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Tuesday, July 29, 2014         

AFTERMATH: JAPAN 8.9 EARTHQUAKE


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Powerful science lurks in deep

A multitude of variables come into play when dissecting the genesis of a titanic undersea upheaval

By Jim Borg

POSTED:


It's a famous old parlor trick: Yank a tablecloth so fast that the plates stay in place.

Pull the cloth too slowly, however, and the plates collide and chip - or, worse, crash to the floor.

That first scenario happens every day on the table known as the Pacific Basin.

An unusually violent collision of sea-floor plates resulted yesterday in a magnitude-8.9 earthquake, one of the strongest in history, which unleashed the explosive force of many thousands of Hiroshima bombs.

The force was so strong that it moved the island of Honshu eight feet to the east, said Ken Hudnut of the U.S. Geological Survey in Pasadena, Calif.

The quake occurred at a depth of 15 miles in the Japan Trench. There the plates that form the earth's crust are on a slow-motion collision course.

Hawaii rides on the Pacific plate, creeping northwest at just over 3 inches per year - about the same rate that fingernails grow.

At the Japan Trench, the Pacific plate subducts, or slides, under the North American/Eurasian plate, a process that is messy at best and cataclysmic at worst. When the plates get locked for a time, pressure builds. When the blockage clears, it releases massive amounts of energy.

Scientists say yesterday's quake ruptured a 250- to 300-mile-long segment of the plate boundary.

The result was an uplift of the sea floor - and the ocean - by several yards.

The Japan Trench subduction zone has generated nine earthquakes of at least magnitude 7 since 1973.

Yesterday's quake was about 25 miles southwest of a magnitude-7.2 event that occurred two days earlier. Scientists now consider that a foreshock.

But they can be forgiven for not recognizing it at the time. No quake of equivalent size to yesterday's had occurred on that plate boundary since the year 869, as confirmed by geological research.

In the past few days, many large aftershocks have occurred, the largest being a magnitude-7.1. Geophysicists say aftershocks could continue for months.

ALL QUAKES, however, do not a Pacific-wide tsunami make.

Magnitude is a factor, but so is the directionality of the energy, scientists say.

The biggest tsunamis to hit Hawaii have been generated not from Japan, but from Kamchatka (1952), the Aleutian Islands (1946 and 1957), Chile (1960) and our own backyard (1975).

Wave harmonics also come into play.

Gerard Fryer, a geophysicist with the Pacific Tsunami Warning Center, compares it to someone sloshing around in a bathtub half full of water.

If the sloshing is timed right, the bather can make the water splash out of the tub. If the timing is not right, he says, the movements of the bather and the water cancel each other out.

Both those phenomena happen as a tsunami encounters islands or other land masses along its route.

As the tsunami approaches shore at the speed of a jetliner, it slows down. Its energy has to go somewhere, so the wave builds.

But how it builds depends on bathymetry - the contours of the tub. The wave can break up into smaller ripples that, like the bather and the bath water, can reinforce each other or cancel each other out.

That would explain the range of tsunami heights experienced around the islands and along the West Coast - 6.6 feet in Crescent City, Calif., only 2.5 feet in Santa Barbara.

Yesterday morning it was nature playing tricks.






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