A 32-year slow-motion earthquake – the slowest ever recorded – ultimately led to the catastrophic Sumatra earthquake in 1861, researchers at Nanyang Technological University in Singapore (NTU Singapore) found.

The NTU research team says their study highlights potential missing factors or mismatching in global earthquake risk assessments today.

Slow-motion earthquakes or slow-motion events refer to a type of long, drawn-out phenomenon of stress release in which the earth’s tectonic plates slide against each other without causing severe ground shaking or destruction. They typically include movements between a few cm / year to cm / day.

The NTU team made the surprising discovery when they examined the historical sea level on the island of Simeulue off the coast of Sumatra using ancient corals known as “micro-pots”. The disc-shaped coral microtolls grow both sideways and upwards and are natural recorders of changes in sea level and land elevation due to their visible growth patterns.

The NTU team used data from the micro-polls and combined them with simulations of the movement of the Earth’s tectonic plates. It found that the southeastern island of Simeulue sank into the sea faster than expected from 1829 until the Sumatra earthquake in 1861.

This slow slip event was a gradual process that relieved pressure on the shallow part where two tectonic plates met, the NTU team said. However, that stress was transferred to a nearby deeper segment, resulting in a massive 8.5 magnitude earthquake and tsunami in 1861, causing enormous damage and death.

The discovery marks the longest slow-slip event ever recorded and will change global perspectives on the length of time and mechanics of the phenomenon, says the NTU team. Scientists used to believe that slow-slip events only last for hours or months, but NTU research shows that they could actually last for decades without triggering the catastrophic tremors and tsunamis seen on historical records.

The study’s lead author, Rishav Mallick, a PhD student at the NTU Asian School of Environment, said, “It’s interesting how much we were able to discover from just a handful of ideally located coral sites. Thanks to the long span of the ancient corals, we were able to investigate the secrets of the past and find answers to them. The method we used in this article will also be useful for future studies of other subduction zones – locations prone to earthquakes, tsunamis, and volcanic eruptions. Our study can therefore contribute to better risk assessments in the future. “

Co-author, Assistant Professor Aron Meltzner of the Earth Observatory in Singapore at NTU, said, “When we first found these corals more than a decade ago, we knew from their growth patterns that something strange must have been going on while they grew. Now we finally have a workable explanation. “

The results were published in the journal Natural geosciences In May, the authors suggested that current earthquake risk assessments in the observations may overlook ongoing slow-slip events and therefore not properly account for the potential of slow-slip events to trigger future earthquakes and tsunamis.

Possible earthquake in slow motion on Enggano Island

The shallower part of the subduction zone is far from land below kilometers of water and is usually “quieter” and does not cause as many earthquakes. The remote location also makes it difficult for land based scientific instruments to detect activity and scientists to understand what is going on.

Many scientists have therefore tended to interpret the “calm” of the flat part of the subduction zone in such a way that the underlying tectonic plates slide along steadily and harmlessly.

While this might be true in some cases, the NTU study found that this sliding is not as smooth as believed and can occur during slow slip events.

Responding to their findings, Rishav said, “Because such slow-slip events are so slow, we may have missed them as current instrumental recordings are generally only up to ten years long.”

He added, “If similar behavior is observed leading to earthquakes elsewhere, this process could potentially be identified as an earthquake precursor.”

The NTU team drew on their methodology in the research and also highlighted a potential ongoing lengthy slow-slip event on the Indonesian island of Enggano, which is about 100 km southwest of Sumatra.

Asst Prof. Meltzner said, “If our results are correct, it would mean that the communities living near this Indonesian island may be at greater risk of tsunami and earthquakes than previously thought. This suggests that models of risk and mitigation strategies need to be updated. ”

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