2005/02/10 14:04:53 35.748N 90.233 10 4.2 Arkansas
Arrival time list
USGS Felt map for this earthquake
USGS Felt reports page for Central and Southeastern US
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:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 335 55 45 4.01 0.4790 WVFGRD96 1.0 335 55 45 4.02 0.4822 WVFGRD96 2.0 140 80 5 3.92 0.5007 WVFGRD96 3.0 140 75 0 3.95 0.5203 WVFGRD96 4.0 140 75 0 3.96 0.5317 WVFGRD96 5.0 140 75 0 3.98 0.5391 WVFGRD96 6.0 140 80 5 3.99 0.5464 WVFGRD96 7.0 140 80 5 4.00 0.5566 WVFGRD96 8.0 140 80 5 4.01 0.5669 WVFGRD96 9.0 140 85 5 4.03 0.5756 WVFGRD96 10.0 140 80 5 4.04 0.5837 WVFGRD96 11.0 140 80 5 4.05 0.5893 WVFGRD96 12.0 140 85 5 4.07 0.5936 WVFGRD96 13.0 140 85 10 4.08 0.5961 WVFGRD96 14.0 140 85 10 4.08 0.5968 WVFGRD96 15.0 140 85 10 4.09 0.5958 WVFGRD96 16.0 140 85 10 4.10 0.5935 WVFGRD96 17.0 140 80 10 4.10 0.5908 WVFGRD96 18.0 140 80 15 4.11 0.5871 WVFGRD96 19.0 320 75 5 4.12 0.5836
The mechanism correspond to the best fit is
The best fit as a function of depth is given in the following figure:
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 focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
NODAL PLANES STK= 54.99 DIP= 79.99 RAKE= -165.00 OR STK= 322.33 DIP= 75.23 RAKE= -10.35 DEPTH = 15.0 km Mw = 4.11 Best Fit 0.6359 - P-T axis plot gives solutions with FIT greater than FIT90
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion MPH 158 75 eP_+ PVMO 33 88 eP_+ UTMT 61 140 eP_- OXF 151 156 iP_+ UALR 241 220 eP_X SIUC 22 236 eP_X FVM 356 249 eP_X CCM 341 272 iP_+ SLM 360 321 P MIAR 247 333 eP_+ USIN 42 336 eP_X LRAL 134 423 eP_X WCI 51 446 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.
|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) MPH 158 75 PVMO 33 88 UTMT 62 140 OXF 151 156 PLAL 113 214 UALR 241 220 SIUC 22 236 FVM 356 249 CCM 341 272 SLM 360 321 MIAR 247 333 USIN 42 336 LRAL 134 423 WCI 51 446 GOGA 111 673 WMOK 264 786 JFWS 360 796 HKT 221 829 CBKS 295 908 AAM 36 923 NHSC 105 969 MCWV 62 1013 AMTX 266 1027 ERPA 48 1134 DWPF 133 1186 SSPA 60 1209 EYMN 356 1359 SDCO 284 1380 ISCO 293 1426 MNTX 257 1472 PAL 63 1538 HRV 59 1776 LAO 317 1804 DGMT 325 1820 BW06 301 1836 TUC 264 1932 HWUT 296 1961 LKWY 306 1970 HLID 300 2240 MSO 309 2323 NEW 311 2606 MOD 294 2685 SAO 282 2795
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 0.04 3 lp c 0.15 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: