Big Earthquakes, Different Outcomes

On December 26, 2004, a magnitude 9.1 earthquake struck in the Indian Ocean off the coast of Sumatra, triggering a 100-ft (30m) tsunami that swamped the coastlines of Indonesia, Thailand, Burma, Malaysia, India, Sri Lanka, Somalia, Kenya, and dozens of islands. The earthquake and tsunami took more than 230,000 lives in the countries bordering the Indian Ocean.

A little more than 7 years later, on April 11, 2012, a magnitude 8.6 earthquake struck on the Indian Ocean seafloor not far from the epicenter of the 2004 quake, but the height of the tsunami was only 3 ft (1m). The islands and nations bordering the Indian Ocean reported very little, if any, damage resulting from the small wave. Five deaths were reported, but three were from heart attacks and two from shock. Although the 2004 earthquake was stronger, an 8.6 is powerful enough to start a major tsunami under the right conditions.

Why the big difference? It comes down to epicenter location and type of fault. The 2004 quake was produced by a thrust fault rupture in the Sunda Trench, the boundary between the oceanic Indian-Australian plate and the continental Eurasian Plate, where the slip action was vertical and violent. When the fault ruptured, a 1,000-mile (1600km) section of the Indian-Australian plate thrust underneath the Eurasian Plate, and the Eurasian plate lifted upward 50 ft (15m), displacing millions of tons of ocean water in a matter of minutes, and creating the massive tsunami that started rolling toward land at speeds up to 600 mph (1,000kph).

Although the two earthquakes were not far apart in distance, the 2012 earthquake epicenter was on a strike-slip fault within the Indian-Australian Plate, where the slip action was horizontal instead of vertical. The movement on one side of the fault was north northwest, while the movement direction on the other side was south southeast. When the fault line stress released, the sudden horizontal slippage caused heavy shaking, but resulted in very little seafloor deformation, very little water displacement, and a very small tsunami.