ALBERT

Description: Slow slip events (SSEs) at the northern Hikurangi subduction margin, New Zealand, are among the best-documented shallow SSEs on Earth. International Ocean Discovery Program Expeditions 372 and 375 were undertaken to investigate the processes and in situ conditions that underlie subduction zone SSEs at the northern Hikurangi Trough. We accomplished this goal by (1) coring and geophysical logging at four sites, including penetration of an active thrust fault (the Pāpaku fault) near the deformation front, the upper plate above the SSE source region, and the incoming sedimentary succession in the Hikurangi Trough and atop the Tūranganui Knoll seamount; and (2) installing borehole observatories in the Pāpaku fault and in the upper plate overlying the slow slip source region. Logging-while-drilling (LWD) data for this project were acquired as part of Expedition 372, and coring, wireline logging, and observatory installations were conducted during Expedition 375. Northern Hikurangi subduction margin SSEs recur every 1–2 y and thus provide an ideal opportunity to monitor deformation and associated changes in chemical and physical properties throughout the slow slip cycle. In situ measurements and sampling of material from the sedimentary section and oceanic basement of the subducting plate reveal the rock properties, composition, lithology, and structural character of material that is transported downdip into the SSE source region. A recent seafloor geodetic experiment raises the possibility that SSEs at northern Hikurangi may propagate to the trench, indicating that the shallow thrust fault (the Pāpaku fault) targeted during Expeditions 372 and 375 may also lie in the SSE rupture area and host a portion of the slip in these events. Hence, sampling and logging at this location provides insights into the composition, physical properties, and architecture of a shallow fault that may host slow slip. Expeditions 372 and 375 were designed to address three fundamental scientific objectives: Characterize the state and composition of the incoming plate and shallow fault near the trench, which comprise the protolith and initial conditions for fault zone rock at greater depth and which may itself host shallow slow slip; Characterize material properties, thermal regime, and stress conditions in the upper plate directly above the SSE source region; and Install observatories in the Pāpaku fault near the deformation front and in the upper plate above the SSE source to measure temporal variations in deformation, temperature, and fluid flow. The observatories will monitor volumetric strain (via pore pressure as a proxy) and the evolution of physical, hydrological, and chemical properties throughout the SSE cycle. Together, the coring, logging, and observatory data will test a suite of hypotheses about the fundamental mechanics and behavior of SSEs and their relationship to great earthquakes along the subduction interface.

Description: Subaqueous slopes are susceptible to a broad range of failure mechanisms and deformation styles, many of which are not well characterised. We undertook novel laboratory-based testing using a Dynamic Back-Pressured Shearbox on samples collected from an area subject to ongoing slope failures, situated on the upper slope of New Zealand's Hikurangi Margin, to determine how increases in pore water and gas pressures generate shallow mass movement. Using both water and nitrogen gas we observed similar responses in both cases, indicating that behaviour is dominated by the normal effective stress state regardless of pore-fluid phase. Shear-strain accumulation, representing landslide movement, shows a slow episodic pattern, in common with many shallow terrestrial landslides. Our results are relevant for landslides occurring in shallow near surface sedimentary sequences but have implications for deep-seated landslide behaviour. They suggest that once movement initiates at a critical effective stress, its rate is regulated through dilation and pore expansion within the shear zone, temporarily increasing effective stress within a narrow shear band and suppressing rapid shear. Consequently, under certain conditions, shallow submarine landslides (e.g. spreading failures) can undergo slow episodic movement which allows them to accumulate large shear strains without accelerating to catastrophic movement even when they are unconstrained.

Description: Highlights • We report on methane seeps found at shallow depths on New Zealand's Hikurangi Margin. • Tools have been applied to measure bubble characteristics from video footage and to estimate the gas flow rate using acoustic data. • We estimate that the entire Tuaheni seep field produces somewhere in the range of 30–2415 t of methane per year. • The density of seeps at this location is far greater than anything else observed on the Hikurangi Margin. Abstract We analyse an area of high density submarine methane gas seeps situated on the shelf to slope transition (130–420 m water depth) on the northern region of New Zealand's Hikurangi margin, off Poverty Bay. Multibeam and singlebeam echo sounder data collected in 2014 and 2015 revealed 〉600 seeps, at much greater density than any previously mapped areas of seepage on the Hikurangi margin. To broadly constrain the output of methane from these seeps, we have estimated the flow of methane at individual seeps, utilising perspective-measurements applied to still frames from a deep towed camera system to measure the dimensions of rising bubbles. We combine bubble size and rise-rate distributions with singlebeam acoustic data to estimate gas flow rates at six selected seeps sites. Our results predict a wide range (3.0–2249 mL/min) of methane release into the water column. If we assume that the six seeps we analysed are representative of the entire seep population, and that gas flow is constant, we can extrapolate across the seep field and infer a gas release of 30 to 2415 t of methane per year into the ocean.