A new survey suggests the fault line beneath Tokyo is miles closer to the surface than seismologists realised
Hiroshi Sato of the University of Tokyo's Earthquake Research Institute sent vibrations into the ground using explosives and then examined the pattern of the waves that bounced back to determine the composition of the sub-surface rocks.
The seismologists determined the fault line is between two miles and 16 miles below Tokyo. Previous estimates placed the fault line depth at 12-to-25 miles.
The new findings might mean an earthquake occurring at the fault line might cause more extensive damage than previously thought.
Shinji Toda, a Japanese geologist, has discovered a new tectonic plate under the Tokyo area, a finding that may force the government to review its quake preparation plans. Japan endures 20 per cent of the world's powerful earthquakes as the country lies at the crossing of four tectonic plates -- the Eurasian, North American, Philippine and Pacific plates. If a quake measuring 7.3 on the Richter scale were to hit the Tokyo region in the evening rush hour, it would kill 13,000 people and inflict more than $1 trillion in damage, according to a government estimate released in February. The geological structure of the Kanto region, which includes Tokyo, is complex with the interaction of the Philippine, Pacific and Eurasian plates, and the portion of a plate believed to be part of the Philippine plate now seems to be an independent tectonic plate.
Toda, chief researcher at the Active Fault Research Centre of the National Institute of Advanced Industrial Science and Technology, said he analysed data on 150,000 quakes with magnitudes above 2.0 between 1979 and 2004 in the Kanto region. If Toda's findings are confirmed, Japan may need to review its earthquake-related policies around Tokyo, since such polices are made on the assumption the Philippine plate is a single tectonic plate. "We need to fundamentally review our understanding of earthquake mechanisms, including the structure of tectonic plates." - Shinji Toda. The previous big quake that hit the Kanto region occurred in 1923, leaving 142,807 people dead or unaccounted for. In January 17, 1995, the western Japanese city of Kobe experienced a pre-dawn tremor, registering 7.3 on the Richter scale, which killed 6,433 people. The Kobe quake was the most devastating in modern times to hit a city in the developed world.
