tsunamis

tsunamis The word ‘tsunami’ is derived from the Japanese words meaning ‘harbour wave’. Often described as tidal waves, although they have nothing to do with tides, tsunamis are in fact generated by offshore earthquakes, submarine landslides, and occasionally by undersea volcanic activity. In each case, water disturbance is created by large-scale underwater displacement of sediment or rock on the sea bed, usually as a result of a fault or a landslide. The initial water movement is often characterized by a rapid drawdown or lowering of the sea surface at the coast as the water moves into the area of sea-bed displacement. Thereafter, large kinematic waves are propagated outwards from the zone of sea-bed disturbance, travelling across the ocean at very high velocities, often in excess of 450 km per hour, and possessing very long wavelengths and periods. At the coast, the flood level or run up associated with a tsunami is partly a function of the dimensions of the propagated waves, but it is also greatly influenced by the topography and bathymetry of the coastal zone. The waves can reach considerable elevations and can cause widespread destruction and loss of life.

The geological processes associated with tsunamis are poorly understood. Remarkably, most Earth science textbooks do not consider the geology and geomorphology of tsunamis. Tsunamis, however, produce unique coastal landforms that are characterized by the effects of erosion and deposition of large magnitude. They frequently deposit boulder accumulations, and in many coastal areas, run-up processes commonly result in the deposition of continuous and discontinuous sheets of sediment. Many earthquake-generated tsunamis are also associated with the flooding of coastlines caused by subsidence associated with the earthquakes (coseismic subsidence). On the sea bed, tsunamis can also result in the remobilization of sediments and the formation of distinctive strata (so-called tsunamites).

Until recently, knowledge of the long-term frequency of tsunami flooding along individual coastlines was based simply on whether or not the occurrence of former tsunamis was described in historical records. Thus, for example, the record of documented tsunamis for Hawaii extends only to 1850 ad. Before this date, the return frequency of tsunamis is not known, since no historical record exists, yet many tsunamis are likely to have occurred. In recent years, it has proved possible to investigate the occurrence of prehistoric tsunamis for individual areas because many ‘palaeotsunamis’ have been associated with the deposition of sediment in coastal areas subject to flooding. In this way it has been possible to calculate the number of tsunamis that have taken place in particular areas over timescales of 10⁴–10⁵ years. For example, geological research in Japan has provided a chronology of past tsunami flooding for the last 4000 years.

During the early 1990s a number of large tsunamis occurred in various parts of the world. Studies of the geomorphological processes associated with these floods have provided a unique opportunity to understand processes of tsunami erosion and deposition. The most important tsunamis that have taken place during this period include a major tsunami that struck the Pacific coastline of Nicaragua and Costa Rica during 1991 and a very destructive tsunami on the island of Flores, Indonesia on 12 December 1992. More recently, destructive tsunamis have struck on two occasions in Hokkaido in northern Japan and in the neighbouring Kurile Islands, Russia, as well as on three occasions in Java, Indonesia, and in the Philippines. Data on coastal flooding resulting from these tsunamis has bee used to develop models of individual tsunamis. In most instances, this type of numerical modelling has been used to reconstruct the properties of the offshore earthquakes and sea-bed faulting that generated the tsunami waves. In general, these studies have shown that the observed flood run-up at the coast is much greater than the run-up values predicated by the mathematical models.

Not all tsunamis are generated by offshore earthquakes. As mentioned above, many are triggered by submarine sediment slides. One of the world's largest areas of submarine slides is located west of the coast of Norway in the Norwegian Sea. This area, known as the Storegga area, has been the site of three exceptionally large underwater slides during the past 30 000 years. Each of these submarine slides is believed to have generated an exceptionally large tsunami. The best known of these is a slide that took place approximately 7000 years ago and involved the movement of approximately 1700 km³ (cubic kilometres) of debris from the continental slope to the abyssal plain east of Iceland. The tsunami generated by this slide produced flooding up to 10 m above former sea level along parts of the west coast of Norway; it also caused severe flooding along the northern and eastern coastlines of Scotland and as far south as the present location of Amsterdam.

What was probably the world's largest tsunami took place in the Pacific basin approximately 105 000 years ago. This tsunami was generated by a large underwater slide south of the island of Lanai in Hawaii. The tsunami is presently believed to have caused flooding up to approximately 360 m above former sea level on Lanai. It has also been proposed that the tsunami waves reached up to 20 m, above sea level along the New South Wales coastline of eastern Australia.

Recognition that underwater slides are capable of generating large tsunamis has, not surprisingly, rendered the task of mathematical modelling of tsunamis much more difficult. More importantly, it has made it very clear that it is a mistake to assume that all major tsunamis are solely the products of offshore earthquakes. The clearest example of this is the well-known meteorite impact that took place in central America approximately 65 million years ago (the so-called K/T impact) that is commonly associated with the extinction of the dinosaurs. A number of Earth scientists believe that this meteorite impact also generated an extremely large tsunami that caused severe flooding throughout many coastal areas. Some scientists have gone so far as to suggest that this tsunami might have caused flooding throughout the entire world at this time.

As a result of the loss of life and damage to property caused by tsunamis in the Pacific, attempts have been made to develop a tsunami warning system, the purpose of which is to alert the public in advance of the arrival of individual tsunamis. Tsunami warning systems did not exist for the Pacific prior to the Aleutian trench earthquake of 1 April 1946. A tsunami warning system network was eventually established by the United States Coast and Geodetic Survey with its headquarters on the Island of Oahu, Hawaii. The centre, known as the Pacific Tsunami Warning Centre (PTWC), became operational in 1948 and is now linked to over 30 seismological stations throughout the Pacific Basin. These provide data on Pacific earthquakes whose magnitude and epicentres make them tsunamigenic (capable of producing tsunamis). Once an earthquake has taken place, the PTWC issues a ‘tsunami watch’ to all receiving stations. In addition, the initial registration of a tsunami on tide gauges is relayed to the PTWC and, once the estimated times of tsunami arrival are computed for different coastal areas, a tsunami warning is issued.

The accuracy of PTWC tsunami warnings is exemplified by the Chilean earthquake and subsequent tsunami of 21 May 1960. Once the earthquake epicentre had been determined and tide-gauge data analysed, it was predicted that the tsunami would reach the Hawaiian islands 14 hours 56minutes after its generation off the Chilean coast. The prediction was that the first wave would strike Hilo, Hawaii at 9.57 p.m. It arrived one minute later.

Because tsunamis are less frequent outside the Pacific region, it might be argued that the cost-effectiveness of separate tsunami warning systems for other areas is very low. Until recently, this was the case in Portugal. On 1 November 1755 ad, one of the highest-magnitude earthquakes documented for Europe led to the complete destruction of the city of Lisbon, as well as causing widespread damage and loss of life across a coastal zone stretching from northern Portugal to southern Morocco. In Lisbon alone, some 50 000 people were killed. Most of them were drowned by the resulting tsunami, which reached a height of 17 to 20 m in many areas. In recent decades, two minor tsunamis have struck the coast of Portugal, one on 25 November 1941 and the other on 28 February 1969. The Portuguese authorities, aware of the risk posed by tsunamis, subsequently decided to instal seismometers and wave recorders west of Portugal with the aim of providing advance warning of any future tsunami comparable to the one that struck Lisbon in 1755. In most other areas of the world (apart from the Pacific), however, coastal populations have no protection from tsunamis. They rely instead on the expectation that no tsunamis will ever strike the particular coastlines in which they live.

Alastair G. Dawson