Tsunami Waves Recorded on Japanese Coasts

Small tsunami waves have been reported on Japanese coasts following the magnitude 7.5 earthquake recorded in Papua, Indonesia at 11:34 NZDT this morning.

Sunday 4th January 2009

The Japan Meteorological Agency issued a tsunami warning for southern coasts of the Japanese islands of Kyushu, Shikoku and Honshu at 2:32 p.m. NZDT, estimating that waves could raise the sea level by about half a metre.

The waves began arriving at Japanese wave gauges at about 2 o’clock New Zealand time, reaching heights of between 10 and 40 cm at various locations.

This is in accordance with data supplied in a bulletin from the Pacific Tsunami Warning Centre at 3:11 p.m. NZDT stating that small waves had been detected at various pacific locations. Sea level changes of between 4 cm at Yap and 15 cm at Shikoku, Japan were reported.

The latest bulletin from the Japan Meteorological Agency advises that succeeding waves may be higher, but the forecast remains for wave heights of about half a metre. The interval between the waves has been measured at 14 to 18 minutes in Japan.

Warnings have not been issued for New Zealand or western coasts of the United States. Based on data released after the first of the deadly pair of earthquakes this morning, the New Zealand arrival time of the first of any tsunami waves will be about 12 hours after the earthquake, close to midnight tonight. While no warning is in effect, the timing lessens the chances of swimmers and recreational fishers experiencing a change in sea level.

Tsunami waves travel at about 700 km/h in the open ocean and, in deep water, may be only centimetres high. They slow dramatically as they near the coasts of land, and the rising sea-floor causes the wave to increase in height. The shape of the coast where the wave makes landfall determines the final height of each wave as it comes ashore, with some locations amplifying waves arriving from a particular direction while others attenuate the waves, reducing their effect.

This variability makes the likely effect of any particular tsunami event difficult to accurately predict. However, methods are improving, with scientists combining evidence of old events with historical observations and data being supplied from the growing networks of wave gauges.

[Compiled from data supplied by the U.S. Geological Survey and its contributing agencies, the U.S. National Oceanic & Atmospheric Administration’s Pacific Tsunami Warning Centre, and the Japan Meteorological Agency.]

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