Indian Ocean Earthquake and Tsunami

About the Indian Ocean Earthquake and Tsunami

Basics of Tsunami

What are Tsunamis?
Tsunamis are large sea waves typically produced by an earthquake or submarine volcanic eruption. Tsunami waves are produced when the seabed is ruptured, causing vertical movement of the water mass in the epicentral region. Tsunami waves differ from the common ocean waves in many aspects. Common ocean waves are a kind of surface waves called Rayleigh waves that are confined to the surface layer of the sea and tend to have wavelengths on the order of tens of meters and periods of tens of seconds. Tsunami waves can attain wavelengths over hundreds of kilometers and oscillatory periods of several minutes. The energy associated with the tsunami waves can propagate across the ocean at velocity several hundreds kilometers per hour. When the tsunami waves arrive at the coast, wave height can increase to over ten meters and even several tens of meters, causing devastating damage to the coastal regions.

What caused the Indian Ocean Tsunami?
The massive earthquake (Magnitude 9) centred on Aceh, NW Sumatra, occurred above a several thousand kilometer-long subduction zone in which the floor of the Indian Ocean is advancing northwards at around 7 cm per year and being forced beneath the Southeast Asia continental plate. Scientists estimate 25-30 m of fault rupture at the earthquake’s epicenter, which occurred around 30 km beneath Earth’s surface within the Burma micro-plate. Essentially, several hundred years' of accumulated strain was catastrophically released in seconds-minutes during the earthquake event. The resulting large-scale displacement of the ocean floor, along a huge length of the Java-Sumatra-Burma subduction zone, triggered the massive tsunami that devastated vast swathes of coastline rimming the northern Indian Ocean.

Are we at risk in South China?

Animation of Indonesian Tsunami (credit: NOAA)

As tsunamis are disasters that can be generated in all of the world's oceans, inland seas, and in any large body of water, any coastal area is at risk from such event. While the effects of at least four Pacific Ocean tsunamis have been recorded in Hong Kong over the past 25 years these events only had a minor effect on HK tide gauges. Nevertheless the potential for a disastrous tsunami along the coast of the South China Sea is real and the potential tsunami risk to Hong Kong may have been under-estimated in the past. The magnitude 9 earthquake which generated the December 26th, 2004 Indian Ocean Tsunami occurred along the subduction zone between the Indian and Burma plates. A similar plate tectonic setting is present in our region where oceanic crust of the South China Sea is being subducted along the Manila Trench in the western Philippines. A very large magnitude earthquake underwater along this plate boundary would have the potential to generate a tsunami which could inundate the Hong Kong and south China coastline within a few hours of the tremor. However, if an appropriate warning system was to be set up combined with increased public education about natural Earth hazards and how to respond to such events then many lives might be saved.

Additional information
Please download the file (in English or Chinese) for additional technical information on earthquakes and tsunamis prepared by Dr LS Chan in the Department of Earth Sciences.

Presentation on Tsunami

The Department of Earth Sciences at the University of Hong Kong organized a free public lecture on Tsunami between 5:00 and 6:30pm, 13th of January, 2005 in Rayson Huang Lecture Theatre, HKU. The lecture was jointly given by Professors John Malpas, Jonathan Aitchison and Drs Lung Chan and Jason Ali. Dr Chan's lecture materials can be downloaded from here.

Links on Earthquake and Tsunami

Links on Post-tsunami humanitarian situation

Return to the Department of Earth Sciences, HKU

Last Updated January 3, 2005