14 DAY TRIAL // Japan is a country well-known for its frequent ground motions, almost one every week, and the inhabitants of the The Land of the Rising Sun have got used to this phenomenon.
But what nobody knew was that the fault responsible for the earthquakes is much closer to the surface then it was thought. A study published in the Science journal revealed that that the Tokyo-Yokohama area is sitting on three tectonic plates found at a depth ranging from 4 to 26 kilometers, not at 40 kilometers as it was originally thought l.
According to National Geographic, temblors nearer the surface are more likely to generate strong ground motion, which raises the potential for damage. Japan's deadliest quake, a 7.9 magnitude temblor in 1923, and subsequent fires, killed 143,000 people and razed Yokohama and most of central Tokyo. A quake in October last year that killed 40 in Niigata, north of Tokyo, originated about 10 kilometers below the surface and swayed buildings in central Tokyo.
The four-year study published in Science was funded by the Japanese government. It was conducted by Sato and his colleagues at the Earthquake Research Institute at the University of Tokyo. He said the fault zone between two key tectonic plates just south of Tokyo seems to be rising to the surface.
Ground motion is difficult to predict, Sato said. For the study, the team used air guns and explosives to send seismic signals into the earth, then read them with seismic reflection equipment from the University of Texas at El Paso, modified it to send slower waves to the fault, which when returned, created a clear image.
"The shallow point is the source where seismic waves are titillated," Hiroshi Sato, lead author of the study which will be presented to the public later today in Tokyo, said in an interview. "This is not good news for people in the metropolitan area."