Location

1996/11/29 05:41:32 35.97N 90.00W 5 4.3 Arkansas

Arrival Times (from USGS)

Arrival time list

Felt Map

USGS Felt map for this earthquake

USGS Felt reports page for Central and Southeastern US

Focal Mechanism

The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
Figure 1. Location of broadband stations used to obtain focal mechanism

Preferred Solution

The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is

      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.

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:

hp c 0.04 3
lp c 0.10 3
br c 0.12 0.2 n 8 p 2
taper W 0.1
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   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
Figure 1. Waveform inversion focal mechanism

The best fit as a function of depth is given in the following figure:

Figure 2. Depth sensitivity for waveform mechanism

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
Figure 3. Waveform comparison for depth of 8 km
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.

Surface-Wave Focal Mechanism


  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

First motion data

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

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.

Data preparation

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.
Best mechanism fit as a function of depth. The preferred depth is given above. Lower hemisphere projection

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.

Love-wave radiation patterns

Rayleigh-wave radiation patterns

Broadband station distributiuon

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

Waveform comparison for this mechanism

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

Discussion

The Future

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.

Acknowledgements

Dr. Harley Benz, USGS, provided the USGS USNSN digital data.

Appendix A

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.

Quality Control

Here we tabulate the reasons for not using certain digital data sets

Last Changed Tue Jun 7 15:21:27 CDT 2005