NOAA 2000-515
Contact: Jana Goldman


Two years after a tsunami killed more than 2,000 people in a Papua New Guinea village, scientists are offering the suggestion that some of the largest tsunamis may be enhanced by underwater landslides and slumps, in addition to the movement of the ocean floor, because of an underwater earthquake.

"We've spent a lot of time looking at the possible cause of the Papua New Guinea tsunami," said Eddie Bernard, director of NOAA's Pacific Marine Environmental Laboratory in Seattle, Wash., which among other things, studies tsunamis. "There was just something wrong with the timing and size which made us take a look at other causes."

Earthquake-generated tsunamis travel at known speeds. Sensors registered a magnitude 7 earthquake at 6:49 p.m. on July 17, 1998. However, by the accepted formula, the wave that hit the north shore of the country was not only about 10 minutes too late, it was also too big.

An international scientific team went quickly to Papua New Guinea, located north of Australia, to begin to find clues as to the tsunami's origin. Communications are often very basic to non-existent in PNG, and it was critical to get eyewitness accounts and collect evidence in place before survivors relocated or material was removed by storms or scavengers.

Despite conflicting information about the wave's arrival time, the team was able to determine that the first wave hit shore a few minutes after 7 p.m. It is often the second tsunami wave that does the most damage, as curious spectators, believing that the worst is over, wade out into the receding water or climb down from a safe spot on high ground,

Noting many landslides on shore, the survey team suggested that the tsunami may have been triggered by an offshore slide, which would account for the timing delay and the larger-than-expected wall of water. A marine survey consisting of multi-beam bathymetric surveys and visual examinations with manned submersibles was conducted that found evidence of a slump, which starts and stops, and reacts to shifts in the ocean floor. Landslides, on the other hand, accelerate and just keep on going.

Earlier this year, researchers from the United States Geological Survey found evidence of a similar land formation off the southern California coast. A team from the Woods Hole Oceanographic Institution also discovered faults in the ocean floor on the east coast, an area not usually associated with tsunamis, most likely caused by eruptions of gas trapped under layers of sediment.

"These discoveries are drawing our attention to other causes of tsunamis, besides the traditional tectonic earthquake," said Bernard. "The more we learn about possible causes, the better we can know when to issue warnings."

There are two U.S.-operated tsunami warning centers, one each in Alaska and Hawaii and run by NOAA's National Weather Service. These two centers use seismometers to detect the occurrence of earthquakes within minutes of the start of an earthquake. They cannot, however, detect underwater landslides or slumps with these instruments.

A series of buoys anchored along the Pacific coast from Alaska to Monterey, Calif., can detect the presence of a tsunami. So even if a slump or landslide generates a tsunami, tsunami detectors exist to warn people of its dangers.

As with any natural hazard, the more informed the public is, the better are the chances for their survival. For instance, after the 1998 PNG tsunami, a team went to Vanuatu, a group of islands in the Pacific, and showed videos of earthquakes, tsunamis, and volcanoes to villagers. When a tsunami stuck there last December, only three people died. According to the research laboratory team, the others remembered the lessons of the video and headed for high ground when the earth began to shake and the water began to recede.