Pacific Coastal and Marine Science Center
Tsunamis and Earthquakes
Tsunami Generation from the 2004 M=9.1 Sumatra-Andaman Earthquake
The tsunami from the 2004 M=9.1 Sumatra-Andaman earthquake was primarily caused by vertical displacement of the seafloor, in response to slip on the inter-plate thrust fault (see tectonics).
During the 2004 Sumatra-Andaman earthquake, the sea floor on the overriding Burma plate deformed vertically, uplifting seaward toward the trench and downdropping (subsiding) landward toward the coastline. The line in map view where offshore uplift gives way to subsidence near the coast is termed the hingeline.
Vertical displacement of the water column approximates that of the sea floor below, resulting in the initial "N-shaped" tsunami wave that is typically generated by subduction earthquakes. This N-wave then splits in two, resulting in a pair of N-waves traveling in opposite directions. One, the distant tsunami, propagates outbound across the Bay of Bengal towards India and Sri Lanka, eventually reaching the Atlantic and Pacific Oceans. The other, the local tsunami, travelled towards Indonesia, Thailand, and nearby islands in less than an hour.
The length of the fault that ruptured during this earthquake was massive, extending from northwest Sumatra north to the Andaman Islands. In tsunami models, because of its size, we have to include the time it takes for the 2004 earthquake to unzip along the fault. Like a propagating crack in a frozen lake, the rupture front for this earthquake moved at a high speed of approximately 2.5 km/s, typical for subduction zone earthquakes. It took approximately 8 minutes for the rupture front to propagte from the hypocenter to the end of its journey 1200 km away (approximately the length of California!). In tsunami models for smaller magnitude earthquakes, the displacement of the seafloor caused by an earthquake is assumed to occur instantaneously, since tsunami waves move more slowly than fault rupture.
Shown below is an animation of both the northward propagation of the rupture front and an exaggeration of the vertical movement of the seafloor. (The water has been removed in this animation.) The seafloor is colored red where it moves upward and colored blue where it moves downward. Islands that straddle the regions of upward and downward movement tilt about the hingeline as shown below. Coral reefs on the up thrown side are permanently exposed, similar to what was observed from field studies after the March 28, 2005 northern Sumatra earthquake.
Animation: Vertical movement of the seafloor over the 2004 Sumatra-Andaman earthquake, view to the northwest, water removed
This animation shows a model of vertical seafloor displacement caused by the December 26, 2004 Sumatra-Andaman earthquake. Both the propagation of the rupture front northward and an exaggeration of the vertical movement of the seafloor are shown. The water has been removed in this animation. The seafloor is colored red where the seafloor moves upward and colored blue where it moves downward.
Now we can add water onto the simulation above to see how the tsunami waves move away from earthquake source region. As shown in the animation below, by the time the earthquake rupture reaches the Andaman Islands, the tsunami waves generated at the initiation of rupture have traveled part way into the Bay of Bengal.
Animation: Tsunami wavefield approximately 6 minutes after rupture initiation (origin time), view to the northwest
This animation shows the evolution of tsunami waves caused by the December 26, 2004 Sumatra-Andaman earthquake. Because it takes approximately 8 minutes for the entire fault to break, tsunami waves generated near the epicenter have propagated part way into the Bay of Bengal by the time the earthquake has just started to generated tsunami waves in near the Andaman Islands. Tsunami waves then cross the Andaman Sea toward Thailand.