The 2004 Sumatra-Andaman Earthquake and Regional Tectonic History

The Sumatra-Andaman region lies along the Sunda subduction zone, where the Indo-Australian plate converges with the Eurasian plate at rates of approximately 5–6 cm per year. This tectonic setting produces frequent megathrust earthquakes and has shaped the geological evolution of the Andaman Sea and western Sumatra over millions of years. The trench extends more than 5,000 km from Myanmar southward, accommodating oblique subduction that contributes to both seismic and volcanic activity in the arc. On 26 December 2004, at 00:58 UTC, a moment magnitude 9.1 earthquake struck with a hypocentral depth of 30 km. The rupture initiated near the northern tip of Sumatra and propagated northward along the Andaman segment for roughly 1,300 km. This event remains one of the largest instrumentally recorded earthquakes and generated a trans-oceanic tsunami that affected coastlines across the Indian Ocean. Since 2000, several additional earthquakes of magnitude 7 or greater have occurred within 60 km of the 2004 mainshock epicenter. These include an M 7.4 event on 2 November 2002 located 53 km northwest of Sinabang, an M 7.4 on 20 February 2008 located 58 km northwest of Sinabang, an M 7.2 on 9 May 2010 located 50 km south-southwest of Meulaboh, and the 2004 mainshock itself. These events illustrate the continued seismic productivity of the plate boundary following the great rupture. Post-2004 geodetic and seismic monitoring has documented widespread afterslip and viscoelastic relaxation along the megathrust. The region continues to experience both interplate thrust events and intraslab normal-faulting earthquakes, reflecting stress redistribution. Updated seismic hazard assessments incorporate these observations to refine recurrence intervals for future great earthquakes along the Sunda Trench.

References

USGS Earthquake Catalog (event parameters for 2004 Sumatra-Andaman mainshock and listed regional events since 2000).
Global CMT Catalog (moment magnitudes and focal mechanisms for listed earthquakes).