14 July 2005Michael Hopkin
The fault line that runs beneath Tokyo is kilometres closer to the surface than seismologists realized, according to a new survey. That could be worrying news to residents waiting for the next 'big one' to hit the city, as quakes occurring at a shallow depth tend to cause more shaking and damage to buildings than deeper ones. Tokyo is among the most densely populated earthquake zones in the world, with some 33 million people living and working in the conurbation. In 1923, around 105,000 people in the city were killed by a huge magnitude 7.9 quake.Occasional quakes continue to plague the city, thanks to the fact that it sits on a fault line where two rocky plates meet: one holds Honshu Island, while another - the Philippine Sea plate - slides underneath it.
Ready for the jolt
But knowing more about the fault will certainly help in predicting the effects of future quakes, Sato adds. "Before our survey, in spite of the sophisticated monitoring system, the basic structure was not well determined," he says. "Now we know its precise geometry, we will be able to better analyse its behaviour."The study may even help to predict the timing of the next large jolt, Sato hopes. By calculating how far the fault slipped in the 1923 event, and comparing this to the Philippine plate's average speed of advance and rate of stress build-up, researchers could estimate the average frequency of quakes."We are advancing step by step," Sato says.
http://www.nature.com/news/2005/050711/full/050711-10.html
