General Colloquium:
April 27 - 4:00pm Phys 223
(Coffee at 3:30p.m. in room 242)

Professor Xavier Le Pichion
Chair of Geodynamics
College de France, Paris

Title: "The Deformation of Japan as Measured from Space and its Implications on the Seismic Cycle"

Abstract
The ability to measure positions at the surface of the Earth with a precision of a few millimeters has introduced a new era in the understanding of the processes of deformation of the Earth. Plate Tectonics in the sixties had produced a quantitative kinematic model of the permanent motion of a few large essentially undeformed plates, but this model was obtained with measurements of relative velocities averaged over a few millions of years, along the oceanic plate boundaries. Now, with space geodesy, in a years time, velocity vectors can be obtained on any point of a continent, thus giving access both to the rigid plate velocities and to the horizontal components of the tensor of deformation within tectonic zones and plate boundaries. The space geodesy measurements have confirmed the long term kinematic plate tectonic model but they have also opened the possibility to study for the first time in detail the transient deformations related in particular to the seismic cycle along plate boundaries.

The seismic cycle is related tot he elasticity of the plate. The elastic deformation builds up along a plate boundary until the limit of friction on the boundary fault is exceeded. Then, the elastic deformation is released nearly instantaneously during an earthquake. In Japan, a network of 2000 GPS permanent stations record in real time the deformation tensor over the whole surface of the islands. We can thus monitor the build up of the elastic deformation due to the subduction of the oceanic plates below the Japanese islands. These data then can be used to study whether the fault surface is fully locked and how the resulting transient strain is accumulating. The results can be compared to the displacements occurring over the large subduction earthquakes every 100 years or so. Measurements over three years already give some remarkable results that challenge some of the models proposed for the seismic cycle. The implications of these results are discussed.