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Japan 'slow earthquake' zones may curb massive damage: researchers

(Mainichi)

KYOTO -- Slow earthquakes, in which faults slip more slowly than standard earthquakes and do not cause ground shaking, may act as a barrier against major fault ruptures and curb massive damage, new research suggests.

Conversely, slow earthquakes are rarely seen in areas where major earthquakes have occurred, researchers including members of Kyoto University's Disaster Prevention Research Institute found.

The team, whose findings were published in the Aug. 23 online edition of the American journal Science, noted that fault ruptures had stopped expanding in an area in the Pacific Ocean off eastern Japan prone to "slow earthquakes" during the 2011 Great East Japan Earthquake. In the Japan Trench off the coast of eastern Japan, the oceanic Pacific plate is subducted under the continental plate, building up tension. The massive 2011 earthquake occurred when this built-up tension caused a fault rupture.

The phenomenon of slow earthquakes was discovered around 20 years ago. In slow earthquakes, plates slip at a speed of about one-10 millionth of a standard earthquake. They do not cause ground shaking, but have attracted researchers' attention for their relationship to the mechanism of major earthquakes.

The research team mapped the distribution of slow earthquakes with data from the Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S-net), which was installed in 2016, and data collected between 1991 and 2018 by the microearthquake network at the University of Tokyo and elsewhere.

The researchers found slow earthquakes had been distributed in a wide area running north to south outside of an epicentral area where fault ruptures occurred in the 2011 earthquake. The epicentral area stretches approximately 450 kilometers from north to south, and approximately 200 kilometers from east to west. Almost no slow earthquakes had taken place within this area. The researchers' findings indicate that fault ruptures were limited in areas where there were large numbers of slow earthquakes. Had the ruptures not ceased, the size of the 2011 earthquake may have exceeded magnitude 9, the researchers say.

It also emerged that other earthquakes with a magnitude of 7 and over that occurred along the Japan Trench since 1930 did not take place in areas that had high numbers of slow earthquakes. The research team says that areas with a high distribution of slow earthquakes may possibly act as a barrier preventing the spread of fault ruptures, and points out that the geological conditions of areas where slow earthquakes occur and those where major earthquakes occur may be different.

(Japanese original by Ryo Watanabe, Osaka Science & Environment News)

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