Mechanics of icequakes (La Girose glacier, French Alps)

Agnès Helmstetter, Jean Robert Grasso, Bruno Hernandez, Michel Bouchon, Michel Dietrich
Laboratoire de Géophysique Interne et Tectonophysique, Université Joseph Fourier, Grenoble, France

the Girose Team 2000
Laboratoire de Géophysique Interne et Tectonophysique, Grenoble, France
Laboratoire de Glaciologie et Géophysique de l'Environnement, CNRS , Grenoble, France
Laboratoire de Géophysique, Pau, France

Abstract. During the 2000 summer a geophysical experiment was perform on a French alpine glacier, la Girose, Oisans, in order to study the glacier deformation style. This experiment is part of a program that tries to investigate glacier behaviour in time and space and energy domain. Deformation measurements that use GPS sensor are compared to fracturing processes that are recorded by seismic station.
In this study we focus on the seismic monitoring of a 50 m by 30 m zone. We used an array of 24 sensors, positioned 10m apart from each others. The area was located in the upper part of the glacier, up stream from an open crevasse area. In addition to opening displacement, crevasse geometry exibits a clear normal faulting componant and a weak strike slip displacement. The crevasse orientation i s homogeneous and perpendicular to the ice flow direction. Some surface cracks, with a maximum few meter length, and a less to 1 mm aperture, exist within the seismically monitored area. Using standard seismological techniques we first derive a velocity model from acti ve seismic profile. Secondly, we locate icequakes and we test hypocenter accuracy using calibration shots. Icequake locations define horizontal alignments parallel to downstream crevasses and perpendicular to ice flow direction. Hypocenter depths ranging from 0 to 12-m agree with the depth of neighb ouring crevasses. Preliminary results suggest that the recorded icequakes do not fit the double coup le model that applies for most earthquakes. Recorded signals are well described by a mode I aperture source, on a plane parallel to the neighbouring crevasse direction. We use inversion methods to further constrain the source geometry.