Update: Mark Drechsler has provided some images of the Kaiapit landslide, which are now integrated into the text below. Thanks to Mark for this – it is much appreciated. All images copyright of Mark Drechsler. In addition, I have placed a copy of the Drechsler et al (1989) paper on the International Landslide Centre website here. It provides a fascinating level of detail about the slide. I recommend this paper over and above the alternative.

Original text: Thanks for Mark Drechsler of Parsons Brinkerhoff Australia and the University of Adelaide for reminding me of the Kaiapit Landslide, which is surely one of the largest landslides of the last two decades (can anyone think of any larger?). Even though its impact was not enormous (74 deaths in three villages) the scale of this landslide is staggering. The landslide was described in a paper by Dreschler et al. (1989) and one by Peart (1991), from which these details are taken.

The slope failed at 10:43 am on 6th September 1988 when approximately 1.8 cubic kilometres of mountian collapsed, leaving a scar 1600 m high. The debris ran out over a distance of 6.5 km, covering an area of over 11 square kilometres. The estimated velocity is up to 180 km/hour. The slide inundated three villages and formed four valley-blockage lakes.

What is particularly interesting about this slide is the lack of an obvious trigger. Peart (1991) reported that the time of initiation was not the rainy season (indeed it was towards the end of the dry season) and that the period in question was not particularly wet. No seismic activity was recorded. The area has not been glaciated, so over-steepening and loss of support cannot be a cause. Clearly the area is affected by high levels of uplift, which may have allowed the formation of over-steepened slopes. These are probably a causal factor, but do not explain the triggering.

Peart (1991) sagely suggested that there may have been a time-related factor in the initiation of the landslide. I am convinced that time-dependent behaviour is important for large-scale landslide triggering, which means that the time of failure might lag significantly behind the trigger event. This is not always the case, but quite often very large landslides do not have an obvious trigger event. Other examples include the Mount Cook failure in New Zealand and the Leyte landslide in the Philippines.

Sadly the Google Earth imagery of the slide site is covered by clouds. Comments are welcome as ever.

Reference
Drechsler, M. Ripper, I., Rooke, E. and Warren, E. 1989. The Kaiapit Landslide, Papua New Guinea. In: Engineering Geology in Tropical Terrains, Universiti Kebaangsan Malaysia.
Peart, M. 1991. The Kaiapit landslide: events and mechanisms. Quarterly Journal of Engineering Geology, 24, 399-411.

Hat-tip: Mark Drechsler from Parsons Brinckerhoff and University of Adelaide.