The December 2004 M7.2 Earthquake West of Sabang, Indonesia

On 26 December 2004 at 04:21 local time, a magnitude 7.2 earthquake struck 284 km west-northwest of Sabang, Indonesia, at a focal depth of 39.2 km. This event occurred within the tectonically active 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 along the interface of this megathrust system, a region long recognized for its capacity to generate significant seismic activity. Sabang lies on Weh Island at the northern tip of Sumatra, positioning it directly above one of the world’s most seismically productive subduction segments. Geological records indicate that the broader Aceh–Andaman region has experienced repeated great earthquakes, with paleoseismic evidence pointing to major events in the 14th and 16th centuries. These historical ruptures demonstrate the persistent strain accumulation and release along the plate boundary. The 2004 earthquake contributed to the ongoing seismic sequence in the area. Its moderate magnitude and intermediate depth are consistent with intraslab or interface faulting typical of the subduction environment. Regional monitoring shows that strong earthquakes have continued to affect the zone since 2000, underscoring the need for sustained hazard assessment. Post-event studies have refined understanding of crustal deformation in northern Sumatra. GPS measurements reveal ongoing post-seismic relaxation and interseismic locking patterns that influence future rupture potential. The combination of historical recurrence intervals and modern instrumental data supports the characterization of this segment as capable of producing both moderate and great earthquakes. Local geology features a complex arrangement of accretionary prism sediments, forearc basins, and volcanic arcs shaped by millions of years of subduction. These structures influence ground-motion amplification and tsunami generation potential during large events. Continued seismic monitoring by regional networks provides updated catalogs that help refine probabilistic hazard models for coastal communities near Sabang. Preparedness measures, including early-warning systems and building-code enforcement, remain central to reducing risk in this geologically dynamic setting.

References

USGS Earthquake Catalog (event parameters)
Global CMT Project (focal mechanism context)
Scientific literature on Sunda megathrust paleoseismology