Tsunami, Harbor Wave, 津波, Цунами, 海啸, 海嘯

Map of the Sendai earthquake and aftershocks

The main earthquake was preceded by a number of large foreshocks, beginning with a 7.2 M_W event on 9 March approximately 40 km (25 mi) from 11 March quake, and followed by another three on the same day in excess of 6 M_W in magnitude. One minute prior to the effects of the earthquake being felt in Tokyo, the Earthquake Early Warning system connected to more than 1,000 seismometers in Japan sent out warnings on television of an impending earthquake to millions. This was possible because the damaging seismic S-waves, traveling at 4 kilometers per second, took about 90 seconds to travel the 373 km (232 mi) to Tokyo. The early warning is believed by the Japan Meteorological Agency to have saved many lives.

The earthquake occurred at 14:46 local time in the western Pacific Ocean, 130 km (81 mi) east of Sendai, Honshu, Japan. Its epicenter was 373 km (232 mi) from Tokyo, according to the United States Geological Survey (USGS). Multiple aftershocks were reported after the initial magnitude 9.0 quake. A magnitude 7.0 aftershock was reported at 15:06 local time, 7.4 at 15:15 local time and 7.2 at 15:26 local time. Over five hundred aftershocks of magnitude 4.5 or greater have occurred since the initial quake.

Initially reported as 7.9 by the USGS, the magnitude was quickly upgraded to 8.8 and then to 8.9, and then again to 9.0. This earthquake occurred where the Pacific Plate is subducting under the plate beneath northern Honshu; which plate this is is a matter of debate amongst scientists. The Pacific plate, which moves at a rate of 8 to 9 cm (3.1 to 3.5 in) a year, dips under Honshu's underlying plate releasing large amounts of energy. This motion pulls the upper plate down until it breaks. The break 130 kilometers (81 mi) off of the coast of Sendai was estimated to be several tens of kilometers long and only 32 kilometers (20 mi) deep, and caused the sea floor to spring up several meters, causing the earthquake. A quake of this size usually has a rupture length of at least 480 km (300 mi) and requires a long, relatively straight fault line. Because the plate boundary and subduction zone in this region is not very straight, earthquake magnitudes are usually expected not to exceed 8.5; the magnitude of this earthquake was a surprise to some seismologists. The hypocentral region of this earthquake extends from offshore Iwate to offshore Ibaraki Prefectures. The Japanese Meteorological Agency said that the earthquake may have ruptured the fault zone from Iwate to Ibaraki with a length of 500 km (310 mi) and a width of 200 km (120 mi). Analysis showed that this earthquake consisted of a set of three events. The earthquake may have had a mechanism similar to that of another large earthquake in 869 with estimated magnitude M_s 8.6, which also created a large tsunami.

The quake registered the maximum of 7 on the Japan Meteorological Agency seismic intensity scale in Kurihara, Miyagi Prefecture. Three other prefectures—Fukushima, Ibaraki and Tochigi—recorded an upper 6 on the JMA scale. Seismic stations in Iwate, Gunma, Saitama and Chiba Prefecture measured a lower 6, recording an upper 5 in Tokyo.

Energy

This earthquake released a surface energy (M_e) of 1.9±0.5×10¹⁷ joules, dissipated as shaking and tsunamic energy, which is nearly double that of the 9.1-magnitude 2004 Sumatran earthquake that killed 230,000 people, and flung the 2,600 ton Apung 1 ship 2 to 3 km (1.2 to 1.9 mi) inland. "If we could only harness the [surface] energy from this earthquake, it would power [a] city the size of Los Angeles for an entire year," USGS director Marcia McNutt said in an interview. The total energy released (M_w) was more than 200,000 times the surface energy and was calculated by the USGS WPhase Moment Solution at 3.9×10²² joules, slightly less than the 2004 Sumatra quake. This is equivalent to 9.32 teratons of TNT (approximately 600 million times that of the Hiroshima bomb, or 186,400 times as powerful as man's largest-ever explosive device, Tsar Bomba.)

Geophysical impact

Soil liquefaction in Koto, Tokyo