The main tremor occurred at 7.58 local time on 26 December 2004 (00.58 GMT) of magnitude 9 ‘mega thrust’ at Aceh, Sumatra.
Asian tsunami disaster
The main tremor occurred at 7.58 local time on 26 December 2004 (00.58 GMT) of magnitude 9 ‘mega thrust’ at Aceh, Sumatra. The scale of slippage was quite heavy, 30 km, from seafloor at 1200 km a stretch of Indian Plate thrust up to 20 m under the Burmese plate, raising the seafloor by several metres as shown in Fig. 16.12. The Indian Plate and Burmese Plate rubbed each other and about 1200 km of the latter was snapped off. Historically this was the fourth biggest earthquake in the last century. The others are given in Table 16.10.
The epicentre of this earthquake lay 250 km south-east of Banda Aceh, Indonesia and 1600 km north-west of Jakarta. The energy released was
equivalent to 475 000 kilotonnes of TNT or 23 000 Hiroshima-sized atom bombs. The earthquake caused a tsunami. Tsunami is a Japanese word, ‘Tsu’ meaning harbour and ‘nami’ meaning wave. Some people prefer to use the term tidal wave, but this is a misnomer. Scientists call this a seismic sea wave. Seismic sea waves are due to an earthquake but tsunami waves may be due to non-seismic activity such as a landslide or meteorite impact. Wind waves have a period of 10 s with a wavelength of 150 m, and shallow tsunami water waves have a wavelength of 100 km and T = 1 hour. A wave behaves as a shallow water wave if the ratio between water depth and wavelength gets very small.
where h = 1000 m and v = 200 m/s or 700 km/h.
The rate at which a wave loses its energy is inversely related to its wavelength. As a tsunami leaves the deep water of the open ocean, it travels into shallow water near the coast it transforms. As water depth decreases, the tsunami slows and because of the shoaling effect, the tsunami magnitude grows several metres in height. Thirteen countries, including Indonesia, Sri Lanka, India, Thailand, Myanmar, Malaysia, Maldives and Sumatra were affected. It is estimated that more than 250 000 people died and many were injured. Table. 16.11 gives the equivalent tsunami magnitude corresponding to the earthquake magnitude.
1 Unfamiliarity with warning signs
In the minutes preceding a tsunami strike, the sea often recedes temporarily from the coast. The total energy released = 2.0 exajoules (12 1018 joules), enough to boil 150 litres of water for every person on Earth. The 2004 tsunami resulted in an oscillation of the Earth’s surface of about 20–30 cm. This was felt across the planet up to Oklahoma. It has shortened the day by 2.68 microseconds. The smaller islands of Sumatra have moved by 20 m. A tsunami in deep water waves forms only a small hump that is barely noticeable and harmless, which travels at a very high speed 500–1000 km/h. In shallow
water near the coastline a tsunami slows down to only 10 km/h. In doing so,
it forms large destructive waves. Because the 1200 km of the fault line affected by the earthquake were in nearly north–south direction, the greatest strength of tsunami waves were in east–west direction. The tsunami reached the Indian Ocean in 2 hours. In South Africa, which is 8500 km from Sumatra, a 1.5 m high tide surged on shore after 16 hours.
Damage mechanisms due to earthquakes
Very large inertial forces develop in a structure due to earthquake ground motion and could cause collapse.
Earthquakes instigate landslides and other surficial movements.
They induce soil consolidation or liquefaction beneath the foundation.
Sudden fault displacement in close proximity to a structure is hazardous to pipelines, canals and dams.
Tsunamis are seismically induced water waves that may threaten coastal regions.
Earthquakes may induce fire and explosions.
Ground shaking poses the most potentially hazardous damage mechanism to structures and its effects may be widespread. Hence the response of structures to earthquake ground motion should be addressed.