At 2.46 pm local time on Friday last week, Japan shook like never before. The magnitude 9.0 earthquake wrenched the main island of Honshu 2.5 metres closer to the US and nudged the tilt of Earth's axis by 16 centimetres. At the epicentre, 130 kilometres offshore, the Pacific tectonic plate lurched westwards, and a 10-metre-high tsunami sped towards the coastal city of Sendai and the surrounding region.
The devastation caused by the events is difficult to exaggerate - estimates suggest the number of fatalities could top 10,000. One of the few consolations is that quakes of magnitude 8.5 and above are rare: the Sendai earthquake is in the top 10 of the highest-magnitude quakes of the last 100 years.
Yet three of these - the December 2004 Sumatra quake, the February 2010 Chile quake, and now Sendai - have struck in just over six years. This presents a horrifying possibility: that there is a link between these megaquakes and that, as a result, more could strike.
Most geologists say that the number of megaquakes is too small to be able to make a statistically convincing case for a link. "You will get a lot of different answers from different people, but inevitably the ability of any one of those to convince everyone else that they're right is going to depend on the statistics of small numbers," says Ross Stein of the US Geological Survey (USGS) in Menlo Park, California, "and we're never going to get anywhere."
A handful, however, feel there must be a link between recent events. "What is clear is that for the 6.2 years since 2004, there have been more great earthquakes around the world than in any 6.2-year period throughout the 110-year history of seismic recordings," says Thorne Lay at the University of California, Santa Cruz. His colleague Emily Brodsky goes further: "The recent spurt of magnitude-8-plus earthquakes may be an extended aftershock sequence of the 2004 Sumatra earthquake."
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