Tsunamis, Volcanoes and Earthquakes

Dr. Philippe Blondel, University of Bath, on 14 June 2002

At 9.30 am on 1 Nov 1755 an earthquake hit Lisbon killing 900 people directly. Forty-five minutes later, a 5 metre high wave hit the town, flooding the river Tagus and killing more than 10,000. It is now estimated that the cause was an earthquake centred about 200km off the Portuguese coast in the Atlantic measuring about 8.5 on the Richter scale and the strongest ever recorded in Western Europe. The tsunami was felt along the whole of the coastline, being over 15m high at Cabo de São Vicente, and 2m at Gibraltar and Cornwall. There are still layers of sediments in the dunes on the Isles of Scilly that were deposited then.

Although they are rare in Europe, tsunamis are common in the Pacific, one well-studied example occurred in 1960 off the coast of Chile and reached Japan. Most of these are caused by earthquakes occurring in the subduction zones where one tectonic plate is moving below another and triggering a submarine landslide close to the epicentre. Occasionally, like the one off the Canary Isles about 10,000 years ago, the tsunami is the result of a massive landslip into the sea. In all cases, there is a sudden displacement of a large volume of water that then spreads in a ring. Where the ocean is deep this might be hardly noticeable, but when it approaches shallower water, the volume being displaced has a much greater effect, giving rise to these large destructive waves. The USA has now placed a number of buoys across the Pacific to give them good clear warning. In Hawaii these have already proved their worth as they have 8 hours warning from an earthquake in Chile, and 10 hours from Alaska. This is sufficient to enable people to get to higher ground and reduce the death toll, even if not the damage to houses and other infrastructure.

Fifty years ago, very little of the sea floor was mapped, but with the aid of new equipment this is being remedied. New ideas for using sonar are being introduced. By using more beams, an area can be examined more rapidly, and especially useful is towing a device close to the sea floor that gives sonar readings to left and right. This sidescan sonar provides a broad strip of readings that can be processed later to give an accurate map of the area. These techniques show that at the margins of the continental shelves, there are very steep slopes leading down to the abyssal plain about 4 - 6km deep. These are much steeper than land slopes; for example, off Almeria, southern Spain, the drop is from 80m deep to 3000m in about 30km horizontal distance. On this slope, we now know there are canyons formed from erosion of mud and sand, in many cases brought down by major rivers running off the land; and across the slope a fault line is visible.

After the 1755 quake, Marques Pombal issued a questionnaire to the population of Portugal, and thus was the first person to see the value of collecting accurate data for analysis. In his honour, a fault line has been named which it is now believed was the site of the earthquake. Data from this area has been thoroughly mapped in a marine expedition led by Dr. E. Gracia (CSIC Barcelona, Spain) in autumn 2001. Its results are still being examined, but there are clear signs of relatively recent landslides that would explain the 1755 event. These slides displaced huge amounts of water, being tens of kilometres in extent both in length and breadth.

Scientific data collection is still in its early days. Most of the known slides in the world have been found more or less by accident. Off the Norwegian coast, there are a number of vast slides found by oil prospectors, one of which is 120km long.

It is now clear that these slides and corresponding tsunamis are a fairly common occurrence in historical terms. On the northwest coast of the U.S. archaeologists have found a number of Indian firepits separated by layers laid down by incoming waves. Tsunamis are the only obvious explanation for seashells found at a height of 130m in Hawaiian cliffs.

With more information and monitoring equipment, we can give more warning to protect populations that might be at risk. More mapping is necessary, and governments are now funding this work, especially in their local areas. The European Union is particularly active in this domain, with more funds and less restrictions than most national programmes. The next decades will see important progress in the mapping and the understanding of tsunami risks all over the world.

Andy Pepperdine