In New Zealand, the boundary between the Australian and Pacific plates is defined by the Alpine Fault, a major strike-slip feature considered to have a high probability of rupturing in a large earthquake (of moment magnitude greater than 8) within the next 50 years. To gain insight into the physical processes controlling a large fault near the end of its seismic cycle, an international team drilled two boreholes through the Alpine Fault’s central segment as part of the Deep Fault Drilling Project.
As part of this investigation, Boulton et al. analyzed the rocks retrieved from both holes. Both successfully penetrated the fault, including its core, which hosts the principal zone of slip where most earthquake displacement occurs. The boreholes also pass through the surrounding “damage zone,” where the rocks are highly fractured from deformation associated with their journey along the fault. Here any given rock has undergone about 35 kilometers of vertical displacement and more than 200 kilometers of horizontal displacement over the past 5 million years.
The researchers identified eight characteristic rock units within the cores, the most distinctive of which is a gouge—a pulverized, claylike mixture formed along the principal zone of slip. The gouge contains abundant clay and white mica minerals, including smectite, the presence of which indicates that low-temperature alteration occurred within this unit. This alteration, the authors report, has resulted in the principal zone of slip having a fabric of slippery, sheeted minerals in which sliding preferentially occurs.
Unlike in a number of other faults, whose rocks display volume losses of up to 90%, the Alpine Fault material gained volume in and around its core zone as smectite, other clay minerals, and calcite gradually precipitated. Over time, these minerals sealed off fractures, decreasing the fault rock’s porosity and permeability and increasing the fault’s strength. Ultimately, the researchers argue, these changes allow the fault to build up strain and release this energy in a large earthquake.
The energy released during the rupture then refractures the surrounding rock, leading to increases in permeability, fluid migration, and mineral alteration that begin the cycle anew. In light of these results, the researchers argue, observations of fluid flow throughout the seismic cycle are needed to improve models of how rock-fluid interactions influence Alpine Fault seismicity. (Geochemistry, Geophysics, Geosystems, https://doi.org/10.1002/2016GC006588, 2017)
—Terri Cook, Freelance Writer