Last Updated on 16 seconds ago by TodayWhy Editorial
At about 2 a.m. on Sunday, August 23, 2026, phones across greater Tokyo lit up with emergency earthquake alerts. Seconds later, a magnitude 5.9 quake centered in southern Ibaraki Prefecture — roughly 60–70 km northeast of central Tokyo and about 70 km deep — sent shaking across the capital region.
The Japan Meteorological Agency recorded intensity “lower 5” on the shindo scale in parts of Ibaraki, Saitama, Chiba and Tokyo’s Adachi Ward. At least 37 people were injured, mostly from falls or collisions with furniture. There was no tsunami risk. Yet high-rise towers more than 60 km from the epicenter kept swaying after the ground-level shaking had already stopped.
Why did this Tokyo earthquake happen where it did, and why did magnitude alone not explain the prolonged sway?
What actually happened in the Tokyo earthquake
The Japan Meteorological Agency recorded the quake at magnitude 5.9, striking at around 2am local time with its epicentre in southern Ibaraki Prefecture, just north of Tokyo. At least 37 people were injured across the region. Shaking reached “lower 5” on Japan’s seven-point shindo intensity scale in Tokyo’s Adachi Ward and across Ibaraki, Saitama and Chiba prefectures — strong enough to knock objects off shelves and send people reaching for something to hold onto. There was no tsunami threat.
Prime Minister Sanae Takaichi posted a warning on social media within an hour, urging residents in areas that felt strong shaking to stay alert for a follow-up earthquake of similar size. That’s a standard precaution after a shallow-to-moderate quake in this region, not a sign that a bigger event is confirmed to be coming — but it reflects how seriously Japanese authorities treat the days immediately following any quake of this size.
Magnitude, shindo, and long-period motion — three different things
| Measure | What it describes | This quake |
|---|---|---|
| Magnitude | Energy released at the source | About 5.9 (preliminary); JMA later listed a moment magnitude near 5.8 |
| Shindo (seismic intensity) | How strongly the ground shook at a specific place | Lower 5 in parts of Tokyo, Ibaraki, Saitama, Chiba |
| Long-period ground motion | Slow, rolling waves that can resonate with tall buildings | Enough to make skyscrapers sway for minutes after the main jolt |
Magnitude answers “how big was the earthquake.” Shindo answers “how hard did it feel here.” Long-period motion answers “why did the towers keep moving.”
Why the Tokyo earthquake happened where it did
This is the part most earthquake coverage skips entirely. Roughly 150 miles southeast of Tokyo, according to UC Berkeley’s Seismology Lab, three tectonic plates — the Pacific, the Philippine Sea, and the plate carrying the Eurasian continent — meet at a single point called a triple junction. That alone wouldn’t be unusual; triple junctions exist elsewhere in the world.
What makes this one different is that it’s a triple trench junction — the only confirmed example on the planet where all three plate boundaries meeting at that point are active subduction trenches simultaneously, with one plate diving beneath two others at once. North of the junction, the Pacific plate dives under the Eurasian and North American plates along the Japan Trench. To the south, that same Pacific plate continues subducting, but now beneath the Philippine Sea plate, along the Izu-Bonin Trench. Southwest of the junction, the Philippine Sea plate is itself sliding beneath the Eurasian plate along the Nankai Trough. Three subduction zones, one junction, nowhere else like it has been identified anywhere on Earth.
Tokyo and the Kanto region sit almost directly on top of this arrangement. It’s the underlying reason Japan logs roughly 1,500 earthquakes a year, and specifically why the Kanto region has such a complicated mix of shallow and deep seismic activity compared with most other major cities.
Why the Tokyo earthquake made skyscrapers keep swaying
This is the detail that made Sunday’s quake feel more dramatic in central Tokyo than its magnitude alone would suggest. Japanese authorities recorded “Class 2” long-period ground motion across Tokyo’s 23 wards and parts of Saitama and Chiba — a separate measurement system from the standard shindo intensity scale, specifically designed to capture how tall buildings respond to earthquakes.
Long-period ground motion consists of slow, rolling seismic waves that can travel much further than sharp, high-frequency shaking without losing much energy. Ordinary low-rise buildings barely notice them. Tall buildings are a different story: every building has a natural oscillation period, and taller buildings sway more slowly and over a longer period than short ones. When a long-period seismic wave happens to match a skyscraper’s natural sway rhythm, the building resonates — amplifying the motion rather than damping it, in the same way a swing goes higher if you push it at exactly the right point in its arc each time.
