Possible sources
of tsunami being
reconsidered

Little work has been done on
the impact to Hawaii of a large
temblor to the west

After the Dec. 26 Indian Ocean tsunami, scientists are raising questions about the threat of a potential tsunami from a "100-year, 9.0 magnitude earthquake" originating west or southwest of Hawaii.

In a paper published in the online journal Science of Tsunami Hazards, Oahu Civil Defense tsunami adviser Daniel Walker suggests further study is needed to prepare for a tsunami from areas where large seismic waves have not hit Hawaii in the recorded past.

An important question, Walker wrote in an e-mail to the Star-Bulletin, is whether no major tsunami has hit Hawaii from the regions because of protection from island chains and shallow water between Hawaii and earthquake zones, or because an earthquake large enough to generate a tsunami has not yet occurred.

"Regarding the Western Pacific, the largest tsunami recorded since 1900 was from an 8.4 earthquake (11 feet high on the Big Island). A 9.0 or greater could present a far more dangerous situation," the retired UH-Manoa senior seismologist said.

The 1933 tsunami originated in Japan, just northwest of Hawaii. Other deadly or destructive tsunamis in recorded history originated in the north or northeast, from the Aleutian Islands and Alaska; the southeast from Chile; or were locally generated off the Big Island.

"That only means we haven't had one in the last 200 years, not that we're not going to get one," said Walter Dudley, a tsunami researcher at UH-Hilo.

"We are in the middle of the Pacific. We can receive tsunamis from anywhere in the Pacific Rim or anywhere in the interior, and we generate our own tsunamis," he said.

Some possible sources include the Marianas Trench, directly west of Hawaii, and the Tonga and Kermadec trenches, southwest of the state between New Zealand and Tonga.

But because deadly or destructive tsunamis haven't been generated from those areas, scientists aren't certain what would happen if a large earthquake occurred there.

The tsunami maps in the phone book don't reflect that danger, and there are no computer models to help predict what would happen.

Current evacuation plans for areas like Kailua-Kona, which has seen a lot of oceanfront development in the last few years, may not be adequate in the event of a tsunami from an unexpected direction, Dudley said.

In the event of a large earthquake, the Pacific Tsunami Warning Center should be able to get a warning out to Hawaii and other areas that could be affected, said Charles "Chip" McCreery, the geophysicist in charge at the center.

There are tidal gauges near Japan, Wake Island and elsewhere in the Pacific that should be able to measure a tsunami generated, he said. Hawaii would have at least a three-hour warning.

After the Dec. 26 Indian Ocean tsunami, funding was made available to add new tidal buoys between Hawaii and west Pacific faults, and to the south of the islands, McCreery added, although there's no timetable yet on when the new buoys will be in place.

McCreery said they also are working on new computer models and tsunami inundation zone maps.

But because a major tsunami hasn't come from those areas, "data is limited."

Former UH tsunami scientist Gerard Fryer said the Marianas Trench hasn't been very active, but it has the potential to generate a large tsunami. Because of its location, there aren't major land masses between it and Hawaii.

"We basically pooh-poohed the hazards from the Marianas, and that could be a mistake," Fryer said.

While there's no recorded history of a large tsunami from the south, a sinkhole dug on Kauai has produced geologic evidence of a huge tsunami that hit the south shore of that island 500 years ago.

The question is where that tsunami came from, Dudley said. Was it locally generated, or from a large earthquake in the southwestern Pacific?

In 1901, an 8.0 earthquake on the Tonga Trench sent a 4-foot tsunami to Kailua-Kona that caused minor damage, Fryer said.

A 9.0 earthquake on the Richter scale would release about 31 times more energy than an 8.0 earthquake.

The fault is home to a chain of underwater volcanoes that have the potential to create underwater landslides, in addition to the tsunami threat from earthquakes.

"You've got a whole large oceanic plate descending, which creates a line of volcanoes," said John Wiltshire, one of several UH-Manoa scientists who are studying the trench and underwater volcanoes there.

"We can tell there's been smaller slumps and movements, which also implies that there are larger slumps and larger movements," Wiltshire said.

"The Tonga Trench is not aimed the right way," said Fryer, who added that South America may be more in danger from an earthquake there. Even if a tsunami were generated from the Tonga Trench, Fryer said, islands and coral reefs between the fault and Hawaii would slow and absorb most of the tsunami energy before it reached our shores.

Still, Fryer notes, most scientists did not think there was much risk of a tsunami in the Indian Ocean until it happened.

Wiltshire agreed that a tsunami is more likely to come from the Aleutian fault. But, he added, "the study of the earth is full of surprises."