March 28, 2005 Sumatra Earthquake

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Posting Date: March 28, 2005, 12:00:00 PM

A great earthquake struck 203 km off the west coast of northern Indonesia today, March 28, 2005, at 16:09 UTC. The USGS has estimated the earthquake’s magnitude at 8.7. The earthquake occurred a bit south of, and on the same fault line as the M9.3 earthquake that caused a tsunami that killed more than two hundred thousand people in the region on December 26, 2004. The USGS has classified this earthquake as an aftershock of the December 26 earthquake. Focal depth was estimated at about 30 km. The epicenter was located off the coast of the island of Sumatra, about 205 km west northwest of Sibolga, Sumatra, 245 km southwest of Medan, Sumatra, and 535 km west southwest of Kuala Lumpur, Malaysia. The earthquake was felt as far away as Singapore and the Malaysian coastal city of Penang. The island of Sumatra sits at the convergence boundary between the Sunda and the Australian and Indian tectonic plates. The Australian and Indian plates are obliquely subducting beneath the Sunda plate at the Sumatran subduction zone at a rate that varies from about 60 mm/year in Southern Sumatra to about 52 mm/year in northern Sumatra. Because of the arced shape of the subduction zone in this region, the direction of convergence is not totally perpendicular to the plate boundary. This tends to create rather complex motions within the subduction zone and crustal faults with strong lateral components to accommodate the oblique plate convergence. The epicentral location of today’s earthquake is about 190 km southeast of the epicentral location of the M9.3 December 26 2004 earthquake. In 1861 a segment of the subduction zone, south of the epicentral location of the current earthquake, ruptured with a reported magnitude of 8.4. Although the earthquake was extremely strong, the epicenter was at moderate depth and it occurred well offshore. Initial reports of structural damage from earthquake shake are fairly localized. One report from Nias, an island off the coast of western Sumatra, describes considerable building damage. There are also reports of dozens of fatalities and heavy building damage on Simeulue Island, Indonesia. Tsunami warnings have been issued for the region and low lying coastal areas have been evacuated in Sumatra, Malaysia, Thailand and Sri Lanka. It is still in the very early aftermath of this event. The AIR earthquake team is analyzing ground motion records as they come in from seismic networks around the world and will post additional information as soon as that analysis is complete.

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Posting Date: March 28, 2005, 12:00:00 PM