Seismic Hazards in the Sinabang Region of Indonesia

The area surrounding Sinabang on Simeulue Island lies within one of the world's most active subduction zones. Here the Indo-Australian Plate converges with the Eurasian Plate along the Sunda megathrust at rates exceeding 5 cm per year. This tectonic setting produces frequent large earthquakes and has shaped the island's steep topography and narrow coastal plains.

On 2 November 2002 at 01:26 local time, a magnitude 7.4 earthquake struck 50 km northwest of Sinabang at a focal depth of 30 km. The event occurred within the overriding plate or along the plate interface, generating strong shaking across Simeulue and triggering minor landslides in the island's central highlands. No widespread tsunami was reported, consistent with the moderate depth and strike-slip component often observed in this segment of the margin.

The 2002 earthquake belongs to a sequence of strong events that have affected the region since 2000. A magnitude 7.4 shock occurred on 20 February 2008 roughly 56 km northwest of Sinabang, only 14 km from the 2002 epicenter. The great magnitude 9.1 Sumatra-Andaman earthquake of 26 December 2004 ruptured a much longer segment of the megathrust, with its southernmost aftershocks located approximately 53 km from the 2002 hypocenter. These events illustrate the segmented nature of the Sunda megathrust, where adjacent patches can fail independently or trigger neighboring sections.

Geological studies of Simeulue reveal repeated great earthquakes over the past several centuries. Coral microatoll records document uplift during events in 1797, 1833, and 1861, indicating that the megathrust beneath the island has produced magnitude 8+ ruptures at intervals of roughly 200 years. The 2002 and 2008 events represent moderate release of strain within the same locked zone that later participated in the 2004 rupture.

Post-2004 geodetic measurements show that the megathrust segment west of Simeulue remains partially locked, accumulating elastic strain at rates comparable to those before 2004. This continued locking, combined with the historical recurrence pattern, indicates that the region retains the potential for future large earthquakes. Local authorities therefore maintain enhanced seismic monitoring networks and tsunami early-warning systems to reduce risk to the island's population and infrastructure.

References

USGS Earthquake Catalog (events 2002-2008)
Prompt-provided event parameters (2002 M7.4, 2008 M7.4, 2004 M9.1)