Abstract
Abstract. Porosity reduction in rocks from a fault core can cause elevated pore fluid pressures and consequently influence the recurrence time of earthquakes. We investigated the porosity distribution in the New Zealand's Alpine Fault core in samples recovered during the first phase of the Deep Fault Drilling Project (DFDP-1B) by using two-dimensional nanoscale and three-dimensional microscale imaging. Synchrotron X-ray microtomography-derived analyses of open pore spaces show total microscale porosities in the range of 0.1 %–0.24 %. These pores have mainly non-spherical, elongated, flat shapes and show subtle bipolar orientation. Scanning and transmission electron microscopy reveal the samples' microstructural organization, where nanoscale pores ornament grain boundaries of the gouge material, especially clay minerals. Our data imply that (i) the porosity of the fault core is very small and not connected; (ii) the distribution of clay minerals controls the shape and orientation of the associated pores; (iii) porosity was reduced due to pressure solution processes; and (iv) mineral precipitation in fluid-filled pores can affect the mechanical behavior of the Alpine Fault by decreasing the already critically low total porosity of the fault core, causing elevated pore fluid pressures and/or introducing weak mineral phases, and thus lowering the overall fault frictional strength. We conclude that the current state of very low porosity in the Alpine Fault core is likely to play a key role in the initiation of the next fault rupture.
Highlights
Fault mechanics, fault structure, and fluid flow properties of damaged fault rocks are intimately related (e.g., Gratier and Gueydan, 2007; Faulkner et al, 2010)
It can be noted that the lower-cataclasite sample (DFDP-1B 69_2.57) has twice as much pore space (Fig. 3d) as any of the other samples
The expected maximum pore volume was estimated to be largest in the principal slip zone (PSZ)-2 sample (DFDP-1B 69_2.54), reaching 862 μm3 (Fig. 3c)
Summary
Fault structure, and fluid flow properties of damaged fault rocks are intimately related (e.g., Gratier and Gueydan, 2007; Faulkner et al, 2010). Post-seismic recovery mechanisms (gouge compaction and pressure solution processes) result in reductions in porosity, permeability, and fluid flow (Renard et al 2000; Faulkner et al, 2010; Sutherland et al, 2012). These processes may cause elevated pore fluid pressures within fault cores and trigger frictional failure (e.g., Sibson, 1990; Gratier et al, 2003; Zhu et al, 2020). The state of porosity within rocks from fault cores can play a key role in fault slip.
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