April 2011 Fukushima earthquake

What Happened

At 17:16 JST (08:16 UTC) on 11 April 2011, a strong earthquake struck the Hamadōri region of Fukushima Prefecture, in northeastern Japan. The event registered a magnitude of 6.6 on the moment magnitude scale (Mw), with a shallow focus of 13 km (8.1 mi). The epicentre lay inland, approximately 36 km (22 mi) west of the city of Iwaki, and the shaking was widespread, with strong to locally severe ground motion reported across the region.

The earthquake was an intraplate event, meaning it occurred within a tectonic plate rather than at a plate boundary. It resulted from normal faulting to the west of Iwaki, a mechanism in which the hanging wall drops down along a dipping fault plane. The rupture reactivated two geological faults in an area generally characterised by low seismicity, and the strong ground movements triggered numerous landslides across adjacent mountainous terrain.

Immediate impacts included a few fires and the loss of electricity for 220,000 households. Localised tsunami alerts were issued by officials, though no significant waves were generated. Despite the intensity of the shaking, structural damage remained limited. Four people were killed and ten others were injured as a direct result of the earthquake.

Background

The April 2011 Fukushima earthquake did not occur in isolation. It was triggered by the 2011 Tōhoku earthquake, a massive megathrust event that had struck the same region on 11 March 2011, exactly one month earlier. That earlier earthquake, centred offshore, had generated a devastating tsunami and caused widespread destruction along the Pacific coast of northern Honshu. The April event was the strongest aftershock of the Tōhoku sequence to have its epicentre located inland.

The Hamadōri region, where the earthquake struck, is the southeastern coastal area of Fukushima Prefecture. It sits between the Abukuma Highlands to the west and the Pacific Ocean to the east. The region's geology includes a series of active faults, but prior to 2011 it had not been known for frequent or large earthquakes. The low background seismicity made the reactivation of the two faults during the April event a notable occurrence for researchers.

Because the earthquake occurred in an area with relatively few historical seismic events, the strong ground movements prompted extensive surveys in the region. Researchers sought to map the newly reactivated faults, measure surface rupture, and understand how the stress changes from the March Tōhoku earthquake had triggered this inland event. The findings contributed to a broader reassessment of seismic hazard in inland areas of northeastern Japan.

Why It Mattered

The April 2011 Fukushima earthquake mattered for several reasons. First, it demonstrated that the Tōhoku earthquake's influence extended far beyond the offshore rupture zone, capable of reactivating dormant faults deep inland. This challenged prior assumptions about the limits of aftershock zones and the stability of inland fault systems.

Second, the event highlighted the vulnerability of mountainous terrain to earthquake-triggered landslides. The numerous landslides across the adjacent mountains caused significant local disruption, even where buildings survived the shaking. This underscored the need for landslide hazard mapping in regions with steep slopes and active fault systems.

Third, the earthquake's occurrence in an area of low seismicity raised questions about how well seismic hazard assessments account for fault reactivation. The extensive surveys that followed provided new data on fault behaviour and helped refine models for future earthquake risk in similar geological settings. The event also served as a reminder that even moderate-magnitude earthquakes, when shallow and inland, can produce severe shaking and casualties.

The Name in Japanese

The Japanese name for this earthquake is 福島県浜通り地震 (ふくしまけんはまどおりじしん, Fukushima-ken Hamadōri jishin). In romaji, it is written as Fukushima-ken Hamadōri jishin. The name translates roughly as "Fukushima Prefecture Hamadōri Earthquake."

Breaking down the name character by character:

• 福 (ふく, fuku) — "good fortune" or "blessing." This is the first character of Fukushima.
• 島 (しま, shima) — "island." Together, 福島 (Fukushima) means "blessed island" or "fortune island."
• 県 (けん, ken) — "prefecture." This is the administrative division suffix used for Japan's prefectures.
• 浜 (はま, hama) — "beach" or "shore." This character refers to the coastal nature of the region.
• 通 (どお, dōri) — "road" or "way." In this context, it indicates a route or passage. Together, 浜通り (Hamadōri) means "beach road" or "coastal route," and is the traditional name for the southeastern coastal strip of Fukushima Prefecture.
• 地 (じ, ji) — "ground" or "earth." This is the first character of the word for earthquake.
• 震 (しん, shin) — "to shake" or "tremble." Together, 地震 (jishin) means "earthquake," literally "earth shaking."

Note that the reading of 通 in this word is dōri, not the more common tōri, due to the compound context. The full name, 福島県浜通り地震, thus identifies the event by its location — the Hamadōri coastal region of Fukushima Prefecture — and by its nature as an earthquake.

Key Figures

• Date and time: 11 April 2011, 17:16 JST (08:16 UTC)
• Magnitude: 6.6 Mw
• Depth: 13 km (8.1 mi)
• Epicentre: Approximately 36 km (22 mi) west of Iwaki, in the Hamadōri region of Fukushima Prefecture
• Coordinates: 36.9450000° N, 140.6716667° E
• Casualties: 4 killed, 10 injured
• Infrastructure impact: 220,000 households lost electricity; a few fires broke out
• Tsunami: Localised alerts issued; no significant waves generated
• Triggering event: 2011 Tōhoku earthquake (11 March 2011)
• Fault mechanism: Normal faulting, with reactivation of two geological faults
• Japanese era: Heisei period (1989–2019), year 23 of the Heisei era (平成)