Thursday, September 2, 2004

Clues to ancient
tsunami found

UH scientists report that a
landslide once sent waves four
miles inland on the Big Isle

A giant tsunami caused by the collapse of part of Mauna Loa volcano 120,000 years ago surged more than four miles inland and deposited coral at the 1,600-foot level of the Big Island, according to University of Hawaii scientists.

Gary McMurtry, an associate professor in the Department of Oceanography, and Gerard Fryer of the Hawaii Institute of Geophysics and Planetology studied a deposit in Kohala of coral, smashed-up marine shells, lava rock and soil on shore cemented together by what was once coralline sand. The researchers tied the deposit to similar material at a coral terrace now 1,400 feet below sea level.

The scientists believe the coral deposit is evidence of a so-called megatsunami. Their findings are reported in an article this month's Geology research journal.

The deposit is 120,000 years old, the same age as the underwater coral terrace and what is known as the Alika 2 Landslide.

The scientists theorize that the landslide, which sent 120 cubic miles of material into the ocean from Mauna Loa, created a megatsunami that swept miles inland. In contrast, the material from the 1980 Mount St. Helens eruption and landslide was less than one cubic mile, the scientists said.

"These giant landslides seem to occur during periods of higher-than-normal sea level -- like we have now," said McMurtry. "They pose a hazard not just in Hawaii, but at all big oceanic volcanoes worldwide."

However, "the chances of it happening during the lifetime of anyone presently alive is so small that it isn't something that you should lose sleep over," Fryer noted.

These gigantic landslides occur in Hawaii about every 200,000 years, Fryer said. The largest giant landslide occurred several hundred thousand years ago off Oahu when half of the Koolau Volcano fell off, creating what is now the Windward side of Oahu.

While it is not likely to happen any time soon, Fryer said it is important to study these giant landslides and megatsunamis because of the potential for destruction.

If a similar event were to occur today on the Big Island, a megatsunami would flow over the isthmus of Maui, wipe out Honolulu and could reach Wahiawa, Fryer said.

The theory of megatsunamis is controversial among geologists, Fryer said.

Other scientists believe that while a giant landslide would have generated a tsunami, they disagree about how large the tsunami was.

Some scientists believe the coral deposits were created not by a giant tsunami, but by the rising and sinking of the islands.

Scientists believe that as the Big Island grows, it is sinking, but the sinking is creating a seesaw effect on the earth's crust that is raising other islands like Oahu.

Fryer said there is a coral deposit at the 1,000-foot elevation level on Lanai that they would like to compare and date to coral deposits offshore. If they can date the coral deposit to the Alika 2 Landslide, they believe they will be able to show that the megatsunami also hit Lanai.

UH Dept. of Oceanography



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