The Casita landslide revisited

One of the most deadly hurricanes of modern times was Hurricane Mitch, which tracked across Central America in late October 1998. Many of the tens of thousands of victims were killed by landslides. Perhaps the most notable event was a lahar (a volcanic landslide) that swept down from near the summit of Casita volcano in Nicaragua, killing about 2500 people over the course of its 6 km path (and some more in the hyper-concentrated flow (debris rich flood) events that travelled a further 10 or so kilometres from the toe of the slide. Unfortunately, despite the magnitude of this event the amount of published literature about it has remained quite limited. It is therefore terrific that a paper has just been published by Graziola Devoli and her colleagues (Devoli et al. 2009) that seeks to summarise the published and unpublished reports about this remarkable landslide.

Wikipedia has a very decent image of the upper track of the landslide, which gives a pretty good idea of the scale of this event:

Whilst MDA have a great overview image of the source, track and runout zone:

Complex landslides such as this are poorly understood. In particular, as in the landslides that I highlighted in Sichuan, the mechanisms of initiation and movement are quite intricate. Devoli et al. (2009) have used a range of geological, geotechnical and analytical techniques to get a better idea of what happened.

The landslide was triggered by very heavy rainfall - they suggest that about 750 mm (that's about a years worth for where I live) of rain fell in a little over 80 hours. Interestingly, they conclude that the landslide can be divided into three key phases:

1. Failure started in a fractured and altered volcanic breccia in the northern area of the scarp which released a volume of about 260,000 cubic metres. The flow that developed from this failure swept downslope and entrained colluvium deposits at the toe of the slope in the southern part in less than about 40 seconds.
   
2. The rapid removal of the colluvium on the slope triggered a second failure. This also originated in the scarp shown on the image above. In this phase about 640,000 cubic metres of volcanic breccia slipped over a unit of clay-rich pyroclastic deposits. It is unclear as to whether this flow joined the first one or occurred separately. Either way, blocks in this flow travelled 9 km or more downslope.
   
3. The third and final stage consisted of a as a sudden debris / rock avalanche that originated in the uppermost section of what is how the landslide scar. This failure, with a volume of 690,000 cubic metres, appears to have occurred very soon after the first two events.

In the aftermath there has been considerable concern raised about the likelihood of failure of the entire flank of Casita Volcano (indeed, even I have published on this theme - see van Wyck de Vries et al. 2000). The paper concludes that under conditions of high groundwater instability of these flanks can occur, although as we understand these very large failures so poorly I would be cautious in the interpretation of this particular result. This is a slope that needs an active monitoring programme for sure.

References
Devoli, G., Cepeda, J. and Kerle, N. 2009. The 1998 Casita volcano flank failure revisited — New insights into geological setting and failure mechanisms. Engineering Geology, 105, 65-83.

van Wyk de Vries, B., Kerle, N., Petley, D., 2000. A sector collapse forming at Casita
volcano, Nicaragua. Geology 28, 167–170.

Volcanic flank collapse and tsunamis

A few years ago the media got rather excited about a paper that suggested that there was the potential for a giant flank collapse on the Cumbre Vieja volcano in the Canary Islands. A model of the resultant tsunami suggested that it could be sufficiently large to cause huge losses throughout the coastal areas of the North Atlantic. Most scientists now believe that this tsunami was something of an exageration of a worse case scenario, (see for example this critique) but the potential for a flank collapse to generate a local tsunami event is very real, albeit comparatively rare. As a result, there is considerable interest in trying to understand these events better.

At the GSA Annual Meeting this week, Matthew Hornbach from the University of Texas presented a rather interesting study that has just been undertaken on the island of Tongatapu in Tonga. Tongatapu is located in the South Pacific with a number of active volcanoes nearby (Fig 1).

Fig. 1: Google Earth image of the island of Tongatapu showing the location of three unnamed submarine volcanoes to the west.

Hornbach found that on Tongatapu there are seven coral boulders located up to 400 m from the current coastline (Fig. 2). The largest of these boulders has a mass of about 1200 m. The boulders have clearly been lifted from the coral that fringes the island - Science News reports that for the largest boulder the point in the reef from where it has been removed is clear.

Fig. 2: Science News image of one of the boulders on the island of Tongatapu.

Now, this island is flat, meaning that the boulders cannot have rolled there. Hence, Hornbach has concluded that these boulders were emplaced by a huge tsunami, which they have provisionally dated to have occurred in the Holocene (i.e. in the last 10,000 years). This is potentially the largest tsunami deposit so far recorded - the wave must have had a run up of >40 m vertical distance. Hornback believes that the source of this tsunami was a submarine volcano located to the west of the island (Fig. 1). They report that bathymetric surveys of this feature show that it rises to to the water surface - some reports suggest within 65 m. This large volcano apparently has a section missing from the east side, which Hornbach interprets as being the result of a flank collapse - i.e. a giant landslide in which the side of the volcano collapses. Flank collapses undoubtedly can cause very large, localised tsunamis.

As an aside, it is interesting to note that the major population centres are on the sheltered north and east side of the islands (Fig. 3) . I wonder if this is a coincidence?

Fig. 3: Google Earth image of Tongatapu .