Quantitative Characterisation of Fracturing Around the Damage Zone Surrounding New Zealand's Alpine Fault Using X-ray CT Scans of DFDP-1 Core

Abstract:

X-ray computer tomography (CT) scans of core recovered from the first phase of the Deep Fault Drilling Project (DFDP-1) through the Alpine Fault provide an excellent opportunity to analyse brittle deformation around the fault. In particular, assessment can be made of the heavily fractured protolith constituting the damage zone. Damage zone structures are divided into two types that result from two distinct processes: (1) “off fault damage” formed by stress changes induced by the passage of a seismic rupture and (2) “off fault deformation” that represent structures, which accommodate strain around the fault that was not localised on the principal slip zone (PSZ). The distribution of these damage zones structures within CT scans of the recovered core was measured along a scanline parallel to the core axis and assessed using a weighted moving average technique to account for orientation bias. The results of this analysis reveal that within the part of the fault rocks sampled by DFDP-1 there is no increase in density of these structures towards the PSZ. This is in agreement with independent analysis using Borehole Televiewer Data of the DFDP-1B borehole. Instead, we consider the density of these structures to be controlled to the first order by lithology, which modulates the mechanical properties of the fault rocks such as its frictional strength and cohesion. Comparisons of fracture density to p-wave velocities obtained from wireline logs indicate they are independent of each other, therefore, for the cores sampled in this study fractures impart no influence on the elastic properties of the rock. This is consistent with the observation from core that the majority of fractures are cemented. We consider how this might influence future rupture dynamics.