Geohazard Risk and Prediction
Posted by Science Oxford on August 25, 2011 | comments
A Science Oxford Live talk by Mike Clare, a geohazard specialist from Fugro GeoConsulting Ltd., based in Wallingford.
By Blanka Sengerová.
Risk assessments tend to lead to a glazing over and a bit of muttering about health & safety gone mad in many instances. Not so for Mike Clare, whose day job involves creating risk assessments. Not as you might know them though, relating to spending your day at a computer screen or putting together your folding chair or flatpack furniture, but assessing the potential for earth processes to endanger clients’ projects, which include such large-scale things as oil rigs and similar constructions.
Put simply, part of Mike’s job is to advise clients on hazards associated with earth processes, including earthquakes, volcano eruptions and landslides. Although no geologist can predict exactly when volcanoes are going to erupt or earthquakes are going to occur, for instance, they can use the geological past and the current layout of the ground to try and predict with more or less certainty, whether such events might occur.
What is a geohazard then? It is an event or feature related to Earth processes which has the potential to cause harm to humans, property or the environment. So if a landslide happens somewhere offshore, away from oil platforms or shipping routes, it is still a slide (i.e. a geological occurrence), but not a geohazard. The issue has been raised that there appear to have been more earthquake related deaths in the recent past, and Mike argued that this is likely related to the fact that these days the city-dwelling population is much bigger, making earthquakes in populated areas much likelier to have fatal effects. Whereas earthquakes in the past still happened, they weren’t a geohazard because they had little or no impact on human life. Perhaps obviously so, but it is important to note that there is a distinct risk between a ‘hazard’ and a ‘risk’. A hazard is the potential to cause harm, whereas the risk involves the probability and its consequences.
Why do geohazards occur then? Earthquakes and volcano eruption occur due to energy being built up at depth and subsequently released. There are two main reasons why energy needs releasing in the case of earthquakes. The first involves two continental plates pushing together, leading to the subduction of one of them and the subsequent build up of energy below the surface; the second involves energy build-up due to the sideways ‘shuffling’ motion of two plates relative to each other.
Mike then talked about the difference between the primary and secondary hazards of earthquakes. The primary hazards, often the ones most reported on, are things such as the high magnitude of an earthquake or the high ground acceleration, and these are known about straight after the event. It turns out that it is mostly the secondary hazards, including landslides, rockfalls, loss of agricultural land, and changes in aquifers disturbing the local hydrogeology, that are the ones leading to death. The effects of these, however, are often not identified until 6-12 months after the event, by which time the money from the World Bank Aid is no longer available (this needs to be drawn within three months of the natural catastrophe). An example quoted was the 2003 Bam earthquake in Iran where the fact that most of the buildings were made of mud meant that the relatively low-magnitude earthquake led to some horrendous destruction.
How do you go about geohazard assessment for a project, such as an off-shore oil-rig then? Firstly, you think about where it is based, and whether it is close to any fault lines. Secondly, you look at how big a potential earth movement event might be, using historical records as a guide. Thirdly, you try to estimate when the next big event is likely to take place, but this obviously works only for repeated geohazards, not new ones. Put simply, what you have to do is try to “infer the past, understand the present and predict the future”, which you can do by improving the quality and comprehensiveness of the underlying knowledge in order to reduce uncertainties, for instance by ocean floor mapping. Once you’ve assessed the risks you look for a solution. You either accept the risk (if there’s fertile soil on the slopes of a volcano, you may be willing to take the risk and set up villages there because there is no other option), you avoid it (by moving away from the slopes of the volcano) or, most likely, you mitigate against it.
In terms of off-shore oil rigs, for instance, risk mitigation may be the only solution, because when you are drilling for oil you have to make a pipeline, perhaps through a landslide prone area, because this option is cheaper than not building the pipeline at all. However, you may be able to mitigate the risk by building the pipeline parallel with the likely direction of a landslide, which would simply cause it to stretch, and not perpendicular to it, which would more readily break the pipeline. A specific off-shore landslide area mentioned was the Storegga slide off the coast of Norway. Some oil companies will traverse this area by pipeline. Questions to be asked include whether it is better to go across the route of the existing landslide (on the basis that it won’t happen in the same place again) or to route the pipes along the adjacent slope. A key to making this decision involves trying to work out what the conditions were like when the original landslide happened.
Of course, no current talk on geohazards could go without mentioning the recent Japanese Sendai earthquake and tsunami that led to such devastation at the Fukushima nuclear plant. The problem here was that the off-shore earthquake caused a tsunami and although the waves were relatively low (1-10 feet) offshore, they got much bigger (closer to 40 feet) nearer the coast, flooding the power station. The situation can be used to exemplify the ‘system definition’ aspect of risk assessment. The questions to ask are: i) what is there (e.g. the mud huts in Bam were much more readily collapsed than earthquake proof buildings would have been), ii) is there an evacuation route (if the only way out is through the mountains, up one road, then evacuation will be very difficult, if not impossible), iii) what’s the topography of the land (a cove/harbour like coastal shape with a flood plain on the edge of the sea surrounded by mountains makes the land readily ideal for flooding by a tsunami), iv) what’s the vulnerability of the area like (e.g. are there roads parallel or perpendicular to the wave direction) and v) are there any objects at particular risk from flooding (e.g. an oil refinery or said nuclear power station). A defined system can then be dealt with in terms of the risk management.
Apart from historical data that can be used to predict earth movement events in certain areas, there are other techniques that companies such as Fugro use for improving their knowledge base. One of these involves taking core samples of earth and measuring small scale features within these. And whilst landslides on terrestrial areas can be dated using things as simple as sweet wrappers (e.g. their use by dates or the availability of certain chocolate bars in certain eras), mapping on the ocean floor is carried out using specialised autonomous underwater vehicles (AUVs). Such AUVs are used to map the ocean floor in terms of the topography/bathymetry, as well as sub-bottom profiling, i.e. imaging below the sea floor – typically 50-200m (this is perfect for geohazard identification, but no good for detecting oil). Importantly, the erosion is much slower in the marine compared to the terrestrial environment, which makes landslides that happened hundreds of years ago look relatively fresh. In the end, using such data, it becomes more realistic to reconstruct geohazards, such as landslides, of the past and hopefully predict those of the future. The data is collated in something called a hazard and vulnerability map, which together can be interrogated to give a risk map, which allows for considered future developments and design of mitigation strategies.
The lively Q&A session after the talk demonstrated a keen interest in things surrounding the geological mapping of the earth, and rounded off a very stimulating talk…
What do you think?