411 Dive Log Australasia April 2025

Sumatra’s far side

M ankind will never forget the Indian Ocean Earthquake and tsunami that struck Banda Aceh, 20 years ago on December 26th, 2004 at 8 am. Known as the Sumatra-Andaman Earthquake, with an epicenter off the west coast of north Sumatra, it reached a magnitude of 9,3 on Richter’s scale. text and photos by Pierre Constant

The undersea megathrust was caused by a rupture along the fault between the Burma Plate and the Indian Plate. There is a subduction zone in the Sunda trench between the Indian Plate and the Burma Plate, where the former subducts under the latter. Extension of the Eurasian Plate, the Burma Plate comprises the Andaman and Nicobar Islands, and a portion of northwestern Sumatra. A large extension of the boundary between the Burma Plate and the India Plate slipped, causing the megathrust over 1600 Km. The boundary underwent thrust faulting, the Burma Plate shifting up 5m vertically and 11m horizontally over the Indian Plate. The rapid rise of the seafloor over 7 minutes generated a massive tsunami with waves up to 30 m high, that killed about 229,800 people in 14 countries along the coasts of the Indian Ocean. Banda Aceh reported the greatest number of deaths, at 61000 (25% of its population). It was the most powerful earthquake in the 21st century and the third most ever recorded in the world. The hypocentre was north of Simeulue Island, 160 Km off the western coast of Sumatra at a depth of 30 km below sea level.

During the last glacial maximum, 18000 to 20000 years ago, Sundaland was one large landmass uniting the Asian mainland, including the Malay Peninsula, Borneo, Bali, Java, and Sumatra. A ‘land bridge’ allowed animals to move freely from southeast Asia to the nearby islands of today. Sea depths over the Sunday shelf rarely exceed 50 m. This Sunda landmass was separated from the Sahul landmass – further south and east – by the ‘Wallace Line’ which isolated Sulawesi, the Lesser Sunda Islands, and Australia/New Guinea, which had a different fauna and flora. The sea level rise during the meltwater pulse 14600 to 14300 years ago changed the landscape. Pleistocene paleo-river systems of the Sunda Shelf were submerged by present sea levels. Having left Africa 1,8 million years ago for its long migration, “ Homo erectus ” took advantage of the sea level regression during the last ice age, and of a land bridge across the Sunda Shelf, to reach Java in the early Lower Pleistocene. Gayo highlands. The Leuser Mountain Plateau lies to the southeast of Banda Aceh, in the highlands.

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DIVE LOG Australasia #411- April 25

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