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Tiny tsunami strikes Japan

November 15, 2006 By Nicola Jones This article courtesy of Nature News.

Giant quake generates alert but only small waves.

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A gigantic magnitude-8.1 earthquake in Japan today triggered warnings of a tsunami and a call for limited evacuation, although the waves that hit land only measured tens of centimetres at their highest.

The quake struck at 20:14 local time in the Kuril Islands, which stretch northeast from Hokkaido to the Kamchatka peninsula. Earthquakes are extremely common in Japan, but a quake of this size is rare. As this story went to press, there were no reports of major damage from the quake itself.

The Japan Meteorological Agency issued an alert at 20:29 calling for people to evacuate the seashore immediately on the Sea of Okhotsk coast and the eastern part of the Pacific coast of Hokkaido an area that is not densely populated. Waves were at that point forecast to be potentially "up to 2 metres" high.

At roughly the same time the Pacific tsunami warning centre similarly issued an alert noting a quake of magnitude 7.7 and warning of the possibility of a tsunami, although at that time no data were available to confirm whether or not a wave had formed, they said. This was updated 45 minutes later when Pacific scientists upgraded the magnitude to 8.1.

Prominent broadcast

Japan is very well equipped to assess tsunami risks and communicate these to the public. Seismic monitors, computer simulation programs and round-the-clock staff are capable of detecting a quake and forecasting warnings within minutes of an event. Almost all television stations in Japan switched to news during this evening's potential crisis, and hosted a map of the country showing areas of high risk along with a scrolling list of some 100 places with projected times and heights of wave arrival. Police, fire engines and sirens alerted people to danger.

Local governments started putting out calls for evacuation by 20:30, and some trains were also stopped. About 1,800 people were under a compulsory evacuation order, and 130,000 were advised to head for higher ground. As of 23:00, 3,900 people had evacuated, causing minor traffic jams.

But whether or not a tsunami forms depends not just on the size and location of an earthquake, but also on the details of how the earth moved a sudden upwards slip of land underwater, for example, is much more likely to produce a wave than a slow sideways slide. These factors are much more difficult to determine.

"It takes a while to work out what type of quake it is," says Simon Boxall, an oceanographer at Southampton University, UK. "So the safest thing is to issue alerts based on the quake's magnitude and location, out of caution."

The first report of tsunami activity provided by the Pacific centre was of a small wave of 30 centimetres at Hanasaki Hokkaido, at 21:43 local time. Other reports confirmed similarly small waves. Waves of this height are unlikely to cause any damage in an area like Japan where the normal tidal range is in the order of metres, notes Boxall, but could prove more problematic in places with a smaller tidal range, such as the Mediterranean and the Indian Ocean.

Japan downgraded its alert from a 'warning' to an 'advisory', with anticipated wave heights of less than half a metre, at 23:30. The advisory was cancelled altogether at 1:30 Thursday morning.

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