2018 Hokkaido Eastern Iburi earthquake

What Happened

At 3:08 a.m. Japan Standard Time on 6 September 2018, a magnitude 6.6 earthquake on the moment magnitude scale struck Iburi Subprefecture in southern Hokkaido, the northernmost of Japan's four main islands. The epicentre lay near the city of Tomakomai, at a depth of 35.0 kilometres. The Japan Meteorological Agency recorded a magnitude of 6.7 on its own scale and assigned the quake a maximum intensity of 7 on the shindo (震度) scale — the agency's seven-step measure of how strongly the ground shakes at a given place, where 7 is the highest level ever recorded.

Shaking was felt strongly across Hokkaido and in Aomori Prefecture, at the northern tip of Honshu, and was detected as far south as the Kantō region, the plain around Tokyo. Long-period ground motion, the slow swaying that affects tall buildings and large structures, reached class 4 on the JMA's scale for that phenomenon.

The earthquake cut electrical service across Hokkaido, leaving 5.3 million residents without power. Forty-one people were confirmed dead and 691 were injured.

Background

The event falls in the Heisei period (1989–2019), the era name of the reign of Emperor Akihito, and is dated to Heisei 30, the thirtieth year of that era. Japan's era names, or nengō (年号), are still used alongside the Western calendar in official documents, newspapers and government records, so the same earthquake carries two official dates: 2018 in the common era and Heisei 30 in the Japanese reckoning.

Iburi Subprefecture occupies the southern-central part of Hokkaido. Subprefectures are administrative divisions unique to Hokkaido, which is too large and too sparsely populated to be governed from a single capital in the way other prefectures are. Tomakomai, the city nearest the epicentre, is one of Hokkaido's principal port and industrial centres.

Hokkaido sits at the junction of several tectonic plates, and the region experiences frequent seismic activity. The 2018 quake struck in the small hours of the morning, when most residents were asleep indoors — a timing that shaped both the casualty figures and the emergency response.

Why It Matters

The most consequential effect was not the shaking itself but the loss of power. The earthquake knocked out the island's entire electricity supply, a blackout affecting 5.3 million people. Hokkaido is connected to the main grid on Honshu by a single high-voltage direct-current link with limited capacity, so the island could not simply draw replacement power from the south. Restoring service took days and required the restart of thermal plants that had themselves been shut down by the quake.

The death toll of 41 and the 691 injured made this one of the deadliest earthquakes in Japan in the years between the 2011 Tōhoku disaster and the present. Many deaths were caused not by collapsing buildings but by landslides, which swept down slopes in the hills around Atsuma and other communities after heavy rain had left the ground saturated.

The earthquake also became a reference point in Japanese public life for the vulnerability of the country's infrastructure to a single point of failure. The complete isolation of an entire island's power grid had not been anticipated at that scale, and the event prompted reviews of backup capacity and of the Hokkaido–Honshu interconnector.

The Name in Japanese

The earthquake's Japanese name is 北海道胆振東部地震, read in kana as ほっかいどういぶりとうぶじしん and romanised as Hokkaidō Iburi tōbu jishin. Its official designation in full is 平成30年北海道胆振東部地震 (Heisei san-jū-nen Hokkaidō Iburi tōbu jishin), "the Hokkaido Eastern Iburi Earthquake of Heisei 30".

The name is built from place names stacked in front of the word for earthquake, a standard pattern for naming Japanese seismic events. Breaking it down:

北 (ほく, hoku) — north. 海 (かい, kai) — sea. 道 (どう, dō) — road, way, or, in this context, a large administrative circuit. Together 北海道 (Hokkaidō) means "Northern Sea Circuit", the name given to the island in the nineteenth century. Note that the final 道 is voiced in compounds, shifting from tō to dō.

胆 (い, i) — courage, nerve. 振 (ぶり, buri), from the verb 振る (furu), to shake or swing. Together 胆振 (Iburi) is the name of the subprefecture. The characters were chosen for their sounds rather than their meanings, a practice called ateji (当て字), in which kanji are assigned to a place name purely for phonetic value. The name derives from the Ainu language, spoken by the indigenous people of Hokkaido, and its original meaning is not represented by these characters.

東 (とう, tō) — east. 部 (ぶ, bu) — part, section. Together 東部 (tōbu) means "eastern part", specifying that the quake struck the eastern portion of Iburi Subprefecture.

地 (じ, ji) — ground, earth. 震 (しん, shin) — shake, quake. Together 地震 (jishin) is the ordinary Japanese word for an earthquake, used in everyday speech as well as in official reports.

Read as a whole, the name tells a Japanese reader exactly where the earthquake happened — the eastern part of the Iburi district of Hokkaido — without any further explanation.

Key Figures

Magnitude 6.6 — the earthquake's size on the moment magnitude scale, which measures the energy released at the source. The Japan Meteorological Agency assigned a magnitude of 6.7 on its own scale, a slight difference arising from different calculation methods rather than disagreement about the event.

Depth 35.0 kilometres — how far below the surface the rupture occurred, as recorded for the epicentre near Tomakomai.

Shindo 7 — the maximum seismic intensity recorded on the JMA's shindo scale, the top of the seven-level range.

LPGM class 4 — the maximum long-period ground motion intensity recorded, on the JMA's scale for slow, large-amplitude shaking.

5.3 million — the number of Hokkaido residents left without electricity after the quake disrupted the island's power supply.

41 dead, 691 injured — the confirmed human cost of the disaster.

3:08 a.m., 6 September 2018 — the moment the earthquake struck, in Japan Standard Time.