The recent tsunami in the Solomon Islands is a reminder of a particularly destructive form of natural disaster that could well have occurred on our own doorstep
Early on Monday, the Solomon Islands were hit by a tidal wave, a tsunami, which caused severe devastation along the coasts of the western provinces of the South Pacific island nation. The trigger was a seaquake. The event has shocked the entire region, but it is also a reminder to coastal states around the globe of the dangers posed by what is happening at the bottom of the oceans of our troubled planet. Particularly on the Atlantic, where historical memories of past disasters are very weak, there are practically no precautions so far. This is all the more alarming because, in addition to plate tectonics, climate changes can also trigger tsunamis.
In the Solomon Islands, on Tuesday (local time), it was possible to have a general idea of the extent of the devastation. Several tsunamis with heights of up to ten meters hit the coastline in the west of the country. The waves were triggered by a seaquake of magnitude 8.1 on the open Richter scale, just off the coast of New Georgia, one of the islands of the rocky archipelago. At least 13 villages and larger parts of the town of Gizo, located just 50 kilometers from the epicenter, were destroyed. The number of dead was given as 24 by Radio New Zealand on Tuesday morning. Many people are still missing.
The epicenter is marked by the rough cross. Image: NOAA
The Solomon Islands lie northeast of Australia in the tropical western Pacific below the aquator. To the west, the island nation borders Bougainville, the easternmost of Papua New Guinea’s islands. The islands form what is known as an island arc, which means that they consist of a series of volcanoes that were formed at a subduction zone, an area where one plate of the Earth’s crust dips below another. In this process, a part of the crust diving into the hot interior of the Earth melts and returns to the surface as magma, where it forms chains of volcanic islands. The Philippines, Japan or the Aleutian Islands are examples of this.
The region of the Solomon Islands and neighboring areas is characterized by a large number of unusually small plate fragments sandwiched between the coarse Pacific plate and the Australian plate. Most of the islands in the group have high volcanoes. The highest peak is Mount Makarakumburu on Guadalcanal Island in the southeast of the group, at 2447 meters. This topography apparently prevented worse on Monday, because the tsunamis probably simply swept over shallow island atolls, which are not uncommon in the South Pacific, without the inhabitants having had a chance to get to safety
The destruction appears to have been largely confined to the immediate vicinity of the epicenter. From the island of Bougainville, located about 100 kilometers to the northwest, the local administration reports that there have been no injuries or destructions. Even further afield in Australia, there was some commotion, but no damage. The National Weather Service had ied an indefinite tsunami warning immediately after the quake, whereupon many beaches, schools and other facilities along the eastern coast were evacuated as a precautionary measure. In Sydney, the navigation in the harbor was stopped.
Afterwards, there was criticism that the authorities had overreacted. The governor of the state of Queensland, Peter Beattie, did not want to know anything about that. Queensland occupies the northeast of the country, so was closest to the epicenter. Beattie told the Australian press that he was frustrated with the information available to him. His government had received a warning that a tsunami was imminent, but no one had been able to tell him anything about the extent of it. One had therefore "shot blindly". He called for the federal government in Canberra to establish a national tsunami warning system. This is already under construction and should be ready for use in 2009, but Beattie thinks this will take too long. Australian newspapers, meanwhile, report confusion and partial panic prevailing in some areas of Australia’s eastern seaboard.
What is a tsunami?
Since the severe tsunami disaster that struck the country on 26. December 2004 on the coasts of the Indian Ocean, this particularly dangerous at of natural disaster is known to most people in this country as well. The term comes from the Japanese – probably because Nippon suffers particularly often from the phenomenon – and means "rough wave in the harbor".
As a rule, tsunamis are triggered by seaquakes. The plates of the earth crusts, which slide on the plastic earth mantle, shift insistently against each other. Since they are rigid, they usually become entangled until the pent-up stress is released in a quake. Especially when one plate dips below the other, such a quake has a significant vertical component. This means that the upper plate jerks forward and the thrust continues in the water, if the quake takes place under the sea.
Banda Aceh on 21. January 2005. Photo: USGS
The speed of the waves generated is high and can reach 800 to 900 kilometers per hour in the deep ocean. On the high seas, the waves are only a few centimeters high. But because they cover the entire water column, unlike the waves normally generated by wind and storms, they build up in the shallow coastal waters to form water fronts many meters high. These tsunamis have a much greater impact than conventional hurricane waves, not only because of their height, but also because of their high speed and the much coarser mass in motion. It can be up to 8000 times. So the problem is not only the inundation caused by the wave, but also its destructive force.
By far the most frequent tsunamis occur in the Pacific region, where they can wreak havoc over many thousands of kilometers in the event of major earthquakes. This happened, for example, in 1960 during an earthquake off the coast of northern Chile, which caused waves that raged in Hawaii and across the Pacific in Japan, among other places. More than 50 percent of all tsunamis occur in the Pacific Ocean. This is due to the fact that this is the coarse ocean basin and that there are also numerous island arcs and other earthquake zones along its edges. Since 47 B.C., there have been historical accounts of over 1200 Pacific tsunamis. At around ten percent, the Mediterranean is also a tsunami-prone region. 67 tsunamis are known from Italy in the last 2000 years. In Messina, Sicily, a tsunami killed 8 people in 1908.000 people.
