April 18, 2016 8:20 pm JST

Unusual quake cluster worries Japan

TOKYO -- Seismic activity in southern Japan is mystifying geologists and keeping the nation on edge.

     The island of Kyushu has been struck by a series of significant earthquakes, with the epicenters moving progressively further inland. The cluster started with the deadly quakes that hit Kumamoto Prefecture last Thursday and Saturday. Temblors subsequently rocked the Mount Aso region and neighboring Oita Prefecture. 

     There is a known concentration of faults in the area. Still, experts say it is highly unusual to have a string of quakes measuring around magnitude 6 and stretching over such a vast area. The epicenter of the Oita jolt was about 100km away from the first Kumamoto quake.

     "I don't quite understand what is happening with the recent earthquakes, because it's an unfamiliar phenomenon," said Yoshihisa Iio, a professor at Kyoto University's Research Center for Earthquake Prediction.

     The Japan Meteorological Agency said it is unprecedented to have a group of large quakes in these three parts of Kyushu. Experts are divided over how far the shaking will spread and whether it could prompt more quakes centered elsewhere.

Linked faults

The Beppu-Shimabara graben -- a type of geological formation -- stretches east to west across Kyushu, through Oita and Kumamoto prefectures. A number of faults run underground. Scientists believe such concentrations of faults increase the chances of what they call earthquake swarms. When one fault shifts, causing an earthquake, it can add to the strain on other faults, triggering more tremors.

     The government's earthquake research committee attributed the magnitude-6.4 quake that hit Kumamoto last Thursday evening to a shift in the northern part of the Hinagu fault zone. The magnitude-7.3 quake that struck in the wee hours of Saturday morning occurred in the Futagawa fault zone, which runs just north of the Hinagu zone, the Geospatial Information Authority of Japan said.

     Part of the Futagawa fault zone, about 27km in length, slid by around 3.5 meters, according to the GSI.

     The government committee met on Sunday and agreed that the Futagawa zone was the culprit in the main quake. This zone, it turns out, is longer than previously thought and stretches close to Mount Aso's caldera. The committee warned local residents to brace for more aftershocks.

     Indeed, aftershocks continue in the Kumamoto, Aso and Oita regions. According to the Meteorological Agency, Kumamoto has seen the second-highest number of inland earthquakes on record, after those set off by the earthquake that hit the northwestern prefecture of Niigata in 2004.

     Meanwhile, the GSI said the main Kumamoto quake unleashed 40% more seismic energy than the devastating 1995 earthquake. Saturday's quake "may have impacted nearby faults," said Hiroshi Yarai, director of the GSI's crustal deformation research division. Signs point to the quake nudging the Beppu-Haneyama fault zone in Oita, which lies northeast of the Futagawa zone.

     The Beppu-Haneyama zone, in turn, is linked in the east to the Japan Median Tectonic Line -- a huge fault structure that extends through western Japan, including the island of Shikoku and the Kii Peninsula in Wakayama Prefecture. This raises the possibility that the Kumamoto earthquakes could cause a broader chain reaction across the Bungo Channel in the Shikoku region.

     For now, though, a Meteorological Agency official said that "the Median Tectonic Line doesn't seem to have been activated yet." 

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