At Class 2 on Japan’s four-level long-period ground motion scale, people inside affected buildings typically feel a strong enough sway that they want to hold onto something, and hanging objects can swing dramatically even after the ground itself has gone still. It’s not a hypothetical risk: after the 2011 Tohoku earthquake, long-period ground motion shook high-rise buildings in Osaka — roughly 700 kilometres from the epicentre — badly enough to crack interior fittings, damage fire doors, and trap people in stopped elevators, even though Osaka itself barely felt that earthquake at ground level. Sunday’s Tokyo earthquake produced the same effect on a smaller scale, just much closer to home.
Why the quake’s depth mattered as much as its magnitude
The Tokyo earthquake’s 70-kilometre-deep epicentre is considerably deeper than the shallow, highly destructive earthquakes that make global headlines. Depth generally works in a city’s favour, since it gives seismic energy more rock to travel through before reaching the surface, spreading and weakening it along the way.
But depth cuts differently depending on the kind of wave involved. The sharp, high-frequency shaking that damages buildings directly does fade with depth. Long-period waves are far more persistent over distance, which is exactly why a magnitude 5.9 quake 70 kilometres down was still able to set skyscrapers swaying dozens of kilometres away, even as the immediate ground shaking near the epicentre stayed in a moderate, largely non-destructive range.
Why authorities are watching for aftershocks
Earthquakes like the Tokyo earthquake are frequently followed by smaller aftershocks in the surrounding days, and occasionally by a second event of comparable or greater size, particularly in a seismically active zone like Kanto. That’s the specific risk Prime Minister Takaichi’s warning was addressing — not a prediction that a larger quake is coming, but a reminder that the fault system involved hasn’t necessarily finished releasing stress after a single event.
The Japan Meteorological Agency advised people in areas that felt strong shaking to stay alert for aftershocks of similar intensity for roughly a week. As of the hours after the mainshock, no larger follow-up event had been confirmed, and no tsunami warning was issued.
For official numbers — magnitude, depth, intensity maps, and aftershock guidance — treat JMA as the primary source. Preliminary magnitudes are often refined as more station data arrive.
How this quake compares with a feared “Tokyo inland” scenario
This August 23 event was a moderate, intermediate-depth quake. It injured dozens and disrupted trains, but it was not the much-discussed magnitude-7-class inland earthquake under the capital.
Japanese government damage estimates for a severe Tokyo inland earthquake still point to a far larger national-scale impact — potentially tens of thousands of deaths and economic losses in the tens of trillions of yen in a worst-case setting. That risk remains a planning baseline. The 5.9 Ibaraki quake is a reminder of everyday seismic exposure, not a replacement for those long-term scenarios.
Frequently asked questions
How strong was the Tokyo earthquake on August 23, 2026?
Preliminary magnitude was about 5.9, with a hypocenter near southern Ibaraki at roughly 70 km depth. Maximum reported intensity reached lower 5 on Japan’s shindo scale in parts of the Tokyo region. Moment magnitude was later reported near 5.8.
Why did Tokyo skyscrapers sway if the magnitude was only 5.9?
Tall buildings respond strongly to long-period seismic waves. Even when high-frequency shaking at street level is moderate, those slower waves can match a tower’s natural period and keep it moving after the initial jolt fades.
Was there a tsunami from the Tokyo earthquake?
No. JMA did not issue a tsunami warning. The depth and inland/near-coast setting of this event did not favor a destructive tsunami.
Why do earthquakes like this happen near Tokyo so often?
The greater Tokyo region sits above one of Earth’s most complex plate boundaries. Offshore, the Boso (or Boso-Oki) triple junction marks where the Pacific, Philippine Sea, and overlying continental plates meet in a rare trench–trench–trench configuration. Dual subduction and compression under the Kanto plain help explain why moderate quakes — including events under southern Ibaraki — occur relatively often and can still be felt widely across Tokyo.
Why did buildings sway even though the earthquake wasn’t that powerful?
The quake produced Class 2 long-period ground motion, a slow, rolling type of seismic wave that resonates with tall buildings’ natural sway rhythm, amplifying and prolonging the motion in skyscrapers well after ground-level shaking had stopped.
Was there a tsunami risk from this earthquake?
No. The Japan Meteorological Agency confirmed there was no tsunami threat from Sunday’s earthquake.
Is a bigger earthquake expected to follow?
Not confirmed. Prime Minister Takaichi’s warning reflected standard post-earthquake caution about aftershocks or a possible follow-up event of similar size, not a specific prediction of a larger quake.
How is long-period ground motion different from a regular earthquake intensity rating?
Japan’s standard shindo scale measures shaking intensity at ground level. Long-period ground motion is a separate four-class scale specifically tracking how much tall buildings sway due to resonance, since that effect doesn’t necessarily correlate with how strong an earthquake feels on the ground.