USGS Felt map for this earthquake
USGS Felt reports page for Central 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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| Strike = 165 | Strike = 74 |
| Dip = 85 | Dip = 75 |
| Rake = 15 | Rake = 175 |
| Mw = 4.02 | Depth = 2 km |
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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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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 np 2 lp c 0.10 np 2
This focal mechanism is compared to others in the region in focal mechanism comparison . The interesting point is that the nodal planes of this mechanism line up with seismicity but that the orientation of the P ressure axes (white quadrants) is NW-SE instead of a previously expected E-W.
The waveforms recorded in the Mississippi embayment exhibit a long duration signal which is related to waves trapped in the deep sediments near the surface. The are especially prominent at OXF, MPH, PVMO and UTMT. Other stations exhibit anomalous dispersion and spectral amplitudes - HKT which is affected by the Gulf Coastal Plain.
A waveforms inversion was attempted but the results were not used because special Green's functions would be needed for stations within the embayment.
Won-Young Kim obtained a similar focal mechanism from a inversion of waveforms. His solution can bee seen by clicking on this link.
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 CUS earth model
was used to define the Green's functions.
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