April 2011 Fukushima earthquake

A potent magnitude 6.6 Mw intraplate aftershock occurred at 17:16 JST (08:16 UTC) on 11 April, in the Hamadōri region of Fukushima, Japan. With a shallow focus of 13 km, the earthquake was centred inland about 36 km west of Iwaki, causing widespread strong to locally severe shaking. It was one of many aftershocks to follow the 11 March Tōhoku earthquake, and the strongest to have its epicentre located inland.

The earthquake occurred as a result of normal faulting to the west of Iwaki, and triggered numerous landslides across adjacent mountainous areas. A few fires broke out, and 220,000 households lost electricity. Officials issued localised tsunami alerts, though no significant waves were generated. The earthquake caused little structural damage, but killed four people and injured ten others. The strong ground movements triggered the reactivation of a nearby geological fault, prompting researchers to conduct extensive surveys in the region.

Geology
The magnitude 6.6 Mw Fukushima Hamadōri earthquake occurred inland on 11 April 2011 at 08:16 UTC at a focal depth of 13 km, about 36 km west of Iwaki, Fukushima, or 161 km north-northeast of Tokyo. To the east of the epicentre, the oceanic Pacific Plate is subducted beneath the continental Okhotsk Plate, on which much of Honshu's Tōhoku region is situated. Building stress near the resultant plate boundary has led to the development of shallow inland faults through crustal deformation and folding along the east coast of Tōhoku. This intraplate earthquake occurred in the vicinity of the Idosawa Fault – a shallow crustal fault in the Hamadōri region near Tabito town, Iwaki city, that had previously been inactive.

Surveys near the epicentre revealed a surface rupture of about 11 km and numerous fault scarps, with general vertical displacements of 0.8 to 1.5 m; a maximum displacement of 2.3 m occurred at the small village of Shionohira. Localised right-lateral slip of 30 cm was observed at the subsiding west side of the rupture. The segments of the Idosawa Fault associated with this surface feature were classified as the "Shionohira Fault" in 2011. The nearby Yunodake Fault, a normal dip-slip fault northeast of the Shionohira Fault that had been dormant for 120,000–130,000 years, also ruptured during the quake. These observations indicated that the earthquake occurred as a result of normal dip-slip faulting with some strike-slip component.

Although it was centred near a different fault zone, the earthquake was classified as an aftershock of the 11 March Tōhoku earthquake, which occurred offshore about 235 km to its northeast. The magnitude 9.0 Mw earthquake triggered widespread seismic activity, and its aftershock sequence includes well-over 67 earthquakes of magnitude 6.0 Mw or greater. Apart from the Fukushima Hamadōri earthquake, four of the aftershocks measured magnitude 7.0 Mw or higher. The Fukushima Hamadōri earthquake, however, was the strongest of the aftershocks to have its epicentre located inland. Early estimates placed the strength of the earthquake at a magnitude of 7.0–7.1, but the United States Geological Survey (USGS) lowered the magnitude to 6.6. The Japan Meteorological Agency (JMA) assessed a magnitude of 7.0 Mj and a depth of 6.4 km.

The Fukushima Hamadōri earthquake was succeeded by a number of smaller tremors; that same day, at least 11 earthquakes of magnitude 3.5 Mj or higher were recorded near its epicentre. Of the series, the strongest registered at a magnitude of 5.5 Mj and occurred within 3.5 hours after the initial quake. A shallow magnitude 6.0 Mw (6.4 Mj) earthquake and several smaller tremors struck the region on 12 April.

Effects
The earthquake struck in the late afternoon near a moderately populated region of the Fukushima Prefecture, although most structures around the epicentre were resistant to earthquake shaking. Focussed at an unusually shallow depth, the earthquake generated significant shaking throughout many adjacent prefectures. The strongest ground motion registered at severe (MM VIII) in Ishikawa town on the Mercalli intensity scale. Strong shaking (MM VI) spread through Iwaki, Sukawaga, Kuroiso, Ōtawara and Kitaibaraki, with light tremors (MM IV) felt in areas up to several hundred kilometres from the epicentre, including Tokyo and Yokohama. The earthquake cut electricity to about 220,000 households, with most of the cuts reported in Iwaki city. Workers at the Fukushima Daiichi power plant – distanced 70 km from the epicentre – evacuated to safety, and external power to the plant was cut. The outage briefly disrupted cooling water injections into three of the reactors, but services to the plant were restored by 18:05 JST. Authorities at Tokyo International Airport closed all runways momentarily, while NTT DoCoMo restricted voice calls in 14 prefectures following the quake. East Japan Railway Company temporarily suspended its services to restart four of five bullet-train lines; other Shinkansen bullet trains in the region were also halted.

The earthquake sparked several fires in Iwaki, with one fire breaking out in Asakawa town. Fire engines extinguished a blaze in a liquefied natural gas tank at Daiichi Sankyo's Onahama Plant. Most of the structural damage was due to scattered rock- and landslides along hillsides in the vicinity of Iwaki. A landslide crushed two vehicles and buried three homes in the city, trapping a number of the inhabitants. The incident resulted in two immediate deaths. Four people were critically injured and taken to hospital; one of them was later pronounced dead. The Iwaki Ibaraki Route 14 interchange of the Jōban Expressway, which runs from Misato, Saitama, to Tomiya, Miyagi, was cut off to traffic by a large landslide of 120 x. In Tabito town, very close to the epicentre, a 170 x landslide resulted in the formation of a quake lake – a natural damming of a river by mass wasting – with a water level of 15 m and a storage volume of 1,000–2,500 m3 (35,000–90,000 cu ft). Significant land deformation with traces of uplift was observed in and around town, affecting local roads but largely sparing its structures.

A total of seven people from other regions near the epicentre, including southern Ibaraki, Tochigi and Kanagawa prefectures, suffered minor injuries. Another person was injured during the magnitude 6.0 (Mw) aftershock of 12 April. In a report from July 2011, the Fire and Disaster Management Agency confirmed a death toll of four from the earthquake.

Response
Japan's Earthquake Early Warning system was activated upon the detection of primary waves – seismic waves that forego an earthquake's perceivable ground motions – giving residents 6.8 seconds to seek cover before the main shock. At the risk of a tsunami – which reach their destructive wave heights near shallow coastal waters – local fishing boats along coastlines were shown heading out to sea on national news broadcasts. A warning for a localised tsunami of up to 2.0 m was issued by the Japan Meteorological Agency; however, no significant waves were recorded, and the warning was cancelled soon thereafter. In response to the earthquake, the fire department dispatched search and rescue teams and emergency crews for relief efforts and damage assessments throughout the affected area. Six medical crews in pairs of two were also sent to Kanagawa, Chiba and Gunma prefectures. Former Prime Minister Naoto Kan postponed a press conference scheduled for 17:50 JST marking the one-month anniversary of the catastrophic Tōhoku earthquake and tsunami.

The Fukushima Hamadōri earthquake occurred in a region with historically low levels of seismicity; studies showed that the recent activity near the fault zone had been triggered by the Tōhoku earthquake. Ever since the earthquake triggered their reactivation, the Shionohira and Yunodake faults have provided essential data for local geological surveys on regional land deformation, sedimentary rock distribution and landslide vulnerability. In the earthquake's aftermath, Professor Yagi Hiroshi from the Faculty of Education, Art and Science noted that "a possibility exists for widespread aftershocks of the same size to occur in the near future."