Debris flow-slide initiation mechanisms in fill slopes, Wellington, New Zealand

Abstract

Subaqueous slopes formed on active continental margins are subject to a variety failure styles, but their movement mechanisms during earthquakes remain poorly constrained as few submarine landslides have been directly sampled for detailed investigation. We conducted a series of dynamic shear experiments on samples recovered from the base of the Tuaheni Landslide Complex, located off the east coast of the North Island of New Zealand, to explore its potential behaviour during earthquakes. Our experiments suggest that while liquefaction could occur in marginally stable subaqueous slopes, particularly in shallow sediments on steep slopes or sediments with high initial pore fluid pressures, this is not a likely failure mechanism for pre-existing deep landslide complexes such as the Tuaheni Landslide Complex. Subaqueous landslides occurring along deeper, shallow angled shear zones are likely to experience episodic displacement when pore water pressures are sufficiently elevated during large earthquakes. The observed behaviour provides a credible mechanism through which subaqueous landslides in similar materials and active tectonic settings may be subject to episodic movement without undergoing catastrophic failure.

SAR and optical images correlation illuminates post-seismic landslide motion after the Mw 7.8 Gorkha earthquake (Nepal)

“…No anthropogenic forcing, like road construction, could explain the initiation of this landslide motion in the first few days after the earthquake. The time-dependent behavior, in particular the observed time-delay of large movement initiation after the main shock, suggests either a fluid effect (as for instance a strong supply of groundwater on the sliding surface or a densification that makes the material more prone to brittle failure in response to subsequent elevations of pore water pressure 35 , 36 ) or a time-dependent fracture process of the landslide shear zone (like progressive failure and maturation of the sliding surface 37 ). This last hypothesis is hardly plausible due to the sharp increase of velocity at the landslide initiation, and due to the geomorphology of all the detected landslides, that are mostly reactivations of ancient thick slides with many pre-existing signs of instability.…”

“…Another hypothesis could be a co-seismic landslide compression that can lead to an internal landslide deformation and cracking, and then can favour new water pathways, and makes the landslide more sensitive to further rainstorms or monsoons 35 , 36 . However the little effect of subsequent monsoons and individual rainfalls (Figs.…”

Controls on Post‐Seismic Landslide Behavior in Brittle Rocks

“…Rates and patterns of post-seismic landsliding are at least in part controlled by the nature of coseismic ground shaking at a specific location and how this influences the strength and rheology of slope-forming rocks. Our findings form part of a wider and growing body of work that demonstrates the importance of lithology, local-to regional-scale geomorphology, stress history and landslide mechanism in controlling post-seismic hillslope behavior and variability therein (Brain et al, 2017;Carey et al, 2021;Hu et al, 2018;Kincey et al, 2021). As such, invoking a range of poorly constrained, extrinsic and universal "healing" mechanisms to explain observed trajectories and temporal trends in the rate of new landslides may not be necessary or appropriate at all spatial scales.…”

“…Lawrence et al, 2009). However, isolating the dominant mechanisms, and combinations thereof, that control the postseismic evolution of hillslope strength is not straightforward across landscapes characterised by considerable variations in geomorphic setting, landslide type and morphology (Fan et al, 2018;Hu et al, 2018;Kincey et al, 2021), and substrate lithology, rheology, structure and stress history (Bontemps et al, 2020;Brain et al, 2017;Carey et al, 2021Carey et al, , 2017Gischig et al, 2015;Hu et al, 2018;Lacroix et al, 2014;Samia et al, 2017a;Viles et al, 2018). Both mapped landslide inventories and local field studies reflect landscape and landform response to a specific earthquake event (Rosser et al, 2021).…”

“…The mechanisms responsible for the latter are poorly constrained but have been postulated to result from a range of "healing" processes that include the re-establishment of plant-root cohesion (e.g., Jacoby, 1997;Yunus et al, 2020) and the reversal of dilation experienced during an earthquake as rock and soil masses settle and re-establish frictional contacts (e.g., Lawrence et al, 2009). However, isolating the dominant mechanisms, and combinations thereof, that control the post-seismic evolution of hillslope strength is not straightforward across landscapes characterized by considerable variations in geomorphic setting, landslide type and morphology (Fan et al, 2018;Hu et al, 2018;Kincey et al, 2021), and substrate lithology, rheology, structure and stress history (Bontemps et al, 2020;Brain et al, 2017;Carey et al, 2017Carey et al, , 2021Gischig et al, 2015;Hu et al, 2018;Lacroix et al, 2014;Samia et al, 2017a;Viles et al, 2018). Both mapped landslide inventories and local field studies reflect landscape and landform response to a specific earthquake event (Rosser et al, 2021).…”