Shallow seismic velocity structure of the Chicxulub impact crater from modelling of Rg dispersion using a genetic algorithm

A 20 element seismic array was deployed onshore across the 65 Ma. Chicxulub impact crater, Yucatan, Mexico as part of an integrated onshore-offshore s eismic experiment. Fundamental and higher mode group velocity dispersion cu rves have been calculated through the application of multiple filter and ph ase match techniques to high frequency (0.2-5 Hz) Rayleigh waves recorded o n the array and originating from local quarry blasts. Several 1-D shear wav e velocity-depth models across the crater have been produced through the op timisation of these dispersion curves using a genetic algorithm. These mode ls are resolved to a depth of circa 3.5 km and provide information on the v elocity structure of the upper crater sequences and post-impact sedimentati on, placing constraints on crater morphology and post impact sedimentation patterns. Shear wave velocities of 0.8-2.2 km s(-1) are modelled within the interpret ed Tertiary sedimentary sequence. An inverted velocity gradient is observed over the upper 300 m of the sediments with the exception of a circular reg ion across the crater centre, suggesting the crater influenced sedimentatio n throughout the Tertiary. The base of the Tertiary sequence is modelled at circa 1-1.5 km depth where a thin layer with velocities up to 30% higher t han the overlying sediments is observed. Immediately below this high veloci ty layer a circa 200 m thick low velocity zone with velocities of 1.0-2.3 k m s(-1) is interpreted as a layer of suevitic impact breccia. A shallowing to the base of this layer to circa 1.3 km depth between 30 and 40 km radius from the crater centre would be consistent with the existence of a peak ri ng occurring as a topographic high above the crater floor. (C) 2001 Elsevie r Science B.V. All rights reserved.