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Rob Parker (University of Cardiff) and Dave Petley
The Mw = 7.8 earthquake on Tuesday in Iran was the largest event in that country for about 50 years. Fortunately, the depth of the earthquake (82 km) and the low population density in the affected areas meant that loss of life was low for an event of this size. Indeed, reports suggest that only one person died in Iran, although there are reports of 40 deaths in Pakistan. This single fatality in Iran was the result of a landslide, and one of the images on the BBC reports about the earthquake also seems to show landslides:
Over the last three years or so, we have been working with our colleagues Alex Densmore and Nick Rosser, funded by the Willis Research Network, to develop a model that will allow us to make an initial assessment of landslide impacts in earthquakes. Rob recently submitted his PhD, and has now moved to a post-doctoral position at Cardiff. However, we thought that this event would be an interesting first application of the model, which has been produced through a statistical (logistic regression) analysis of spatial patterns of landslides (with areas larger than 11,000 square metres) triggered by four large earthquakes in the USA, New Zealand, Taiwan and China. The model provides a first-order prediction of the probability of hillslope failure across the region affected by seismic shaking, based on the strength of ground motions and the gradient of hillslopes. Areas likely to have experienced high levels of landslide activity are shown in red, and while areas we expect to be less affected by landslides are shown in green and then blue. Here, landslide probability has been estimated using preliminary ground motion data published by the USGS and hillslope gradients derived from the ASTER global elevation model. Read more
Yesterday I was deeply honoured to present the AQA Annual Lecture at the Geographical Association annual conference in Derby. I talked on the topic of Future Trends in Natural Hazard Losses - essentially a review of what we expect to see in the next few decades on a global basis. I started the talk by looking at recent trends, and then looked forward to what we might expect. I finished with an example of one of the mega-quakes that we fear – in this case a large earthquake in Western Nepal.
The slides are on Authorstream – you can download the Powerpoint file there – and should also appear below:
Accounting for changes in landslide patterns with time is very difficult. Collecting the underlying datasets is problematic in itself (still requiring mapping by hand in most cases) and, of course, landslides result from a combination of a whole range of natural and human factors, all of which change with time. It has been frequently postulated that one of the underlying causes of the increase in landslides in mountain areas in less developed countries is road building – indeed in a paper that I wrote with some colleagues a few years ago (Petley et al. 2007 – drop me a line if you want a copy) we proposed that inappropriately engineered road construction might account in large part for the increase in landslide impacts in Nepal over the last 20 years or so.
As part of the Earthquakes Without Frontiers project we will once again be working in Nepal over the next few years. I am genuinely delighted about this – Nepal is a quite wonderful country, but the landslide problem there is severe. The main aspect of our work will be to think about the threat posed by earthquake-induced landslides, and a major issue here is the way that road networks will be disrupted when a large earthquake occurs. So, this morning I was looking at the ways that road building has changed the landscape in Nepal in recent years, and came across this really interesting example from Central Nepal. The area in question lies close to the Prithvi Highway, which is the main access road from the southern plains through the mountains to Kathmandu. The rural road in question, which is in the very southeast of Gorkha District, is a small road that links to the main Prithvi Highway and passes through a settlement called Ghyalchok before climbing over the ridge, ultimately joining another road near a settlement called Darbung Phant (see Figure 1 below). The mountains here are large, but they are not the huge snow-capped high Himalaya to the north. The road, which starts at about 300 m above sea level and climbs to about 1300 m, was presumably constructed to link the many ridge-top communities to the main road network, and as such is typical of thousands of roads being built in Nepal. The road, which is almost certainly unsurfaced, can be seen as a thin brown line on Fig 1.
The unusual spell of very prolonged and intense rainfall across the UK is continuing, with the Met Office forecasting more to come in the coming days. Reports of landslides continue to flood in from around Britain – for example in the Highlands of Scotland the A890 is blocked once again at Stromeferry – this is the same section of road that caused so many problems last year. Yesterday, the BGS and the Met Office took the highly unusual step of issuing a landslide warning for SW England, primarily because conditions are now so saturated that landslides are inevitable. There is particular concern around coastal cliffs, which in the UK are often mantled with landslides. Beaches are popular with walkers, meaning that there is a danger of rockfalls onto people, whilst cliff tops in the UK are the sites of highly popular coastal footpaths. The dangers are obvious.
Meanwhile, it is good to see that these extreme conditions are driving the media to provide increased coverage to the changes that we are now seeing in the climate. Although climate change denialists will undoubtedly disagree, there is no doubt that extreme conditions are becoming increasingly normal. Unfortunately, this is just the start. Read more
One of the legacies of the Christchurch earthquake sequence in New Zealand has been both increased levels of rockfall hazard and increased awareness of the level of hazard that predated the seismic events. The most seriously affected area is the Port Hills area to the southeast of the city, where large numbers of houses were affected by rockfalls during the earthquakes, and may more were left at risk in the aftermath. To assess the level of hazard properly, Christchurch City Council commissioned GNS Science to undertake a quantitative life risk assessment for the affected areas. This has been a huge piece of work, undertaken in the public spotlight on a very short timescale. The aim has been to generate the best possible assessment of the risk to life; this assessment can then be used as the basis for decisions on the viability of individual properties. Read more