2009 Shizuoka earthquake

What Happened

At 05:07 local time on 11 August 2009, a magnitude 6.4 earthquake struck Shizuoka Prefecture in the south of Honshū, Japan's main island. The epicentre lay at coordinates 34.778°N, 138.276°E, a point in the sea off the prefecture's coast. In UTC terms the rupture occurred at 20:07 on 10 August, so international bulletins carried the earlier date while Japanese clocks already read the morning of the 11th.

The shaking arrived in the early morning, when most residents were still at home. The event is recorded in Japan's official earthquake catalogue as the 2009 Shizuoka earthquake, and in Japanese as 駿河湾地震 — the Suruga Bay earthquake.

Background

Shizuoka Prefecture occupies a stretch of the Pacific coast roughly midway between Tokyo and Nagoya, and it sits on one of the most seismically active junctions in the world. Three of the great tectonic boundaries that converge beneath Japan meet in this region: the Philippine Sea Plate pushes northwest beneath the Eurasian Plate, and the Suruga Trough — the undersea valley that gives Suruga Bay its shape — runs directly toward the prefecture's coastline.

The bay itself is a deep, steep-sided inlet, and the fault structures beneath it are capable of producing very large ruptures. Japanese seismologists have long watched the Suruga Trough in particular, because it forms the eastern end of the Nankai Trough system, the source of repeated magnitude 8-class earthquakes throughout recorded Japanese history. A magnitude 6.4 event in the bay is therefore not an isolated curiosity but one entry in a long, well-documented sequence of activity in the same crustal neighbourhood.

The earthquake fell in the Heisei period (1989–2019), the era name of Emperor Akihito's reign, and specifically in the twenty-first year of that era, written in Japanese as 平成21年.

Why It Mattered

The 2009 Shizuoka earthquake mattered less for its own damage than for where it happened. A magnitude 6.4 rupture in Suruga Bay is a direct reminder that the same trough system is capable of far larger events, and Japanese disaster planning treats the region as a priority zone. The earthquake therefore fed straight into the ongoing public conversation about preparedness for a future Nankai Trough earthquake, the scenario that Japanese authorities have long regarded as one of the country's most serious natural-hazard risks.

The timing also placed the event in a crowded period of Japanese seismic history. It came in the same year that the Heisei era entered its third decade, and it was followed within months by other notable earthquakes in the country, keeping seismic safety in the news. For residents of Shizuoka, the practical consequences were local: early-morning shaking, disruption to the start of the day, and the routine but vital checks on infrastructure that follow any significant quake in Japan.

For students of Japanese, the earthquake is also a useful linguistic landmark. Its name preserves an older geographic word — Suruga — that no longer appears on modern maps as a political unit, showing how place names survive inside disaster terminology long after the administrative boundaries that produced them have disappeared.

The Name in Japanese

The Japanese name of the event is 駿河湾地震. It is read in kana as するがわんじしん, and romanised as Suruga-wan jishin. Breaking it down:

駿 — read する (suru) in this word. This is the first character of 駿河 (Suruga), the historical name of the district along this stretch of coast. The character is otherwise rare in modern Japanese and appears chiefly in names.

河 — read が (ga) here, though its usual reading is かわ (kawa), "river". It is the second character of 駿河. The voiced ga rather than kawa is a standard sound change when characters combine into a compound name.

湾 — read わん (wan), meaning "bay" or "gulf". This is the character that turns the old place name into a body of water: 駿河湾 is Suruga Bay, the inlet where the earthquake's epicentre lay. The same character appears in names like 東京湾 (Tōkyō-wan), Tokyo Bay.

地 — read じ (ji), meaning "earth" or "ground". It is the first half of the word for earthquake.

震 — read しん (shin), meaning "shake" or "tremor". Together, 地震 (jishin) is the ordinary Japanese word for "earthquake", and it is the standard suffix attached to the place name of any quake: 駿河湾地震, 東日本大震災, and so on.

So the whole name reads literally as "Suruga Bay earthquake" — place, then phenomenon. This pattern is the default in Japanese: name the location, add 地震, and the event is identified. Readers who learn to parse 駿河湾地震 can parse the name of almost any Japanese earthquake they will meet.

Key Figures

• Date: 11 August 2009 (local time); 10 August 2009 in UTC
• Local time: 05:07
• UTC time: 20:07
• Magnitude: 6.4
• Epicentre coordinates: 34.778°N, 138.276°E
• Location: Shizuoka Prefecture, Honshū, Japan
• Japanese name: 駿河湾地震
• Kana reading: するがわんじしん
• Romaji: Suruga-wan jishin
• Period: Heisei era 平成, year 21 (2009)