22 April 2009

EGU Day 3

Posted by Dave Petley

I am just going to write up one session from today’s meeting as it contained the most interesting talks that I heard. This was an annual session on landslides induced by volcanoes and earthquakes, the first three talks of which focussed on the Wenchuan event. Regular readers will know that I am very interested indeed in this event (see here for example).

So first up was Ed Harp and two colleagues from USGS. They provided a pretty general overview of the landslides triggered by the earthquake, but supplemented with some very nice satellite imagery of key sites. He highlighted the death toll associated with the earthquake-induced landslides (20,000+) and the huge number of dams that needed clearing (he quoted 33 that required mitigation). The tour included Beichuan, Tangjiashan, etc. I guess there wasn’t much new or scientifically-challenging here, but it was a good start. The final part of the talk highlighted the collaboration between USGS and the China Geological Survey, which is going to allow transfer of techniques for seismic landslide hazard analysis, data collation to test the USGS PAGER model and quantification of the sediment flux. In questioning Ed said that the planned seismic hazard analysis tool is Newmark Displacement. I do wonder whether this is the right tool in this part of China – it is probably appropriate for the initiation of the slides, but most of the failures that I saw have lower sections that are very complex, with massive entrainment of slope debris and colluvium. This is where the people and infrastructure are, so it seems to me that without substantial modification Newmark is going to be quite problematic.

Second up was Gorum and his colleagues from ITC. The poor chap had the misfortune of giving his first international conference talk to a packed house about a set of landslides that he had not visited! In that context he did very well indeed. He gave a slightly broader overview, making use of some of the data collected by the Chengdu University of Technology from their mapping. In particular, he highlighted that the landslide are focused close to the fault trace (NB this focus is not on the epicentral region), with many slides on the lower gorges. Using satellite imagery they have mapped an initial 11,308 landslides, compared with 1,638 before the earthquake. They also noted that they had mapped 256 valley blocking slides, with the highest concentration being on the fault trace. They are now working with the Chinese to understand the landslide distribution (very challenging) and to undertake a multi-hazard analysis. I cannot quite see how the latter will be done – the presenter seemed to imply that they will use the existing landslide distribution to drive a modelling exercise. I hope that this is not the case as the seismically-induced landslides will not give a good indication as to where rainfall-triggered slides will occur in a post-earthquake landscape.

The final talk from Wenchuan was by Chigira and his colleagues from Kyoto, with substantial co-authorship from China. This was the best judges of the three, provising a nice summary of the key points issues, well-illustrated using good images. They highlighted the role of dissolution in raising landslide susceptibility – the point being that dissolving limestone beds creates voids that allows drainage of groundwater, reducing susceptibility to rainfall induced slides, but creating point-to-point contacts that increases susceptibility to earthquake induced sliding. This is a nice point. He concluded by looking at some of the very largest slides, concluding that the geomorphology before the earthquake showed depressions and dips on the big slides that indicated that they were potentially unstable. Thus, the biggest slides were considered to be predictable. I am not sure that I agree with the latter point completely (unless all slopes with these features failed, which I don’t think is the case), the observations about the morphology are well-made.

The penultimate talk upon which I will comment was by Niels Hovious from Cambridge, with co-authors from Taiwan and elsewhere. Niels used the 1999 Chi-Chi earthquake in Taiwan to examine the distribution of landslides that are generated, and then to look at the production of sediment. First, Niels argues that the landslides closely map onto the distribution of ground shaking, with which I agree, but then argued that the highest landslide density occurs around the epicentre. This may well be true for Chi-Chi, but it was not for one of jis other examples (Northridge) and it was not true for Pakistan, where the highest densities are at the fault rupture. Most importantly, it is also not true for Wenchuan, where again the highest densities lie close to the surface expression of the fault rupture and not around the epicentre. Niels then showed that the density of landslides increased remarkably in the aftermath of the Chi-Chi earthquake – in the Chenyoulan ctachment that they studied the number of landslides before the earthquake was 8123, with a further 3,800 being triggered in the event. However, in the nearly ten years since a further 48,370 landslides have been triggered. However, Niels showed that the sediment concentration in the rivers is now close to base level again, suggesting that the earthquake’s impacts are now reducing. This is good stuff but, given that the landscape is affected by typhoons that are exceptionally extreme events one wonders how applicable it is to other places. It seems to me that they need to work in China! An important aside is that this talk does highlight the importance of being prepared for massive sediment production in China.

The final talk that I shall briefly mention is that of Merri and his colleagues from Italy, who are using a finite difference model (FLAC3D to simulate the impacts of magma intrusion on the stability of the Stromboli volcanic edifice. The presentation was quite nice, but the model seems a little flawed. First, it assumes that the volcano is geologically homogenous – volcanoes certainly are not, and given that deposits are layed down in slope parallel layers, this heterogeneity can be a big factor in slope instability. Second, the model appears to ignore pore pressure affects (I asked whether pore pressures are being modelled – the presenter ducked the issue by talking about over-stress. Given that injections of hot material cause increases in pore pressure for certain, and these may well be very important in understanding slope stability – this is a substantial omission.

Comments welcome as ever, especially from the presenters and other attendees. Do feel free to comment if you disagree with what I have written. Finally, apologies for typos, spelling mistakes, etc. The spell check function isn’t working and I don’t have time to check.