20 January 2009

Earthquakes, landslide disasters and the collapse of civilisations

Posted by Dave Petley

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!