Earthquakes, landslide disasters and the collapse of civilisations

There is a very interesting paper being published next week in the Proceedings of the National Academy of Sciences by Dan Sandweiss, Ruth Shady, Mike Moseley, Dave Keefer and Charles Ortloff. This paper, which is reported in a long press release from the University of Florida, combines archaeology, geology and geomorphology to look at the collapse of the Supe civilisation in Peru, which survived for a couple of millennia in the coastal areas of North Peru before it appears to have abruptly collapsed and disappeared about 2000 years ago. The civilisation is intriguing because they built large and complex pyramids, some over 30 m high, as shown in the Google Earth image below. To be able to build such structures the society must have been complex and organised, so why did it disappear so abruptly?


The paper suggests that about 3,600 years ago the area was struck by a massive earthquake, an idea that is not unreasonable given that we know that this a highly seismically active area. The earthquake caused widespread destruction to the towns, but given the length of time that the civilisation survived in a seismically-active area, this would not have been the first time that this happened in all probability. The key factor was the triggering by the earthquake of massive landslides on the adjacent hills, which moved large amounts of debris into the valleys (as happened in the Wenchuan earthquake last year). This debris was then mobilised by floods triggered by heavy rainfall (again, as happened recently in Sichuan), probably associated with strong El Nino events, which bring heavy rain to this part of the world. This mobile debris would have been a hazard in itself, but more importantly it washed out to sea, whereupon it was redeposited on the coast to form a feature called the Medio Mundo, which is a large coastal bar (see below - click for a better image in a new window):

The Medio Mundo sealed off coastal bays, and thus eliminated a major source of food. Furthermore, sand from the ridge blew inland on the prevailing winds, swamping farms and communities. The huge resultant sand deposits, with the wind sculpted features, can be seen on the image above. Within a few generations the civilisation collapsed, never to recover in that form.

All-in-all a very interesting story that shows how combinations of events can lead to catastrophes. There are lessons to be learnt by modern societies from this!

Costa Rica earthquake - landslides have proven to be a major threat

News continues to flow in regarding the impact of the earthquake in Costa Rica. It is increasingly evident that landslides have probably been the major problem, which is unsurprising given the topography and soils. The landslides have clearly caused substantial loss of life and are also hampering rescue operations.

The Washington Post is reporting, quite rightly, that there are now concerns about the stability of the slopes in the affected area in the next wet season. The Reuters image below shows seriously deformed ground above the crown of a seismically-triggered landslide at San Rafael de Vara Blanca:


I found the diagram below showing rainfall in San Jose, which is just a few tens of kilometres away:


Fortunately the period January to April is pretty dry, but problems will certainly start in May. Action will be needed to address this as the level of hazard will be high in many areas. At the moment the biggest problems seem to be around the village of Cinchona, on the flank of the volcano, were a family restaurant was buried (see AP images below of Cinchona):


There are certainly lots of other landslides too, as this AP image shows:


Finally, the Washington Post is carrying a slightly intriguing report:

"A wave caused by the earthquake damaged the Cariblanco hydroelectric plant and it will take at least a year to repair, local media quote a senior official as saying."

A bit of digging shows that AP are reporting that "Officials reported meanwhile that the country's leading Cariblanco hydroelectric plant will be out of operation for about a year, after its generators were buried by tons of mud from flooding and landslides unleashed by the earthquake."

According to this site, Cariblanco appears to consist of two 40 MW turbines. It came into operation in 2007 after construction costs of US$170 million. This would appear to be a very expensive set of landslides for such a new project! Unfortunately, our experience elsewhere (e.g. Taiwan) is that sediment delivery increases in the rainy season as further landslides activate, so the operators of this site would be well advised to get some landslide advice.

An update on landslides in Beichuan

At the time of the Wenchuan earthquake I posted on numerous occasions about the landslides around the town of Beichuan. One of the concerns has always been the likely behaviour of the landslides during heavy rainfall events.

As part of the forthcoming Sinorock 2009 conference in Hong Kong there will be a tour to the earthquake-affected area to look at and discuss the landslides. The organisers have put together a web page about this tour (see here). Interestingly, thy have posted three images of Beichuan:

First, before the earthquake:

Second after the earthquake:


And third, most importantly, after a very intense rainfall event on 24th September 2008:


Note the extensive movement of sediment, which has caused the river bed to agrade and bury many structures. This is very similar to what happened in parts of Taiwan after the 1999 Chi-Chi earthquake. Such behaviour represents a serious challenge for the authorities.