The February 2018 Mangapoike landslide, New Zealand: an intriguing failure mechanism?

4 November 2020

The February 2018 Mangapoike landslide, New Zealand: an intriguing failure mechanism?

On a morning in which the media is full of a landslide that fail to materialise, I can find solace in the real thing. On 24 February 2018 an 8 million cubic metre landslide dammed the Mangapoike River in a remote upland area of eastern North Island in New Zealand. I blogged about this landslide at the time – it is a rather beautiful example. The height of landslide dam was managed through blasting, although the landslide remains intact, leaving interesting questions about the cause of the failure. This is a Google Earth image of the landslide:-

Google Earth imagery of the 24 February 2018 Mangapoike landslide, New Zealand.

An interesting paper has been published in the journal Landslides about the Mangapoike landslide (McGovern et al. 2020). This is provides a detailed description and analysis of the feature, which was a wedge failure in weak rock sandstones and mudstones, with the slide surface being defined by a smoothed, northwest-dipping bedding plane. The elegant diagram below, which is an exemplar of how to use graphics to present landslide information, shows the source area of the slide:

The source area of the February 2018 Mangapoike landslide, New Zealand. Diagram from McGovern et al. (2020).

The landslide mass itself partially disaggregated into blocks, which of course meant that the river was more effectively dammed.

The really interesting aspect of this paper though is its consideration of the causes and triggers of the Mangapoike landslide. The underlying causes are clear (and are well-explained), but what triggered the failure? McGovern et al. (2020) demonstrate clearly that there was no unusual rainfall event prior to the failure. Indeed, they are remarkably definitive about this:

“…it seems implausible that rainfall and elevated groundwater were important triggering factors”.

Instead McGovern et al. (2020) propose that fluid overpressure might have played a role. As they put it:

“The Makaretu Sandstone is a known hydrocarbon reservoir formation within the Tolaga Group, with overlying mudstone units acting [as] seals”.

There is geodetic evidence that prior to the landslide the region underwent approximately 15 mm of vertical movement, which may indicate that there was a fluid overpressurisation event occurring. The authors postulate that gas migrated vertically though the sandstones but was trapped by the mudstones at the base of the landslide, reducing the normal effective stress and triggering the failure.

This mechanism is a hypothesis, and indeed the authors state that:

“Whether fluid overpressure was a contributing factor to the Mangapoike landslide is equivocal.”

There have been previous suggestions that gas overpressurisation might be a factor in certain landslide failures. The interesting thing about this one is that the data is so good that it invites detailed modelling to understand the potential mechanism, and its efficacy, better.