Tsunami Historical Series: Sumatra - 2004

The magnitude 9.1 Great Sumatra-Andaman Earthquake of December 26, 2004, spawned the deadliest tsunami in history, killing more than 230,000 people in 14 countries around the Indian Ocean. More than half of those killed had lived in Acheh Province, Sumatra, where the tsunami rose as high as 30 m (100 ft.) and traveled more than 4 km (2.5 mi.) inland in this low-lying region.

This earthquake began at its epicenter near northern Sumatra and moved the earth's crust an average of 15 m (50 ft.) as it ruptured northward for at least 1200 km (750 mi.) almost to the coast of Myanmar (Burma) over an 8-minute period. This distance is at least 200 km (125 mi.) longer than the length of fault that moved during the largest earthquake ever recorded, the magnitude 9.5 Great Chile Earthquake of 1960.

This animation shows why this south-to-north rupture is important for understanding the behavior of this tsunami, and why such "progressive" rupture needs to be considered for future tsunami forecasting. If the earthquake had moved the fault along its entire length all-at-once it would have sent the largest tsunami waves perpendicular to the fault and so they would have passed south of Sri Lanka. The earthquake motion, however, started in the south and moved northward along the fault so the tsunami began radiating from near Sumatra before it could be generated near Myanmar, thus causing the largest tsunami waves to strike Sri Lanka and Somalia directly, consistent with the tsunami waves actually observed in those countries.

The Pacific Tsunami Warning Center (PTWC) can create an animation of a historical tsunami like this one using the same tool that it uses to determine tsunami hazards in real time for any tsunami today: the Real-Time Forecasting of Tsunamis (RIFT) forecast model. The RIFT model takes earthquake information as input and calculates how the waves move through the world’s oceans, predicting their speed, wavelength, and amplitude. This animation shows these values through the simulated motion of the waves and as they travel through the world’s oceans one can also see the distance between successive wave crests (wavelength) as well as their height (amplitude) indicated by their color. More importantly, the model also shows what happens when these tsunami waves strike land, the very information that PTWC needs to issue tsunami hazard guidance for impacted coastlines. From the beginning the animation shows all coastlines covered by colored points. These are initially a blue color like the undisturbed ocean to indicate normal sea level, but as the tsunami waves reach them they will change color to represent the height of the waves coming ashore, and often these values are higher than they were in the deeper waters offshore. The color scheme is based on PTWC's warning criteria, with blue-to-green representing no hazard (less than 30 cm or ~1 ft.), yellow-to-orange indicating low hazard with a stay-off-the-beach recommendation (30 to 100 cm or ~1 to 3 ft.), light red-to-bright red indicating significant hazard requiring evacuation (1 to 3 m or ~3 to 10 ft.), and dark red indicating a severe hazard possibly requiring a second-tier evacuation (greater than 3 m or ~10 ft.).

Toward the end of this simulated 48 hours of activity the wave animation will transition to the "energy map" of a mathematical surface representing the maximum rise in sea-level on the open ocean caused by the tsunami, a pattern that indicates that the kinetic energy of the tsunami was not distributed evenly across the oceans but instead forms a highly directional "beam" such that the tsunami was far more severe in the middle of the "beam" of energy than on its sides. This pattern also generally correlates to the coastal impacts; note how those coastlines directly in the "beam" are hit by larger waves than those to either side of it.

You can view a YouTube version of this animation here.