Gravimetric satellite data highlight deep deformations prior to the 2011 Tohoku earthquake
Giant earthquakes, such as the one off the coast of Tohoku, Japan, in 2011, are an extreme manifestation of plate tectonics. They most often occur in subduction zones, where an oceanic plate meets a continental plate and plunges into the Earth's mantle.
Publication date: 18/04/2018
Press, Research
Related teams :
Geodesy
Related themes : Natural Hazards
Giant earthquakes, such as the one off the coast of Tohoku, Japan, in 2011, are an extreme manifestation of plate tectonics. They most often occur in subduction zones, where an oceanic plate meets a continental plate and plunges into the Earth’s mantle. Along certain sections, the movement of the plunging plate deforms the continental plate continuously throughout the inter-seismic period. These deformations are measured in great detail using surface geodetic measurements. Unfortunately, however, such measurements do not allow us to follow the deep deformations within the mantle, or to predict with sufficient accuracy the size and timing of earthquakes.
Satellite observation offers new perspectives. Gravity measurements obtained by the GRACE satellites provide a more complete picture of plate movements in the vicinity of subduction zones. Gravity field measurements can be used to detect the redistribution of mass at depth, linked to these tectonic movements. Gravity field variations are mapped every month with homogeneous spatial coverage at up to 400 km resolution. They provide unparalleled information for detecting deep movements within our planet at intermediate spatial and temporal scales.
By analysing satellite gravity field data, a team of researchers from the IPGP, IGN and Géosciences Environnement Toulouse have been able to highlight the sequence of mass transfers associated with the giant Tohoku earthquake (Mw 9.0, March 11th 2011) and place them in a global scenario, demonstrating in particular that they were initiated at depth a few months before the rupture. In addition to the extreme gravity variations concentrated in the vicinity of the epicentre, they detected precursory changes in the Earth’s gravity over more than 2,000 km along the subduction of the Pacific and Philippine plates beneath the Eurasian plate. These signals show us that the giant rupture is part of a regional-scale deformation that migrates from depth to the surface throughout the subduction system, over periods of a few months to a few years. The anomalies preceding the rupture probably reflect a stretching of the Pacific plate to a depth of around 250 km as it plunges into the mantle. The spatial dimension of the precursor movements is echoed in the co- and post-seismic gravity variations, which suggest an increase in the velocity of two oceanic plates, the Pacific and Philippine, after the rupture, over a 2000-km section along the plate boundaries.
These results demonstrate the potential of satellite gravity to study seismic hazard in the context of subduction processes. They provide unique information on periods of the order of a month at regional scales (between 500 and 2000 km). It is now necessary to investigate whether such variations in gravity also exist before the other giant earthquakes that have occurred since GRACE was launched, opening up new prospects for the detection of precursors to giant ruptures.
Ref: Migrating pattern of deformation prior to the Tohoku-Oki earthquake revealed by GRACE data, Panet, Isabelle, Bonvalot, Sylvain, Narteau, Clément, Remy, Dominique, Lemoine, Jean-Michel, Nature Geoscience, 2018 – doi.org/10.1038/s41561-018-0099-3
