'Slow tremor' off Japan could trigger massive Nankai Trough quake: experts

Nankai Trough (Mainichi) Earthquake Assessment Area

TOKYO – Following a powerful earthquake off the coast of Miyazaki Prefecture in southwestern Japan on Aug. 8, which triggered the country’s first major earthquake warning, the possibility of a major quake centered in the Nankai Trough hitting the Japanese archipelago is a major concern.

“There is no data indicating an anomaly at the plate boundary at this time,” the government’s earthquake research committee says. But something noteworthy has been observed: A few hours after the August 8 quake, intermittent shaking, alternating between weak and strong, began in the shallow waters of the Hyuga Nada Sea and has continued ever since.

This phenomenon, known as a “slow earthquake,” occurs when faults slip at a slower rate than a normal earthquake. The movement itself does not cause significant shaking, but the process releases the same amount of tectonic stress as a normal earthquake.

A type of slow earthquake known as a slow-slip occurred after a previous magnitude 7.3 earthquake on March 9, 2011, the Great East Japan Earthquake of 2011, and is believed to have been linked to the massive magnitude 9.0 earthquake that struck the area two days later.

Along the Nankai Trough, a slow earthquake will occur in the area where the boundary between the continental plate and the Philippine Sea plate does not adhere strongly.

Slow earthquake activity has been observed at a shallow site on the plate boundary since Aug. 8 and is currently increasing, said Yusuke Yamashita, associate professor of seismology at the Disaster Prevention Research Institute at Kyoto University.

The slow seismic activity detected by Yamashita included two types of weak vibrations: low-wavelength microseisms, with cycles every 0.1 to 0.5 seconds, and very low-frequency quakes every 10 to 20 seconds. Pinpointing the exact location of the activity would require seismometers installed on the seafloor, but it is believed to be occurring in an area with a radius of about 100 kilometers on the ocean floor at least 50 kilometers southeast of the coast of Miyazaki Prefecture.

Since slow earthquakes can be triggered by magnitude 7 earthquakes, which occur in the Hyuga Nada Sea once every few years, the current phenomenon cannot be immediately described as an anomaly. However, Yamashita warns that interactions with slow earthquakes could lead to more powerful quakes.

Major earthquakes have struck the southern part of the Hyuga Nada Sea about every 30 years, with the most recent being magnitude 6.9 and -6.7 quakes in October and December 1996. Before that, there was a magnitude 7 quake in February 1961. It is believed that a magnitude 8 quake in October 1662 may have triggered a tsunami about 4 to 6 meters high in the Miyazaki Plains, where the current capital of Miyazaki Prefecture is located.

“The remnants of the Aug. 8 earthquake alone could cause a magnitude-7 earthquake, and through interaction with a slow earthquake it could turn into a magnitude-8 earthquake. The high activity of slow earthquakes indicates that the plate boundaries are still unstable, and it will take about a month for the boundaries to stabilize,” Yamashita said.

According to Takuya Nishimura, a professor of geodesy at the same institute, slow earthquakes do not usually cause major earthquakes because stress is released when they occur. However, they are said to cause larger earthquakes, such as what was observed in the massive 2011 earthquake. Nishimura stressed the importance of monitoring, noting that “the trends of slow earthquakes provide information that helps determine whether they will lead to a major earthquake.”

(Japanese original by Yurika Tarumi, Lifestyle, Science & Environment News Department)

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