How the quake has moved Japan

Japan coast before and after quake Coastal areas has been transformed by the quake and tsunami

Japan's coastline may have shifted by as much as 4m (13ft) to the east following Friday's 8.9 Magnitude earthquake, according to experts.

Data from the country's Geonet network of around 1,200 GPS monitoring stations suggest a large displacement following the massive quake.

Dr Roger Musson from the British Geological Survey (BGS) told BBC News the movement observed following the quake was "in line with what you get when you have an earthquake this big".

The quake probably shifted Earth on its axis by about 6.5 inches (16.5cm) and caused the planet to rotate somewhat faster, shortening the length of the day by about 1.8 millionths of a second.

Japan's meteorological agency has proposed updating the magnitude of the earthquake to 9.0.

This would make it the joint fifth biggest quake since instrumental records began, but other agencies have not yet followed suit.

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

The dense rock making up the Pacific Ocean's floor is being pulled down (subducted) underneath Japan as it moves westwards towards Eurasia.

Dr Brian Baptie, also from the BGS, explained that the quake occurred on the subduction zone along two tectonic plates, the Pacific plate to the east and another plate to the west, which many geologists regard as a continuation of the North American plate.

Infographic

As the Pacific plate moves westwards underneath Japan, it drags the North American plate downwards and westwards with it.

As an earthquake occurs, the upper plate lurches upwards and eastwards, releasing strain built up as the two plates grind against one another.

In the most recent case, this movement gave a kick to the seabed, displacing a large amount of water and leading to the tsunami waves which devastated coastal areas in the Sendai region.

"The Pacific plate has moved a maximum of 20m westwards, but the amount of movement will vary even within the fault," said Dr Musson.

"That doesn't mean the whole country has shifted by that amount because the actual displacement will decay further from the fault."

Geonet is operated by Japan's Geographical Survey Institute (GSI). Work on the array began in 1993, and it has now grown into the largest GPS network in the world, according to the GSI.

Its data show a movement eastwards of up to 4m in coastal areas of Japan.

Dr Ken Hudnut, a geophysicist at the US Geological Survey (USGS) in Pasadena, California, told MSNBC that information resources linking GPS readings to maps, such as driving directions and property records, would have to be changed as a result of the shift.

"Their national network for property boundary definitions has been warped," he explained. "For ships, the nautical charts will need revision due to changed water depths, too (of about 3ft). Much of the coastline dropped by a few feet, too, we gather."

Paul.Rincon-INTERNET@bbc.co.uk

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