The earthquake happened just off the coast of the Indonesian island of Sumatra and may have turned deadly in 1861.
Researchers from Nanyang Technological University in Singapore identified the earthquake, which they’re calling a “slow-slip event” or a “slow-earthquake”.
This happens when a fault line in the Earth’s crust is moving but not quick enough for a quake to be noticeable.
According to National Geographic, study author Emma Hill said: “I wouldn’t have believed that we would find a slow-slip event so long, but here we found it.”
The researchers think a “slow-slip event” started off the coast of Sumatra in 1829 and slowly moved for over 30 years before ending in a big disaster.
The scientists have published a study in the journal Nature Geoscience explaining how they spotted signs of an earthquake event that happened for decades and linked it to the large Sumatra earthquake of 1861.
The magnitude 8.5 quake caused a giant tsunami that ravaged over 300 miles of Indonesian coastline and could have killed thousands of people.
The aftershock is said to have continued for months.
Scientists identified the slow-earthquake event by studying coral along the the Sundra megathrust fault line on the sea floor near the Indonesian island of Simeulue.
Fault line movements can leave a coral imprint that shows how the ground has moved overtime.