M7.1 Earthquake Strikes Offshore Sumatra Near Teluk Dalam, Indonesia

On April 24, 2023, at 20:00 local time, a magnitude 7.1 earthquake occurred 171 km south-southeast of Teluk Dalam on Nias Island, Indonesia. The event registered at a focal depth of 34 km. This earthquake forms part of the ongoing seismic activity along the Sunda subduction zone, where the Indo-Australian Plate converges with the Eurasian Plate at rates of approximately 5–6 cm per year. The hypocenter location places the rupture within the megathrust interface offshore western Sumatra. Regional tectonics feature oblique subduction that produces both thrust and strike-slip faulting. Nias Island and the surrounding Mentawai Islands sit above the locked portion of the plate boundary, accumulating strain that is periodically released in large events. Historically, the area has experienced several major earthquakes. The 2004 Mw 9.1–9.3 Sumatra-Andaman event generated a devastating Indian Ocean tsunami. One year later, the 2005 Mw 8.6 Nias–Simeulue earthquake ruptured a 400 km segment of the megathrust directly beneath Nias Island, producing intense shaking and a modest tsunami. Additional significant events include the 2010 Mw 7.8 Mentawai earthquake and multiple Mw 7+ aftershocks that have followed the 2005 rupture. The April 2023 earthquake occurred within the aftershock zone of these earlier events, consistent with continued post-seismic relaxation and stress transfer along the plate interface. At a depth of 34 km, the event likely involved thrust motion on the subduction megathrust rather than shallower crustal faulting. Ground shaking was felt across Nias and parts of mainland Sumatra, though no major tsunami was generated due to the moderate magnitude and focal depth. Seismic hazard assessments for the region classify the Sunda megathrust as capable of producing Mw 8.5+ earthquakes with return periods of several decades to centuries. The 2023 event underscores the persistent risk to coastal communities on Nias and the Mentawai Islands, where rapid population growth and limited infrastructure resilience remain concerns. Ongoing monitoring by Indonesia’s BMKG and international networks continues to track aftershock sequences and deformation measured by GPS stations across the forearc. These data help refine models of plate coupling and improve probabilistic forecasts for future large earthquakes in the Sumatran subduction system.

References

USGS Earthquake Catalog BMKG Indonesia Seismic Reports Sieh et al., 2008, Earthquake Supercycles Inferred from Sea-Level Changes Recorded in the Corals of West Sumatra, Science Chlieh et al., 2008, Geodetic and Geological Constraints on the 2005 Nias Earthquake, Journal of Geophysical Research