The geography of Japan: a mountain range in the sea that is watched around the clock

What the archipelago is made of

Japan runs more than 3,000 kilometres from the Sea of Okhotsk in the north to the Philippine Sea in the south, between 20° and 45° north and 122° and 153° east. The five main islands are Hokkaido, Honshu, Shikoku, Kyushu and Okinawa, and the Japanese government classifies everything else as remote islands.

How many islands that is was revised sharply and recently. In 2023 a government recount using digital mapping raised the total from 6,852 to 14,125, of which 430 are inhabited. Nothing appeared or disappeared; the resolution of the survey changed, which is a useful reminder about what a geographic figure is measuring.

The Geospatial Information Authority of Japan measures the national territory annually. As of 1 July 2021 it stood at 377,973.89 square kilometres, of which 364,546.41 was land and 13,430 water. That figure grows. Volcanic eruptions add territory, the islands expand naturally, and land is reclaimed, so a country's area here is a measurement with a date on it rather than a constant.

Honshu, Kyushu and Shikoku are separated only by the narrow straits of the Seto Inland Sea and function as one landmass. Okinawa and the Ryukyus run southwest towards Taiwan, and the Bonin, Volcano and Nanpō islands run out into the Pacific.

Why so few places are habitable

About 73% of Japan is mountainous, with a range running the length of each main island, and forest covers 68.55% of the country, because the mountains are wooded. The high point is Mount Fuji at 3,776 metres.

What is left is coastal plain, valley floor and river mouth, and that is where everyone is.

Most of that Honshu population is on the Pacific side. The coast facing the Sea of Japan is markedly less dense, which is a consequence of both terrain and weather rather than of history. With so little level ground, hills and lower mountainsides around towns are cultivated where they can be.

For administration the country is divided into eight regions and, within them, prefectures. Hokkaido is the exception: the region and the prefecture are the same thing.

Where the flat ground is

Scattered plains and intermontane basins cover only about 27% of Japan, and the location of that 27% has decided where the country's power sat in every period of its history.

The Kantō Plain is by a wide margin the largest, centrally placed, and historically the most agriculturally productive, which is to say the most taxable. The Tokugawa established their government at Edo on it in 1603 and it became Tokyo in 1868. Kyoto sits in the Yamashiro Basin of 827.9 square kilometres and Nara in the Nara Basin of 300, which is a fair measure of how much room the ancient capitals had.

Hokkaido is the agricultural exception in every direction. It holds nearly a quarter of Japan's arable land and 22% of its forest, and the average farm there was 26 hectares in 2013 against a national average of 2.4, which is roughly eleven times the size. The rest of Japanese farming is small because the land is.

Around 0.5% of the country is reclaimed land, a practice begun in the twelfth century by diking river deltas and terracing paddies into mountainsides. Most of it happened after the Second World War, during the economic miracle, when between 80 and 90% of all tidal flatland on the archipelago was reclaimed. Kansai and Chūbu Centrair airports sit on artificial islands in Osaka and Ise bays, and Port Island, Rokkō Island and Kobe Airport were all built out from Kobe.

Why the rivers do not carry ships

Japanese rivers are steep, short and fast. Most run under 300 kilometres and the longest, the Shinano at 367 kilometres, crosses from Nagano to Niigata and reaches the Sea of Japan. Few are navigable except in their lower reaches.

What they do instead is generate power and demand flood control. The rapid drop from the mountains makes them useful for hydroelectricity, and the seasonal swing in their flow has driven flood defence works throughout the country's history. Coastal shipping, especially around the Seto Inland Sea, does the work that river navigation does elsewhere.

The largest freshwater lake is Biwa, 670.3 square kilometres northeast of Kyoto, and it is an unusual body of water by any measure. Formed by plate tectonics in an active rift zone, it dates back at least four million years, reaches 104 metres deep, and has therefore never filled with sediment. It holds more than 1,000 species and subspecies, including 46 native fish of which 11 species and 5 subspecies are endemic or nearly so, and about 5,000 water birds arrive each year.

How the sea sets the boundaries

Japan's coastline runs 29,751 kilometres, and its exclusive economic zone covers 4,470,000 square kilometres, more than eleven times its land area. Territorial sea extends 12 nautical miles.

There is a deliberate exception worth noting. In four international straits, La Pérouse, Tsugaru, Ōsumi and Tsushima, Japan claims 3 nautical miles rather than 12, which leaves an international corridor of open water through each. A country that had claimed the full extent everywhere would have closed those passages.

The distances to the continent explain a great deal of Japanese history. Kyushu lies 190 kilometres from the southern tip of the Korean peninsula, nearly six times the width of the English Channel, and the main islands lie about 800 kilometres from China across open sea. Kyushu was historically the gateway, because it was the only short crossing.

