2004/11/07 11:20:25 32.97N 87.90W 5 4.0 Alabama
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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NODAL PLANES STK= 246.32 DIP= 54.37 RAKE= -70.48 OR STK= 35.00 DIP= 40.00 RAKE= -114.99 DEPTH = 3.0 km Mw = 4.25 Best Fit 0.7933 - P-T axis plot gives solutions with FIT greater than FIT90
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 0.02 3 lp c 0.06 3
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 270 30 -45 4.17 0.4932 WVFGRD96 1.0 270 40 -50 4.13 0.4981 WVFGRD96 2.0 265 50 -65 4.14 0.4958 WVFGRD96 3.0 90 40 55 4.08 0.4928 WVFGRD96 4.0 95 40 65 4.09 0.4867 WVFGRD96 5.0 110 35 80 4.08 0.4686 WVFGRD96 6.0 105 90 -60 4.10 0.4584 WVFGRD96 7.0 285 90 55 4.08 0.4562 WVFGRD96 8.0 105 90 -55 4.08 0.4545 WVFGRD96 9.0 105 90 -55 4.08 0.4519 WVFGRD96 10.0 290 85 50 4.08 0.4504 WVFGRD96 11.0 290 85 45 4.08 0.4555 WVFGRD96 12.0 290 85 45 4.09 0.4608 WVFGRD96 13.0 290 85 45 4.09 0.4647 WVFGRD96 14.0 290 85 45 4.10 0.4682 WVFGRD96 15.0 290 85 45 4.10 0.4716 WVFGRD96 16.0 290 85 45 4.11 0.4749 WVFGRD96 17.0 290 85 45 4.12 0.4780 WVFGRD96 18.0 290 80 50 4.12 0.4812 WVFGRD96 19.0 290 80 50 4.12 0.4845
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.
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The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion LRAL 85 85 e+ OXF 321 221 X MPH 322 304 X WVT 1 351 X UTMT 347 384 X PVMO 337 416 X UALR 297 457 e+ SIUC 348 540 i+
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. |
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) LRAL 85 85 OXF 321 221 MPH 322 304 WVT 1 351 UTMT 347 384 PVMO 337 416 UALR 297 457 SIUC 348 540 MIAR 290 554 USIN 2 555 WCI 14 601 FVM 338 602 CCM 333 641 NATX 260 650 SLM 342 663 BLO 10 699 NHSC 87 721 DWPF 129 821 ACSO 27 918 WMOK 284 1025 MCWV 42 1035 KSU1 313 1038 AAM 19 1101 CBN 56 1116 JFWS 350 1123 ALLY 33 1182 SDMD 51 1223 ERPA 32 1232 SSPA 44 1232 CBKS 305 1248 BINY 42 1466 SADO 28 1513 PAL 50 1529 KGNO 35 1594 SDCO 293 1682 NCB 40 1703 ISCO 301 1757 GAC 33 1769 HRV 49 1783 ULM 344 2029 TUC 274 2144 BW06 306 2185 DGMT 326 2195 MVU 292 2276 HWUT 301 2297 AHID 305 2302 LKWY 310 2329 BOZ 312 2469 HLID 305 2586 TPNV 288 2617 MSO 313 2687 TPH 290 2707 BMN 296 2733 DAC 286 2737 MNV 291 2791 FCC 353 2906 NEW 314 2972 CMB 290 2985 EDM 326 3028 SAO 287 3080 DRLN 45 3096 COR 303 3304 OCWA 309 3449
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
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.
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files: