disaster." Earthquake The magnitude 9.1 (Mw) undersea megathrust earthquake occurred on 11 March 2011 at 14:46 JST (05:46 UTC) in the north-western Pacific Ocean at a relatively shallow depth of , with its epicenter approximately east of the Oshika Peninsula of Tōhoku, Japan, lasting approximately six minutes. The earthquake was initially reported as 7.9 Mw by the USGS before it was quickly upgraded to 8.8 Mw, then to 8.9 Mw, and then finally to 9.0 Mw. On 11 July 2016, the USGS further upgraded the earthquake to 9.1. Sendai was the nearest major city to the earthquake, from the epicenter; the earthquake occurred northeast of Tokyo. The main earthquake was preceded by a number of large foreshocks, with hundreds of aftershocks reported. One of the first major foreshocks was a 7.2 Mw event on 9 March, approximately from the epicenter of the 11 March earthquake, with another three on the same day in excess of 6.0 Mw. Following the main earthquake on 11 March, a 7.4 Mw aftershock was reported at 15:08 JST (6:06 UTC), succeeded by a 7.9 Mw at 15:15 JST (6:16 UTC) and a 7.7 Mw at 15:26 JST (6:26 UTC). Over eight hundred aftershocks of magnitude 4.5 Mw or greater have occurred since the initial quake, including one on 26 October 2013 (local time) of magnitude 7.1 Mw. Aftershocks follow Omori's law, which states that the rate of aftershocks declines with the reciprocal of the time since the main quake. The aftershocks will thus taper off in time, but could continue for years. The earthquake moved Honshu east, shifted the Earth on its axis by estimates of between and , increased Earth's rotational speed by 1.8 µs per day, and generated infrasound waves detected in perturbations of the low-orbiting Gravity Field and Steady-State Ocean Circulation Explorer satellite. Initially, the earthquake caused sinking of part of Honshu's Pacific coast by up to roughly a metre, but after about three years, the coast rose back and then kept on rising to exceed its original height. Geology This megathrust earthquake was a recurrence of the mechanism of the earlier 869 Sanriku earthquake, which has been estimated as having a magnitude of at least 8.4 Mw, which also created a large tsunami that inundated the Sendai plain. Three tsunami deposits have been identified within the Holocene sequence of the plain, all formed within the last 3,000 years, suggesting an 800