Scientists drawing maps to outline tsunami danger zone

Posted: Monday, November 19, 2001

HOMER (AP) -- Scientists will draw new maps this winter outlining the tsunami danger zones of Homer and other Kachemak Bay communities.

The maps will be made by feeding the latest charts of ocean-bottom contours and new models of wave behavior into a supercomputer.

The effort is part of a federally funded program to assess tsunami risks in 80 exposed coastal communities in Alaska. The first such hazard maps were introduced this month for Kodiak, where the worst-case wave generated by the computer turned out to be the wave that devastated the town's waterfront in 1964.

After Homer comes Seward, where sonar mapping of Resurrection Bay has been under way this year. Other priority communities are Sitka, Sand Point, Unalaska, Yakutat and Whittier.

Tsunamis are waves generated by earthquakes or landslides. In the ocean, where subsea buoys now send warnings if a wall of water rushes past, a 500 mph tsunami may only create a swell a few feet high. But when the waves hit land, they can become deadly forces of nature.

The objective of the mapping program is to plan evacuation routes and increase public awareness, said state officials involved in the project.

''If you have an earthquake where you can't stand up, head to high ground. Don't wait for a warning,'' said Scott Simmons with the state Division of Emergency Services, who met with Homer officials last week.

Anchorage is not considered high risk because upper Cook Inlet is so long and shallow. A tsunami generated in deep water would tend to spread out and lose force as it moved north, said scientists with the Alaska Earthquake Information Center at the University of Alaska Fairbanks, who are preparing the new maps.

Tsunamis don't approach land as a towering curl of surf, said state seismologist Roger Hansen. A tsunami is more like an incoming tide, only one that races across the tidal flats as fast as 40 mph, faster than anyone can run away. Sometimes it can be fronted by a small wall of water known as a bore tide, Hansen said.

Geophysicists have learned to project the course of a tsunami by studying the shape of the ocean bottom. In Kodiak, for example, big channels under the sea would funnel waves in certain directions, increasing the impacts in some places and diminishing them elsewhere.

University scientists mapped out several tsunami scenarios for Kodiak based on different earthquake fault lines in the region. They had a lucky chance to test their theories by plugging in data from the 1964 Alaska earthquake and then comparing their theoretical results with the actual damage from the tsunami that struck Kodiak. The results were very close, Hansen said.

In fact, that wave proved to be the most damaging of the scenarios tested.

Homer's situation will be different. Kachemak Bay suffered damage in 1964 from sinking land forms, not from waves. The likeliest danger for Homer would be a tsunami generated by a landslide on Augustine Volcano, an island 60 miles away, the geophysicists said.

Homer is a concern because the low-lying Homer Spit is heavily populated with fishery workers and tourists in summer.

A wave from Augustine would probably reach the Homer Spit in an hour and twenty minutes, said geophysicist Elena Suleimani, who based her study on an 1883 volcanic eruption and wave that hit the village of Nanwalek.

The bad news is there would probably be little warning of a wave generated by a distant landslide. No major earthquake would have been felt to trigger an evacuation.

The good news is that Suleimani calculated the sea rose only three feet when the Augustine wave hit the then unpopulated Spit in 1883. Homer city officials said the Spit road is built to withstand waves bigger than that from occasional severe storms.



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