.5815
WVFGRD96   11.0   155    65    70   3.83 0.5926
WVFGRD96   12.0   155    65    70   3.85 0.6115
WVFGRD96   13.0   155    65    70   3.86 0.6238
WVFGRD96   14.0   155    65    70   3.87 0.6302
WVFGRD96   15.0   155    65    70   3.89 0.6326
WVFGRD96   16.0   155    65    70   3.90 0.6299
WVFGRD96   17.0   140    75    55   3.91 0.6245
WVFGRD96   18.0   140    75    55   3.92 0.6184
WVFGRD96   19.0   140    75    55   3.93 0.6085

The mechanism correspond to the best fit is
Figure 1. Wavefrom inversion focal mechanism

The best fit as a function of depth is given in the following figure:

Figure 2. Depth sensitivity for waveform mechanism

The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted. Each observed-predicted componnet is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. The number in black at the rightr of each predicted traces it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed and because the velocity model used in the predictions may not be perfect. A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The bandpass filter used in the processing and for the display was

hp c 0.07 3
lp c 0.10 3
Figure 3. Waveform comparison for depth of 8 km

Surface-Wave Focal Mechanism


  STK=     164.99
  DIP=      85.00
 RAKE=      45.00
  STK=      70.00
  DIP=      45.22
 RAKE=     172.94
DEPTH = 15.0 km
Mw = 3.98
Best Fit 0.7472 - P-T axis plot gives solutions with FIT greater than FIT90

First motion data

The P-wave first motion data for focal mechanism studies from the broadband stations are as follow:

Sta Az(deg)    Dist(km)   First motion
PVMO      325   37 eP_X
UTMT       67   58 iP_D
MPH       201  120 eP_X
WVT        90  147 iP_D
SIUC        7  176 iP_C
OXF       178  180 iP_D
PLAL      135  180 iP_C
FVM       338  222 iP_C
USIN       38  258 iP_C
SLM       346  286 eP_X
UALR      241  302 iP_D
LRAL      146  412 iP_C
BLO        37  425 eP_X

Mitch Withers, CERI, University of Memphis provided the first motion solution from the dense short period network. This solution is

All focal mechanisms, the waveform inversion, the selected surface-wave radiation pattern and the dense local network share the nodal plane striking roughly southeast and steeply dipping tot he southwest. The other nodal plane dips to the east or south at a much shallower angle. Each solution has its own limitations. the waveform inversion and first motion solutions rely on a good curstal velocity model for prediction of waveforms and p-wave take-off angles. The surface-wave solution is hampered by the fact that this particular mechanism is not a good generator of long-period surface waves.

The preferred solution is the waveform inversion solution.

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)   
WVT	   90	  147
SIUC	    7	  176
OXF	  178	  180
PLAL	  135	  180
FVM	  338	  222
USIN	   38	  258
SLM	  346	  286
UALR	  241	  302
LRAL	  146	  412
BLO	   37	  425
KSU1	  300	  711
ACSO	   49	  727
WMOK	  262	  860
NHSC	  109	  915
MCWV	   62	  931
CBKS	  291	  956
ALLY	   50	 1013
CBN	   74	 1097
AMTX	  264	 1100
JCT	  240	 1148
DWPF	  137	 1168
SDMD	   68	 1169
MVL	   66	 1227
BINY	   56	 1343
SDCO	  282	 1438
PAL	   64	 1457
NCB	   52	 1561
HRV	   60	 1694
BAR	  269	 2522
COR	  299	 3000

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 0.06 3
lp c 0.14 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 Thu Jun 2 15:05:19 CDT 2005