Constraining Microfractures in Foliated Alpine Fault Rocks With Laser Ultrasonics

Abstract

Quantifying the amount and alignment of microfractures is important to understand the geomechanics, fluid flow, and seismic imaging of fault zones. At the Alpine Fault, New Zealand, the preferred alignment of minerals, foliation, and fractures results in elastic wave anisotropy. We have designed a unique laser-ultrasonic laboratory setup to study Alpine Fault rock samples at upper crustal conditions. Combined with differential effective medium modeling, we distinguish microfracture porosity and orientation from mineral alignment, as a function of distance to the principal slip zone (PSZ). Nearest to the PSZ, the cataclasite has the lowest P wave anisotropy with the most (randomly oriented) fractures. Next, the ultramylonite exhibits the greatest P wave anisotropy (∼45%) with 40% of its fractures aligned with foliation. Further from the PSZ, P wave anisotropy is 14-19% on average, due to 20-30% of the fractures being oriented in the same direction as mineral alignment. Plain Language SummaryThe movement on faults causes fractures and alignment of minerals in the adjacent rocks. This causes seismic waves to travel fastest parallel to these features, a phenomenon known as anisotropy. Quantifying the amount and orientation of the fractures that contribute to this anisotropy is important for understanding fault zones and the processes which could influence the nature of earthquakes. However, this requires a method that can separate the effects of the fractures from those of the minerals. We have designed a system that uses laser ultrasonics to measure the speed of waves through rocks under pressure at spatially dense locations. We combine these measurements with numerical modeling, allowing us to accurately quantify the amount and orientation of fractures in rocks from the Alpine Fault, New Zealand. We find that the total amount of fractures increases toward the fault. Additionally, the amount of fractures aligned with the fault plane (up to 40%) increases toward the fault until the fractures become almost completely randomly oriented for the cataclasite in the core of the fault.

Seismic Anisotropy and Its Impact on Imaging the Shallow Alpine Fault: An Experimental and Modeling Perspective

“…We use a modified version of the DEM GassDem code by Kim et al. (2019) that mixes parallel to foliation and randomly distributed microfractures (Simpson et al., 2020). The DEM builds on the MTEX porosity‐free stiffness tensor derived from EBSD.…”

“…We model the fast and slow elastic wave velocities by following the DEM approach of O'Connell and Budiansky (1974) and Bruner (1976) (Figure 7). We use a modified version of the DEM GassDem code by Kim et al (2019) that mixes parallel to foliation and randomly distributed microfractures (Simpson et al, 2020). The DEM builds on the MTEX porosity-free stiffness tensor derived from EBSD.…”

“…Recently, Simpson et al. (2020) measure with a unique technique multidirectional wavespeeds to characterize seismic anisotropy and fractures on Alpine Fault rocks up to 1 km in depth. The objective of our study is to estimate elastic wave anisotropy on heterogeneous micaceous quartzofeldspathic mylonite samples from the Alpine Fault by studying the contributions of CPO, texture, and microfractures.…”

“…Almqvist et al (2013) combine numerical modeling of EBSD, experimental wave speeds, and microfracture imaging to study and point at the possibility that microfractures could remain open at high effective pressures. Recently, Simpson et al (2020) measure with a unique technique multidirectional wavespeeds to characterize seismic anisotropy and fractures on Alpine Fault rocks up to 1 km in depth. The objective of our study is to estimate elastic wave anisotropy on heterogeneous micaceous quartzofeldspathic mylonite samples from the Alpine Fault by studying the contributions of CPO, texture, and microfractures.…”

“…Because earthquakes in the Alpine Fault happen at shallow depths (<12 km), we set out attention to characterizing fractures and their influence on LVZ and reflectivity in the upper crust. Few studies have targeted quantifying microfractures in shear zone rocks by combined laboratory ultrasonic data and elasticity modeling based on EBSD and differential effective medium (DEM) (e.g., Almqvist et al, 2013;Kern & Wenk, 1990;Simpson et al, 2020;Sun et al, 2012). Based on EBSD, Zhong et al (2014) compare experimental ultrasonic wave speeds to FEM for a phlogopite-bearing harzburgite with a fracture, while Morales et al (2018) compare mantle conditions laboratory ultrasonics to MTEX and DEM modeling for an antigorite-olivine schist, though the analysis was not performed on the exact same sample.…”