A 32-year slow-motion earthquake – the slowest on record – eventually led to the catastrophic 1861 earthquake in Sumatra, researchers at Nanyang Technological University in Singapore (NTU Singapore) found. .
The NTU research team says their study highlights potential missing factors or poor modeling in earthquake risk assessments around the world today.
“Slow motion” earthquakes or “slow sliding events” refer to a type of long and prolonged stress release phenomenon in which the Earth’s tectonic plates slide against each other without causing jolting or destruction. major soil. They generally involve movements of a few cm / year to cm / day.
The NTU team made the surprise discovery by studying historic sea levels using ancient corals called “microatolls” on Simeulue Island, located off Sumatra. Growing both laterally and upward, disc-shaped coral microatolls are natural recorders of sea level changes and land rise, thanks to their visible growth patterns.
Using data from microatolls and combining them with simulations of the movement of Earth’s tectonic plates, the NTU team discovered that from 1829 until the Sumatran earthquake in 1861, the southeast of the island de Simeulue was sinking faster than expected into the sea.
This slow sliding event was a gradual process that relieved stress on the shallow end of where two tectonic plates met, the NTU team said. However, this stress was transferred to a nearby deeper segment, culminating in the 8.5 magnitude earthquake and tsunami in 1861 which resulted in enormous damage and loss of life.
The discovery marks the longest slow-slide event on record and will change the global outlook on the duration and mechanisms of the phenomenon, says the NTU team. Scientists previously believed that slow-slide events only happened over hours or months, but NTU research shows they could, in fact, last for decades without triggering the disastrous tremors and tsunamis seen in the documents. historical.
Lead author of the study, Rishav Mallick, a doctoral student at NTU Asian School of Environment, said: “It is interesting to see how much we were able to discover from a handful of conveniently located coral sites. ancient corals, we were able to probe and find answers to the secrets of the past. The method we adopted in this article will also be useful for future studies of other subduction zones – places prone to earthquakes, tsunamis and eruptions. Our study may therefore contribute to a better risk assessment in the future. “
Co-author, Assistant Professor Aron Meltzner of Singapore Earth Observatory at NTU, said: “When we first discovered these corals over ten years ago, we knew from their growth patterns, that something strange had to happen while they were growing up. Now we finally have a viable explanation. “
The results, published in the peer-reviewed scientific journal Geoscience of nature in May, led the authors to suggest that current earthquake risk assessments may ignore ongoing slow slip events in observations, and therefore fail to properly consider the potential for slow slip events to trigger future earthquakes and tsunamis.
Possible slow motion earthquake in progress on the island of Enggano
Located far from land under miles of water, the shallower part of the subduction zone is generally “quieter” and does not produce as many earthquakes. Due to its remoteness, it is also difficult for terrestrial scientific instruments to detect activity and for scientists to understand what is going on.
Many scientists have therefore tended to interpret the “calm” of the shallow end of the subduction zone to mean that the tectonic plates below slide smoothly and safely.
While this may be correct in some cases, the NTU study found that this slip is not as smooth as assumed and can occur during slow slip events.
Elaborating on their findings, Rishav said, “Because such slow-sliding events are so slow, we might have missed them because current instrumental recordings typically only last ten years.”
He added: “If similar behavior is observed leading to earthquakes elsewhere, this process could possibly be recognized as an earthquake precursor.”
Drawing on their methodology in the research, the NTU team also highlighted a potential prolonged slow slide event on the island of Enggano, Indonesia, located about 100 km (60 miles) to the southwest. from Sumatra.
According to Professor Meltzner, “If our conclusions are correct, it would mean that communities living near this Indonesian island are potentially facing a higher risk of tsunami and earthquake than previously thought. This suggests that risk models and mitigation strategies need to be updated. “