Damage signatures and imaging of crustal fault zones

Tom M. Mitchell - University College London

Crustal fault zones (FZ) have been studied by field geologists, seismologists and experimentalists for decades, and are mapped directly at the surface, and inferred from geophysics at depth. In a FZ, fault slip is commonly hosted in a narrow fault core, surrounded by a fracture damage zone of variable size up to 100s of metres in width. This damage is accrued by a combination of quasistatic and coesismic processes. With increasing displacement and fault maturity, FZs increase both size and complexity, due to overprinting of incremental damage. Fracture damage in FZs imparts a fundamental control on earthquakes.

Firstly, FZs rocks are generally more permeable than intact rocks, and hence play a key role in the migration of crustal fluids. Damaged rocks have reduced elastic moduli, cohesion and yield strength resulting in reduced elastic wave velocity, causing attenuation and potentially non-linear wave propagation effects during ruptures. The amount and spatial variation of these reductions can directly modify rupture dynamics, leading to the generation of slip pulses that can accelerate the transition to supershear rupture. Significant reductions of velocity within a FZ results in the structures trapping seismic waves that can continuously perturb stresses on the fault during earthquakes. At the same time, these low velocity zones can be used to infer damage structure at depth.  Finally, the dynamic generation of damage as the rupture propagates can itself influence the dynamics of rupture propagation; by increasing energy dissipation, modulating the rupture velocity and modifying the size of the earthquake, changing the efficiency of weakening mechanisms such as thermal pressurisation of pore fluids, and even generating additional seismic waves. All of these effects imply that a feedback exists between the damage imparted immediately after rupture propagation, at the early stages of fault slip, and the effects of that damage on subsequent ruptures dynamics.

To complicate matters, with increased pressure and temperature at depth, the structure, mechanical, and hydraulic characteristics of a FZ are subject to constant change (e.g. healing and/or sealing) during the seismic cycle as the fault evolves. This can complicate interpreting geophysical data when some damage can heal rapidly, and be subsequently invisible to many geophysical techniques. Additionally, the structure and physical properties of damage depends not only on fault maturity, but also on the scale of observation and the types of techniques used to map and infer fault structure at depth.  As techniques and technology improve, we are quantifying FZ structure and damage in the field and laboratory from the kilometre to metre scale, down to the nano-scale, and combining this with advances in seismic event location and high resolution tomography we have increasingly better ideas of how these structures evolve with depth. This presentation addresses the state-of-the-art of our understanding of FZ structure, and reflects on how we image and interpret fault structures at the surface and below.