As the world remembers the Boxing Day tsunami of 2004 on its tenth anniversary, it’s worth looking behind the devastation at the earthquake that caused it and reflecting on the possibility of such an event occurring again.
Just three months prior to the event, in September 2004, an article in the Geoscience Australia’s newsletter raised the issue of potential damage resulting from an Indian Ocean earthquake and ensuing tsunami, noting: “There is an international tsunami warning system for the Pacific Ocean, but none for the Indian Ocean,” and asking: “How vulnerable is Australia to the risk of tsunami, and are we leaving our western coastal communities exposed?”
In the event the scenario which they had considered (a repeat of an 1833 earthquake off Sumatra) came very much more quickly than perhaps expected and didn’t, after all, affect Australia, though it did reach many Indian Ocean coasts.
The Boxing Day Earthquake: Statistics
The bare statistics of the Boxing Day earthquake are breathtaking. At M9.1, it was the third largest tremor recorded since 1900 (effectively, when reliable records began), exceeded only by an M9.5 off Chile in 1960 and an M9.4, the Great Alaska Earthquake of 1964.
Computer modelling reported by the United States Geological Survey indicates the scale of the event: “the fault moved upwards of 15-20 m, and slipped over an area approximately 1300 km long (along-strike) by 150 km wide (in the down-dip direction)”.
It was followed by a spectacular series of aftershocks. The USGS recorded: “over 5,000 events of M 4 or larger, several dozen events of M 6 or larger, and two events between M 7-8” in the two years following the mainshock.
Such large physical movements have significant consequences for human populations. While both the Chilean and Alaskan events also produced tsunamis, their toll of death and damage was tiny by comparison.
Once more we turn to the USGS, which reminds us that in all probability more than a quarter of a million people lost their lives (the exact number is uncertain) and around 1.7 million lost their homes in 14 countries around the Indian Ocean.
“The tsunami,” concludes the USGS, “caused more casualties than any other in recorded history and registered nearly worldwide on tide gauges in the Indian, Pacific and Atlantic Oceans.”
Tectonic Setting and Seismic History
The earthquake occurred just off northern Sumatra, on one of the planet’s major subduction zones. Here the Australian plate is moving against the Sunda plate at the relatively rapid rate of around 57 mm each year (Yeats) and is being forced downwards beneath the island of Sumatra.
This type of convergence produces all of the planet’s great earthquakes and Sumatran earthquakes have the dubious distinction of occupying four out of the 15 top spots in the USGS’ list of major earthquakes since 1900. Three, all at least M8.5, occurred in the northern part of the subduction zone.
Most earthquake lists go back only to the development of reliable instrumental measurement (i.e. the seismometer); but there’s plenty of evidence from written and geological sources (such as corals and sedimentary deposits) for significant and major tremors in the past.
Yeats lists several historic events with estimated magnitudes comparable with these large events from before the 2004 event — in 1797, 1833, 1861, 1881, 1937, 1941 and 1994. Between them, these events have ruptured much of the boundary between the two tectonic plates.
Nor is 2004 the most recent large earthquake. The map shows that it was followed by an M8.6 in early 2005; an M8.5 in September 2007; and a pair of earthquakes at M8.6 and M8.2 on the same day in 2012. While the first of these may have been part of the 2004 aftershock and the latter two were not subduction related, it’s nevertheless clear that the Sumatran margin has long been active and remains so.
Can We Expect the Same Again?
Yeats suggests that the observed incidence of mega-earthquakes off the west coast of Sumatra “may be part of a local cluster of giant earthquakes.” What is certain is that the Sumatran subduction zone has been frequently and dramatically active in both the recent and the more distant past.
Bearing that in mind, we can reasonably expect that it will be similarly active in the future — and we must hope that lessons learned from in the ten years since Boxing Day 2004, in particular the implementation of new tsunami warning systems, will, even though they cannot prevent a repeat of the earthquake and tsunami, help in hazard management and hopefully save many lives.