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Japanese quake: Why this one is so destructive

This tremblor was a “megathrust” or “up-and-down quake” — the kind of earthquake that can send monstrous waves travelling hundreds of miles an hour for some time after the actual geological disruption under Earth’s surface.

Houses burn at night following an earthquake in Natori City, Miyagi Prefecture, northeastern Japan
Houses burn at night following an earthquake in Natori City, Miyagi Prefecture, northeastern Japan.

The devastation and dread caused by Japan’s 8.9-magnitude earthquake could have been confined to Japan and its near neighborhood had it been a different kind of quake.

But this tremblor was a “megathrust,” the kind of earthquake that can send monstrous waves travelling hundreds of miles an hour for some time after the actual geological disruption under Earth’s surface.

Some people who have lived through call them “up and down quakes” as opposed to the side-to-side variety. Both can be deadly destructive, as Haiti learned last year.

But a megathrust quake often threatens a vast area. That’s why Pacific coastal towns — from Australia to Russia, Taiwan to Oregon — are bracing for the aftermath of this latest earthquake.

A mighty snap
In this case, the quake occurred on a fault line between two of the Earth’s tectonic plates which are huge and constantly shifting slabs of the Earth’s crust. One plate — the Pacific plate — had been sliding beneath another — the North American plate. The massive plates bend, building up tremendous pressure until one breaks loose with a mighty snap that lifts the ocean floor and slaps the nearby earth too.

When that happened under the Indian Ocean in 2004, the 9.1-magnitude quake was so powerful that it set the planet vibrating in oscillations as if it were a ringing bell. Even under Minnesota, the then-frozen ground moved up and down nearly a half-inch as seismic waves spread around the globe. And the resulting tsunamis killed some 230,000 people in 14 countries.

It’s too soon to predict the toll from the tremblor near Japan. The quake itself took place 15 miles beneath the Earth’s surface, about 80 miles east of the Japanese island of Honshu and some 230 miles northeast of Tokyo, according to the U.S. Geological Survey.

The fact that it was a relatively shallow quake elevates the tsunami danger for thousands of miles along Pacific shores.

Haiti: a strike-slip fault
The disruption beneath the ocean floor in Haiti was a different geological event. It came along what’s known as a strike-slip fault. Two of the gigantic plates that form the Earth’s crust ground against each other in a side-to-side motion.

Think of setting two bricks on a table and trying to grind them slowly past each other under a lot of pressure. They catch on each other at times and then abruptly break loose, shaking anything you might set on top of them.

There was a risk Haiti would see large waves caused by underwater landslides, but not of killer tsunamis.

One reason the 7.0-magnitude quake in Haiti was so destructive was that it happened close to the surface — about 8 miles underground — and very close to the heavily populated area of Port-au-Prince. In all, eight Haitian cities and towns — including the capital of 3 million — suffered violent to extreme shaking, even though that quake was less powerful.  

Japan vulnerable
The 8.9-magnitude quake near Japan was the most powerful ever recorded in that country and the fifth most powerful to hit the world in the past century, Reuters reported

Chile suffered the most powerful quake on record, a magnitude 9.5 in 1960, which set tsunamis crashing onto shores in Japan, Hawaii and the Philippines. In all, 13 quakes of magnitude 7 or larger have hit Chile’s coastline since 1973.

A quake as strong as the one that hit Haiti happens somewhere on the planet about 20 times a year. Indeed, a magnitude 7 quake rumbled with little notice near Japan’s Ryuku islands last winter.

Japan is particularly vulnerable to earthquakes. It lies along the volatile Pacific Ring of Fire, a narrow zone around the Pacific Ocean where a large chunk of Earth’s earthquakes and volcanic eruptions occur, according to the online publication LiveScience.

“Roughly 90 percent of all the world’s earthquakes, and 80 percent of the largest ones, strike along the Ring of Fire,” LiveScience said.

Magnitude not the full story
Scientists calculate earthquake magnitudes on a logarithmic scale. This means that an earthquake of, say, magnitude 7.2 would produce 10 times more ground motion and release about 32 times more energy than one of magnitude 6.2.

But a quake’s magnitude doesn’t tell the full story of death and destruction.

A magnitude 9.2 quake in Prince William Sound, Alaska, killed 128 people in 1964. Compare that with the magnitude 7.9 quake in Eastern Sichuan China where at least 68,000 people were killed in 2008.

Haiti’s poverty and flimsy buildings were factors too in the terrible toll the quake took on that country.

Japan is far wealthier, with strict building codes and a sophisticated understanding of handling seismic catastrophes. What happened in Haiti brings to mind the observation that earthquakes don’t necessarily kill people. Collapsing buildings are the culprit.

Even so, Japan is suffering severely. The hundreds of bodies already found are just the beginning of the toll this quake has taken — and could continue to take in aftershocks and tsunamis.

“Stunning TV footage showed a muddy torrent of water carrying cars and wrecked homes at high speed across farmland near Sendai, home to one million people and which lies 300 km (180 miles) northeast of Tokyo. Ships had been flung onto a harbor wharf, where they lay helplessly on their side,” Reuters said.

“A ship carrying 100 people had been swept away by the tsunami,” it continued. “One train was derailed and another unaccounted for. In Tokyo, residents who had earlier fled swaying buildings jammed the streets trying to make their way home after much of the city’s public transportation was halted.”

It will be weeks or months before the economic costs are known. Electronics giant Sony Corp, one of the country’s biggest exporters, shut six factories, as air force jets raced to the northeast coast to determine the extent of the damage, Reuters said.

Sharon Schmickle writes about national and foreign affairs and science. She can be reached at sschmickle [at] minnpost [dot] com.