1026 Manju earthquake

What Happened

On 10 June 1026, a tsunami struck the Sea of Japan coast of Iwami Province (石見国, Iwami no kuni), a province covering roughly the western part of present-day Shimane Prefecture. The waves reached heights of about 10 metres (33 ft) at the site of present-day Masuda (益田), a city on the same coast. The event is counted among the largest tsunamis known in the Sea of Japan.

The destruction was severe. More than 1,000 people were killed and 3,000 homes were destroyed. Off the coast, an island was reportedly sunk beneath the waves.

The cause has been debated. Some accounts attribute the tsunami to an earthquake; others to an undersea landslide. The epicentre coordinates associated with the event, 34.8°N 131.8°E, place the source in the Sea of Japan off the Iwami coast.

Background

The event falls in the Heian period (平安時代, Heian jidai), the span from 794 to 1185 during which the imperial court sat at Heian-kyō, present-day Kyoto. By the eleventh century the court had developed a refined aristocratic culture, but the provinces along the Sea of Japan remained exposed to the sea and largely dependent on coastal fishing and farming.

Iwami Province lay on the western end of Honshu's Japan Sea coast, facing the open water that separates Honshu from the Korean peninsula. The region's settlements clustered along the shoreline, which left them with little protection when the sea rose.

The date falls in Manju 万寿 (まんじゅ, Manju), a Japanese era name (nengō 年号) used from 1024 to 1028. The event occurred in the third year of that era, which is how the sources date it.

Why It Mattered

The 1026 tsunami is remembered as one of the largest in the Sea of Japan, and the figures attached to it — over a thousand dead, three thousand homes lost — mark it as a catastrophe on a scale rarely recorded for the period. The reported sinking of an offshore island is a striking detail: a permanent change to the coastline, not merely damage to what stood on it.

The event also matters for how the region's seismic history is understood. The debate over whether an earthquake or an undersea landslide generated the waves reflects a broader problem in the study of the Sea of Japan: the sea has produced destructive tsunamis whose sources are not always obvious from the historical record alone. The 1026 event is one of the earliest well-attested cases on this coast, and it sits alongside later tsunamis in the same sea as evidence that the region's coasts face a recurring hazard.

For the Japanese language, the event preserves a snapshot of Heian-era dating practice: it is named for the era in which it occurred, not for a modern calendar year.

The Name in Japanese

The Japanese name is 万寿地震, read まんじゅじしん in kana and written Manju jishin in romaji.

The name breaks down into three characters:

• 万 (まん, man) — "ten thousand". A common character in Japanese numerals and in words expressing great number or longevity.
• 寿 (じゅ, ju) — "longevity", "long life", "congratulations". It is the second character of the era name Manju.
• 地震 (じしん, jishin) — "earthquake". The first character, 地 (ち, chi), means "earth" or "ground"; the second, 震 (しん, shin), means "quake" or "shake". Together they form the standard Japanese word for an earthquake.

The first two characters, 万寿, are the era name (nengō 年号) Manju, in use from 1024 to 1028. Era names were adopted from Chinese practice and were changed to mark auspicious or significant events; each era name is a pair of characters chosen for their auspicious meaning, and 万寿 — "ten thousand years of life" — is a wish for long prosperity. Japanese era names are still used today alongside the Western calendar, which is why a disaster from the eleventh century carries a name built from an era that lasted only a few years.

Note the reading of 寿. In isolation the character is often read kotobuki when it means "congratulations" or appears in names, but inside the era name Manju it takes the Sino-Japanese reading ju. Similarly, 万 is read man here rather than its native Japanese reading yorozu. Compound words built from Chinese characters typically use these Sino-Japanese readings, which is why the era name is pronounced Manju rather than something built from native Japanese words.

The full name, then, is literally "Manju earthquake" — the earthquake of the Manju era. Japanese disaster names frequently work this way: an era name or a place name attached to the type of disaster, giving a compact label that fixes the event in time or space. Readers who know the era names of the Heian period can place 万寿地震 in the 1020s immediately.

Key Figures

No individual is named in connection with the event. The record of the 1026 tsunami is a record of dates, places and numbers: the third year of Manju, the Iwami coast, the drowned island, the dead and the destroyed houses.