April 2011 Fukushima earthquake

Northern Honshu lies on the Okhotsk plate, beneath which the Pacific plate is subducting along the line of the Japan Trench, the surface expression of this convergent boundary. As a result of this convergence, much of the Okhotsk plate is in a state of west–east oriented compressive stress, leading potentially to a mix of strike-slip and reverse faulting. In coastal parts of southern Hamadōri and northern Ibaraki, however, the maximum horizontal stress is oriented approximately north–south, consistent with the presence of active normal faults in this area. ==Earthquake==

The 6.6 Fukushima Hamadōri earthquake occurred inland on 11 April 2011 at 08:16 UTC at a focal depth of , about west of Iwaki, Fukushima, or north-northeast of Tokyo. 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 two zones of surface rupture. Along the Izodawa fault, about of rupture were recorded and numerous fault scarps, with general vertical displacements of ; with maximum observed displacements in the range to the small village of Shionohira. The nearby Yunodake Fault, a normal dip-slip fault northeast of the Idosawa Fault, that had been dormant for 120,000–130,000 years, also ruptured during the quake, with a maximum observed vertical offset of . The magnitude 9.0 Mw earthquake triggered widespread seismic activity, including over 67 earthquakes of magnitude 6.0 Mw or greater. Apart from the Fukushima Hamadōri earthquake, four of the related events measured magnitude 7.0 Mw or higher. The Fukushima Hamadōri earthquake, however, was the strongest such event to have its epicentre located inland. The Japan Meteorological Agency (JMA) assessed a magnitude of 7.0 Mj and a depth of . From variations in observed displacement on the two surface ruptures and the geometry of fault branching, it has been inferred that rupture propagation began near the southern end of the Itozawa fault, close to the epicentre. The rupture then propagated to the north for 14 km until it approached the northwestern end of the Yunodake fault. The rupture then jumped the 2.5 km gap to that fault, before propagating southeastward for a further 16 km. The magnitude of the earthquake estimated from geological parameters taken from these two surface ruptures is in the range 6.5–6.8 . The occurrence of active normal faulting on the Pacific coastal area of Honshu, within a zone presumed to be under west-east compression, led to the suggestion that the Tohoku earthquake temporarily altered stress state around the epicentral area. However, a study of microearthquakes over the preceding several years demonstrate that the area has been subjected to long-term extension. 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. 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. – 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. 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. 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 . In Tabito town, very close to the epicentre, a landslide resulted in the formation of a quake lake – a natural damming of a river by mass wasting – with a water level of and a storage volume of 1,000–2,500 m3 (35,000–90,000 cu ft). ==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. – local fishing boats along coastlines were shown heading out to sea on national news broadcasts. 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. 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." == See also ==