Acknowledging the earthquake as the warning, long-term planning and education are key factors the territory must consider in order to lessen the local tsunami risk, says Dr. Guy Gelfenbaum, an oceanographer with the U.S. Geological Survey.

In presenting the findings of several groups of scientists who were in American Samoa to study the Sept. 29 tsunami’s impact, Gelfenbaum closed his presentation with this: “there is no easy solution to the local tsunami risk.”

“The earthquake is the warning. Long-term planning is essential. Education is the tool,” reads one of his final slides. “Individuals must know what to do.”

More than 100 people — college students, members of the academic community and concerned citizens — attended the presentation held at the American Samoa Community College Lecture Hall, Thursday, Oct. 22.

To reduce losses in future tsunamis, Gelfenbaum says tsunami education programs must be promoted. In addition, he urges “keep the memory of the Sept. 29 catastrophe”. The tsunami killed more than 100 people in Samoa, American Samoa and Tonga.

SCIENTIFIC STUDY

The goals of the tsunami study were to:

— Measure tsunami wave heights, run-up and inundation distances;

— Understand magnitude and pattern of inundation; and

— Improve models of tsunami inundation to help prepare against future tsunami hazards.

In order to accomplish these goals, scientists took eyewitness accounts from villagers, measured water levels at the beach, inland and run ups; flow directions; inundation distance; topographic profiles, sediment deposits; subsidence and coastal change.

Some traces that indicate how high the wave got include dead plants, debris and broken tree branches.

He said in studying tsunamis, timing is key as scientists must collect data quickly. In speaking with witnesses, they found that there was minor structural damage due to the earthquake and that the tsunami arrived between 15-20 minutes after the earthquake. The second wave was the largest.

Understanding the tsunami impact, said Gelefenbaum, is important for hazard mitigation in order to prepare for future incidents. Pointing to two sites, he said in Tula, in northeastern Tutuila, homes were destroyed and floods were about waist high because Tula is a low lying coastal plain. He said scientists found something like they had never seen elsewhere — clumps of land thrown onto the land by the wave.

In Poloa, which was completely wiped out save for one home and a church, Gelefenbaum said the wave was high and had a lot of force, the height of the wave in Poloa about 30-35 feet.

As reported previously in the Samoa News, the tsunami hit at 6:48 a.m. in the Samoa Islands Region, off the Tongan Trench. Gelfenbaum said Thursday the Sept. 29 tremor occurred at the subduction zone where the Pacific Plate is sliding, or, converging under the Australia Plate, the plates moving into each other 1.5 inches every year.

“This is quite fast,” he said. “The Pacific Plate is sliding under the Australia Plate. Stresses build up and the release of stresses causes a large earthquake.”

He said the Samoa Islands Region is a complicated region that has lots of earthquakes, 12 in the last 100 years. There have been a number of relatively large earthquakes in the past, the largest before the Sept. 29 tremblor taking place June 17, 1917 —an 8.5 magnitude earthquake which generated a tsunami that destroyed homes and a church in Pago Pago Harbor. Run up, or the height of the tsunami was 4 ft.

A tsunami forms when an earthquake vertically displaces the seafloor and the water above it and tsunami waves differ from wind waves, Gelfenbaum continued, noting that the wavelength of a tsunami decreases in shallow water while its height increases in shallow water.

“When the wave gets to shore it responds differently...it doesn’t just wash up and wash out,” he said. “A tsunami wave keeps flowing on shore, it moves like a fast rising tide.”

The earthquake of Sept. 29, that registered 8.0-8.3 on the Richter scale, Gelfenbaum said, resulted in a tsunami that “traveled all across the Pacific Ocean” to the west coast U.S. But he added the further the wave travels, its height dissipates.

In their studies, scientists found that witnesses saw an initial drawdown (water receding), and villagers counted differing numbers of waves. Some villagers told Samoa News they saw between three to five waves and others say there were up to seven. Scientists determined that were three large waves in Samoa’s Sept. 29 tsunami, each coming in about 12 minutes apart.

In the vicinity of village channels (auvai), they found higher run up as though the channels fueled the tsunami. In their model, scientists found that even with different reef widths, run ups were about the same.

In his conclusion, Gelfenbaum says tsunami education programs must be promoted and that the memory of Sept. 29 should be kept alive.

“People on both sides of the island had training last year and they knew what to do after the earthquake...in Tula...they had training one or two weeks before the tsunami...it helps to know what to do,” he said. “Finding ways of keeping the memory of the event so information is passed down. There might be a tsunami in a week or in a hundred years...people need to be prepared.”

Reconstruction he says must be planned so that routes and paths easily lead people to safe areas. He says critical structures such as schools and hospitals should be built out of inundation zones.

“As government plans reconstruction, if you’re going to have to rebuild and it is possible to do so, don’t locate them in inundation zones,” he said. “There is not much time...it’s only a few minutes...individuals need to know what to do...there won’t be any time for governments to warn people.”