Ostermeijer, Giles A.; (2020) Multiscale high-resolution mapping of fracture damage around seismogenic faults. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The three-dimensional structure of a fault zone, composed of a central core and surrounding damage zone, evolves over a fault’s lifetime through a combination of quasistatic and coseismic processes. The damage zone consists of a complex network of brittle structures (e.g. fractures) with scales ranging from microscopic to kilometres long, and has been shown to grow wider and more complex with increased fault displacement. Recent investigations have shown this width increases to a limit of a few hundred metres at displacements below one kilometre. Still, little is known regarding the distribution of internal complexities, or how these might evolve as a fault matures. To quantify damage zone heterogeneity and address its evolution, I performed detailed mapping of damage at centimetre, outcrop, and kilometre scales along three seismogenic faults which have accrued 3, 11, and 21 km of displacement. Damage zone widths do not significantly increase in size with increasing displacement, supporting the existing width limits. I show that heterogeneities in both micro-fracture and meso-fracture damage have systematic trends with distance from the main fault cores. Along less mature faults, 3-11 km displacement, heterogeneity in damage decreases with distance from the fault, reaching background heterogeneity at distances shorter than the width of the damage zone. Spatial analysis shows that this complexity is controlled by damage clustering around the tips of larger faults and fractures. I show that even after the damage zone limit is reached, damage continues to evolve internally. Faults coalesce to form large subsidiary structures, while damage closest to the core transitions towards smaller scales. Along very mature faults dynamic pulverisation drives damage to microscopic scales, becoming macroscopically more homogeneous. These results provide insights into how damage evolves with increasing fault maturity, allowing us to better understand the processes that control the growth and evolution of fault zones.

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