Modeling the complexity of frictional interfaces in subduction zones

sujet de stage M2R-TS

The diving of subduction slabs beneath convergent plate margins is at the origin of the largest and most destructive earthquakes, as illustrated by the 2011 MW 9.0 Tohoku earthquake in Japan. At the contact between the subducting and the overriding slabs, slip can occur either in an unstable way with an elastic stress accumulation abruptly relaxed through a sliding instability, or in a stable way, i.e. involving slip velocities that are slow compared to earthquake propagation velocities [Scholz, 1998]. The relative distribution of stable (“creeping”) and unstable (asperities) zones over the contact area is thermally controlled, and defines a seismogenic zone between the ocean trench and typically 40 km depth. The lateral, along-trench structure is generally less understood, as it differs from one zone to the next, and requires the analysis of inter-seismic displacements from land-based GPS stations. The seismic coupling coefficient, which quantifies the relative amount of long-term slip accommodated by earthquakes, is estimated in the most favorable cases (e.g. Japan) with a spatial resolution of 50 km.
There is growing evidence for a complexity of the frictional subducting interface at small scales unresolved by GPS measurements. As an example, numerous small earthquakes are recorded within regions which are supposed to be completely blocked during inter-seismic periods [Marsan et al., 2013], whereas non-volcanic tremor is observed at great depths, below the seismogenic zone [Ide et al., 2007]. A fundamental question is therefore to understand to what extent the large-scale mechanical behavior of the interface, as estimated from GPS data, is controlled by small, unresolved asperities. Understanding the link between the small-scale complexity of the frictional behavior and the large-scale mechanics of subduction zones is therefore a key challenge to reconcile seismic and geodetic observations and to understand the dynamics of the diving slab.
In this context, the goal of this Master project will be to develop a simple generic model for a frictional interface within a subduction zone, considering unstable asperities spatially distributed over an otherwise stable interface. Once an asperity will reach its frictional strength and slip, static elastic stresses will redistribute. The associated stress redistribution around the broken asperity could either be relaxed progressively within stable regions, or trigger cascades of slipping unstable events. This will allow the analysis of the fundamental, non-trivial problem of upscaling the complexity of the frictional interface at small scale to the large-scale behavior of the subduction zone.
The numerical model, based on a cellular automaton scheme, will consider a spatial discretization of the sliding interface (upper bound of the subducting slab), mechanically forced by a constant displacement rate at the base of the slab. Slip will be considered as being stable (“creeping”) over the interface, except at the asperities characterized by a distribution of yield strengths. For this Master project, the local mechanical rules will be considered as simple as possible : neither strengthening nor weakening mechanisms of the asperities after failure, nor aging, i.e. no state-dependence ; constant stable creep rate proportional to the stress rise for stable elements. Numerical simulations with different spatial density of asperities will be performed in order to study the dependence of the global behavior of the interface (either stable/”creeping”, or unstable) on this parameter.

This Master project aims at being pursued during a PhD, during which the physics of the interface will be progressively made more complex, and the results of the simulations compared with seismic and high-resolution GPS data in Japan.

Ben-Zion Y. (2012), Episodic tremor and slip on a frictional interface with critical zero weakening in elastic solid , Geophys. J. Int., 189, 1159–1168.
Dublanchet P., P. Bernard, P. Favreau (2013), Creep modulation of Omori law generated by a Coulomb stress perturbation in a 3D rate-and-state asperity model, Journal of Geophysical Research : Solid Earth, 118(9) : 4774-4793.
Ide S., D. R. Shelly, G. C. Beroza (2007), Mechanism of deep low frequency earthquakes : Further evidence that deep non-volcanic tremor is generated by shear slip on the plate interface, Geophys. Res. Lett., doi 1029/2006GL028890,
Marsan D., T. Reverso, A. Helmstetter, and B. Enescu (2013), Slow slip and aseismic deformation episodes associated with the subducting Pacific plate offshore Japan, revealed by changes in seismicity, J. Geophys. Res., 118, DOI : 10.1002/jgrb.50323
Scholz C. H. (1998), Earthquakes and friction laws, Nature, 391, 37-42.