An earthquake of Magnitude 7.2 struck off the east coast of the Japanese island of Honshu on 9 March 2011. Although significant in size, and despite causing tremors which were felt throughout Japan, no casualties had been reported at the time of writing and the forecasts by the United States Geological Survey indicated that the likelihood of damage or casualties was low.
The Honshu Earthquake of March 9 2011
The earthquake report issued by the USGS (“Magnitude 7.2 - Near the east coast of Honshu, Japan” earthquake.usgs.gov) indicated that the quake, which struck at just before midday local time, off the east coast of Honshu and some 400 km to the north east of the capital, Tokyo, occurred at a relatively shallow depth of around 14km. Several aftershocks greater than M5.0 were also recorded.
Submarine earthquakes of this magnitude are capable of generating tsunamis, or tidal waves. Although a local tsunami warning was issued, identifying possible damage along coasts within 100km of the epicentre (NOAA Pacific tsunami warning bulletin, weather.gov), reports indicate that the ensuing wave was small and caused no damage (Sydney Morning Herald, “Earthquake Rocks Japan by Glenda Kwek, smh.com.au).
Japan’s Tectonic Setting
Because of its location on the earth’s surface, Japan is subject to regular seismic events. Part of the Pacific Ring of Fire (a seismically active belt, characterised by earthquake and volcanic activity, surrounding the Pacific Ocean), the archipelago which makes up the country owes its existence to complex tectonic processes arising from the movement of the earth’s plates.
Japan lies in the western Pacific and is located close to the junction of four of the earth’s tectonic plates. The most significant motion is between the Pacific Plate, which is moving roughly north-west at the relatively rapid rate of around 8mm per year, and the Eurasian Plate. The Philippine and North American plates lie to the south and north respectively (NUMO, Tectonic Setting and Evolution of Japan, numo.or.jp).
The relative movement of these four plates – sometimes direct and sometimes oblique - causes deep and complex stresses within the earth. Off the eastern coast of the Japanese island arc, the Pacific Plate, which is relatively dense, is being forced beneath the Eurasian Plate – a process which is apparent on the sea floor in the form of deep trenches.
The stresses generated by these large-scale and conflicting movements build up to a point at which they are periodically relieved by earthquakes. These tremors take place regularly and are often of significant size: the USGS listing of historic earthquakes indicates 20 events greater than M7.0 in Japan since 1891, five of them occurring off the east coast of Honshu since 2000.
The March 9 earthquake, though large, was typical of recent earthquakes in the area in that it has caused little damage and no casualties (as reported at the time of writing). Historically, however, Japan has suffered significantly from earthquakes, with the most deadly recorded being the Kanto event of 1923, which killed almost 150,000 (USGS).
Although earthquakes have the potential to cause immense destruction, improved building techniques and preparations have probably contributed significantly to the relatively limited number of earthquake fatalities to occur in Japan in recent years. The country’s death toll stands at, with fewer than 6,000 deaths since 1960 – the majority of them in the 1995 Kobe earthquake, which killed 5,502 people (USGS).
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