News M7.1 quake hits Miyazaki, megaquake alert declared

The agency said it has started investigating whether there is a link between the temblor and a potential massive trench-type earthquake along the Nankai Trough stretching from off western to central Japan. The tsunami advisories were issued for Kochi, Ehime, Oita, Miyazaki and Kagoshima prefectures. Tsunami with heights of around 50 centimeters and 20 cm was observed in Miyazaki and southwestern Kochi, respectively, the agency said.

Click to expand...

In 1999, the likelihood of the occurrence of a great earthquake in the Tokai area in the 2000-2010 period was estimated to be in the range of 0.35–0.45. Despite the uncertainty of when such an earthquake will occur, local authorities are already taking action to prepare residents for what they regard as an inevitability. The Japanese government estimates that a major earthquake on the Nankai Trough would cause 169.5 trillion yen in direct damage and 50.8 trillion yen in economic losses for the following year. A study by the Japan Society of Civil Engineers in 2018 estimated that the long-term damage from the earthquake could result in 1,240 trillion yen in economic losses over a 20-year period. It is predicted that the economic damage is likely to be 10 times higher than for the 2011 Tōhoku earthquake and tsunami. A death toll as high as 230,000 has been suggested for such an event.

Click to expand...

Information from the agency on Nankai Trough megaquakes is delivered at two levels — an alert and a higher warning. The weather agency issued the lower level alert on Thursday, urging people to be prepared for cases requiring evacuation. No evacuations are required for the alert level. A warning means people would need to be on higher alert, and officials would urge those who wouldn't be able to evacuate quickly to do so before any large quake occurs.

Click to expand...

If a megaquake occurs in the area, the damage is expected to be devastating. For example, the height of a tsunami triggered by the earthquake could exceed 30 meters in areas on the Pacific Ocean side near the epicenter. The JMA called for residents to be prepared for possible earthquakes for at least one week. A trough is a seabed trench at the boundary between two tectonic plates. The Nankai Trough stretches around 700 kilometers from Suruga Bay off Shizuoka Prefecture to the Hyuganada Sea off Miyazaki Prefecture.

Click to expand...

A type of slow earthquake known as a slow-slip occurred after a March 9, 2011, magnitude-7.3 foreshock to the 2011 Great East Japan Earthquake, and was thought to have been related to that magnitude-9 megaquake two days later. Along the Nankai Trough, a slow earthquake will occur in the region where the boundary between the continental and the Philippine Sea plates are not firmly attached. According to assistant professor of seismology Yusuke Yamashita at Kyoto University's Disaster Prevention Research Institute Miyazaki Observatory, slow quake activity has been detected at a shallow location on the boundary of the plates since Aug. 8 and is currently picking up. The slow earthquake activity detected by Yamashita has included two types of weak shaking: low wavelength micromotion, with cycles of every 0.1 to 0.5 seconds, and ultra-low frequency earthquake every 10 to 20 seconds. To determine the exact location of the activity will require the installation of seismometers on the sea floor, but it is believed to be occurring in a roughly 100-kilometer radius area on the ocean floor at least 50 kilometers southeast of the Miyazaki Prefecture coast.

Click to expand...