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More substantial inland quakes could follow Osaka tremor: experts

(Mainichi)
(Mainichi)

"That area is a concentrated-stress zone where geological pressure easily builds up underground," points out Kyoto University associate professor Takuya Nishimura, a geodesy specialist.

The earthquake with a magnitude of 6.1 occurred in the northern part of Osaka Prefecture on the morning of June 18 appears to have been the simultaneous movements of faults.

One was the east-west horizontal movement of a strike-slip fault, while the other a reverse dip-slip fault which shifts in vertical space. The epicenter of the quake was south of the active Arima-Takatsuki fault zone (roughly 55 kilometers long) that runs east-west along the north of the prefecture.

The eastern portion of the Arima-Takatsuki fault zone is believed to have seen activity three times over the last 3,000 years. In 1596, the area was shaken by the Keicho Fushimi earthquake with an estimated magnitude of 7.5. The government Headquarters for Earthquake Research Promotion calculated the possibility of a 7.5-maginitude tremor occurring along that fault line within the next 30 years as "almost 0 to 0.03 percent."

At a meeting of the headquarter's Earthquake Research Committee on June 18, a fault line along which the Osaka quake occurred was not identified. This was because a magnitude of 6.1 is relatively small, and the displacement of the fault at the epicenter did not appear on the surface.

In fact, around the epicenter of the tremor, various active fault zones are all concentrated, including the roughly 42-kilometer Uemachi fault zone cutting through Osaka Prefecture north to south, the approximately 71-kilometer Rokko-Awaji Shima fault zone stretching all the way from the north of Osaka Prefecture to Awaji Island in Hyogo Prefecture, including the epicenter of the 1995 Great Hanshin Earthquake, the Ikoma fault zone, running some 38 kilometers north-south through the west of the area around Mount Ikoma and others.

Water pipes that ruptured due to the earthquake fill a hole in the road with water in Takatsuki, Osaka Prefecture, on June 18, 2018. (Mainichi)

"Earthquakes with magnitudes registering in the sixes can occur all across Japan, but especially in the Kansai region, it's no surprise when one occurs," says Nishimura. Tohoku University professor Shinji Tooda, a researcher of earthquake geology, also called for caution. He cited the example of the 2016 Kumamoto Earthquake, when a foreshock was followed by the main quake two days later, creating even more destruction. "This earthquake could be the spark for activity along other fault lines in the area," he said.

Amid talk of an approaching quake along the Nankai Trough, there are experts that consider that the activity has reached the stage where inland earthquakes will begin to increase.

The Nankai Trough runs from Tsuruga Bay in Shizuoka Prefecture in the north all the way to the open sea off the coast of Japan's southern island of Kyushu. In the last 90 to 150 years, ocean trench earthquakes with magnitudes in the eight range have occurred along with tsunami. According to the Japan Meteorology Agency, since the 1944 Tonankai earthquake, which registered a maximum of 6 on the Japanese 7-point seismic intensity scale, there have been 58 earthquakes around Japan that were a lower 6 or higher on the scale, including the June 18 tremor. Of those, 50 have occurred following the Great Hanshin Earthquake in 1995.

"As the Philippine Sea Plate continues to slide under the Eurasian Plate where the Japanese archipelago is located, the pressure on inland areas of Japan increases, and it is believed that this will lead to an increase in earthquakes along active fault lines," explains Yasuhiro Umeda, professor emeritus of seismology at Kyoto University.

"We can't say definitively at this point if the (Osaka) earthquake this time is directly related to an earthquake along the Nankai Trough," says University of Tokyo Earthquake Research Institute professor and head of the Division of Disaster Mitigation Science Takashi Furumura. "The epicenter was shallow at 13 kilometers. There is a chance that aftershocks will occur, so caution is necessary."

(Japanese original by Shimpei Torii, Ryo Watanabe and Koki Matsumoto, Osaka Science & Environment News Department)

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