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Experts: Soft soil added to intense shaking of quake on Osaka plain

(Mainichi)

OSAKA -- The soft soil beneath the Osaka Plain may have been responsible for the extreme shaking felt in the area when a magnitude 6.1 earthquake hit northern Osaka Prefecture and the surrounding areas on June 18, experts say.

The epicenter of the latest quake was south of the Arima-Takatsuki fault zone and about 13 kilometers below the surface -- a relatively shallow depth. The amount of energy released by the earthquake was well below one-thirtieth of the magnitude 7.3 1995 Great Hanshin Earthquake, but the seismic intensity recorded near the epicenter was a lower 6 on the 7-point Japanese scale near the epicenter. An intensity of 5 was also recorded in many locations, including nearby southern Kyoto Prefecture and southeastern Hyogo Prefecture. The observation has serious implications for other densely populated major cities in Japan, such as Tokyo and Nagoya, which also sit on soft-soil plains created by rivers.

Naoshi Hirata, who heads the government's Earthquake Research Committee, told a press conference on June 18 that seismic intensities tend to increase in areas with thick deposits of sediment, that is, soil. "Unfortunately, that was the case with the latest quake," Hirata said.

The Osaka Plain, where the intense shaking was observed, was formed in part by sediments from the Yodo River and its three tributaries -- the Katsura, Kizu and Uji rivers -- over the last 6,000 years. Records of the magnitude 7.5 Keicho Fushimi earthquake of 1596 indicate that the present-day prefectures of Osaka, Kyoto, Hyogo and Nara suffered major damage.

Professor Hiroshi Sato of the University of Tokyo's Earthquake Research Institute, a specialist in structural geology, explained that soil conditions are a major factor in affecting seismic intensities when an earthquake is triggered by the movement of active faults at a depth of around 10 kilometers. "Plains at the mouth of rivers tend to have layers of deposited clay, and when water is mixed with such soil, it amplifies earthquake tremors and extends the oscillation time," Sato explained.

Japan has a number of plains where major damage was caused by earthquakes, such as the Kanto Plain, where Tokyo is located, and the Nobi Plain, on which Nagoya sits. In the case of the Kanto Plain, the magnitude 7.0-7.1 Ansei Edo earthquake of 1855 and the magnitude 7.9 Great Kanto Earthquake of September 1923 triggered massive structural damage and loss of human lives in areas with soft soil, such as near a river mouth or Tokyo Bay.

Itsuki Nakabayashi, professor emeritus at Tokyo Metropolitan University, who is well versed in urban disaster prevention, called for better preparation for the possibility of a major earthquake hitting a metropolitan area.

"A large number of casualties can result when disasters occur in cities, even when structural damage is limited," explained Nakabayashi. "Recovery can take time in metropolitan areas due to the disruption of lifelines and distribution systems."

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

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