20 years have passed since December 26, 2004when in theIndian Ocean off the northwestern coast of Sumatra, a very violent event occurred earthquake of magnitude 9.1. It was the third strongest earthquake in 60 years, followed by a tsunami destructive, with waves up to 30 meters. These events devastated the northern part of the Indonesian archipelago, but also territories thousands of kilometers away, causing in total approximately 230,000 victims22,000 missing and damages amounting to tens of billions of dollars. The tsunami spread throughout the Indian Ocean, even reaching Somalia. Let's see what exactly happened and what the consequences were.
9.1 magnitude earthquake with epicenter in the Indian Ocean: what happened in 2004
Il Sumatra earthquake (or Sumatran – Andaman) occurred at 7:58 am (local time) inIndian Ocean: the hypocenter was identified a 30 km deepenter theepicenter a few kilometers from the coast western Sumatra island. The earthquake, of magnitude 9.1it lasted 8 minutes. To understand why an earthquake of this magnitude could have occurred, we need to know the geological context of the area. In fact, it occurred at the marginsaid convergentalong which the Indian plate flows below that of Sunda. Here the plates are subjected to enormous compressive stresses, due to which the rocks accumulate large quantities of energy, which they sometimes suddenly release in the form of seismic waves. In the case of the 2004 earthquake, the release of energy occurred with the breaking of one fault 1200 km long.
Waves over 30 meters high reach the coasts of Indonesia in 20 minutes: the tsunami
An earthquake which, as in this case, has a hypocenter below the ocean floor, can displace the latter, creating a “step”. The consequence is that the column of water above is raised, giving rise to a wave tsunami. This is exactly what happened on December 26, 2004: a seabed was created on the seabed difference in altitude with a maximum height of approximately 20 metreswhich triggered tsunami waves that reached one in the open sea speed of approximately 700 km/h. The waves were partly directed towards the nearest coasts, the Indonesian ones (tsunami locale) and partly towards the open sea throughout the Indian Ocean (distant tsunami).
As they proceed towards the coasts, the waves slow down and grow in height: in 2004 they hit the coasts of northern Indonesia just 20 minutes after the earthquake, reaching aheight up to 30 m. The most affected area was the Indonesian province of City of Acehwhere the waters entered inland for more than 4 km. The waves that headed offshore, however, reached 2 hours after the earthquake Sri Lanka, India e Thailand (here the sea first retreated for about 20 minutes, revealing a few hundred meters of beach, and then inundating the coasts with waves up to 18 m high). After 3 and a half hours the Maldivesafter 6 hours the Seychelles and after about 8 hours even the coasts of Somaliamore than 5000 km away from the epicenter.
230,000 victims, the tragedy 20 years ago
The earthquake itself caused much damage and victims, but the most serious consequences were attributed to the tsunami. The victimsin total, were approximately 230.000 (of which a third are children), while i 22,000 missing. In Indonesia the deaths were approximately 174,000, in Sri Lanka 41,000, in India 10,7000, in Thailand 5300, in Somalia 300.
Such a high number of victims can also be explained by the fact that a tsunami warning system for the Indian Ocean, which could have warned the populations of his arrival. In this case, however, even if the system had been active, the alarm could not have been issued in time to warn the inhabitants of Sumatra, where the waves arrived too quickly (about 20 minutes after the earthquake). In other more distant locations, however, the alert could have saved many lives.
For the damages caused by these events, amounting to some tens of billions of dollarsthe UN member countries have allocated approx 2.6 billion dollars. The numerous public and private donations, over time, have contributed to the rebirth of the most affected cities, such as Banda Aceh.
The 2004 tragedy, if nothing else, encouraged the development of monitoring systems of earthquakes that occur at sea and warning for tsunami risk. Also in Italy, following this event, the Tsunami Warning Center of the National Institute of Geophysics and Volcanology (INGV).
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