The Chile Earthquake and Pacific Tsunami Alert
The Chilean earthquake occurred Saturday (27/2/10) at 5.34 pm Sydney time. The earthquake was a magnitude 8.8 the largest in years and far more powerful than last month’s Haiti earthquake. It occurred at a depth of 55 km just off the southern coast of Chile.
12 minutes after the earthquake the Pacific Tsunami Centre in Hawaii released its first Pacific-wide warning, still too early to know if a tsunami had been generated.
18 minutes after the quake the first tsunami reached the Chilean coast with the highest waves reaching 2.4 m while most of the coast received a wave less than 1 m high.
Fortunately, the very severe earthquake only generated a relatively small tsunami, ranging from 0.3 to 2.4 m in height along the Chilean coast and less elsewhere. The tsunami propagated out across the Pacific at a speed of about 700 km/hr. It reached Hawaii 15 hours later with a wave 1 m high on Maui, 1.5 m high on New Zealand’s Chatham Island, 0.5 m at Norfolk Island, 0.4 m at Vanuatu and only 0.1 m along the Tasmanian and NSW coast, where the tsunami arrived 15 hours after the quake. The tsunami finally reached Alaska 18 hours later where it was 0.1 m high.
Many people like me headed to the coastal vantage points to wait and watch, and I am sure most like me didn’t see much, as a 0.1 m tsunami is essentially undetectable to the naked eye. Outside of Chile no severe damage has been reported
The reason for the large earthquake but only relatively small tsunami can be explained by the nature of the earthquake. To generate a tsunami the quake must also cause a displacement in the seafloor and/or ocean surface, either up or down. The huge 8.9 scale 2004 Boxing Day tsunami for instance, generated a sudden 10 m rise in the seafloor along a 1000 km long section of the seabed off Sumatra. This produced the massive and devastating Aceh tsunami.
It appears the equally large, but deep Chilean earthquake generated a much smaller shift in the seabed and hence a smaller tsunami, as there is no direct correlation between the size of an earthquake and size of any tsunami. This is why the Pacific Tsunami Warning Centre can detect the earthquake but cannot say if a tsunami has occurred until one is actually recorded at the coast or by a deep ocean tsunami detection buoy.
Meanwhile so many people turned to Coastwatch.com to find out what was happening the site crashed at 7.15 am Sunday, a victim of the tsunami!!
For more information about the Australian Tsunami Warning systems go to http://www.bom.gov.au/tsunami/about_atws.shtml
Prof Andy Short, OAM
Senior Coastal Scientist
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