2005/05/01 12:37:32 35.83N 90.15W 9.7 4.1 Arkansas
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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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:
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 0.5 305 65 20 4.09 0.6740 WVFGRD96 1.0 305 80 40 4.14 0.6864 WVFGRD96 2.0 305 80 30 4.13 0.7083 WVFGRD96 3.0 310 70 25 4.14 0.7188 WVFGRD96 4.0 310 65 20 4.16 0.7291 WVFGRD96 5.0 310 65 20 4.16 0.7388 WVFGRD96 6.0 310 65 20 4.17 0.7439 WVFGRD96 7.0 310 65 20 4.17 0.7464 WVFGRD96 8.0 310 65 15 4.18 0.7466 WVFGRD96 9.0 310 65 15 4.18 0.7450 WVFGRD96 10.0 310 65 15 4.20 0.7422 WVFGRD96 11.0 310 65 15 4.20 0.7371 WVFGRD96 12.0 310 70 15 4.20 0.7304 WVFGRD96 13.0 310 70 15 4.20 0.7220 WVFGRD96 14.0 310 70 15 4.21 0.7151 WVFGRD96 15.0 310 70 15 4.21 0.7056 WVFGRD96 16.0 310 70 10 4.22 0.6981 WVFGRD96 17.0 310 70 10 4.22 0.6898 WVFGRD96 18.0 130 70 15 4.23 0.6825 WVFGRD96 19.0 130 70 15 4.23 0.6787The 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 0.02 3 lp c 0.06 3
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The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion PVMO 32 76 iP_D MPH 166 81 iP_C UTMT 63 129 iP_D OXF 155 161 iP_C PLAL 116 211 iP_C WVT 80 212 iP_D UALR 240 231 iP_D FVM 354 240 eP_- CCM 339 266 iP_D SLM 359 312 eP_X OLIL 29 370 eP_X LRAL 136 424 eP_+ WCI 51 434 iP_D BLO 40 490 eP_X
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) PVMO 32 76 MPH 166 81 UTMT 64 129 OXF 155 161 PLAL 116 211 WVT 80 212 UALR 240 231 FVM 354 240 CCM 339 266 SLM 359 312 USIN 42 324 LRAL 136 424 WCI 51 434 BLO 40 490 LTL 186 590 GOGA 112 669 KSU1 304 677 WMOK 264 794 ACSO 50 796 CBKS 294 912 NHSC 105 964 MCWV 62 1002 AMTX 265 1035 JCT 239 1077 ERPA 48 1122 CBN 73 1166 DWPF 134 1187 SSPA 60 1197 SDMD 67 1239 MVL 65 1297 SADO 40 1362 LTX 244 1457 MNT 256 1482 MNTX 256 1482 KGNO 47 1490 PAL 63 1528 NCB 52 1631 KAPO 20 1636 GAC 44 1650 ULM 346 1668 VLDQ 33 1710 HRV 60 1765 LAO 317 1802 BW06 301 1838 WUAZ 275 1918 AHID 300 1958 HWUT 296 1964 LKWY 306 1971 MVU 285 1977 DUG 290 2042 LMQ 45 2092 BOZ 308 2107 GGN 55 2220 HLID 300 2241 ELK 292 2254 GLA 269 2282 MSO 309 2323 LMN 54 2396 ICQ 44 2399 GSC 276 2410 TPH 284 2416 BMN 290 2418 WALA 315 2429 BAR 270 2460 MNV 285 2496 MWC 274 2544 ISA 278 2553 WVOR 295 2557 FCC 355 2567 NEW 310 2606 EDM 324 2649 MOD 294 2688 CMB 284 2693 HAWA 305 2712 SCHQ 33 2768 SAO 281 2801 PNT 311 2819 WDC 290 2869 YBH 293 2886 HUMO 295 2912 HOPS 287 2920 COR 299 2962 YRTN 358 3006 LLLB 313 3023 DRLN 50 3036 SHB 310 3122 OZB 308 3217 CBB 310 3239 KNDN 343 3336 EDB 309 3356 MGTN 343 3371 IHLN 342 3372 BOXN 343 3390 SNPN 342 3392 PHC 311 3400 MLON 343 3406 CAMN 342 3411 YKW1 338 3412 NODN 342 3434 FRB 19 3437 QILN 3 3441 GLWN 344 3463 BBB 313 3490 EKTN 343 3493 FNBB 327 3504 ACKN 343 3527 YMBN 343 3534 COWN 344 3561 LUPN 344 3607 CTLN 339 3626 GALN 338 3642 DLBC 324 3841 WHY 325 4189 SFJD 24 4285 RES 358 4338 INK 337 4501
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 0.02 3 lp c 0.10 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:
BLA
In addition we did not use the following stations because data were not consistent with the solution either due to response problems or data problems: ISCO,JFWS, LTL, MIAR, NCB, MIAR, AHID, EYMN, SNCC, AHID
