2008 Iwate earthquake

What Happened

On 24 July 2008, an earthquake struck off the northern coast of Iwate Prefecture in north-eastern Japan. Its moment magnitude was 6.8, and its focus lay at a depth of 115 km. Because the rupture occurred so deep, the seismic energy travelled upward through the crust and spread across a wide area of northern Honshu rather than concentrating at a single point near the surface.

The Japan Meteorological Agency measures shaking on its own scale, the shindo (震度, しんど, seismic intensity scale), which runs from 0 to 7 and describes how strongly the ground moves at a given place rather than how much energy the earthquake released. This event produced a maximum intensity of shindo 6− (read "roku-kyū", 6弱), recorded in both Aomori and Iwate Prefectures. The scale's 6 is split into 6− and 6+, and 6− denotes shaking strong enough to make it difficult to remain standing and to topple poorly anchored furniture.

One reading was initially reported as 6+ in the town of Hirono, Iwate. A later examination concluded that the instrument producing that reading had been improperly installed, and the value was revised downward. The earthquake occurred during the Heisei period (1989–2019), in the twentieth year of the Heisei era.

Background

Japan sits on the boundary of four tectonic plates, and the Japanese archipelago experiences a substantial share of the world's large earthquakes. The Tohoku region, which includes Iwate and Aomori, lies where the Pacific Plate descends beneath the Okhotsk Plate along the Japan Trench. That subduction zone generates earthquakes at a range of depths, and seismologists classify them by where the rupture occurs.

Shallow earthquakes break close to the surface and tend to concentrate their damage in a narrow zone. Intermediate-focus earthquakes, by contrast, rupture at depths of roughly 70 to 300 km, within the descending slab of oceanic plate. This earthquake at 115 km belongs to that intermediate category. Deep ruptures release their energy far below the surface, so the waves fan out and reach the surface over a broad region. The result is an earthquake felt across hundreds of kilometres, with comparatively moderate peak shaking at any one location, rather than a sharp local jolt.

Iwate Prefecture occupies the north-eastern coast of Honshu, facing the Pacific Ocean. Its coastal towns have long experience of earthquakes and of the tsunamis that subduction-zone events can generate. The 2008 event, occurring at intermediate depth, did not produce a destructive tsunami, but it was felt widely across the region.

Why It Mattered

The earthquake's significance lies chiefly in what it demonstrated about how intermediate-focus events behave. A magnitude 6.8 rupture at 115 km depth shook a broad swath of northern Honshu, reaching maximum intensity 6− in two prefectures. For communities far from the epicentre, the shaking was a reminder that damaging ground motion does not require a nearby shallow fault.

The event also drew attention to the reliability of Japan's dense seismic observation network. The initial reading of 6+ at Hirono, Iwate, was the highest value recorded anywhere for this earthquake. When the instrument behind it was found to have been improperly installed, the reading was corrected, and the maximum intensity for the earthquake was set at 6−. The episode illustrated how much weight rests on the correct siting and maintenance of the sensors that feed the shindo scale, since those readings inform public alerts and emergency response in the minutes after a quake.

The earthquake occurred three years before the 2011 Tohoku earthquake, which would devastate much of the same coastline. The 2008 event stands on its own record as a well-documented intermediate-depth earthquake in the northern Iwate coastal region.

The Name in Japanese

The Japanese name of this earthquake is 岩手県沿岸北部地震. Its kana reading is いわてけんえんがんほくぶじしん, and its romaji is Iwate-ken Engan Hokubu Jishin. The name is a compound that describes exactly where the earthquake occurred, and breaking it into its parts makes the logic visible.

岩 (いわ, iwa) — rock, boulder. 手 (て, te) — hand. Together, 岩手 (いわて, Iwate) is the name of the prefecture. The combination is traditionally explained as "rock hand," and it functions here purely as a place name.

県 (けん, ken) — prefecture. Japan's top-level administrative divisions are the 都道府県 (とどうふけん, todōfuken): the metropolis of Tokyo (都), the northern territory of Hokkaido (道), the two urban prefectures of Osaka and Kyoto (府), and the remaining 43 prefectures (県). 岩手県 (いわてけん, Iwate-ken) is therefore "Iwate Prefecture."

沿 (えん, en) — to run alongside, to follow a course. 岸 (がん, gan) — shore, coast, bank. Combined, 沿岸 (えんがん, engan) means "coast" or "coastal waters," the strip of land and sea where they meet. The reading of 岸 shifts from its standalone kishi to gan in this compound.

北 (ほく, hoku) — north. 部 (ぶ, bu) — part, section. Together, 北部 (ほくぶ, hokubu) means "northern part." Note that 北 is read hoku here rather than kita, the reading it takes when standing alone.

地 (じ, ji) — ground, earth. 震 (しん, shin) — shake, quake. Combined, 地震 (じしん, jishin) is the standard Japanese word for "earthquake."

Read as a whole, 岩手県沿岸北部地震 parses as "Iwate Prefecture coastal northern earthquake" — the northern coastal part of Iwate Prefecture. The name follows the standard Japanese convention for naming earthquakes by their location, the same pattern seen in names such as 新潟県中越地震 (Niigata-ken Chūetsu Jishin) for the 2004 Niigata earthquake.

Key Figures

• Date: 24 July 2008
• Moment magnitude: 6.8
• Depth: 115 km (intermediate-focus earthquake)
• Maximum JMA seismic intensity: shindo 6− (6弱), recorded in Aomori and Iwate Prefectures
• Coordinates: 39.8020° N, 141.4640° E
• Location: Iwate Prefecture, Japan
• Period: Heisei period (1989–2019), year 20 of the Heisei era
• Notable revision: an initial reading of 6+ at Hirono, Iwate, was later attributed to an improperly installed instrument and corrected