At 4:27 p.m. on the 28th, a magnitude 7.1 earthquake swept through Kumamoto, Japan, which suffered aftershocks through the night into the early hours of the 29th. In less than a day since an intensity 7 was recorded, tremors strong enough for people to feel continued more than 100 times. The Japan Meteorological Agency said there is a possibility that a larger earthquake could follow and asked people to stay on alert for about a week.
The Japan Meteorological Agency tallied that from 4:27 p.m. on the 28th (local time), when an intensity 7 was observed, to 10:50 p.m. the same day, there were 100 aftershocks. Of these, three exceeded intensity 5-lower. Unlike magnitude, which indicates the absolute strength of an earthquake, intensity is the scale the Japan Meteorological Agency uses to quantify the shaking felt in an area. The agency's intensity scale is divided into 10 levels from 0 to 7. At 5-lower, dishes and books on shelves fall, and most people feel the need to hold on to something.
In Kumamoto, a magnitude 6.5 earthquake at 9:26 p.m. on Apr. 14, 2016, 10 years ago, recorded intensity 7 in the Mashiki-cho area. At 12:03 a.m. the next day, a magnitude 6.4 earthquake followed. And at 1:25 a.m. on the 16th, about 28 hours after the first quake, a stronger magnitude 7.3 quake struck Kumamoto again. At that time, intensity 7 was measured in Mashiki-cho and Nishihara-mura. It was the first time in the agency's observation history that intensity 7 was recorded twice in the same area over two days.
Foreshock, main shock, and aftershock are relative labels applied only after seismic activity ends. It is impossible to know in advance whether the first intensity 7 earthquake that occurred in the afternoon of the 28th is a foreshock or a main shock. In Korea as well, in Sept. 2016 in Gyeongju, a magnitude 5.8 earthquake followed 48 minutes after a magnitude 5.1 quake, leading the earlier quake to be classified as a foreshock.
The site of this earthquake borders a section that the quake 10 years ago did not touch and left behind. In Kumamoto, the Futagawa fault zone extends west of Mount Aso, and from there the Hinagu fault zone branches to the southwest and continues to the Yatsushiro Sea. The 2016 quakes occurred as the Futagawa fault zone and the northern part of the Hinagu fault zone moved in succession. The National Institute of Advanced Industrial Science and Technology in Japan analyzed that as the released stress shifted to the surrounding area, the southern section of the remaining Hinagu fault zone became prone to generating an earthquake.
Uki, the epicenter of this quake, also sits on that Hinagu fault zone line. Sakai Shinichi, a professor at the University of Tokyo's Earthquake Research Institute, told NHK on the 28th that the southern part of this Hinagu fault zone may have moved. However, how much of the accumulated stress the magnitude 7.1 quake released and whether there is room for further movement along adjacent sections will likely be determined after an official assessment by the government's Earthquake Research Committee.
Even if a larger earthquake does not come, it is expected to take considerable time for Kumamoto to recover from the damage from this quake. According to Japan's fire authorities, reports have come in that 12 dwellings collapsed in Kumamoto Prefecture, and collapses were observed at 27 spots on the stone walls of Kumamoto Castle, a key tourist site. In Yatsushiro, where the hardest-hit Aeon Mall is located, part of a bridge deck subsided into the river. Experts said, "Buildings and roads that have cracked once can collapse further even in magnitude 4 to 5-class quakes."
The Korea Meteorological Administration also noted that during the 2016 Kumamoto earthquakes, buildings that withstood the first quake collapsed with the second shaking two days later, and urged people not to approach damaged buildings and cliff areas in regions that experienced strong shaking and to watch out for additional quakes and rain-induced landslides for about a week.