At 2 a.m. on the 23rd, a magnitude 5.9 earthquake struck southern Ibaraki Prefecture, Japan.
Although the epicenter was in southern Ibaraki Prefecture, strong shaking spread as far as Tokyo, Saitama, and Chiba, and it had been 5 years since shaking of at least lower 5 on the Japanese seismic intensity scale was observed in Tokyo’s 23 Wards.
Shaking was also detected in Miyagi and Shizuoka prefectures. Why did such a wide area shake?
3-Line Summary
1. The magnitude 5.9 earthquake in Ibaraki shook a wide area because its source was deep
2. The focus was about 70㎞ deep, and lower 5 intensity in Tokyo occurred for the first time in 5 years
3. The places to check are not the news but the Japan Meteorological Agency and railway service information
It struck southern Ibaraki Prefecture at 2 a.m.
At 2 a.m. on the 23rd, a magnitude 5.9 earthquake occurred in southern Ibaraki Prefecture, Japan. The Japan Meteorological Agency estimated the depth of the focus at about 70㎞. According to a Financial News report, shaking of lower 5 was observed in 17 cities, wards, and towns, including Ibaraki, Saitama, and Chiba prefectures, as well as Adachi Ward in Tokyo. There was no tsunami.
Lower 5 is a level at which most people feel fear, and dishes on shelves or books on bookcases may fall. Intensity 4, one level lower, was recorded across a wide area of the Tokyo metropolitan region, including Tokyo’s Chiyoda and Setagaya wards, Yokohama, and Saitama. It had been 5 years since shaking of at least lower 5 was observed in Tokyo’s 23 Wards, following the earthquake in northwestern Chiba Prefecture in October 2021. Shaking reached Miyagi, Niigata, Nagano, and Shizuoka prefectures at intensity 3.
22 people injured; no abnormalities at nuclear plants
According to a Financial News report, 22 people were injured in the Kanto region: 9 in Saitama Prefecture, 8 in Kanagawa Prefecture, 4 in Chiba Prefecture, and 1 in Ibaraki Prefecture. Most injuries involved people falling, falling from beds or stairs, or being cut by broken glass. In Yokohama, a woman in her 80s suffered a serious back injury after falling from her bed.
Damage to facilities also followed. At the Ushiku City government office in Ibaraki Prefecture, part of the ceiling, approximately 50㎝ wide and 2m long, fell, but no one was injured. In Tokyo’s Koto Ward, water flowed onto the road after a water pipe burst, and some areas of Adachi Ward experienced a water outage. Train services stopped on sections of the Joban Line, the Utsunomiya and Takasaki lines, and the Musashino Line, while the Shinkansen also experienced a temporary power outage. However, no abnormalities were confirmed at Fukushima Daiichi and Daini nuclear power plants or Ibaraki Prefecture’s Tokai Daini Nuclear Power Plant.
Deeper than its magnitude: It occurred about 70㎞ below ground
The point experts highlighted in this earthquake was not its magnitude but the depth of the focus. Professor Shinichi Sakai of the University of Tokyo’s Earthquake Research Institute told NHK that “southern Ibaraki Prefecture is an area where earthquakes occur frequently” and explained that “because this earthquake occurred at a relatively deep location, it appears that strong shaking was transmitted across a wide area.” In other words, the shaking that began deep underground spread sideways instead of being concentrated only around the epicenter.
Southern Ibaraki Prefecture also experienced earthquakes accompanied by a maximum intensity of lower 5 in April and June this year. This time, long-period ground-motion class 2, which causes high-rise buildings to sway slowly and for a long time, was also observed in Tokyo’s 23 Wards, Saitama Prefecture, and Chiba Prefecture. At this level, it is difficult to walk without holding onto something, and objects on shelves may fall. This is why shaking can feel stronger and last longer on upper floors, even in the same building.
Aftershocks for a week, compounded by wet ground
The Japan Meteorological Agency urged people to prepare for aftershocks reaching a maximum intensity of lower 5 over the next week, particularly during the next 2~3 days. Aftershocks are not the only concern. According to a Herald Economy report, rainfall exceeding 120㎜ per hour fell in Tokyo’s Itabashi Ward and elsewhere the previous day, while ground in parts of Chiba Prefecture had already weakened because of record heavy rain. The agency warned that when earthquakes strike weakened ground, they can lead to landslides or earth-and-sand collapses.
Prime Minister Sanae Takaichi instructed relevant ministries and agencies to quickly assess the damage, and a contact office for the Crisis Management Center was established at the Prime Minister’s Office. Prime Minister Takaichi said on social media, “Please continue to remain alert to the possibility of another earthquake of a similar magnitude.”
What should you check if you are there now?
By 6:30 a.m., the Tokyo Fire Department had received 39 rescue and emergency-dispatch calls, many reporting that people were trapped in elevators or unable to leave rooms because refrigerators and wardrobes had toppled over. This means that actual damage resulted more from people falling and becoming trapped indoors than from the earthquake itself. During the several days when aftershocks are expected, it is safer not to place heavy objects beside the bed or near exits, and to take the stairs instead of an elevator when you feel shaking.
If you have travel plans, there are specific places to check before departing. Because trains repeatedly stop and resume service by route, it is quicker to check the relevant line directly through JR East Train Status Information. Aftershock and heavy-rain alerts are consolidated at Japan Meteorological Agency Disaster Information. If your accommodation is in a high-rise building, it is also worth considering that long-period ground motion may cause it to shake longer than lower floors.
The number to note in this earthquake was not the magnitude 5.9 but the focus depth of about 70㎞. Even at the same intensity, the range of shaking varies depending on where an earthquake occurs, and this time the entire Tokyo metropolitan region fell within that range. Over the next several days, how wet the ground beneath your feet is may be a more important factor than news about aftershocks.
References
- Financial News, eToday, Herald Economy, Hans Economy, Asia Today, Digital Times, Gyeongbuk Maeil
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