Earthquake

The Tōhoku Earthquake was a magnitude-nine earthquake that struck the northeastern coast of Japan. The earthquake was the fifth-strongest since 1900, and the seventh-largest in recorded history. It also was the strongest to ever be recorded in the Tohoku region, as well as the having the largest fault slip ever in human history.

Cause
The earthquake was caused by the subduction of the Pacific tectonic plate under the plate that Japan is on. The fault that lies at the boundary of the two plates is called the Japan trench.

At the Japan trench, the two tectonic plates slipped past each other approximately 5 metres, with other sources saying as far as 80 metres. The earthquake occurred as a result of the built-up tension between the two plates, and possibly aided by the lubricating effects of clay from the Pacific plate, which has been dragged to the region where it mets the Philippine and Eurasian plates, where Japan is located. This proposed clay lubrication is supported by the fact that, during drilling and sampling to discover anomalies in the cause, engineers and scientists found that the earthquake produced a large amount of heat (ranging from (600o to 1200oC).

This earthquake also triggered a tsunami soon after.

Impact
The earthquake caused the tsunami, which was the third mega earthquake-generated tsunami in the decade, following the Sumatra tsunami and the Chile tsunami. The tsunami in-turn caused the Fukushima Nuclear Disaster.

Aside from the tsunami, the earthquake had 10907 aftershocks, most of which contributed to the destruction and damage of buildings and infrastructure.

External Link
A real-time video of seismic sensors