Alan V. Morgan
Most of the world's population added this new word to their lexicons at year end 2004. Peter and I had almost finished the "Fall version" of What On Earth, but as discussions of the massive earthquake near Banda Aceh, Sumatra, and the unprecedented tsunami waves that ravaged the shorelines of southeast Asia dominated the news we thought that we should modify this issue. By now almost all people on Earth are aware of the scale of the disaster, although figures on deaths and damage are still being added to the total. So far more than 150,000 are known to be dead, almost 100,000 in Indonesia alone, with an additional 30,000 in Sri Lanka and the damage undoubtedly runs into billions of dollars. The number of dead in this tragedy ranks the Banda Aceh earthquake and associated tsunamis as the seventh largest recorded in human history. Certainly the almost unprecedented visual coverage provided by the world's media has etched the scale of human suffering and grief on everyone's minds. But what is the geological background to this story, and what were the elements that brought the catastrophe to reality?
The Banda Aceh earthquake was big. It ranked a 9.0 on the Mw seismic scale as the joint-fourth largest earthquake in the last century, rivaling the 9.0 Mw Kamchatka earthquake of November 4th 1952. (The Mw scale is one of four commonly used seismic scales; it reflects the moment of the earthquake that is a measure of the rigidity of the earth times the average amount of slip on the fault times the amount of fault area that slipped). Larger earthquakes include the 9.1 Mw in the Andreanof Islands in Alaska; the 9.2 Mw Prince William Sound (Good Friday) earthquake on the March 23, 1964, and the 9.5 Mw colossus near Valdivia in Chile on May 22, 1960. However, the loss of life was significantly less in each of the other earthquakes. The Chilean earthquake, the largest ever instrumentally recorded, had about 2,800 deaths with less than 250 attributed to the tsunamis that swept across the Pacific to Japan and the Philippines.
So what happened at 0:58.53 on December 26, 2004? At 7:58:53 local time a large slippage took place at the junction of the India Plate and the Burma "Micro- Plate" some 250 km SSE of Banda Aceh in northernmost Sumatra (Figure 1). This type of slippage causes what is known as a "megathrust earthquake". The movement was created as a result of stresses between the India Plate, which is being driven under the Burma Plate. The boundary can be seen in part along the line of the Sunda Trench that runs parallel to, and several hundred km west of, the Sumatran coastline. Here the India Plate is moving about 6cm per year to the northeast plunging beneath the Burma Plate. Rocks at the interface were gradually warped until they ruptured, and the breakage resulted in a tremendous release of energy. The area affected initially was at least 100 km in size and the horizontal displacement seems to have been about 15m at maximum. The tilting of the Burma Plate as it was underthrust and buckled by the India Plate resulted in a seaward uplift of the sea floor by about 2m over a distance of some 400km by 80km. At the same time the landward side of the seafloor was depressed by about 2m in a region about 100 km by 50 km close to the Sumatra coast. Almost immediately the huge shock wave from the initial earthquake started to initiate sympathetic shocks along the whole 1,200km length of the Burma Plate. Many of these would have been catastrophically large in their own right if they had taken place close to large population centres. At the same time a series of tremendous waves was initiated from the shifting configuration of the sea floor (Figure 2).
These tsunami "tidal waves" raced toward Banda Aceh and other small coastal communities. The oscillatory motions within the waves travel at high speed - at about 800km per hour - although if you were at sea you probably would not notice the wave passing under you. However, when the waves reached land the water slowed and piled up. At least five huge waves, estimated at 12m in height crashed into Banda Aceh. The devastation from the earthquake (locally estimated at Magnitude 8) and the associated tsunami waves was terrifying and complete as the water surged several kilometers inland (Figure 3). Many people undoubtedly died because they simply did not realise what was happening.
I always tell my students that if they are on the seashore and the water suddenly starts to move rapidly offshore, don't wander down to see flapping fish and pick up shells, just head for the highest point that you can find furthest from the coast, because a tsunami wave is likely to follow. This is exactly what one ten-year old British girl told her mother. She had just finished a school class on earthquakes and tsunamis before going on vacation in Thailand. She communicated this to her mother who alerted Thai staff at their resort and 100 people ran to safety before the waves swept in. Another survivor in Banda Aceh realized that the earthquake would likely be followed by a tsunami. He gathered his wife and two children, put them all on his motor cycle and headed inland until he ran out of gas. They all survived, watching the waves crashing across Banda Aceh from a hillside outside the town. In both cases some basic geological knowledge allowed the survivors a sufficient time interval that they were able to escape.
Detailed satellite images have allowed us to see "before and after" images of many of the areas affected, and it allows us to appreciate the nature of the tsunami and its results in stunning detail. Space precludes presentation of many of the details and I have inserted several that I feel are appropriate. The comparisons are all provided courtesy of DigitalGlobe "Quick Bird images".
The tsunami waves did not stop on the western coasts of southeast Asia, they also raced across the Indian Ocean and slammed into the coastline of Sri Lanka and southern India. The town of Hambantota was devastated by waves. At least 1,100 are reported dead and 1,000 missing out of a population of about 48,000. Many of the boats in Figure 6 ended up inside the lower part of the town. Similar stories are typical along the northern, eastern, and southern Sri Lanka coastline, where over 30,000 are reported killed with many more still missing. The death toll is estimated at about 15,000 along the southern coast of India. And still the waves rolled on.
The waves were recorded as tidal fluctuations in New Zealand and even along parts of the west coasts on North and South America. As I complete this, more images are coming in of the ravaged coastline of Sumatra. News from my sister is that one of her closest friends and family were killed in Thailand where they had gone for a Christmas vacation.
This is an event that stunned the world. Geologically we expect this to happen, but it is always a shock when the human side of the disaster is revealed. Politicians are vacillating on how and where aid should be delivered. Three billion US dollars has been donated, and a small fraction of this sum in advance could have established a tsunami warning system for the areas most affected. Likely little could be done for the coastline of Sumatra which was so close to the epicentre of a huge earthquake, but even there people would have had enough warning to escape the waves. Certainly in Thailand, Sri Lanka and India there was sufficient warning to have moved people the few kilometers inland. That would have been enough in Khoa Lak to have saved several thousands. A few simple lessons in basic geology would also have helped. And perhaps the most ironic observation is that less than one month before this disaster the UNESCO decided to close its Division of Earth Sciences.
Alan V. Morgan