Earthquake Timeline and Locations
The main earthquake was preceded by a number of large foreshocks, beginning with a 7.2 MW 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 MW 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 (2.5 mi) 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. The quake has been assigned the GLIDE number EQ-2011-000028-JPN, a globally unique identifier, by the Asian Disaster Reduction Center, which provides constant updates. 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.
For current aftershock locations, please see the USGS quake map.
Earthquake Measurements and Geological Background
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 the stress builds up enough to cause a seismic event.
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 (19.9 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, it is unusual for the magnitude of an earthquake 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 Ms 8.6, which also created a large tsunami. Other major earthquakes with tsunamis struck the Sanriku Coast region in 1896 and 1933.
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.
The United States' Nuclear Energy Institute released figures indicating the earthquake generated peak ground accelerations of 0.35g (3.43 m/s²) near the epicenter; with a study by the University of Tokyo indicating some areas experienced accelerations in excess of 0.5g (4.9 m/s²).
Energy Release of the Earthquake
This earthquake released a surface energy (Me) of 1.9±0.5×1017 joules, dissipated as shaking and tsunamic energy, which is nearly double that of the 9.1-magnitude 2004 Indian Ocean earthquake and tsunami 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 city the size of Los Angeles for an entire year," USGS director Marcia McNutt said in an interview.
The total energy released, also known as the seismic moment (M0), was more than 200,000 times the surface energy and was calculated by the USGS at 3.9×1022 joules, slightly less than the 2004 Indian Ocean quake. This is equivalent to 9.32 teratons of TNT, or approximately 600 million times the energy of the Hiroshima bomb.
Geophysical Impact of the Earthquake
The quake moved portions of northeast Japan by as much as 2.4 m (7.9 ft) closer to North America, making portions of Japan's landmass "wider than before," according to geophysicist Ross Stein. Portions of Japan closest to the epicenter experienced the largest shifts. Stein also noted that a 400-kilometer (250 mi) stretch of coastline dropped vertically by 0.6 m (2.0 ft), allowing the tsunami to travel farther and faster onto land.
The Pacific plate itself may have moved eastwards by up to 20 m (66 ft), though the actual displacement will have diminished with greater distance from the site of the fault. Other estimates put the amount of slippage at as much as 40 m (130 ft), covering an area some 300 km (190 mi) to 400 km (250 mi) long by 100 km (62 mi) wide. If confirmed, this would be one of the largest recorded fault movements to have been generated by an earthquake.
According to Italy's National Institute of Geophysics and Volcanology, the earthquake shifted the Earth's axis by 25 centimeters (9.8 in). This deviation led to a number of small planetary changes, including the length of a day and the tilt of the Earth. The speed of the Earth's rotation increased, shortening the day by 1.8 microseconds due to the redistribution of Earth's mass.
The axial shift was caused by the redistribution of mass on the Earth's surface, which changed the planet's moment of inertia. Due to the effects of conservation of angular momentum, such changes of inertia result in small changes to the Earth's rate of rotation. These are expected changes for an earthquake of this magnitude.
Shinmoedake, a volcano in Kyushu, erupted two days after the earthquake. The volcano had erupted in January 2011; it is not known if the later eruption was linked to the earthquake. In Antarctica, the seismic waves from the earthquake were reported to have caused the Whillans Ice Stream to slip by about 0.5 meters (1.6 ft).