New Zealand is tectonically complex, hosting both subduction and strike-slip fault zones associated with the plate boundary between the Pacific plate and the Australian plate. The country is thus geologically active, with many potential natural hazards that require investigation and close monitoring (Figure 1, Barnes et al., 2019). The need for improved mitigation also presents an opportunity to better understand the Earth processes prevalent in active tectonic settings. Between 2017–2018, International Ocean Discovery Program Expeditions 372 and 375 set out to collect data (cores, well logs, and in-situ geophysical measurements) at the Hikurangi Margin in northern New Zealand, with primary goals of better understanding the mechanisms and behaviors of gas hydrate-bearing landslide (Barnes et al., 2019) and slow slip events on subduction faults (Saffer et al., 2019). A combined total of six sites were sampled between the two expeditions, with two in the Tuaheni Landslide Complex (TLC) and four in a transect across the Kermadec Trench near the Hikurangi Margin.
Submarine landslides are often the byproduct of catastrophic destabilization events like earthquakes. If severe, the resulting slope failure can sometimes significantly displace the seafloor, even causing tsunamis (Harbitz et al., 2014). However, the submarine Tuaheni Landslide Complex in the offshore Hikurangi margin is unusual, showing evidence of slow, continuous downslope movement of sediment rather than discrete events. Furthermore, the onset of landslides seem to correlate with the edge of gas-hydrate, with a general shift in the styles of seafloor deformation above and below the depths where the gas hydrates are stable (Figure 2, Mountjoy et al., 2014). The cores and well logs collected from Expedition 372 were used to test several hypotheses on how exactly the gas hydrates are related to the observed gradual landslides.
Hikurangi Margin is also well documented for another tectonic phenomenon that is observed along the subduction zones, called slow slip events (SSEs). Unlike the devastating megathrust ruptures that release strong seismic energy from sudden movements along the fault surface, SSEs release energy gradually, over a course of weeks or even months (Saffer et al., 2019). While not necessarily an immediate natural hazard, the phenomenon is a relatively recent finding and garnered increased attention from the research community in the last two decades as the observational techniques improved and densified. Hikurangi margin hosts a region of well-studied SSEs with regular occurrences, where the fault (Pāpaku fault) is at shallow depths, accessible by drilling (Saffer et al., 2019). The goal of Expedition 375 was to reach and sample the fault zone, through coring, wireline logging, and installment of geophysical observatories (Figure 3).
Recent studies have begun to shed light on the mechanisms behind the above phenomena using the core and geophysical data retrieved by Expeditions 372 and 375. For example, the cores from Expedition 375 reveal highly variable lithologies near the slow slip fault surface, ranging from marine clay and carbonates to volcaniclastic and conglomerate rocks (Figure 4). The large variety of material with different compositions and grain-sizes also means different deformational responses to the stress as they are carried into the Earth on the subducting plate, and by extension, the different slip behavior (Barnes et al., 2020). Computer numerical modeling has also previously shown that fault surfaces with variable materials might favor transient slow slips over large, sudden ruptures (Saffer et al., 2015), compatible with the observations made from the borehole data.
References
Barnes, P. M., Pecher, I. A., LeVay, L. J., Bourlange, S. M., Brunet, M. M. Y., Cardona, S., … & Wu, H. Y. (2019). Expedition 372A summary. Texas A&M Univ. https://doi.org/10.14379/iodp.proc.372A.101.2019
Saffer, D. M., Wallace, L. M., Barnes, P. M., Pecher, I. A., Petronotis, K. E., LeVay, L. J., … & Wu, H. Y. (2019). Expedition 372B/375 summary. Proceedings of the International Ocean Discovery Program, 372, 1-35. https://doi.org/10.14379/iodp.proc.372B375.101.2019
Harbitz, C. B., Løvholt, F., & Bungum, H. (2014). Submarine landslide tsunamis: how extreme and how likely?. Natural Hazards, 72, 1341-1374.
Mountjoy, J.J., Pecher, I., Henrys, S., Crutchley, G., Barnes, P.M., and PlazaFaverola, A. (2014b). Shallow methane hydrate system controls ongoing,downslope sediment transport in a low-velocity active submarine landslide complex, Hikurangi Margin, New Zealand. Geochemistry, Geophysics, Geosystems, 15(11):4137–4156. https://doi.org/10.1002/2014GC005379
Barker, D.H.N., Henrys, S., Caratori Tontini, F., Barnes, P. M., Bassett, D., Todd, E., and Wallace, L. (2018). Geophysical constraints on the relationship between seamount subduction, slow slip and tremor at the north Hikurangi subduction zone, New Zealand. Geophysical Research Letters, 45(23):12804–12813. https://doi.org/10.1029/2018GL080259
Barnes, P. M., Wallace, L. M., Saffer, D. M., Bell, R. E., Underwood, M. B., Fagereng, A., … & IODP Expedition 372 Scientists (2020). Slow slip source characterized by lithological and geometric heterogeneity. Science Advances, 6(13), eaay3314.
Saffer, D. M., & Wallace, L. M. (2015). The frictional, hydrologic, metamorphic and thermal habitat of shallow slow earthquakes. Nature Geoscience, 8(8), 594-600.
