Cave Reveals History of Ancient Tsunamis in Aceh

A Cave Reveals the History of Ancient Tsunamis in Aceh

A cave was found near the epicenter that triggered the tsunami in Indonesia, which contained giant waves up to 7,500 years ago. A rare natural archive that shows when about the next disaster occurs.

This finding shows the longest and most detailed timeline of tsunamis that have occurred off the coast of the western part of the province of Nanggroe Aceh Darussalam. Precisely near the center of the 9.1 magnitude earthquake that triggered 30 meters of waves on December 26, 2004 which killed 230,000 people in a number of countries, half of them in Indonesia.

This limestone cave is located a few meters off the coast of Banda Aceh, one meter above the waves at knee height, and protected from storms and wind. Only large waves can inundate coastal areas capable of spraying inward.

Researchers in 2011 found sand deposits in the seabed that were swept into caves thousands of years ago and ended neatly in layers between bat droppings like geological cakes. Radiocarbon analysis of materials found, including shells and the remains of microscopic organisms, provides evidence of 11 tsunamis before 2004.

Periods vary

The duration of the disaster is uncertain, according to the head of the research team Charles Rubin of the Earth Observatory of Singapore (EOS). The last one before 2004 occurred around 2,800 years ago, but there were four tsunamis that occurred in the 500 year period before that.

And maybe there are other natural disasters. Researchers know, for example, that there were two major earthquakes around the Banda Aceh region around 1393 and 1450. Rubin said a large tsunami could have swept the evidence of another disaster through erosion.

"The conclusion that can be taken is that a major disaster that occurred in 2004 does not mean that it will not happen again in the next 500 years," Rubin said, adding that the cave was discovered accidentally and was not part of the planned fieldwork.

Complete data

The earthquake that led to the 2004 tsunami shocked researchers because the fault that produced a massive earthquake, was inactive for hundreds of years.

And since the last major earthquake more than 500 years before, there has never been an oral history that could help understand such risks.

Since 2004, a lot of research has been carried out to try to understand the history of the west coast of the island of Sumatra by examining sand deposits, lifting coral reefs and GPS data.

"The findings are very significant," said Katrin Monecke, a professor of earth science at Wellesley College in Massachusetts.

He studied tsunami sand deposits found in swamps in the region, but was not involved in cave research, which was presented at the conference of the American Geophysical Union in San Francisco. "The sand layer in the cave recorded in a very long period of time and gave information about the frequency of the earthquake."

Although long-term recordings were protected in the cave, Rubin said the frequency of tsunamis still could not be known with certainty or when a disaster could occur in a short period of time between each other.

Geophysical perspective

Geologist Kerry Sieh, director of EOS who participated in my research, has predicted that a giant earthquake could again shake the Aceh region in the coming decades. They generally come in the form of cycles and the earthquake in 2004 increased more pressure on the fault. But its history is very diverse, making it impossible to give precise predictions.

"By studying the types of tsunamis that have occurred in the past, maybe we can plan mitigation for the next tsunami," said Nazli Ismail, head of the department of physics and geophysics at Syiah Kuala University in Banda Aceh who worked on this project.

The Indonesian archipelago is located on the Pacific Ring of Fire, a series of volcanoes and fault lines that surround the Pacific basin. This is the place for the biggest and most deadly seismic activity in the world.

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