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= 30.00
DIP= 85.00
RAKE= -170.00
OR
STK= 299.12
DIP= 80.04
RAKE= -5.08
DEPTH = 19.0 km
Mw = 4.57
Best Fit 0.9065 - P-T axis plot gives solutions with FIT greater than FIT90
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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, 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. The figure
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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 rpeferred 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. The Each solution is plotted as a vector at a given value of strike and dip witht he 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 P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion SIUC 258 129 i- WCI 77 134 e- BLO 39 173 i+ UTMT 209 219 e- SLM 290 226 e- CCM 273 304 e- JFWS 340 587 e+ EYMN 346 1148 e- DUG 284 2170 ee+
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) SIUC 258 129 WCI 77 134 BLO 39 173 WVT 181 204 UTMT 209 219 SLM 290 226 PVMO 225 257 CCM 273 304 PLAL 184 333 MPH 212 370 OXF 201 410 MYNC 133 459 ACSO 57 486 UALR 230 541 JFWS 340 587 GOGA 141 639 MIAR 236 644 BLA 95 656 MCWV 72 715 NHSC 126 876 SSPA 68 903 SADO 41 1040 WMOK 253 1049 BINY 61 1110 EYMN 346 1148 KGNO 50 1170 DWPF 150 1244 NCB 55 1320 GAC 46 1330 KAPO 17 1340 HRV 65 1467 LBNH 57 1500 ULM 337 1508 RSSD 302 1520 ISCO 283 1557 ANMO 264 1704 LTX 242 1753 LMQ 48 1775 LMN 58 2088 WUAZ 270 2118 DUG 284 2170 TUC 260 2181 HLID 294 2320 FCC 351 2356 ELK 286 2376 WALA 309 2427 SCHQ 33 2450 GLA 266 2500 TPNV 276 2510 EDM 319 2590 GSC 272 2600 PLM 268 2670 DRLN 53 2720 CHF 271 2740 HAWA 300 2760 TOV 271 2810 PNT 307 2832 LON 300 2940 COR 295 3039
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 osberved 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 0.02 2 lp c 0.15 2
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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 0.02 2 lp c 0.50 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: The PEPP stations in Indiana had incorrect gains, which have since been corrected in the IRIS archive.
Also note that the sP arrivals directly following the P-arrival at the nearest stations is a direct indicator of source depth.
