What is considered the longest earthquake in history dates back to 1861. It is still the object of study by experts today to prevent a similar phenomenon from happening again in the future. In Indonesia, near the island of Sumatra, about 160 years ago an earthquake of magnitude 8.5 killed thousands of people by raising a wall of water that hit the coastal area of the island.
What could have looked like an earthquake like so many others turned out to be the culminating phase of a “silent” earthquake that began some 32 years earlier. Earthquakes of this type are called “slow sliding events”, lasting several days, months or even years. But even within this very particular category, the Sumatra earthquake remains an exceptional case, for its thirty-year duration, double the earthquake which, according to scientists from Nature Geoscience, previously held the longest record in history. Emma Hill, a geodesist who works at the Earth Observatory of the Nanyang University of Technology in Singapore, says: “I never thought that such a long slow sliding event could occur, and yet we have found traces of it.”
The scholars are therefore also amazed, who now devote themselves to the study of this phenomenon to try to prevent something like this from happening again in the future. Earthquakes with this characteristic emit the energy accumulated with the movement of the tectonic plates, not in the form of tremors and explosions, but in a more silent and gradual, apparently harmless way. In fact, it is precisely these small movements underground that stress the adjacent areas along a fault, causing much stronger and more dangerous shocks in the surrounding areas.
Concern is already strong for other Indonesian areas, such as the island of Enganno, which according to the scholar Rishav Mallick is sinking very quickly, presenting a series of elements that would suggest a slow earthquake. It seems that the tectonic shifts of the earth are recorded, unexpectedly, by the coral, in particular the “Porites”, which have a narrow and elongated shape, as they tend to elongate reaching the surface of the water. If the water level rises, the coral stretches in height; if the level decreases, the part of coral that exceeds the water surface dies, and the submerged one extends laterally. Therefore, these corals are defined as “natural tidal gauges”, allowing you to keep a track of the change in water level over time.
On the west coast of Sumatra, the tectonic plate slips under the Sunda plate and stops in an area below an arch of Indonesian islands. The collision causes the descending plate to drag the ground above it as well, which can go so far as to raise coastal areas upwards. Following the 8.7 magnitude earthquake that occurred in Sumatra in 2005, geologist Aron Meltzner returned to the island every year to focus on studying corals, and it was thanks to this commitment that, ten years later, Meltzner and his colleagues traced the ground movements that caused the earthquake of 1861, understanding that it was a special case of “slow-motion” earthquake.
It is clear that the Earth continues to reserve experiences never seen or studied before, reminding us how little control we can have over it: “Every time we think we understand how tectonic movements work, the Earth shows us something new and unexpected” – says geodesist Emma Hill – “the more we collect data, over time, the more surprises like this we will discover”.