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Simultaneous Kyushu quakes feared to trigger temblors in Shikoku

A wide swathe of farmland is seen after being ripped open by the Kumamoto earthquakes, on April 16, 2016, in Minamiaso, Kumamoto Prefecture. (Mainichi)

A series of earthquakes that have occurred in three regions of Kyushu are feared to trigger temblors in Shikoku and other regions, say experts.

A magnitude-6.5 earthquake on the night of April 14, which measured 7 on the 7-point Japanese intensity scale in Mashiki, Kumamoto Prefecture, turned out to be a prelude to a larger temblor. In the early hours of April 16, an M-7.3 earthquake, whose energy was about 16 times that of the previous one, occurred, causing further damage.

The quakes that began in the Kumamoto district of Kumamoto Prefecture are believed to have affected faults in other areas, triggering temblors in the prefectural village of Minamiaso as well as in neighboring Oita Prefecture.

"When a large-scale earthquake strikes, it sometimes triggers temblors in other areas. As the seismic activity is intensifying in both the Kumamoto and Oita regions, they are apparently stimulating each other," says Atsumasa Okada, professor emeritus of tectonic geomorphology at Kyoto University.

The Beppu-Shimabara rift zone, which consists of multiple active faults, lies in central Kyushu where the earthquakes are currently occurring. As such, areas from Beppu Bay to the Yatsushiro Sea have drawn attention from seismologists as quake-prone areas.

The M-6.5 first quake that struck at 9:26 p.m. on April 14 occurred at the northern tip of the Hinagu fault zone, where it crosses the Futagawa fault zone.

"The northern tip of the fault zone was hard to break, and prevented activity in the Futagawa fault zone. I think the northern tip broke and became unable to stop the activity of the Futagawa fault zone, allowing it to break," says Hiroshi Sato, professor of structural geology at the University of Tokyo Earthquake Research Institute, as he explains the possible cause of the main quake that struck at 1:25 a.m. on April 16.

Gen Aoki, director of the Japan Meteorological Agency Earthquake and Tsunami Observations Division, has pointed to the possibility that the main quake has triggered seismic activity in surrounding areas.

Okada says, "There is a possibility that earthquakes will occur through mutual reactions between seismic activities in different regions, and attention should be paid to this activity."

Earthquakes are spreading northeast, and the Median Tectonic Line fault zone -- the largest-class fault zone in Japan which stretches from Shikoku to the Kinki regions -- is an extension of this line. Noting that the seismic activity in Kyushu could stimulate the Median Tectonic Line, some experts underscore the need to exercise caution.

The government's Headquarters for Earthquake Research Promotion recognizes an about 360-kilometer-long zone extending from Nara to Ehime prefectures as the Median Tectonic Line, and is analyzing the possibility that the zone will trigger earthquakes.

However, as many researchers regard active faults that stretch all the way into Kyushu as part of the Median Tectonic Line, the seismic activity in Kyushu could have further repercussions. The Ikata Nuclear Power Plant in Ehime Prefecture is situated south of this zone.

Yoshinobu Tsuji, a special visiting researcher at the Building Research Institute, points out that the latest quakes occurred when part of the Median Tectonic Line moved.

Historical records show that large-scale earthquakes repeatedly occurred around the Median Tectonic Line over a short period in 1596. The 1586 Tensho earthquake that caused serious damage to extensive areas in the Kinki and Chubu regions is an example of a temblor that had repercussions on surrounding areas. Following the March 11, 2011 Great East Japan Earthquake, an earthquake struck Nagano Prefecture the next day and an area around Mount Fuji in Shizuoka Prefecture on March 15.

"The latest earthquake could trigger relatively large earthquakes in the Bungo Channel that separates Kyushu and Shikoku. People in these areas, including Shikoku, should exercise caution," says Tsuji.

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