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Towards a better understanding of mega-earthquake triggering mechanisms

In a recently published study, IPGP researchers and their colleague show that fluid pressure variations in subduction zones control, over the long term, the power of mega-earthquakes and the dynamics of deep accretion.

Towards a better understanding of mega-earthquake triggering mechanisms

Publication date: 09/10/2019

Press, Research

The triggering of an earthquake depends, overall, on the stresses linked to the movement of the plates and the frictional forces applied in the contact zone between the two plates. In a subduction context, this contact zone between the plunging plate and the upper plate is not uniform, but divided into segments that are more or less strongly coupled. However, while this heterogeneity crucially controls the dynamics of subduction, the temporal and spatial persistence and distribution of these strongly coupled segments are only weakly constrained.

Seismological and geodetic observations show that, in addition to the geological and structural complexity of the upper and lower plates, variations in interstitial fluid pressure also directly influence this coupling differentiation. These have a major effect on the level of friction at subduction interfaces and therefore potentially on the seismogenic behaviour of major thrust faults.

Although these variations in fluid pressure are difficult to study, previous studies have shown that the phenomenon of accretion at the base of the pre-arc region (‘tectonic underplating’) can be triggered by local variations in coupling between the plates. In the long term, this could shed light on hydro-mechanical processes and hence the distribution of stress along the interface between the plates.

Schémas d’une zone de subduction selon deux modélisations différentes (froide ou chaude). (© A. Menant, IPGP)

In a study published in July in Scientific Reports in the journal Nature, researchers from IPGP and ETH Zürich assess for the first time, on a million-year timescale, the relationships between fluid distribution/transport, stress regime and accretion mechanics in deep forearc regions. Using hydro-mechanical numerical simulations, the authors of this study show that such accretion processes can be used as a good indicator to characterise, over the long term, the zonation of friction distribution on the subduction interface.

By recognising these specific structures, built up over tens of millions of years, it would be possible to identify the subduction segments with the strongest frictional behaviour in the different mega-overthrust zones. This information is crucial for predicting potential ruptures along an interface or for modelling margin dynamics over periods of several thousand to several million years.

Réf: Stress-driven fluid flow controls long-term megathrust strength and deep accretionary dynamics, Menant A, Angiboust S and Gerya T., Scientific Reports, 2019. doi.org/10.1038/s41598-019-46191-y

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