April 2011

Genome Biology: Tsunami

A reprint of a commentary in Genome Biology by ASBMB Past-president Gregory Petsko in response to the December 2004 tsunami that hit Indonesia.*

It began at the bottom of the ocean, off the west coast of Indonesia. From there it spread outward, silently, invisible under the surface of the water. When it came ashore, in forty countries, some as far as 4,000 miles from where it started, it killed upwards of 150,000 people. There have been deadlier catastrophes - the earthquake in China in the mid-1970s killed a quarter of a million in that country, and the cyclone that devastated Bangladesh in the decade before that is thought to have caused half a million deaths - but none that involved so many different nations scattered across so much of the earth’s surface. The tsunami spawned by the magnitude 9.0 earthquake of 26 December 2004 was perhaps the first truly global natural disaster in modern history: a third of the world’s countries were directly affected. The worldwide scope of the destruction reminds us of something that genomics is also starting to make clear: that we are all truly one people, that national and racial differences are artificial and insignificant compared to the common bond of our humanity. Despite all our technological prowess and environmental hubris, we also have yet another grim reminder that Nature, not Man, is still boss of this planet.

The word "tsunami" comes from the Japanese words for harbor (tsu) and wave (nami). It refers to a series of giant undersea waves that travel at high velocity for very long distances, and that crest when they hit a shoreline in the form of a devastating surge, sometimes as much as 30 meters high. Tsunamis are often called ‘tidal waves’ but that’s a misnomer: the phenomenon has nothing to do with the tides. It has its origins, like everything else that involves the earth’s surface, in plate tectonics.

It is hard to imagine that the theory of plate tectonics, which is at the heart of all modern geological science, is only a hundred years old and was not widely accepted until the 1970s. Schoolchildren had noticed for hundreds of years that the facing shapes of South America and Africa could be fitted neatly together like pieces of a jigsaw puzzle to make a single entity (Francis Bacon had noticed it in 1620 but drew no conclusion), but it wasn’t until 1908 that the amateur American geologist Frank Bursley Taylor proposed that the continents had once slid around and that this motion might have thrust up the world’s mountain chains. His theory was taken up by the German planetary astronomer-turnedmeteorologist Alfred Wegener, who in 1912 proposed that all the world’s continents had once been part of a single giant landmass he called Pangaea, which had split apart in a process of lateral motion that was still continuing. Traditional geologists attacked both Wegener and his ideas viciously, and it wasn’t until the decade after his death (he froze to death on a scientific expedition in Greenland in 1930) that the great English geologist Arthur Holmes provided an explanation for how Wegener’s motion could occur. In a textbook published in 1944 he speculated that heat caused by the decay of radioactive elements in the earth’s crust could produce powerful convection currents that could slide the continents around on the earth’s surface. He has probably as good a claim as anyone to be the father of the modern view of continental drift, although, curiously, he often expressed skepticism about his own theory. Harry Hess, a Princeton University geologist, figured out in the 1950s that there were two large plates of land under the floor of the Atlantic Ocean and that their relative motion was responsible for the topography and geology of the seafloor. Finally, in 1963, Cambridge University geophysicist Drummond Matthews and his graduate student Fred Vine used magnetic readings to prove that the seafloor and the continents were in motion. (Canadian geologist Lawrence Morley came up with the same result at the same time but his paper was rejected by the Journal of Geophysical Research.) J. Tuzo Wilson of Toronto showed at about the same time how plate tectonics could explain the behavior of the ocean floor at mid-ocean ridges. Still, even in the 1970s, many textbooks of geology continued to dismiss plate tectonics as physically impossible.

Today we know that the surface of the earth is composed of about a dozen large plates and almost two dozen smaller ones, all moving in different directions. Where they grind against one another (regions geologists call ‘subduction zones’), the tremendous force can be released either slowly and steadily, giving rise to thermally active regions like Iceland, or sporadically and violently, giving rise to earthquakes and volcanic eruptions. That is what happened on 27 August 1883, when subduction along the Java Trench, where the Indo-Australian plate is moving under the Indonesian Island chain, caused the explosive eruption of the volcano on the Indonesian island of Krakatoa that in turn generated waves that reached 41 meters in height, destroying 165 coastal towns and villages along the Sunda Strait between the islands of Java and Sumatra and killing 36,417 people. (Hollywood made a movie about this great disaster in 1969: Krakatoa, East of Java. In case you are ever tempted to equate Hollywood productions with history, let me point out that Krakatoa is west of Java.) That is probably what happened in 1648 B.C., when the entire Minoan civilization on the island of Crete was wiped out, in a single day, as the result of a tsunami created by the explosive eruption of the volcano on Santorini.

And that is what happened on 26 December 2004, when the Indian plate slid underneath the Burma plate (a subduction zone), driving a 600-mile long piece of the earth’s crust 20 to 50 feet upwards on the floor of the Indian Ocean. This sudden rise in the seafloor displaced an enormous volume of water - exactly as if the ocean were a swimming pool and someone had just dropped a large block of concrete into it. The displacement spread outward in all directions, like the ripples that would spread from that block. But because the event occurred underwater, the displacement traveled underwater until it encountered the sharply rising seafloor on the edge of an island or continent. When the undersea waves hit this obstacle, they were pushed straight up, compressed into walls of water that surged over the landmass.

The speed of a tsunami depends on the square of the depth of the ocean: the deeper the water, the faster the displacement travels. The Indonesian tsunami formed in deep water, which meant that the wave velocity reached upwards of 800 km/h, the speed of a commercial jet aircraft. When they encounter the shallow depths of a coastline the speed of tsunami waves slows to perhaps 45 km/h, still fast enough to do tremendous damage. At top speed it took the tsunami less than 7 hours to cross the Indian Ocean and reach the east coast of Africa, where the waves came ashore in Somalia and killed 150 people who cannot possibly have understood that the power that was destroying them had been spawned more than 3,000 miles away.


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