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The Hikurangi Margin (HM) is a subduction zone along the east coast of the North Island of New Zealand where varying instances of slow slip events (SSEs) and earthquakes occur. These SSEs occur at different time scales and depths when comparing the northern and southern ends of the margin. Previous studies show that the rock comprising the accretionary wedge of the northern margin have low permeabilities, which could induce overpressures and modulate the occurrence of SSEs. Permeability rises when an SSE fractures the rocks within the deep wedge promoting fluid flow and thus dissipating the overpressures along and above the décollement. As fractures heal and permeability recovers overpressures build up once again. Although this cycle may explain the occurrence of SSEs along northern Hikurangi, it is not yet clear how intrinsic permeability varies in rocks above the décollement elsewhere along the margin. To better understand the disparity in SSE occurrence, rock samples from the northern and central part of the margin have been tested for permeability and elastic properties. We tested samples from the Weber, Whangai, Dannevirke and Wanstead formations, which are representative of the lithologies above the décollement in the central margin, and range in age from the Cretaceous to the mid to late Paleogene. We found that the Weber (PQ) and Whangai (PO) formation samples from central HM have higher permeability than northern HM rocks from the same formation in the north. This study provides insight into the mechanisms that lead to significantly fewer SSEs along the central HM. In the near future, we plan to conduct a suite of physical experiments that will include permeability recovery after fracturing, compaction, and ultrasonic velocity analysis to help further understand the stark differences in slip behavior observed along the margin. 

The northern part of the Hikurangi margin (HM) regularly experiences shallow slow-slip events (SSEs), possibly extending into the thrust faults of the sedimentary prism. For example, offshore Gisborne SSEs occur every 1-2 years and can last several weeks, during which 5-30 cm of slip may be accommodated. Understanding what controls the timing of such events will help the comprehension of HM deformation and earthquake mechanics in general. One hypothesis for a slow slip mechanism is that the low permeability of the HM prism rocks and the large fluid volumes dragged deep into the subduction zone cause over-pressures along the megathrust and prism splay faults. Overpressure induces SSEs that locally increase permeability. After an SSE, swelling clays and ductile deformation reduce permeability within months, resetting the conditions for developing overpressure. We tested such a hypothesis by measuring the hydraulic permeability of fractured sedimentary rocks making up the core of the accretionary prism. Tests were performed using a newly developed X-ray transparent pressure vessel mounted inside a micro-computed tomography scanner (mCT) that allowed in-situ observation of fracture evolution as a function of confining pressure, time, and exposure to water. The tested rocks were probably subducted to ~7.5 km and are calcareous-glauconitic fine-grained sandstones with a silty matrix from the Late Cretaceous-to-Paleocene Tinui Group containing ~15% vol% of clay minerals. After exposure to high confining pressure and water, the samples regained pre-fracture permeability in tens of days. mCT imagery suggests that fracture clogging, possibly due to clay expansion, controls healing. We propose that slow slip events in the northern HM open fault fractures and allow drainage at the beginning of the slip cycle, followed by fracture clogging due to swelling clays and ductile deformation, with the duration of the cycle regulated by the interplay of these processes. 

The Hikurangi subduction zone exhibits a north-to-south variation in deformation style. The plate interface in the south is locked, and megathrust earthquakes could accommodate the long-term plate convergence. In contrast, the northern megathrust regularly experiences shallow slow-slip events possibly extending into the thrust faults of the sedimentary prism. Understanding such a difference could reveal slip behavior and seismic cycle controls and help earthquake forecasting globally. One hypothesis is that the prism rock properties and fluid pressures affect these different slip behaviors. To test such a hypothesis, we measured the physical properties of rocks from the northern Hikurangi margin, focusing on ultrasonic elastic properties, permeability, and fracture healing. Such lithologies are equivalent to rocks buried to a few km depths within the accretionary prism. We found that all rocks contain >18 vol% of clay minerals. The hydraulic permeability of rock samples that are proxies for the deep part of the prism (i.e., 5 to 10 km depth) is three to four orders of magnitude lower than the values estimated by different authors for the prism as a whole. The results suggest that active faults and fractures in the accretionary prism must play a key role in draining fluids from the base of the prism and potentially from the subducting plate. Tests on a fractured sample show that fractures heal in tens of days, and permeability decreases over a short period relative to slip cycles of just a few weeks. Microphotography and micro-CT images suggest that healing is achieved by clay expansion. The observed healing could be underestimated as achieved under high confining pressure (up to 200 MPa) but at room temperature and humidity. We conclude that slow slip events in the northern Hikurangi margin may have a critical role in briefly increasing permeability at the beginning of the slip cycle, thus regulating pore pressure in the prism and allowing drainage.