Coastal Processes, Hazards, and Society

2004 Sumatran Earthquake


2004 Sumatran Earthquake

The Boxing Day 2004 Earthquake: A Holiday's Worst Nightmare

It is an idyllic morning to be at the beach, the day after Christmas 2004, bright sunshine and balmy temperatures greet vacationers on the tropical island of Phuket in Thailand. Kids run in the waves and build sandcastles, sunbathers relax. The ocean slowly recedes exposing rocks below the low tide line and much further and stranding fish. People venture way out to see this surprising phenomenon. The Earthquake off Sumatra occurred at about 7 AM local time, it was about 500 km away and no one here felt shaking.

A few people had heard about it on TV but no one puts two and two together---the ocean often recedes as a tsunami approaches. A wave appears on the distant horizon, a bright white band on the ocean surface moving slowly towards the land. People stare at it, puzzled. Gradually it dawns on some that the wave is danger but others don’t realize till it’s too late, especially those who had wandered out. The wave is moving really fast. The peaceful beach scene gradually becomes gripped by panic. People flee as fast as they can and run for higher ground. The wave quickly covers the beach and heads for the luxury beachfront hotels. Pools are covered and beach chairs move around like tinker toys. People cling to trees or desperately clamber up to higher floors to escape the wave. Others have already rushed to higher ground inland. The scene at Phuket is similar to many other resorts in Thailand and Malaysia. The wave reached up to 6 meters or 20 feet high here. Up to 300 people lost their lives on Phuket and many more were injured. But while there were places to escape to inland if you were fast enough, that was not an option in the Phi Phi islands, also in Thailand. The lovely beach there is backed by dramatic, steep limestone cliffs; and there were few hotels, just low lying beach cottages. The tsunami was also up to 6 meters at Phi Phi and as many as 4000 people died though the total could be far higher.

The date couldn’t be worse for such an event to occur; with thousands of people on holiday in the region and with very few people working in government offices, it was a recipe for disaster. Tourists from all over the world were vacationing in seaside resorts in Thailand, Indonesia, India, and elsewhere. Unplugged as they were, it was next to impossible to inform them or the residents of the region of the impending hazard.

Banda Aceh, the capital of the Aceh province of Sumatra, was much closer to the earthquake epicenter and people felt the shaking. However, they had very little time to do anything and there were no warnings for what was to come. And to be effective a warning would have had to have gone out immediately. About 20 minutes later three successive waves arrived, the first wave was smaller but the next two were so large that they reached as far as 4 km (2.5 miles) inland. The water was up to 12 m (40 feet) high in the city and the impact was truly devastating. Houses were totally destroyed, and wreckage, cars, and anything else were carried inland on this massive muddy wall of water and debris. Outside of the city, the wave was even higher, up to 30 meters (100 feet) high on the west coast of Aceh province where it traveled 3 miles inland. The highest measured wave reached 51 meters (167 feet) on a hillside near Banda Aceh. A total of 167,000 people are known to have died in Banda Aceh and many more were missing. The video shows footage of the devastating tsunami in Banda Aceh followed by Phuket.

See caption.
Devastation in Banda Aceh
See caption.
Banda Aceh Devastation

Video: 2004 Boxing Day Tsunami (5:57) (Video is not narrated.)

Now let’s take a step back and understand the megathrust earthquake that generated the tsunami. Remember a megathrust earthquake is one that occurs on a subduction zone where one plate is descending under another. These types of motions tend to cause a lot of strain to build up and thus then to produce massive amounts of energy when it is released, the fault ruptures, and the crust moves or slips. The slip can encompass the movement of massive areas, and involve different motions on either side of the plate boundary. In terms of tsunamigenesis, as we have learned, the critical part of the motion is that the megathrust often causes rapid upward motion of crust (and downward motion in some areas). And, of course, these fault zones are almost all under the ocean.

