International Centre for Geohazards, P.O.Box 3930, Ullevaal Stadion, 0806 Oslo, Norway

NORSAR, P.O. Box 53, 2027 Kjeller, Norway

University of Strasbourg, EOST, 5 rue René Descartes, 67084 Strasbourg Cedex, France

now at: University of Nancy, France

University of Oslo, P.O. Box 1047, Blindern, 0316 Oslo, Norway

now at: University of Svalbard, Longyearbyen, Norway

University of Oslo, P.O. Box 1047, Blindern, 0316 Oslo, Norway