1940 Shakotan earthquake

What Happened

At 00:08:22 Japan Standard Time on 2 August 1940, a magnitude 7.5 earthquake struck off the coast of Hokkaidō, the northernmost of Japan's main islands. The epicentre lay at 44.561°N, 139.678°E, in the Sea of Japan west of the Shakotan Peninsula. The shaking reached Shindo 4 on the Japan Meteorological Agency's intensity scale — strong enough to be felt clearly and to rattle objects, but not violent.

The earthquake itself caused comparatively little damage. The tsunami that followed did not. Waves struck the Hokkaidō coast at roughly 2 metres, and the surge killed 10 people and injured 24 on the island. Homes and boats were destroyed along the Sea of Japan coastline. Across the water, the highest waves reached about 5 metres on the coast of Russia, making this a tsunami that crossed national boundaries.

Background

The Shakotan Peninsula juts westward from the southwestern corner of Hokkaidō into the Sea of Japan. The seafloor off that coast is a tectonically active zone where the Eurasian Plate meets the Okhotsk Plate, and the region has a long record of offshore earthquakes and the tsunamis they generate.

In 1940 Japan was in the 15th year of the Shōwa era (昭和, the reign of Emperor Hirohito), a period that had begun in 1926 and would run to 1989. The country was three years into the Second Sino-Japanese War, and the state's resources and attention were heavily committed to the war effort on the Asian mainland. Coastal communities on the Sea of Japan side of Hokkaidō were largely fishing and farming towns, dependent on the sea and on boats that a tsunami could sweep away in minutes.

The Shakotan event belongs to a class of earthquake that seismologists call a tsunami earthquake — a shock whose seismic shaking is modest relative to the size of the wave it produces. The 1940 quake's Shindo 4 rating and light structural damage, set against a tsunami that killed people and destroyed property, fit that pattern.

Why It Mattered

The 1940 Shakotan earthquake demonstrated the danger that a moderate, non-destructive earthquake can pose when it occurs under the sea. Residents who felt only moderate shaking had little reason to expect a lethal wave, and the tsunami arrived with the destructive force that the shaking itself did not have.

The event also showed how far a tsunami generated off Hokkaidō can travel. Waves reached the Russian coast at roughly 5 metres, more than twice the height recorded along Hokkaidō itself, and destruction extended across the Sea of Japan. A single offshore rupture could therefore affect two countries.

For Japan, the earthquake added to a body of experience that shaped later coastal defence and tsunami warning practice. The country's position on the Pacific Ring of Fire means offshore earthquakes are a recurring hazard, and each event — particularly one where the tsunami, not the shaking, caused the casualties — informed how authorities thought about warning coastal populations.

The Name in Japanese

The Japanese name of this earthquake is 積丹半島沖地震, read in kana as しゃこたんはんとうおきじしん and romanised as Shakotan Hantō Oki Jishin.

The name is built from four elements, read as a single compound:

• 積 — read しゃ (sha) here. In this place name the character is used phonetically; its general meanings include "to accumulate," "to pile up," or "to load."
• 丹 — read こ (ko) here. Also used phonetically in this name; its general meanings include "red," "cinnabar," and "sincerity."
• 半 — read はん (han). Meaning "half."
• 島 — read とう (tō). Meaning "island." Together, 半島 (はんとう, hantō) means "peninsula" — literally "half island."
• 沖 — read おき (oki). Meaning "offshore" or "open sea." It refers to the area of water away from the shore, and in earthquake names it marks the epicentre as lying out at sea.
• 地震 — read じしん (jishin). Meaning "earthquake." The first character, 地 (ち, chi), means "ground" or "earth"; the second, 震 (しん, shin), means "to shake" or "tremor."

So the whole name parses as "Shakotan Peninsula offshore earthquake." The first two characters, 積丹, are ateji — kanji chosen for their sound rather than their meaning — to write the Ainu-derived place name Shakotan. The remaining characters carry their ordinary meanings: peninsula, offshore, earthquake. Japanese earthquake names follow this pattern closely, combining the location with 沖 (offshore) when the epicentre lies at sea and 地震 (earthquake) as the standard noun, which is why the same building blocks recur across the names of many Japanese seismic events.

Key Figures

• Date and time: 2 August 1940, 00:08:22 Japan Standard Time.
• Magnitude: 7.5.
• Maximum seismic intensity: Shindo 4 on the Japan Meteorological Agency scale.
• Epicentre: 44.561°N, 139.678°E, off the coast of Hokkaidō in the Sea of Japan.
• Tsunami deaths: 10, on Hokkaidō.
• Tsunami injuries: 24, on Hokkaidō.
• Maximum wave height on the Hokkaidō coast: approximately 2 metres.
• Maximum wave height on the Russian coast: approximately 5 metres.
• Era: Shōwa 昭和, year 15 of the era.