Detection of a gravity signal before the arrival of seismic waves
The Earth's gravitational field is not uniform over the surface of the globe, but depends on the masses (thickness and density in particular) of the various layers present beneath the surface. During an earthquake, ground movements are accompanied by a significant redistribution of these masses, generating significant changes in the earth's gravitational field.
Subduction at the origin of the Tohoku earthquake
Publication date: 24/11/2016
Press, Research
Related teams :
Seismology
Related themes : Natural Hazards
A disturbance of this gravity field long after the occurrence of an earthquake has already been observed, but the quasi-instantaneous change of this field, during the rupture and before the arrival of the seismic waves, had never been observed before.
The Tohoku-oki megathrust earthquake (magnitude 9.0, Japan, March 2011) provided a unique opportunity to detect such a signal. An international team, including several IPGP researchers, used data recorded by the Kamioka superconducting gravimeter in Japan, located around 500 km from the epicentre, supplemented by data from broadband seismometers in the Japanese F-net network. An analysis of these recordings shows, with a statistical significance of over 99%, that a gravity signal linked to the seismic rupture is indeed present. This discovery opens up new fields of application for Earthquake Early Warning Systems (EEWS).
Currently, EEWSs are based on the detection of compressional seismic waves (P), which arrive very slightly before the highly destructive shear waves (S). This difference in arrival times (only a few seconds near the rupture) is used to alert the local population and protect all the equipment and infrastructure at risk (cutting off water, electricity and gas supplies, stopping trains and lifts, etc.).
The gravity field signal concomitant with the rupture could save precious seconds before the arrival of the P and S seismic waves. It could also enable the exact magnitude of an earthquake to be determined more quickly, as soon as the rupture is over, whereas current methods take several tens of minutes.
However, implementing a gravity-based warning system will require the development of new instruments capable of measuring the Earth’s gravity field much more accurately than current instruments. Such instruments could come from fundamental physics.
Ref : Montagner, J.-P. et al. Prompt gravity signal induced by the 2011 Tohoku-Oki earthquake. Nat. Commun. 7, 13349 doi: 10.1038/ncomms13349 (2016).
