EFFECT OF THE UNCONSOLIDATED SEDIMENTS IN THE NEW MADRID SEISMIC ZONE ON THE 3-D VELOCITY MODELS DERIVED FROM INVERSION OF ARRIVAL TIMES

PUJOL, J., Dept. of Earth Sciences, The University of Memphis, Memphis, TN 38152, jpujol@memphis.edu.

In the central New Madrid seismic zone the thickness of the low-velocity post-Paleozoic sediments (Vp about 1.8 km/s, Vp/Vs about 3, on average) increases from about 100 m to 700 m in a roughly NNW-SSE direction over a distance of about 100 km. Because the underlying rocks have high velocities (Vp about 6 km/s, Vp/Vs about 1.73) the lateral velocity variations introduce time delays that can be as large as 0.3 s for P wave arrivals and 1 s for S waves arrivals. Therefore, a 3-D velocity inversion of arrival time data that does not take into account the sloping nature of the sediments is likely to be affected by errors. This question was investigated using synthetic data generated for a model with a low-velocity layer based on the thickness of the post-Paleozoic sediments. The number of events was 279 and the station distribution was similar to that of the PANDA stations deployed in 1989-1992 (Pujol et al., Eng. Geol., 1997). The velocity model was parameterized in terms of blocks with dimensions of 2 km x 2 km horizontally and 0.25 km in depth. The initial velocity model was layered with a 0.65 km thick low-velocity upper layer. The 3-D inversion software of Benz was used. The computed Vp and Vs, and Vp/Vs, show a number of systematic, spurious anomalies. The S wave velocities are the most affected, with relatively lower velocities roughly where the sediments are thicker. In those areas the Vp/Vs ratio is somewhat higher. In addition, the event locations are not well recovered, with the error in depth (up to about 2 km) increasing in the direction of decreasing sediment depth. These results show that the inversion fails to retrieve the actual velocity variations even when very thin layers are used. There are two likely reasons for that. One is that the rays in the sedimentary layer are almost vertical. The other is that the events are deeper than about 5 km.