Chile quake 2010: Tsunami warning system worked as intended
The Pacific basin's warning network detected the Chile quake's tsunami accurately, and computer models are improving, tsunami experts say.
The Chile quake that struck Saturday and triggered a tsunami that traveled throughout the Pacific basin provided the first basin-wide test of the region's tsunami-warning network in decades.
By several accounts, the region-wide warning effort was a success, though in retrospect evacuations in many places far from the Chilean coast appear to have been excessive given the size of the surge that ultimately arrived.
That apparent disconnect highlights the complexity of tsunami hazard responses, specialists say.
Gauging damage, accuracy
The major warning centers in Hawaii and Alaska issue basin-wide warnings, which include estimated time of arrival for the first of what usually are a series of waves. And they offer a projection of the tsunamis' heights as they approach a coast but remain in open water.
It's up to national, state, and local governments, however, to estimate the effects tsunamis can have at specific sites as they come ashore, then issue evacuation orders.
The warning centers in Hawaii and Alaska "did what their mandate called for" and "did a very good job," says Tad Murty, an adjunct professor specializing in tsunamis at the University of Ottawa in Ontario. The warning center estimated that tsunamis would begin to affect the Hawaiian Islands 15 hours after the magnitude 8.8 quake struck. The forecast was off by 5 minutes, he says.
Tsunamis can be tough to track to anyone bobbing on the ocean's surface. They may represent an increase of eight to 20 inches in the average sea-level height as they pass. That brief change can get lost on a surface already rolling with ranks of large, storm-generated swells stretching as far as the eye can see, or wind-driven waves in a storm.
The answer: pressure-sensitive buoys as deep as 20,000 feet on the ocean floor. Since an earthquake such as Saturday's displaces water from the sea-floor up, these buoys sense the changes in bottom pressure tsunamis trigger as they propagate from their source.
As tsunamis sweep past chains of undersea volcanoes or similar natural sea-floor barriers, they either can intensify or weaken, depending on the conditions they encounter.
But the true forecasting challenge comes as the tsunami interacts with the near-shore bottom, Dr. Murty explains.
Valpariso, Chile, roughly 200 miles north of the quake's epicenter, experienced a tsunami topping 8 feet, while Robinson Crusoe Island, 400 miles offshore, was struck by a wave that reportedly killed four people. Eleven others remain missing.
By the time tsunamis began arriving in Hawaii, Hilo experienced tsunamis less that three feet tall.
Size isn't everything
Less than three feet may be puny by James Michener standards. But Eddie Bernard, a tsunami specialist and director of the National Oceanic and Atmospheric Administration's Pacific Marine Environment Laboratory in Seattle, notes that even small tsunamis can inflict damage, particularly in harbors. The repeated surges can smash boats against each other or against docks. And once a tsunami enters a harbor, it can in effect reverberate within the harbor for hours, much as sound waves from a bell reverberate.
Moreover, even small tsunamis can temporarily alter currents in dangerous ways, Dr. Bernard says. That was the sort of response recorded in L.A. Harbor.
"We were lucky the tsunamis came at low tide; the currents were not as strong as they could have been" at the harbor, he says.
But he adds that more needs to be done to improve site-specific forecasts.
One step forward came following the Sumatran earthquake in 2004. With input from countries in the region, NOAA put together a web-based model that allows countries to make site-specific projections. The model, which can be downloaded and run on a typical laptop if the number of sites is small, allows local managers to project potential wave heights and in-harbor reverberations as a tsunami comes ashore.
At this stage, for the US, the model focuses on harbors, Bernard says. The reasons: the high value of ships, cargo, and other assets stored there; and that's where the best tide gauges are, so model projections can be tested against real-world measurements and the models improved.
Over time, as confidence in the models increases, he says he expects that national, state, and local governments in the Pacific region and elsewhere â€“ particularly among developing countries â€“ will be able to extend the use of the model to larger lengths of coastline to include resort areas and smaller coastal cities.