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.