Question: Hi David, I was wondering if you new how big the Earthquake and Tsunami were in Japan and how after shocks occur.

David Pyle answered on 15 Mar 2011:

Hi Jamaicanguy – yes, the earthquake of 11 March is now rated as magnitude 9. This is one of the four largest earthquakes of the past 100 years. It was just a little smaller than the Sumatran earthquake in 2004, which caused the Indian Ocean tsunami.

Earthquake scientists have worked out that the fracture which broke and moved during the earthquake started at a depth of about 25 kilometres below sea level. The part that moved was about 500 kilometres long (that’s about London to Edinburgh), and 100 kilometres wide. The central part of this fault jumped 15 metres during the quake, as the Pacific plate slipped underneath the plate on which Japan sits (the Amur plate). The main event occurred two days after a magnitude 7.2 quake, which happened near by; this earlier quake is now thought to be a ‘foreshock’, as it wasn’t as large as the mainshock. Unfortunately foreshocks can only be recognised as foreshocks with hindsight – it’s just another earthquake. After such a large fault movement, there is some adjustment as rocks which have been broken or stressed during the quake respond. Aftershocks usually happen in about the same place as the fault which broke – which is why there is such a large region affected by aftershocks at the moment. Aftershocks can continue for a long time – certainly up to a year or more for an event of this size, but the number of aftershocks per day will drop steadily with time. They will also tend to get smaller with time: aftershocks are always smaller than the mainshock (by definition), but there may be some aftershocks which are larger than magnitude 7 over the next few months. For more details, have a look here.

The tsunami was also very large, particularly where the run-up occurred along the north-east coast of Honshu. This region has experienced large tsunami before – with a very large event, perhaps of the same size, in about AD 869. There isn’t an easy way to measure tsunami ‘size’, but it was clearly a very significant event, although the effects around the rest of the Pacific ocean were not as damaging as they might have been.