A moderate earthquake measuring 4.5 on the Richter scale shook Tokyo, Ibaraki Ken Nanbu Region at 15:13 p.m. JST 20 August. There are no immediate reports of damage or casualties.

The quake struck at about 36.1N 139.9E with a depth of 50 km, reported by the Japan Meteorological Agency.

The quake was first reported on the popular micro-blogging network Twitter even before the news could be out on any conventional channels.

Ibaraki Prefecture is a prefecture of Japan located in the Kantō region on Honshū island. The capital is Mito. It is the northeastern part of the Kantō region, stretching between Tochigi Prefecture and the Pacific Ocean and bounded on the north and south by Fukushima Prefecture and Chiba Prefecture. It also has borders on the southwest with Gunma Prefecture and Saitama Prefecture. The northernmost part of the prefecture is mountainous, but most of the prefecture is a flat plain with many lakes.

Japan is located on the Pacific Ring of Fire, at the juncture of three tectonic plates, gives Japan frequent low-intensity tremors and occasional volcanic activity. Destructive earthquakes, often resulting in tsunamis, occur several times each century. The most recent major quakes are the 2004 Chūetsu earthquake and the Great Hanshin Earthquake of 1995.

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