Sunday 31 July 2011

Magnitude 6.4 Earthquake off the Coast of Honshu, Japan. 31 July 2011

At 3.54 Japan Standard Time on the 31st July 2011 an earthquake with a magnitude of 6.4 on the Richter scale struck off the east coast of Honshu, Japan, 18 km from the city of Iwaki in Fukushima Coast. The earthquake, which took place at a depth of about 40 km, was felt across eastern Honshu and southern Hokkaido, but does not appear to have caused significant damage to buildings or infrastructure. No tsunami warning has been issued, nor have any deaths been reported. There have been a few reports of minor injuries, mainly to elderly residents of Fukushima Prefecture.

Map of Japan showing areas where the Earthquake was felt.

Japan is particularly prone to earthquakes; the Japan Meteorological Agency reported 21 earthquakes across the country on the 30th of July and another 21 on the 29th. The country has tight building regulations and a long history of planning for earthquakes, so quakes that would cause major problems in other countries often cause only minor damage in Japan. However the Japanese are not immune to earthquake damage, and really big quakes can overwhelm their defenses. On 11 March 2011 a magnitude 9 earthquake and associated tsunami caused widespread devastation, killing over 15 000 people, destroying several cities and causing a meltdown at the Fukushima Daiichi Nuclear Power Plant. The island of Honshu was moved 2.5 m to the east, relative to the rest of Japan.

Aerial footage of the March 2011 Tsunami.

Japan is so prone to earthquakes because it lies on the conjunction of three moving tectonic plates; the Eurasian, Philippine and Pacific. To the northwest the Eurasian Plate is eastward over the Philippine and Pacific Plates. To the south the Philippine plate is moving northward over the Pacific Plate but under the Eurasian Plate. To the east the Pacific Plate is moving west, but being forced under the Eurasian and Philippine Plates.

The movement of the Eurasian, Philippine and Pacific Plates under Japan.

To make matters worse the Eurasian Plate is somewhat fractured under Japan and East Asia. with two small breakaway plates; The Okhotsk Plate, which underlies northeast Honshu, Hokkaido and the Kamchatka Peninsula (Russia) and the Amurian Plate underlying the southwest of Honshu, Korea and Manchuria. The Okhotsk plate is moving southwest relative to the rest of Eurasia, and the Amurian south-southeast.

The Amurian and Okhotsk Plates under Japan.

This also makes Japan particularly prone to volcanic activity. As one plate is forced below another, rocks are melted by the heat of the earth's interior and rise as liquid magma through the overlying plates forming volcanoes. The complicated network of plates underlying japan leads to dozens of volcanoes all over the country, many of which are active.

Volcanoes of Japan.

The Japan Meteorological Agency currently has alerts out on a number of these volcanoes. Kirishimayama and Sakurajima in the southeast are both subject to Level 3 warnings; it is considered unsafe to approach the volcano. Miyakijima in the south and Suwanosejima Satsuma-Iojima in the southeast are subject to Level 2 warnings; it is unsafe to approach the crater. There is also a hazard warning on Ioto and Fukutoku-Oka-no-Ba in the Ogasawara Islands.