<img alt=“samoa quake.jpg” src=“http://blogs.nature.com/news/thegreatbeyond/samoa%20quake.jpg” width=“307” height=“322” align=“right” border=0 hspace=“10px”/>Posted for Quirin Schiermeier and David Cyranoski
A massive earthquake triggered a tsunami that has devastated Samoa and American Samoa killing dozens and perhaps hundreds.
The earthquake, which the Japanese Meteorological Agency measured as a magnitude 8.3, struck at 6:48 local time at a reported depth of 32 kilometres and a distance of 190 kilometres from the Samoan islands. But most of the damage came with the tsunami waves, measuring up to 6 metres in American Samoa, that hit shore shortly afterwards.
Residents in Samoa complained of having little or no warning, some saying they only had 3 minutes. Tide gage records indicate the waves arrived in Pago Pago 8 minutes after the initial warning was issued and in Apia 28 minutes after the warning was issued.
“Clearly, there was very little time for evacuations,” says Costas Synolakis, a tsunami specialist at the University of South California in Los Angeles.
“What is abundantly clear once again is how important public education is for communities at risk, that strong ground shaking IS the warning to evacuate to high ground. The shaking lasted for at least 3 minutes.
“Our mantra is to evacuate if on the coast and if feeling an earthquake that lasts more than 30 seconds, only it is very, very hard to convince local officials to implement public education campaigns, particularly if there hasn’t been a strong event in living memory. With self-evacuation without waiting for warnings, many lives would had been spared.”
Samoa, Tonga, and other island groups in the south-central Pacific, are known to be at risk of tsunamis generated by quakes near the Tonga arc system.
In 1917, a magnitude 8.5 earthquake close to the epicentre of yesterday’s quake, likely triggered a tsunami that was destructive in Samoa but hardly perceptible in the far-field. Hydrodynamic simulations of a tsunami wave triggered by an earthquake near Tonga in 1865 suggest that run-up heights reached 2 metres in Rarotonga and 80 centimetres in the Marqesas Islands, hundreds of kilometres to the east. (Okal, E., Borrero, J. and Synolakis, C. Geophysical J. Int. 157, 164-174; 2004).
“The scenario establishes that Tonga does carry the potential threat of generating a destructive far-field tsunami,” the study concludes. “Such a risk should be taken into account as part of tsunami warning and mitigation procedures for the Cook Islands, French Polynesia and other island groups in the south-central Pacific, especially given the shorter travel time of tsunami waves from Tonga, as compared with other tsunamigenic zones around the Pacific.”
The shallowness of this earthquake, along with its size, caused the big waves, says Shinsaburo Nishi of the Japan Meteorological Administration’s earthquake and tsunami monitoring division. “Some of the energy from an earthquake 100 kilometres or more deep would be absorbed by the earth in between,” says Nishi.
In fact, this one was so shallow, estimated at only some 30 kilometres depth, that Japan [the JMA] did its own tsunami calculations as if the earthquake occurred at sea level. “The difference in calculations is less than the margin of error,” he says. “We did it to be safe,” he adds. The added safety factor gave an estimate of a 50 cm tsunami hitting Japan, but when it finally came, the largest tsunami (in Aikawa on the north-eastern coast) was 15 centimetres.
In Japan, a tsunami warning would have gone out within 2 minutes, says Nishi. “But how quickly you get the message out to people depends on the previous experience with tsunami and infrastructure. In Japan we can get the message out quickly, and most people will know to take refuge.”
The speed of the tsunami depends, among other things on the depth of the water. Roughly speaking a tsunami in 5000 metres depth travels at 800 kilometres per hour (“the speed of an aircraft”), in 500 metres at 250 kilometres per hour (“the speed of the bullet train”), in 100 metres depth at 100 kilometres per hour (“the speed of a car”), says Nishi.
Image top: earthquake location / USGS
Image lower: tsunami simulation by Costas Synolakis and Nikos Kalligeris