Waiting for the next big wave
The remains of the town of Minamisanriku, Japan
25 May 2012 by Hannah Evans, David Tappin and Colm Jordan
Understanding the sediments deposited by devastating tsunamis is essential for identifying where similar disasters have struck, how big they were and how often they can happen – work which could ultimately save many lives. British Geological Survey scientists describe their journey to study deposits left by the 2011 tsunami in Japan.
In March 2011, the magnitude 9 Tohuku earthquake caused a massive tsunami to hit the east coast of Japan. Despite sophisticated warning systems and coastal defences, at the most recent count 15,000 people are known to be dead, nearly 5,000 are still missing and 300,000 were left homeless. We visited Japan just three months after the tsunami to help investigate the sediments the tsunami left behind, to map the areas flooded and to record evidence for the height of the waves.
By comparing the results of our research with evidence from the geological record - the sediments buried beneath the modern land surface - we can improve our understanding of the size of past tsunamis and the timescales on which they occur. This will help create better evacuation procedures and coastal defence measures, and aid redevelopment of the area, potentially reducing loss of life from future tsunamis.
Mapping a modern tsunami
The 2011 tsunami gave us a unique opportunity to investigate deposits from a known catastrophic tsunami and to build up a picture of the type of sediments that these waves leave behind. An area affected by a modern tsunami is a difficult working environment and the east coast of Japan was no exception.
The sheer scale of the devastation that the tsunami caused was unbelievable. A post-apocalyptic landscape is perhaps the best way to describe the scene that greeted us on reaching the tsunami zone. Not only was a huge swath of the Japanese coast affected - some 500km of the eastern shore of Honshu Island - but huge piles of debris, including buildings and lorries, were left strewn across the area.
Mapping the tsunami sediments
Trees were stripped or bent double and concrete blocks some 10m long were thrown inland from the destroyed tsunami defences at the coast. All manner of personal possessions were scattered everywhere as a tragic reminder of the impact on local people.
To gain a better understanding of the extent and characteristics of the tsunami sediments we used a combination of high-resolution satellite images and digital mapping techniques. Our results reveal that the tsunami waves travelled up to 4·5km inland on the low-lying coastal plains. We recorded evidence of the height of the waves as they flowed over the land. This included trees with their branches stripped to around 7m above ground level and debris stranded on three-storey buildings.
In highland regions further north, narrow coastal valleys focused the tsunami waves leading to water heights of up to 40m. This was responsible for the almost total destruction of several towns along the coast, including the town of Minamisanriku.
Our mapping demonstrates that sediments from the 2011 Japan tsunami are up to 30cm thick and are largely composed of fine sand and silt eroded from areas just offshore and near the coast. The tsunami also deposited bigger man-made items like houses and cars.
Tsunamis in the geological record
Historical records such as written archives and local folklore are often used to give scientists an idea of how often tsunamis have affected an area. Such records may contain information on small to medium height waves but aren't likely to include the largest events - these happen so rarely that the last one may have struck an area long before records began. To understand the biggest and most devastating tsunamis we must turn to the geological record.
Mapping recent tsunami sediments, such as those from the March 2011 tsunami, is vital if we are to learn to identify tsunami deposits in the geological record. Once we have recognised similar sediments from the ground beneath the modern land surface, we can date them, and combine these dates with those from historical records to build up a picture of tsunami size and frequency.
The geological record in Japan stretches back some 3,800 years. Within these sediments, evidence has been found for a number of tsunamis. The largest of these occurred in 869AD and is known as the 'Jogan Tsunami'. The sand it deposited extends several kilometres inland and this has been used to estimate the size of the waves. However, our investigation of the March 2011 tsunami revealed that finer deposits laid down by the tsunamis, known as silt, may extend much further inland than the tsunami sand.
The Jogan tsunami is likely to have deposited silt but this is very fragile, and so is easily destroyed by wind and rain and the activity of humans. This means that any silt from the Jogan tsunami may well have been eroded over the 1143 years since it happened. So the Jogan tsunami was probably much bigger than previously believed, with the tsunami waves affecting a larger area of land.
To cause such a big wave, a large earthquake is needed. The earthquake that triggered the Jogan tsunami may therefore have been as large as that of March 2011. This would mean these devastating events could happen more often than previously believed. Research into the geological record in countries prone to tsunamis is, therefore, very important in identifying the size of past tsunamis and how often they hit an area.
Our work is part of extensive research being conducted on last year's disaster in Japan to improve our understanding of tsunami size and frequency. Our ultimate aim is to reduce the loss of life from future events by allowing improved tsunami warning systems, evacuation procedures and coastal defence measures.
Hannah Evans is a coastal and geohazard geomorphologist at BGS. Professor David Tappin is a BGS tsunami researcher. Dr Colm Jordan is a remote sensing geologist and team leader for the BGS Earth & Planetary Observation & Monitoring Team.
This work was funded by the Japan Tsunami Urgency Response NERC Urgency Grant.