A shallow magnitude-7.1 earthquake occurred off the coast of Sumatra, Indonesia, on 25 April at approximately 4am (Singapore time). The major earthquake occurred along a tectonic plate boundary called the Sunda Megathrust, between Tanahbala Island to the north and Siberut Island to the south, at the northern edge of a region known as the Mentawai Seismic Gap. 

The Mentawai Seismic Gap is a 300-km long region between south of Tanahbala Island and North Pagai Island which has not ruptured in a great earthquake in the past two centuries. Over this period, the tectonic plates have been moving towards each other at a rate of about 5.5 cm/yr, but they have been stuck along the region of the Sunda Megathrust called the Mentawai Seismic Gap. This region has therefore accumulated stress and is a region where we expect an even larger earthquake in the future.


Block diagram showing the tectonic setting of the 25 April 2023 Mw 7.1 earthquake and the location of some EOS GPS stations. (Source: Earth Observatory of Singapore)

“Today’s magnitude-7.1 event was strong, but it likely released only a small portion of the stress that the region accumulated over the past 225 years. We are therefore still expecting a great earthquake in the region in the future, although we cannot say exactly when it will happen,” said Assistant Professor Aron Meltzner, a Principal Investigator at the Earth Observatory of Singapore. 

It is not uncommon for the Sunda Megathrust to produce earthquakes with magnitudes greater than 6. For example, a magnitude-6.7 earthquake occurred in the same region on 14 March 2022. “Today’s event is the largest earthquake that has occurred in the region in 88 years,” said Asst Prof Meltzner.

Great earthquakes in this region can produce large tsunamis, such as the magnitude-9.2 earthquake that occurred close to Aceh in 2004. However, smaller earthquakes are much more common. For example, a few earthquakes of magnitude 4 and 5 occurred in the same region as today’s event over the past week. 

Today’s earthquake is a reminder of the constant earthquake and tsunami hazards from the Sunda Megathrust. “The Mentawai Seismic Gap experiences a great earthquake (of magnitude 8 or greater) about once every 200 years. In a typical 3-day window, the probability of a great event in the region is therefore about 1 in 25,000. However, we know from general earthquake studies that earthquakes cluster in time and space. Globally, the probability that any earthquake will be followed within 3 days by a larger event nearby is about 1 in 20. While it is more likely than not that no larger earthquake will happen in the days ahead, the likelihood of such an event in the next few days to weeks is nonetheless elevated compared to a typical week,” said Asst Prof Meltzner. 

Asst Prof Meltzner stresses that a large earthquake can happen at any time. “While we cannot predict when a great earthquake will occur within the Mentawai Seismic Gap, we can be prepared for it,” he added.