Tsunami-forecast model developed by Seattle scientists passes first big test

In its first major test, a tsunami-forecast model developed by Seattle scientists did a good job of estimating the timing and size of the waves that hit Hawaii and other parts of the Pacific basin after the massive earthquake off the Chilean coast on Saturday.

"We nailed it pretty closely," said Vasily Titov, lead tsunami modeler for the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory (PMEL).

The computer model Vasily and his co-workers have been fine-tuning for the past several years predicted maximum surges of about 3 feet in coastal towns across Hawaii. Actual tidal gauge measurements from the islands validate that prediction.

There's still some confusion as to why officials in Hawaii initially warned that waves up to 10 feet were possible.

Staffers at NOAA's Pacific Tsunami Warning System in Hawaii haven't completely integrated the new tsunami model into their operations, said Director Charles McCreery. They also weren't sure how reliable the model would be, he added.

The public warning that led to the evacuation of up to 50,000 people from Hawaii's coastlines was based on many factors — including the state's history of deadly tsunamis originating off Chile and reports that some islands in the tsunami's path were battered by waves up to 12 feet tall, McCreery said.

"We can't really gamble with people's lives."

Even waves of 3 feet or less can create currents that could drag people off beaches or damage boats and buildings, said tsunami expert Eddie Bernard, PMEL's director.

While most communities far away from the earthquake's epicenter escaped major tsunami damage, hundreds of people perished as waves slammed cities and villages along Chile's coast.

Titov received an automatic cellphone alert shortly after the earthquake struck. He raced to NOAA's Sand Point headquarters — and found several members of his team already at work. Within minutes, their model had generated a map of how the tsunami was likely to propagate across the ocean. But initial results are uncertain. The scientists updated their forecasts as more detail on the earthquake arrived.

Tsunamis are triggered when earthquakes shove the seafloor upward, displacing vast amounts of water. The power of the quake and the size and orientation of the seismic slip are key factors in determining how big the tsunami will be and in which direction it will travel.

Once the tsunami is under way, the most valuable data for the forecast model comes from undersea sensors called DART stations, for Deep-ocean Assessment and Reporting of Tsunami. As tsunami waves pass overhead, the pressure-sensitive instruments measure their size and transmit the data.

Since the devastating Indian Ocean tsunami, NOAA has expanded its DART network in the Pacific from six instruments to 32.

A single station sits off the coast of Chile.

It took the wave about three hours to reach that instrument, which almost instantaneously beamed its measurements to the waiting scientists. That single snapshot of the tsunami's size, force and direction was enough to significantly boost the forecast accuracy, Titov said.

Once they had what they considered to be a reliable forecast, the Seattle team shipped the information to the tsunami-warning center in Hawaii. Scientists there were also feeding information into the model and tracking the results — which provided the basis for tsunami warnings across the Pacific.

But the initial model only provides a picture of the tsunami as it propagates through the open ocean. How destructive the waves will be as they hit shore also depends on coastal topography and details like the size and depth of a harbor.

So Titov and his colleagues have gathered that level of detail to develop high-resolution models for 43 communities along the Pacific Coast of the United States, from Hawaii and California to Washington and Oregon. In the coming years they will add another 32 communities to that list.

McCreery hopes the community-level modeling eventually will be expanded to include cities and ports in all Pacific Rim nations, many of which rely on the U.S. for warning.

Bernard, the PMEL director, gives the warning system a grade of "B+" for its performance on the Chilean tsunami. He'd like to have more instruments in the Pacific, a goal Seattle scientists are working toward with the development of smaller sensors that can be more cheaply and easily deployed.

Forecasters making life-and-death decisions still need a better understanding of how much confidence they can place in the models, McCreery added. But as use of the system becomes routine, it should lead to more accurate predictions and fewer false alarms, he said.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com