Case Studies

1. 1946 Aleutian
2. 1960 Chile
3. 1964 Alaska
4. 1992 Nicaragua
5. 1992 Flores Island
6. 2004 Asia
7. 2009 Samoa

 

The Boxing Day 2004, Asian tsunami

Early in the morning of 26 December 2004, a large magnitude earthquake off the coast of Sumatra generated an Indian Ocean wide tsunami that caused more than 283,000 deaths. It was the world’s largest death toll of any recorded tsunami. This focussed global attention on the tsunami hazard and the need for warning systems other than in the Pacific Ocean. What have we learnt from this event? The following represents the latest information we have available.

The major earthquake that generated the tsunami was located 22 km off the western coast of northern Sumatra and had a seismic moment magnitude (Mw) between 9.1 and 9.3 (equating to magnitude 9.0 on the Richter scale). This was the largest seismic event on Earth in more than 40 years. It was the fifth largest earthquake ever recorded. The resultant faulting offset the land by 15 m near Banda Aceh, but northwards towards the Nicobar and Andaman Islands the rapid offset was smaller. However, tsunami and geodetic measurements indicate further slow slip occurred in the north for over another 50 minutes or longer.

The earthquake was obviously triggered by subduction of the Indo-Australian Plate beneath the Andaman Microplate, along a 1300 km long megathrust off the west coast of Sumatra. A further earthquake with an Mw = 8.6 occurred on 28 March 2005, off the island of Nias, and involved a further 300-km-long rupture of the plate boundary to the south.

Clearly the large tsunami that was generated formed by displacement of the sea floor along the plate boundary. Because the plate boundary is oriented north-south, so the greatest strength of the tsunamis was in an east-west direction. Once formed, the tsunami spread out across the Indian Ocean at speeds of 500 km per hour. In Indonesia the tsunami travelled up to 12 km inland, and in Sri Lanka, Thailand and India, waves reached 4 km inland. The maximum height of the tsunami at the northern tip of Sumatra, near Banda Aceh, was almost 35 m (some sources suggest up to 49 m high). Even in the Maldives of the central Indian Ocean, the waves were 3 to 4 m high, yet the maximum land height is only 1.5 m above sea level.

Several web sites model the spread of the tsunami, even to New Zealand!

First visit:

http://walrus.wr.usgs.gov/tsunami/indianocean.html

This site shows the tsunami waves generated during the first 30 minutes in the Indian Ocean.

Then visit:

http://www.bom.gov.au/info/tsunami/tsunami_info.shtml and
http://www.gns.cri.nz/what/earthact/tsunami

Here you can track the spread of the tsunami across the Indian Ocean and around to New Zealand and the Pacific.

To comprehend the scale of the devastation, visit the following site that gives an aerial photographic record of the damage in Banda Aceh:

http://www.unep.org/tsunami/Banda_Aceh_Tsunami_Damage.pdf

Search the other parts of the website dealing with the 2004 tsunami at:

http://www.unep.org/Tsunami

Here you can read about the environmental response to the tsunami, situation reports, and see more maps and graphics. Click on to ‘Information Materials’ and look through the ‘Photo Gallery’.

Then visit the site:

http://www.asiantsunamivideos.com

Here you can select from a wide range of video clips of the tsunami around the Indian Ocean, recording the impact of the tsunami beyond Indonesia.

To gain an understanding of the scientific background to the tsunami disaster, visit:

http://iri.columbia.edu/~lareef/tsunami/

Here you can scan short articles, maps and diagrams explaining what happened. You do not need to read the sections on the 1883 and 1941 earthquakes in the region.

To learn of the impacts of the tsunami visit:

http://www.coe-dmha.org/Tsunami/Tsu122905.htm

For a general account of this catastrophe go to: www3.interscience.wiley.com/cgi-bin/fulltext/118652096/PDFSTART and read the article “The Asian Tsunami Disaster: December 2004” by Tony Waltham in Geology Today 21: 22-26.

View the accompanying video of the tsunami arriving along the Thailand coast.

Samoan 2009 tsunami

On the 29 September 2009 the Samoan region was struck by a Mw 8.1 earthquake which generated a tsunami up to 14 m high in southern Samoa.

Click here to view the accompanying photos immediately after the event at Lalomanu, on the south coast of Samoa, by one of our former students who was on the scene.  Courtesy of W. J. Morrell.”