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Science Frontiers ONLINE No. 115: Jan-Feb 1998 |
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The hilina slump a.k.a. "the big crack"
Huge chunks of the Hawaiian Islands have been sliding into the Pacific Ocean for hundreds of thousands of years. (SF#101) Geologists classify these slides as either "slumps" or "debris avalanches." Slumps move just a few inches a year but are prone to bigger, jerky adjustments. Debris avalanches are fast cascades of rocks and soil. In Hawaii, both varieties of movement can involve massive blocks of real estate. In the huge Nu'uanu debris slide, stone blocks 6 miles across tumbled 30 miles out to sea. Both slumps and debris slides may create colossal tsunamis. (Tsunamis are miscalled "tidal waves," but they have nothing to do with tides and do not behave like tides or wind-driven waves.)
When large pieces of the Hawaiian Islands slip into the ocean, the entire Pacific Rim is smashed by the resulting tsunamis. In New South Wales, Australia, there is geological evidence that part of this coast was scoured by a Hawaiigenerated tsunami 100,000 years ago. The postulated wave started out about 375-meters (�-mile) high in Hawaii. By the time is reached Australia, it was about 40 meters high. (SF#85)
Worse waves may be on tap. A 4,760 cubic mile chunk of the Big Island (Hawaii) is breaking away at the rate of 4 inches per year. This is the Hilina Slump, and it is said to be "the most rapidly moving tract of ground on Earth for its size." The Hilina Slump can move much faster. At 4:48 AM, November 29, 1975, a 37-mile-wide section suddenly dropped 11� feet and slid seaward 26 feet. The result was a magnitude-7.2 quake and a 48-foot-high tsunami. This was a minor of the slump. If the entire 4,760-cubic-mile block decided to break off, it would probably create a magnitude-9 quake and a tsunami 1,000-feet high. All the coast-hugging cities of the Hawaiian Islands would be swept away. And LOOK OUT Australia, Japan, and California.
(Napier, A. Kam; "Landslide," Honolulu, p. 28, February 1997. Cr. H. DeKalb.)
Comment. Tsunamis travel at jet speeds on the deep, open ocean and have such small amplitudes that ships rarely notice them. Only when they reach shallow water do they slow down and reach monstrous sizes.