9.0 Quake was big even for Japan

BBC News, By Jonathan Amos, Science correspondent, 11 March 2011 (11-3-11)

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Another day, another earthquake. Except the Magnitude 8.9 tremor off the coast of Honshu, Japan, will be a standout event for 2011 - perhaps not in terms of the eventual death toll it brings, but certainly in scale.

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There are usually only one or two quakes of this size every year. And even for a country such as Japan, which is very familiar with seismic hazards, this is extraordinary.

The history books show there have been seven quakes rated at 8.0 or greater since 1891 in Japan. And with big tremors come big aftershocks.

Following the initial 8.8 event at 1446 local time (0546 GMT), a sequence of major tremors was initiated - six of them within an hour-and-a-quarter that were all bigger than or all equal to last month's quake in Christchurch, New Zealand (6.3). The largest of the aftershocks was a 7.1.

Some of the early video footage to emerge from Japan was dramatic - city workers hanging on to their desks as everything rocked around them and buildings on fire being swept across farmland as tsunami waters washed inland.

The tectonics in this part of the world are, of course, well-understood. It is one of the most seismically active areas on Earth. The country accounts for about 20% of global quakes of Magnitude 6.0 or greater, and seismometers are recording some kind of event every five minutes, on average.

Japan lies on the infamous "Ring of Fire", the line of frequent quakes and volcanic eruptions that encircles virtually the entire Pacific Rim.

At this location, the dense rock making up the Pacific Ocean's floor is being pulled down underneath Japan as it moves westwards towards Eurasia. The epicentre was well out to sea - some 130km from the city of Sendai; but at a relatively shallow depth below the seabed - just 24km.

This clearly led to a fair degree of vertical upward movement in the bed as the resultant tsunami were soon hitting shorelines.

The US-run Pacific Tsunami Warning Center said amplitudes of up 7.3m were recorded on the coast of Japan. Even out in the deep ocean, the specialist tsunami warning buoys were recording wave amplitudes of a metre, which is considerable.

This means waves will reach out across the Pacific, towards the Philippines, Hawaii and perhaps even to be recorded on the North and South American landmass.

What is likely to interest seismologists will be the association with a number of very strong foreshocks in recent days.

These began on 9 March with a Magnitude 7.2 event just 40km from Friday's earthquake, and continued with a further three earthquakes greater than Magnitude 6 on the same day.

In terms of public awareness and reaction, these foreshocks could turn out to be quite important because they will have reminded people what they are supposed to do in a big quake to protect themselves.

Remember, the scale used to measure earthquakes is not a simple linear one.

Each step in magnitude equates to a 32 times jump in the release of energy. As a consequence, the Friday's 8.9 event was some 250 times more energetic than anything seen on Wednesday this week; and about 1,400 times more energetic than the the Great Hanshin earthquake, or Kobe earthquake, in 1995 (M 6.8).

Jonathan.Amos-INTERNET@bbc.co.uk

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