Deep plate movements prior to a mega-earthquake detected in the Earth’s gravity field
A study led by researchers from IPGP, IGN and Université Paris Cité, shows that deep mass redistributions can be observed prior to a subduction zone mega-earthquake, thanks to gravity field measurements performed by the GRACE satellite. By eliminating hydrological sources from the global gravity signal, the French team shows that the observed anomalies appear to be consistent with a stretching of the plunging plate at around 150 km depth in the mantle, suggesting that the giant Maule rupture in Chile in 2010 may have originated in a surface propagation of this plunging plate deformation.
Publication date: 19/04/2022
Press, Research
Related teams :
Geodesy
Related themes : Earth and Planetary Interiors, Natural Hazards
Subduction earthquakes, at the interface between a continental plate and an oceanic plate plunging into the mantle, are among the most intense in the world. Understanding the processes that trigger them and identifying pre-seismic signals remains a major scientific and societal challenge. But the role played in generating these earthquakes by the movements of the plunging plate as it descends into the mantle remains poorly understood. The ability to continuously monitor seismic and a-seismic deformations of the various components of the subduction system, from the interface between the continental and oceanic plates to the deepest part of the subducted plate’s path, is therefore of paramount importance.
Space geodesy and seismic observations offer extremely fine-grained monitoring of these deformations at shallow depths or at seismic frequencies, but the full spectrum of deformations of the plunging plate, which exerts a tensile force on the subducting oceanic plate, is largely unknown due to the lack of observations at this depth, yet their monitoring is crucial, as they may precede more superficial movements that would result from their propagation towards the surface.
In a study published on April 15th, 2022 in the journal Earth and Planetary science Letters, a team of scientists from the Institut de physique du globe de Paris, Université Paris Cité, the Institut national de l’information géographique et forestière (IGN), and the Géoscience Environnement Toulouse laboratories (CNRS, Université Toulouse 3, IRD, CNES) and Géoscience Rennes (CNRS, Université de Rennes 1), has used satellite gravimetry to try and fill this observational gap, thanks to a unique sensitivity to mass redistribution at all depths and global spatial coverage.
Since 2002, the GRACE and now GRACE Follow-On satellites have been mapping the spatio-temporal variations in the Earth’s gravity field on a monthly basis. These variations are due to mass transfers resulting from the redistribution of water within and between the atmosphere, oceans, polar ice caps and continental waters, for example, as well as from tectonic movements on our planet. These gravity data, with a spatial resolution of up to around 300-400 km, therefore offer highly complementary information to space geodesy and seismic data.
Analysis of these satellite data enabled the French team to identify an anomalous variation in the Earth’s gravity in the months preceding the giant Maule earthquake in Chile (Mw 8.8, February 27th, 2010), which occurred in an area a few hundred kilometers long to the north of the epicentral zone. A detailed study of the hydrological signal in this zone, where the Nazca plate plunges beneath the South American plate, using models and in-situ observations, has led to the conclusion that this signal, equivalent to a movement of 60 km3 of water in 2 months, cannot be explained by water redistributions in the climate system and more likely originates within the solid Earth. Thus, the anomalies observed appear consistent with a stretching of the plunging plate towards 150 km depth in the mantle, perhaps associated with fluid displacements at the level of the subducted plate, and equivalent to a normal fault earthquake of Mw 8.2. The study also notes that this deformation occurs in an area where the dip of the Nazca plate is changing very rapidly.
Finally, the migration of gravity anomalies before and during the earthquake suggests that the giant Maule rupture may have originated in a surface propagation of this plunging plate deformation. These results demonstrate the interest and potential of satellite gravity observations for assessing seismic hazard in the subduction zone, and open up new perspectives for studying the interactions between these deep mass transfers and seismicity at plate boundaries.
Ref: Bouih, M., Panet, I., Remy, D., Longuevergne, L. & Bonvalot, S. (2022). Deep mass redistribution prior to the 2010 Mw 8.8 Maule Earthquake (Chile) revealed by GRACE satellite gravity, Earth and Planetary Science Letters, DOI : https://doi.org/10.1016/j.epsl.2022.117465
