ALBERT

Description: A passive, 20-element, short-period (1 Hz) and broadband seismic array was deployed over the Chicxulub impact structure for c. 100 days in early 1996. The principal objective was to study the shear-wave anisotropy associated with the structure; in particular, to determine the presence (or absence= of radial symmetry which will allow comment on the time variance of that anisotropy. A total of 15 teleseismic, 75 regional, and 100 local events were recorded. Preliminary results from studies of the surface-wave dispersion of the local events, and a receiver function analysis of a single teleseismic event are reported here. Thirty local events have been located, a number of which originated from quarries within the array. Analysis of seismograms from three of these events demonstrates a bimodal distribution; those whose ray-paths cross the outer part of the impact structure show a strong inverse produced by the sedimentary depositional environment, with deeper water sedimentation in the outer part of the post-impact crater basin and shallower water sedimentation over the upraised peak-ring block at the centre. Receiver functions derived for an event originating in Peru are dominated by an efficient mode conversion, simply modelled as a P-S multiple from the base Tertiary boundary. This shows a strong correlation with distance from the centre of the impact structure and implies it has an S-wave radial symmetry. The multiple also has a variable delay probably related to the depth of the conversion boundary. Unfortunately, the Moho conversion occurs at almost exactly the same time as this surface layer sediment multiple, restricting any modelling of Moho topography and its influence on the receiver functions.