Coupling at plate boundaries : insights from laboratory experiments

Abstract

The aim of this Thesis is to study the role of interplate frictional properties on boundary shaping and seismicity associated with subduction zones. This is achieved by means of a multidisciplinary approach, combining two different scales of analogue models, rheometry and worldwide statistics of subduction-related thrust-type earthquakes. Concerning the key problem of long-term frictional resistance to subduction, I relate the peculiar shape observed along Andes at present-day and associated topography to lateral variation in mechanical coupling. Frictional processes acting along the subduction thrust fault play also an important role also on shorter timescale with effect on seismicity. The strategy of the adopted approach is to formulate the simplest setup allowing systematically the separation between different effects. The Andean belt is the result of a favorable combination of plate kinematic parameters. While the Nazca plate, driven by negative buoyancy, undergoes weakened continental South America, mechanical coupling between converging plates is resisting to this motion. The development of trench curvature, shortening of the overriding plate, and topography could be related to lateral variation of the degree of mechanical coupling between converging plates. Here I use laboratory models of subduction in order to qualitatively test this hypothesis (Chapter 3). Most of the global seismic energy is released by discontinuous shear of the frictional interface between the subducting and the overriding plate. First-order importance in controlling the seismic variability of subduction zones is recently attributed to interface roughness (including amount of sediments) and subduction velocity. Here I present a set of spring block-like models, which is known to mimic the seismic behavior, with the aim to explore the role played by the contact roughness, sliding velocity and normal load in friction dynamics (Chapter 5). The experimental setup consists of a viscoelastic gelatin slider - analog of the Earth's crust - moving on sandpaper, a small scale rough interface representative of the interplate contact. These experimental results, combined with worldwide seismic observables, offer the possibility of reconciling within a single model where contact interface is evolving during plate subduction the occurrence of creepingvelocity strengthening and seismic-velocity weakening regions along the subduction thrust plane. This work is supported by a preliminary systematic study of both rheological and physical properties of a wide range of gelatins (Chapter 4) which helped in selecting the right material (pig skin 2.5 wt.% at 10 °C) for a suitable experimental set-up to downscale model for subduction interplate seismicity. I use both tribological and rheological background to built a realistic (wedge shaped) analogue model of subduction thrust-type earthquakes and rupture dynamics including rate- and state- friction and viscoelastic deformation (Chapter 6).