With only two percent of all tsunamis worldwide, the Atlantic is the calmest region of the world in this respect, which means that people there are probably least prepared for the potential dangers. However, there are also historical records of the destructive power of tsunamis from the Atlantic coasts, for example from 1755, when an earthquake destroyed Lisbon. At that time, a tsunami hit the Spanish city of Cadiz, the Atlantic island of Madeira, and even in the Caribbean Sea a seven-meter high wave arrived.
Dangers of a different kind
However, earthquakes are not the only cause that can trigger a tsunami. When large island volcanoes formally explode, the rock masses tumbling into the water can also trigger devastating tidal waves.
The most prominent example of this is the eruption of Krakatoa in the Sunda Strait between Sumatra and Java, which took place on 26 January 2009. August 1883 was torn apart. The force of the explosion was so coarse that the blast was perceived as cannon fire even in Australia, four and a half thousand kilometers away. The lighter components of its ashes spread around the world in the next few weeks and for about three years darkened the sun to such an extent that the global temperature dropped. The tidal wave generated by the collapsing volcanic cone was 30 meters high, destroyed numerous towns and villages on Java and Sumatra, and killed about 36 people.000 people. But such powerful volcanic eruptions are rare. In Europe, the last one of comparable magnitude occurred 3497 years ago, when the island of Santorini exploded in the agais, probably wiping out the Minoan civilization.
Another but related possibility is that a volcanic eruption causes a massive mountainside to slide. This is the threat that Bill McGuire, director of the Benfield UCL Hazard Research Centre in the UK, sees for Cumbre Vieja on La Palma, part of the Canary Islands. The western slope of the volcano had already slid four meters during an eruption in 1949. Should it completely lose its grip, a rolling mass of up to 500 cubic kilometers would plunge into the Atlantic Ocean, presumably triggering a catastrophic tsunami that could ravage coastlines from northern Brazil to the Caribbean, the U.S. East Coast, Newfoundland, Iceland, Western Europe and West Africa. In addition to the direct destruction caused by the waves, an enormous environmental catastrophe was also threatened by destroyed oil tanks, chemical plants, refineries, clearing plants and the like. The draw could be a new volcanic eruption, which is to be expected in 200 years at the latest, but could also occur in the next few decades.
And then there is the possibility, McGuire writes in the British journal New Scientist, that global climate change, in particular the thawing of the Gronland Ice Sheet, could also have triggered tsunamis. To explain this, it is necessary to go back a bit: On the continental margins, where the shallow coastal waters merge into the deep sea, the seabed drops several thousand feet. These mountains consist of more or less steep slopes, which have been formed by sedimentation over millions of years. Every now and then such slopes start to slide, and if really big masses are involved, a tsunami can be triggered.
The most common known event of this type occurred almost 8000 years ago off the coast of Norway. That was the so-called Storegga landslide. In the Shetland Islands and in northern Scotland, the deposits from this period show the traces of several tsunamis, the highest of which must have been 20 meters high. Twenty-seven such events have been recorded in the North Atlantic, McGuire writes, and most were probably due to sea-level rise that began 15.000 years ago.
At this time, the ice sheets in northern Europe and North America began to retreat. and could have triggered the landslides in different ways. For one thing, the continental crust began to rise after the ice melted. Previously, it had printed the load of several kilometers thick ice shells into the plastic subsurface. Scandinavia and the sea floor under the Baltic Sea, for example, are still rising today, so that in the northern part of the Gulf of Bothnia sea level is falling by about one meter per century. Such vertical movements of the crust could well have caused unstable slopes, on which a lot of sediment had accumulated due to the rolling of glaciers and icebergs, to slide.
Another candidate is gas hydrates, that unusual ice of water and methane that cements at least part of the slope. If the hydrates dissolve because the temperature rises or the water prere drops, then the affected slope starts to slide. A third cause could be finally to be looked for in changed superimposed loads in the shelf seas. In shallow coastal seas such as the North Sea, a rising sea level means that the coarser water mass pushes the seafloor further down. This could also have triggered the slipping of slopes.
What worries McGuire now is that, on the one hand, the continental slopes on the edge of Gronland are in a similar state to those of Norway eight or ten thousand years ago. On the other hand, a significant acceleration of iceberg melting and breakup has been observed in Gronland for the last few years. No one knows yet if this is only a temporary phenomenon or if the ice loss will continue in the next decades. Some scientists are even of the opinion that a global warming of two degrees on average – which would be considerably higher in the Arctic – could be enough to cause the Gronland ice to disappear in the long term. In any case, this would be a process that would take centuries and raise sea levels by an estimated seven meters.
Either way, the traces of the prehistoric tsunamis should be a serious warning at least to the coastal inhabitants of Western Europe. On the German North Sea coasts, on the other hand, one can feel quite safe as far as tsunamis are concerned. The North Sea is simply too shallow for this. Before they could reach their brewing crust, they had long since lost most of their energy through turbulence and friction on the sea floor.