The slowest known earthquake

A study of ancient corals in Indonesia has led to the discovery of the slower earthquake of which there is knowledge.

This strange “slow motion” earthquake lasted no less than 32 years. Although, due to its characteristics, it did not cause the damage that conventional earthquakes do, it generated a series of alterations that finally caused the catastrophic 1861 earthquake in Sumatra.

The discovery is the work of Rishav Mallick’s team from Nanyang Technological University (NTU) in Singapore.

Earthquakes “in slow motion” are slow landslides that constitute a type of stress relief phenomenon in which tectonic plates they slide against each other little by little without causing great shaking or destroying the ground. They usually produce ground movements of between a few centimeters a year and a few centimeters a day.

The Nanyang Technological University team made the surprising discovery while studying sea levels in ages past from about ancient corals described as “microatolons“, located in the Simeulue island, located off the coast of Sumatra. Disk-shaped coral microatolls, growing sideways and upward, are natural records of changes in sea level and land elevation. Such changes are seen in the visible markings of their growth patterns.

As part of the study, the team extracted a radial slab from a microatoll and used its data to reconstruct changes in sea level and ground level. (Photo: NTU Singapore)

Using data from microatolons and combining them with simulations of the motion of Earth’s tectonic plates, the Nanyang Technological University team discovered that, from 1829 until the Sumatra earthquake of 1861, the southeast of Simeulue Island sank into the sea faster than expected.

This slow slide was a gradual process that eased the tension in the shallow part of the place where the two tectonic plates met. However, this stress was transferred to a deeper neighboring segment, culminating in the massive 8.5-magnitude earthquake in 1861, accompanied by a tsunami, causing many fatalities and extensive property damage. (Source: NCYT from Amazings)

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