Quake was big even for Japan

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

There are usually only one or two quakes of M 8.0 and above 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 M 8.0 or greater since 1891 in Japan. And with big tremors come big aftershocks.

Following the initial M 8.9 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 (M 6.3). The largest of the aftershocks was a M 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 (subducted) 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.

"The earthquake happened on the Japan Trench which runs roughly north-south and the fault dips shallowly westwards towards Japan at about 15-20 degrees," explained Dr John Elliott from the Centre for the Observation and Modelling of Earthquakes and Tectonics (Comet) at Oxford University, UK.

"Given the size of the earthquake, the fault is likely to have ruptured for about 500km.

"The previous earthquake to rupture probably the same section of fault was the 1933 M 8.4 earthquake (3,000 deaths) and had a large associated tsunami as well."

As slipping occurs along these great lengths, the shifting sea floor lifts a great mass of water along the fault line, launching tsunami - principally along a line perpendicular to the fault, with lesser intensity in the directions along the fault's length.

For Friday's event, that means effects to the east - for instance at Hawaii - will be much more pronounced than those to the north and south.

The US-run Pacific Tsunami Warning Center said amplitudes of up 7.3m were recorded on the coast of Japan.

Japan's Kyodo news agency reported that a 10m wave (33ft) struck the port of Sendai, carrying ships, vehicles and other debris inland.

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 have reached out across the Pacific, towards the Philippines, Hawaii and perhaps even to be recorded on the landmasses of North and South America.

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 M 7.2 event just 40km from Friday's earthquake, and continued with a further three earthquakes greater than M 6.0 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, Friday's M 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 Great Hanshin, or Kobe, earthquake in 1995 (M 6.8).

Japan has had about 10 quakes since 1900 that have resulted in major casualties, typically a few thousand people each time.

"The exception is the 1923 Great Kanto earthquake, Tokyo, which killed 140,000," Dr Elliot said.

"The 1995 Kobe earthquake in southern Japan had about 6,500 deaths, but was different in that it was not a subduction event, and was located under a populated area."

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