The longest earthquake on record ended in 1861 on the Indonesian island of Sumatra. It ended with a sudden rupture that occurred after tectonic plates below the island had been rumbling against each other for 32 years in what was known as a slow-slip event. It was a magnitude 8.5 mega earthquake that had been building up for some time. A tsunami was also triggered.
Scientists have described this event as a "decades-long silent earthquake." It was subtle and unnoticeable. In simple terms, a fault line in the Earth's crust was moving but it was not quick enough to be noticed until its final end.
Scientists only identified the event as a slow earthquake after they studied the activity of the coral growing along the faultline on the sea floor.
An article in the scientific journal, Nature, explains that it was coral scars that provided proof that there was a slow earthquake.
The slow-slip event took decades and the result was that it left its mark on the corals. The researchers studied the growth patterns of the corals. They discovered that the corals experienced vertical movement at the fault starting from the year 1738 to the moment they died in 1861 when the earthquake ended in mega destruction.
Throughout their long lifetime, the corals were periodically exposed to the air as the land was thrust up and down from the movement of the tectonic plates during the decades-long slow earthquake. The corals could not grow when they were exposed to the air so there were layers of dead corals that corresponded with sea levels. In this way, the corals left their imprints for scientists to study.