The archipelago was not always separate. During the Last Glacial Maximum around 20,000 BCE a land bridge joined Hokkaido to Sakhalin and the continent; rising seas removed it around 10,000 BCE. Since then the combination of distance, sea, mountains and steep rivers has made Japan difficult to invade and easy to close, which is exactly what the Tokugawa shogunate did between the 1640s and 1853.

Several maritime boundaries remain disputed. The Senkaku Islands are administered by Japan and claimed by China; the Northern Territories in the Kurils and the Liancourt Rocks are administered by other states and claimed by Japan. This catalogue records that the disputes exist and takes no position on them.

What the plates do

Japan sits in the northwestern Ring of Fire on the meeting point of several tectonic plates, with three oceanic trenches to its east. At the Japan Trench the oceanic Pacific Plate subducts beneath the continental Okhotsk Plate, and that continuous subduction produces the earthquakes, the tsunamis and the stratovolcanoes.

It also produced the country's shape. Subduction pulled the archipelago eastward and opened the Sea of Japan behind it by back-arc spreading about 15 million years ago, separating Japan from the Asian continent.

The volcanic inventory is large and monitored. Japan has 111 active volcanoes, and on average 15 volcanic events including eruptions occur each year. The Japan Meteorological Agency keeps seismographs and related instruments in the vicinity of 50 volcanoes judged remarkably active, sends mobile teams to patrol the rest, and has issued volcanic warnings and forecasts for each active volcano since 1 December 2007. Low-intensity tremors are felt constantly, destructive earthquakes several times a century, and hot springs are everywhere, which the leisure industry has built on for centuries.

How the country watches for the next one

The monitoring effort is on a scale that no other country matches, and the numbers say why.

The Japan Meteorological Agency operates a network of about 200 seismographs and 600 seismic intensity meters, and collects data from a further 3,600 intensity meters run by local governments and the national disaster research institute. Everything feeds in real time into a central system in Tokyo and a second at Osaka.

The response times are the point. When an earthquake occurs, JMA issues its hypocentre, magnitude and observed intensity immediately, and for any event of intensity 3 or greater a Seismic Intensity Information report goes out within one and a half minutes. If a damaging tsunami is expected, a warning is issued for each coastal region within two to three minutes of the quake. The Earthquake Early Warning system announces expected intensity and arrival time in advance of the principal motion, which is enough to slow trains, stop lifts at the nearest floor and give people seconds to move.

Japan's seismic intensity scale has ten degrees, from 0 for imperceptible through 5 lower, 5 upper, 6 lower and 6 upper to 7, and it measures ground motion at a place rather than energy at the source. That distinction matters here more than it does elsewhere, because the same offshore earthquake produces very different shaking along a coastline of this shape.

Why the Nankai Trough is watched separately

One fault system gets its own committee, its own information category and continuous dedicated monitoring.

Along the Nankai Trough, megathrust earthquakes of magnitude 8 or more occur repeatedly, historically on a cycle of roughly 100 to 150 years, though with varying intervals and source regions. What makes them distinctive is that they come in pairs. The Ansei Tōkai earthquake of 1854 was followed 32 hours later by the Ansei Nankai earthquake in the western part of the trough. The Shōwa Tōnankai earthquake of 1944 was followed two years later by the Shōwa Nankai earthquake of 1946.

So an event in one part of the trough raises the assessed likelihood of another elsewhere along it. JMA monitors the region around the clock for seismic activity and crustal deformation, watches for slow slip events on the plate boundary as indicators of changing coupling, convenes an assessment committee when anomalies appear, and issues Nankai Trough Earthquake Extra Information to the public.

The agency is careful about what that information means, and its caution is worth repeating exactly. A Nankai Trough earthquake could occur at any time with no opportunity for advance notice, and information indicating elevated potential does not mean one will strike. Preparation is therefore treated as a daily matter rather than a response to a warning.

Why the climate runs the length of a continent

Twenty-five degrees of latitude give Japan a climate range that most countries its size do not have. The north is humid continental; the centre and south are humid subtropical; the far southern islands reach tropical rainforest.

The archipelago also sits in the typhoon track, which arrives from the southwest in late summer and autumn and is a routine part of the year rather than an exception.

That spread of climate across a mountainous, sea-divided territory is why the flora and fauna are as varied as they are, with endemic species concentrated in the isolated Ogasawara Islands in particular. An island group a thousand kilometres out in the Pacific, never connected to anywhere, ends up with plants and animals found nowhere else, which is the same reason the same thing happened in the Galápagos.