Slowest Earthquake Ever Recorded Lasted Over Thirty Years

AsianScientist (May. 31, 2021) – By studying ancient corals, researchers from Singapore have pieced together the slowest ever recorded earthquake, lasting 32 years, with the event preceding the catastrophic 1861 Sumatra earthquake. Their findings were published in Nature Geoscience.

Located in the Ring of Fire, Indonesia is no stranger to volcanic eruptions and earthquakes. In the mid-19^th century, the island of Sumatra in particular, was rocked by a series of earthquakes of a magnitude of eight and above, with the last one in 1861 resulting in a devastating tsunami with thousands of fatalities.

But while we often think of earthquakes as destructive events, that is not often the case. During ‘slow slip events,’ the Earth’s tectonic plates release stress and slide against each other without major ground shaking or destruction—moving only around a few centimeters over several hours or months, or at least thought researchers.

While studying historic sea levels using ancient corals called microatolls at Indonesia’s Simeulue Island, located off the Sumatran coast, a team from Nanyang Technological University, Singapore (NTU) made the surprise discovery. As they grow both sideways and upwards, the disc-shaped coral microatolls naturally record changes in sea level and land elevation through their growth patterns.

By combining microatoll data with simulations of the motion of the Earth’s tectonic plates, the NTU team found that from 1829 until the Sumatra earthquake in 1861, southeastern Simeulue Island was sinking faster than expected into the sea.

According to the team, this slow slip event was a 32-year gradual process that relieved stress on the shallow part of where the two tectonic plates met. However, this stress was transferred to a neighboring deeper segment, culminating in the massive 8.5 magnitude earthquake and tsunami in 1861 that led to enormous damage and loss of life.

Their discovery marks the longest slow slip event ever recorded—forcing scientists to rethink the timespan and mechanisms of such earthquakes. Instead of hours or months, such earthquakes can go on for decades without triggering the disastrous shaking and tsunamis typically recorded in history.

In fact, slow slip events can even be overlooked by current instrumental records, which are generally up to only ten years long. Applying their newfound methodology, the NTU team identified a potential ongoing slow slip event at Enggano Island, Indonesia, located about 100 kilometers southwest of Sumatra.

“If our findings are correct, this would mean that the communities living nearby this Indonesian island are potentially facing higher risk of tsunami and earthquake than what was previously thought,” said study co-author Assistant Professor Aron Meltzner from NTU. “This suggests that models of risk and mitigation strategies need updating.”

“It is interesting just how much we were able to discover from just a handful of ideally located coral sites,” added lead author Mr. Rishav Mallick, a PhD student at the NTU Asian School of Environment. “The method that we adopted in this paper will also be useful for future studies of other subduction zones—places that are prone to earthquakes, tsunamis, and volcanic eruptions. Our study can therefore contribute to better risk assessments in future.”

The article can be found at: Mallick et al. (2021) Long-lived shallow slow-slip events on the Sunda megathrust.

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Source: Nanyang Technological University, Singapore; Photo: Shutterstock.
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