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Wednesday, January 19, 2005
Commentary: Tsunamis
By Gary Covington
Commentary
Lucky weren't we? So very lucky. That between us and the recent huge offshore earthquake and attendant catastrophic tsunamis there lay the island of Sumatra and the Malay Peninsula.
Regular waves are what we see gently lapping the shores of the Davao Gulf, waves generated by the wind or which are remnants of some distant storm. Tsunamis are waves of energy, shock waves transmitted through water rather than air.
In the deep waters of the high seas a tsunami's energy occupies such an enormous volume of water that it’s passing may go unnoticed. It's just another wave amongst a host of others, a circumstance that changes dramatically as the land is approached and the water shallows.
As the seabed rises, the band of water and its contained energy finds itself being compressed and so seeks a way to expand. It cannot go downwards, the sea floor is in the way. Nor can it push out sideways; a tsunami may extend laterally for tens of miles. That only leaves up and so the energy and its transmitting medium, the water, rear to form a monstrous wave.
Tsunamis are usually generated by sudden and massive displacement of the seabed or by submarine landslides of enormous scale. Both events are triggered by undersea earthquakes or volcanic action, which, like tsunamis, cannot be predicted.
In 1835, aboard HMS Beagle, Charles Darwin sailed into what remained of the Chilean port of Talcahuano. A few days previously there had been a huge submarine earthquake; like that which so recently devastated Sumatra but much nearer to the land. Tremors destroyed the town in six seconds and the ruins were scoured by a succession of gigantic waves.
Offshore the seabed had slumped. Seawater poured into the void and the townspeople witnessed the terrifying spectacle of Talcahuano Bay emptying of water. There remained a muddy seaweed-strewn waste littered with stranded ships lying on their sides.
Half an hour later "with an appalling roar a moving hill of water rose up out of the sea and burst into the bay." The wave swept all before it and then retreated, carrying with it wreckage from the town, cattle, horses and people. Two more waves followed, each bigger than the last.
Volcanoes too can cause tsunamis as did Krakatoa, which used to lie in the Sunda Strait midway between Sumatra and Java. It had lain dormant for centuries until the summer of 1883 when it started to rumble and roar and throw out immense quantities of ash and pumice. This carried on for a couple of months and then, on the morning of August 27th, four stupendous explosions were heard, the last and loudest as far away as Australia.
Most of the island had vanished, replaced by a semicircular bay and a vast hole in the ocean floor. The enormous displacement of material and water gave birth to tsunamis, which exceeded 100 feet in height and moved at 450 miles per hour. The nearby coasts of Java and Sumatra were razed.
The remains of the island are still there, the arms of its bay embracing a new volcano, anak Krakatoa, the child of Krakatoa.
Stand on the roof of the Davao Marco Polo Hotel and look towards the east, towards the Hawaiian Islands. There's nothing in the way. Maybe next time we won't be so lucky.
(Sun.Star Davao)
(January 19, 2005 issue)
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