The NTU research team says their study highlights potential missing factors or mismodelling in global earthquake risk assessments today.
'Slow motion' earthquakes or 'slow slip events' refer to a type of long, drawn-out stress release phenomenon in which the Earth's tectonic plates slide against one another without causing major ground shaking or destruction. They typically involve movements of between a few cm/year to cm/day.
The NTU team made the surprise discovery while studying historic sea-levels using ancient corals called 'microatolls' at Simeulue Island, located off the coast of Sumatra. Growing both sideways and upwards, the disc-shaped coral microatolls are natural recorders of changes in sea level and land elevation, through their visible growth patterns.
Using data from the microatolls and combining them with simulations of the motion of the Earth’s tectonic plates, the NTU team found that from 1829 until the Sumatra earthquake in 1861, south-eastern Simeulue Island was sinking faster than expected into the sea.
This slow slip event was a gradual process that relieved stress on the shallow part of where two tectonic plates met, said the NTU team. However, this stress was transferred to a neighbouring deeper segment, culminating in the massive 8.5 magnitude earthquake and tsunami in 1861 which led to enormous damage and loss of life.
