Readout · Series one · Japan
Living with earthquakes
The events
The five earthquakes
Japan records an earthquake of magnitude 6 or above roughly every four weeks. Most of them pass with little notice. A few become part of the national memory, and those are the ones worth putting side by side.
This story follows five: Kobe in January 1995, Tohoku in March 2011, the two Kumamoto shocks twenty eight hours apart in April 2016, and Noto on New Year afternoon in 2024.
They sit on the same plate boundary, but they are not the same kind of event. Between the smallest and the largest there is a factor of twenty thousand in energy. Their depths differ by a factor of three. What each one did to the people above it does not follow neatly from either number, and the rest of this page is an attempt to work out why.
What this means: The five are spread the length of the country. Kumamoto is on Kyushu in the south west, Tohoku is off the north east coast, and about 1,200 kilometres separate them. The two Kumamoto shocks are 5 kilometres apart and share a pin. Kobe and Kumamoto ruptured directly beneath cities; Tohoku ruptured well out to sea. That difference does much of the work in the charts that follow. Behind them, faintly, is every other event of magnitude 6 and above in the same fifty years.
Strength
How big each one was
Magnitude is a logarithmic scale, which makes it easy to misread. Magnitude 6.2 and magnitude 9.1 look like numbers three apart. Measured as energy they are more than twenty thousand times apart, because every whole step up the scale is about thirty two times more energy than the one below it.
Tohoku is in a different class from the other four. It released roughly a hundred and ninety times more energy than all four of them put together. It does not, however, come top of every measure that follows.
What this means: Each half ring is one earthquake, spreading from a common origin, and the radius is logarithmic. Four of the five sit close together. Tohoku is a long way outside all of them, at more than twenty thousand times the energy of the smallest. That distance is the finding, and it is still that wide after the logarithm has flattened it.
Depth
How deep they were
An earthquake is felt at the surface, not at the point where the rock breaks. The deeper it starts, the further the energy spreads before it arrives. A moderate earthquake a few kilometres beneath a city can deliver more shaking to that city than a far larger one forty kilometres down and well out to sea.
That accounts for much of the difference between Kobe and Tohoku. Kobe was a shallow rupture in the crust, twenty one kilometres down and almost directly beneath a dense urban area. Tohoku was a subduction rupture offshore.
What this means: Kumamoto and Noto are both listed at exactly 10 kilometres, which is the default entered when depth cannot be determined. Two of the five depths on this chart are not observations, so any comparison resting on them is resting on a filing convention.
Shaking
How hard the ground shook
Shaking intensity describes what happened at the surface rather than at the source. It is the closest figure in this record to what somebody standing in the street actually experienced.
Plotted against magnitude across every event in the box, the relationship turns out to be weak. The points form a cloud rather than a line.
The two most intense shakings in fifty years came from a magnitude 6.8 and a magnitude 6.9, both only a few kilometres below cities. The magnitude 9.1 does not appear in the top five.
What this means: Rank the five by size on the left, then by how hard the ground actually moved on the right, and join each one up. If size decided shaking, every line would run flat. Not one of them does. Tohoku is the largest earthquake in the whole record and fourth of five for shaking. Kobe is fourth for size and first for shaking. The switch to All events puts the same question to every event in the box that carries an intensity value.
The record
How the catalogue ranks them
The catalogue includes a significance score. It combines magnitude, shaking intensity, felt reports and estimated impact, which makes it the closest thing in the file to a ranking by how much an event mattered.
By that score, Kobe comes last of these five.
The explanation is in the inputs rather than in the earthquake. Kobe happened in January 1995. There was no way for the public to file felt reports online, and the alert product did not exist yet, so two of the four inputs are blank. A blank is not a zero, but a score built by adding its inputs together cannot tell the difference.
Anyone comparing events across decades runs into this. The older event scores lower because less of it was recorded.
What this means: Kobe scores lowest of the five despite producing the second hardest shaking in the entire fifty year record. Two of the four inputs to its score were never recorded. This affects every comparison that spans decades: older events look smaller because less of them was captured.
Coda
What the comparison shows
Size, depth, shaking and score put these five earthquakes in four different orders. Tohoku comes first by energy and third by significance. Kobe comes last by significance and second by shaking intensity across the whole fifty year record. Two of the five depths are not measurements at all.
None of that is a fault in the catalogue. It is what happens when four columns, each recording something different, are asked to describe the same afternoon.
The same box holds 629 events in total. The dashboard behind this story draws every one of them.
Sources
Two USGS catalogues, pinned and fetched once.
Both catalogues come from the USGS FDSN event service, pinned to explicit start and end times and fetched once on 3 September 2026. The query URLs, the fetch timestamp and the null policy are committed beside the data. Nothing here is live, and that is deliberate: a number drawn from a rolling feed is a fact about an instant rather than about the catalogue, and it cannot be checked later.
Magnitude 6.0 and above, 1 January 1976 to 3 September 2026. 7,182 events, of which 7,128 are earthquakes. Largest is magnitude 9.10, the Tohoku event, and deepest is 688.0 km. Japan accounts for 534 of the earthquakes, 29 of which carry the tsunami warning flag discussed above.
Every recorded event, 4 August to 3 September 2026. 11,247 events, of which 10,993 are earthquakes.
The five named events. Which events carry this story is an editorial choice and is written down in the generator. Nothing about them is. Each one is found in the magnitude 6 catalogue by its USGS event id and its magnitude, depth, coordinates and time are copied across, so an annotation cannot drift away from the chart it annotates. Eight claims the prose makes out loud are asserted before the file is written, including that the Japanese clock reads 05:46 for Kobe and 14:46 for Tohoku, and that the tsunami column is zero on Tohoku. The note beside each event is hand written context and is marked in the file as carrying no source.
Precision. Depth and magnitude are carried exactly as the catalogue publishes them. Rounding depth to a single decimal moved four events onto exactly 10.0 km and turned 1,189 into 1,193, which is a small number and the wrong one, so the build that produces this page's data now asserts fourteen counts against the source files and refuses to write when one disagrees.
Explore more work
Three more case studies about reading a system quickly and correctly: an incident console built for the first five minutes, an interview score you can argue with, and a compliance agent that reports what it could not fix.