2005/06/02 11:35:10 36.14N 89.46W 16 4.0 Tennessee
USGS Felt map for this earthquake
USGS Felt reports page for Central and Southeastern US
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
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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:
The results of this grid search from 0.5 to 19 km depth are as follow:hp c 0.07 3 lp c 0.10 3
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
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The best fit as a function of depth is given in the following figure:
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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
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NODAL PLANES
STK= 164.99
DIP= 85.00
RAKE= 45.00
OR
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
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The P-wave first motion data for focal mechanism studies from the broadband stations are as follow:
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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
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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 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.
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.
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| 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. |
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| 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. |
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
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:
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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
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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.
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.
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Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
