Séminaire IPGS
Intervenant : Aurélien Mordret, MIT, USA
Titre : Environmental seismic noise interferometry
Lieu : EOST, 5 rue Descartes, amphi Rothé
Résumé :
The emergence of seismic noise interferometry permits the continuous monitoring of shear-wave speed changes within the Earth's crust. The sensitivity of the technique allows the detection of relative velocity variations of a fraction of a percent. So far, most studies focus on geologically active targets like volcanoes or fault zones where changes are expected to be large and of a transient nature. More recently, a new related field of research has started to investigate velocity changes due to external forcings where the crust reacts passively to temperature variations, rain falls, etc, i.e., environmental effects.
In this talk, I will present two examples of such studies. In the first example I will show that the current Greenland ice-sheet melt, most likely triggered by global climate change, induces seismic velocity variations is the Greenland's crust. The ice- sheet weight fluctuations, caused by episodes of intense melting in summer and snow falls in winter, create pore-pressure variations in the crust at depth that translate into shear-wave speed variations. This ground-based seismological technique gives the opportunity to monitor in real-time the evolution of the Greenland's ice-sheet with a potentially unprecedented spatial and temporal resolution and can give provide constraints on the future global sea-level rise.
The second example deals with the continuous monitoring of high-rise buildings through seismic interferometry. When subject to environmental effects like temperature or humidity variations, the dynamic characteristics of a building evolve. These changes (of the normal resonance frequencies, for instance) are often small and reversible and can be accurately retrieved by seismic interferometry. Here we use deconvolution interferometry on two weeks of seismic data recorded by an array of accelerometers installed in the Earth Atmospheric and Planetary Sciences department building of MIT. By measuring the speed of the shear-waves travelling up and down in the building we are able to show that the air humidity (more than the temperature) is the main factor controlling the building dynamics. A better comprehension of the environmental effects on buildings can lead to a better and earlier detection of potential damages after an earthquake, a fundamental aspect in safety assessment.
Overall, I would like to emphasize that seismological techniques, in particular seismic noise interferometry, can provide new ways to study the effects of global environmental changes. It can also help to better understand environmental effects on different structures, from the Earth's subsurface to man-made constructions. In short, environmental seismology is becoming a dynamic field of research with many potential applications.