The 26 December 2004 Sumatra Andaman Earthquake took place where the Indian Plate is subducting beneath the Sunda Plate, actually, a small microplate belonging to it, the Burma microplate.

See caption: Plate boundary on Sunda trench. Shock zone slightly south of fault.
Tectonic base map of the Sumatra subduction zone.

The earthquake was the third largest quake ever measured with a magnitude of about 9.1. Two other notable elements of the quake were that it involved eight to ten minutes of shaking which is a very long time compared to most quakes. The rupture took place in several stages with a total of 15 meters or 50 feet of movement over a length of 1500 km (900 miles) and the area of slip was truly massive, about the size of California (as seen in the image below).

Red shaded area to the northwest of Sumatra.
Approximate location of rupture zone shown with large red box and location of epicenters of main earthquake and aftershocks.
Credit: Courtesy of Charles Ammon

The rupture actually involved a complex set of faults with some very rapid upward motion which triggered the tsunami. The initial rupture occurred above the earthquake epicenter then propagated to the north at a speed of 2km/sec or 1.2 m/sec (see image below). The fault that moved was up to 50 km (30 miles) deep and the total amount of motion at the epicenter was about 20 meters or 65 feet. The megathrust motion caused the Burma microplate to move upwards rapidly.

Oceanic plate subducting beneath overriding plate, forcing rock both up and down along hingeline between trench and coastline.
The displacement of rock surrounding the inter-plate thrust is shown in cross-section for the 2004 Sumatra-Andaman earthquake.
Credit: USGS (Public Domain)

The upward motion was enough to make new islands. The animation below shows how the crust moved upwards in propagation from the south towards the north. It is very easy to see how this motion generated a tsunami. The second animation shows how tsunami waves spread out from the area of upward motion. You can see that the waves that hit Banda Aceh in the south of the map came from a different part of the plate boundary from those that hit Thailand in the north.

See preceding paragraph and caption.
Tsunami wave field Bay of Bengal one hour after the earthquake
Credit: USGS (Public Domain)
See preceding paragraph and caption.
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.
Credit: USGS (Public Domain)
See preceding paragraph and caption.
This animation shows the evolution of tsunami waves caused by the December 26, 2004, Sumatra-Andaman earthquake.
Credit: USGS (Public Domain)

The earthquake was so large and the plate motion so significant that the 2004 Sumatra-Andaman earthquake is thought to have triggered other earthquakes in a process known as dynamic including the deadly 2008 Sichuan earthquake in China that killed 69,000 people.

Oceanic Subduction Zone diagram. One plate going under another resulting in a trench and an island arc further inland.
General diagram of an oceanic subduction zone. Sumatra and the Andaman Islands are part of an island arc.
Credit: USGS Pacific Coastal and Marine Science Center, Figure is taken from the online edition of This Dynamic Earth.
Fault slip diagram. Slip results in a dip in one area and a large rise in another resulting in a large wave.
Diagram of tsunami generation from an interplate thrust fault.

Left image of verdant coastal region. Right image of same region appears desolate.
Screen capture from the USGS. Lampuuk on the island of Sumatra before and after the Banda Aceh earthquake on Boxing Day, 2004 (December 26, 2004). Lampuuk is located on the northwestern-most peninsula of the island. The image on the left shows the area prior to the tsunami and, on the right, the area after the tsunami. Tsunami waves swept from west to east across this region in complex patterns. Significant infrastructure including roads, agricultural areas, residential and beachfront areas was obliterated during this event. Moreover, significant erosion pushed the shoreline inland and resulted in subsidence that contributed to continued land loss after the tsunami.

Learning Check Point

After examining the diagrams and maps on the USGS’s Pacific Coastal and Marine Science Center website and watching the animation video of the modeled tsunami waves that were generated by the earthquake. Take some time to think about what you just learned, then consider how you would answer the questions on the cards below. Click "Turn" to see the correct answer on the reverse side of each card.