Two previously unknown active faults were found off the Boso Peninsula in Chiba Prefecture, with one researcher warning that a jolt in the two faults at the same time could trigger an earthquake of magnitude 8 to 9.
The two faults, one at least 160 km long and the other more than 300 km, were found on the floor of the Pacific Ocean around 100 to 200 km southeast of the southern tip of the peninsula, according to a group of researchers from Hiroshima University, Nagoya University, the Japan Agency for Marine-Earth Science and Technology and other parties.
“The faults have been unmarked and uninvestigated. There is a possibility of strong jolts and tsunami reaching the southern Kanto region (including Tokyo) and the Tokai region (central Honshu). It should be promptly investigated in detail,” said research group member Mitsuhisa Watanabe, a professor at Toyo University.
The group will report its findings at a Thursday meeting of the Association of Japanese Geographers in Tokyo.
The group used a bathymetric chart made by the Japan Coast Guard to analyze the geography of the seafloor in detail. It then estimated the location of the active faults by taking into consideration cliffs formed by earthquakes and other elevated features.
According to Watanabe, the two faults were found near a “triple junction,” a point where the boundaries of two oceanic plates and a continental plate meet.
Both north-south faults run parallel. The longer fault to the east has a cliff with a height of more than 2,000 meters, while the other one has a cliff more than 3,000 meters high formed by earthquakes, indicating the high possibility that both have repeatedly caused big quakes, he said.
North of the two faults is a focal region for the 1677 temblor, which had an estimated magnitude of 8.0, and the magnitude 7.4 quake that hit in 1953. But the faults seem unrelated to the two quakes, which were likely caused by movement of another active fault, Watanabe said, adding the past movements of the two faults remain unknown.
Active faults have previously been seen as having little connection to earthquakes that occur near ocean trenches. But the same group confirmed last year the existence of a 500-km active fault on the ocean floor along the Japan Trench, which is believed to have moved when the massive earthquake and tsunami disaster hit the Tohoku region last March.