1996/11/29 05:41:32 35.97N 90.00W 5 4.3 Arkansas
Arrival time list
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.
STK = 120 DIP = 65 RAKE = 15 MW = 3.78 HS = 11
The preferred solution is based on the waveform match. the solution agress with previous solutions in the same area, but is marginal because of the small number of broadband stations in the region at this date.
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:
hp c 0.04 3 lp c 0.10 3 br c 0.12 0.2 n 8 p 2 taper W 0.1The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT WVFGRD96 0.5 100 45 -60 3.79 0.2584 WVFGRD96 1.0 100 50 -65 3.79 0.2632 WVFGRD96 2.0 295 55 -35 3.72 0.2688 WVFGRD96 3.0 305 75 -15 3.66 0.2764 WVFGRD96 4.0 305 75 -15 3.67 0.2854 WVFGRD96 5.0 305 80 -20 3.69 0.2929 WVFGRD96 6.0 305 80 -20 3.70 0.2997 WVFGRD96 7.0 305 80 -20 3.71 0.3046 WVFGRD96 8.0 135 60 25 3.76 0.3085 WVFGRD96 9.0 135 60 25 3.78 0.3141 WVFGRD96 10.0 135 60 25 3.79 0.3179 WVFGRD96 11.0 130 65 15 3.78 0.3192 WVFGRD96 12.0 40 80 -30 3.78 0.3189 WVFGRD96 13.0 40 80 -25 3.78 0.3189 WVFGRD96 14.0 40 80 -25 3.79 0.3175 WVFGRD96 15.0 40 80 -25 3.80 0.3150 WVFGRD96 16.0 40 80 -25 3.81 0.3116 WVFGRD96 17.0 40 80 -20 3.81 0.3073 WVFGRD96 18.0 40 80 -20 3.81 0.3027 WVFGRD96 19.0 40 80 -20 3.82 0.2969
The best solution is
WVFGRD96 11.0 130 65 15 3.78 0.3192
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 bandpass filter used in the processing and for the display was
hp c 0.04 3 lp c 0.10 3 br c 0.12 0.2 n 8 p 2 taper W 0.1
|Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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.|
NODAL PLANES STK= 41.03 DIP= 75.52 RAKE= 164.50 OR STK= 134.99 DIP= 75.00 RAKE= 15.00 DEPTH = 16.0 km Mw = 3.85 Best Fit 0.8959 - 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 MIAR 245 362 eP_- MYNC 99 542 eP_-
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. 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) MIAR 245 362 MYNC 99 542 JFWS 358 771 WMOK 263 809 BLA 78 868 AAM 36 891 CBKS 293 918 MCWV 62 983 GWDE 72 1311 ISCO 292 1436 BW06 300 1841
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 0.10 3 br c 0.12 0.2 n 8 p 2 taper W 0.1
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