[1] British Geol Survey, Kingsley Dunham Ctr, Nottingham NG12 5GG, England
[2] Aspinall Associates, Nottingham NG12 5GG, England
[3] Univ W Indies, Seism Res Unit, St Augustine, Trinidad Tobago
[4] Univ Bristol, Dept Geosci, Bristol BS8 1RJ, Avon, England
[5] Penn State Univ, University Pk, PA 16802 USA