Seismic Hazards in Western Sumatra: The 2005 Nias Earthquake and Regional Tectonics

Western Sumatra lies along the Sunda megathrust, where the Indo-Australian plate subducts beneath the Eurasian plate at rates of approximately 5–6 cm per year. This convergent boundary produces some of the world’s largest earthquakes and has shaped the islands of Nias, Simeulue, and the Mentawai chain through repeated episodes of uplift and subsidence. The region’s geology features a well-developed accretionary prism, forearc basins, and the right-lateral Sumatran Fault that accommodates oblique convergence. On 28 March 2005 at 16:09 local time, an M8.6 earthquake struck 78 km WSW of Singkil at a depth of 30 km. The event ruptured a roughly 400 km segment of the megathrust between Simeulue and Nias islands, generating strong shaking across northern Sumatra and triggering a modest tsunami. It remains one of the largest instrumentally recorded earthquakes in the 21st century and produced measurable coastal uplift of up to 2.5 m on Nias. A subsequent strong earthquake occurred on 6 April 2010, an M7.8 event located 75 km east of Sinabang at a depth of 33 km from the 2005 rupture. This quake filled a smaller patch of the megathrust that had been stressed by the earlier event, illustrating the segmented nature of strain release along the plate boundary. Historical records and paleoseismic studies indicate that the Sunda megathrust has repeatedly hosted great earthquakes. The 2005 rupture overlapped part of the area affected by the 1861 M8.5 event, while the adjacent 2004 M9.1–9.3 Sumatra–Andaman earthquake immediately preceded it. These events demonstrate both spatial and temporal clustering of megathrust ruptures. Current monitoring by the Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) and international networks shows ongoing afterslip and microseismicity along the 2005 rupture zone. Although no M8+ events have occurred since 2010 in the immediate vicinity, the locked portions of the megathrust south of the 2005 rupture remain capable of producing future large earthquakes. Understanding the geometry and frictional properties of the subduction interface continues to improve through GPS networks and seafloor geodesy. These data help refine probabilistic seismic hazard assessments for coastal communities in Aceh and North Sumatra provinces.

References

USGS Earthquake Catalog (event parameters for 2005 and 2010 sequences)
Sieh et al., 2008, Earthquake supercycles inferred from sea-level changes recorded in the corals of west Sumatra, Science
Subarya et al., 2006, Plate-boundary deformation associated with the great Sumatra–Andaman earthquake, Nature