Evolution of asperity contacts during shear failure on frictional interfaces: implications for the initiation of crustal earthquakes

Abstract

Fault zones and associated slip play a key role in the development of structures within the Earth's upper crust. Fault activity controls crustal-scale fluid flow and fault movement is crucial in the accommodation of strain. Although fault slip is broadly classified as brittle-frictional deformation, there is much we do not understand about the physical mechanisms controlling frictional strength, especially under the extreme conditions accompanying earthquake rupture. This thesis presents the results of five experimental studies. These complementary studies have sought to answer fundamental questions about dynamic frictional processes, including 1) how the structure and properties of materials on the fault interface changes with the application of high strain rates and normal stresses during fault slip; and 2) how slip-induced changes in interface properties modify the strength and behaviour of faults. Experimental results are underpinned by the parallel development of a new interferometry-based displacement sensor and the synchronous acquisition of fault strain data. Experiments have been undertaken on a Paterson triaxial apparatus, focusing on SiO2, both in its crystalline (quartz) and amorphous (fused quartz) forms. Results show that over time scales of less than one millisecond and displacements of tens of microns, significant changes occur in the structure of materials on fault interfaces. At high normal stresses and slip velocities less than ~ 0.05 ms-1, the crystalline structure of the quartz partially lost though the process of mechanical amorphization. At higher slip velocities > 0.05 ms-1, enough heat is generated to melt quartz. However, the onset of amorphization or melting does not necessarily drive fault weakening; the amorphous layer must reach temperatures sufficient to cross the kinematic threshold referred to as the 'glass transition' to allow strain to be accommodated through viscous shearing. When melted regions quench at the end of slip, they can weld the fault surfaces together, instantaneously increasing the cohesive strength and changing conditions necessary to reactivate the fault. The molecular structure of these melt-welded regions is shown to be altered to a densified form, resulting from both the rapid cooling rates and exposure to very high pressures. Synchronised data acquisition allows measurement of both the onset of fault rupture and sample shortening resulting from fault slip. Contrary to assumptions made in many previous studies, experimental fault slip differs from natural earthquake slip, with the former occurring only after the entire surface has ruptured. The synchronised sensors also detect the passage of elastic waves propagating from the rupture and reflecting withing the loading assembly. As the waves pass through the sample, they momentarily change the stress conditions on the fault, potentially enhancing slip or contributing to arrest. These results highlight how technological advances have given us a new understanding of the link between mechanical and microstructural processes, while underscoring the underlying link between sample and apparatus behaviour. In the future, these developments will allow us to explore the fundamental physics of fault slip, which in turn, can be applied to macroscopic understanding of the earthquake cycle and the evolution of fault rupture. Show more