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The Sumatran Earthquake of 1861: 32 years in the making

by Mohak Sisodia

The Sumatran Earthquake of 1861: 32 years in the making

June 3, 2021

How did a three-decade-long silent earthquake erupt into a violent tsunami that swept lands and killed thousands?

What does the new study say?

In 1861, an earthquake of about 8.5 struck Sumatra, an Indonesian island and parts of the Dutch East Indies. The quake, eponymously named the 1861 Sumatra Earthquake stirred up a tsunami on the nearby shores and inflicted heavy damage to the areas affected, leading to several thousand casualties.

For years scientists understood this earthquake to be an isolated incident; however, in a recent study by researchers at Nanyang Technological University in Singapore, they found out that it was, in fact, the culmination of the longest earthquake ever recorded, which crept through the subsurface for a whopping 32 years.

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A 32-year long earthquake

The three-decade-long quake, categorized as a “slow-slip event”, is the longest of its kind ever recorded. Usually, these events last for days, weeks or months, very rarely continuing for years. This discovery has baffled scientists who are now eager to find out more about such events and their long-term effects on the movement or our planet.

Unlike the more aggressive quakes, the slow ones do not release energy built up from the shift of tectonic plates in a ground-shattering rupture; instead, they release this stored energy slowly, over more extended periods. Although they are not dangerous on their own, they can lead to shifts of the subsurface, which may strain adjacent zones along a fault, leading to a risk of more violent tremors.

Corals and the sinking islands

Rishav Mallick, the first author of the new study, and the other authors arrived at this observation after Mallick, in 2016, decided to focus on coral data. Coral reefs played a significant part in this study as some corals, like Porites, which grow upward and outward, reaching just below the water surface, and ancient coral structure microatolls, which grow slowly as the water level keeps on increasing.

“They act like natural tidal gauges,” says study author Emma Hill, a geodesist at Nanyang Technological University’s Earth Observatory of Singapore.

When the team from the university analyzed the records of these corals, they found standard growth patterns from 1738 to 1829, indicating the island of Simuelue sank at a rate of one to two millimetres each year during this period. However, while analyzing the data for the years extending from 1829 to 1861, the researchers found out that corals grew at a rate much greater than the standard pattern, peaking at around ten millimetres per year.

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Reasons and Current Scenario

The rise in sea level discussed above can be attributed to many reasons, including climate change-driven factors; however, in this particular case, the reason is believed to be a change in landscape height, leading to the clashing of tectonic plates.

The study highlights the complexity of the subduction zones, and the slow-slip events complicate the scenario further.

More areas in and around Indonesia are showing cause for concern. The southern island of Enganno is “sinking a little too fast”, says Rishav. Although he cautioned that the data is based only on one location, it must not be neglected. It can help researchers figure out if another slow-slip quake is already covering ground in the area.

Understanding the events and future implications

It is comparatively more difficult to keep a tab on, unlike the more violent, rattling quakes. It is critical to understand these slow events to discern the potential peril in further triggering colossal tremors. One of the biggest obstacles in studying these slow events is that they are challenging to identify and track. The lengthy quake in the study crept along a shallow section of the fault, which lies underwater, away from the land and the traditional GPS stations are not helpful on the seafloor as their signals do not penetrate deep into the water. Better instruments available for tracking slow events are expensive.

The deeper we dig into improving our understanding of the planet we inhabit, the more complex it becomes to comprehend. Though it may not be the fanciest thing to monitor such events, it is vital for improving our understanding of our planet.

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