DAY 1
earthquake
Friday, March 11, 2011, 2:46 P.M.
Reactor Status
Reactor 1: Fully operational
Reactor 2: Fully operational
Reactor 3: Fully operational
Reactor 4: Shut down for inspection
Reactor 5: Shut down for inspection
Reactor 6: Shut down for inspection
Standing on Earth, it’s difficult to appreciate its movement. The entire planet spins on its axis at more than 1,000 miles per hour (mph) and hurtles along its orbit through space at 66,660 mph. It’s no wonder, then, that those of us sitting on its surface hardly notice the slow creep of its tectonic plates. But miles beneath the soil and sand, the mountains and oceans, Earth’s lithosphere is broken into a clumsy jigsaw puzzle of rock. The puzzle pieces, called tectonic plates, sit on the asthenosphere, a layer of Earth that shifts and flows.
Rocks in the asthenosphere are under so much pressure that they move in and out of solid form—sometimes they are solid rocks, sometimes liquid magma. Resting on top of this constantly changing layer, the plates creep over, under, and past each other at a rate so slow it would make a snail blush.
More than anything else, Japan has been shaped by the push and pull of plate tectonics. Just off its east coast, deep beneath the seafloor, giant chunks of Earth’s lithosphere and crust are being sucked beneath the country in a process called subduction. Rock from the subducting plates turns to magma when it reaches the mantle, creating hot spots that, over millennia, melt through the crust and break through as lava, forming volcanoes. That upwelling magma is responsible for the breathtaking mountainous landscape of Japan, and those subducting plates cause most of the earthquakes that shake the country.
Japan is an archipelago, a cluster of islands that sit on a particularly active spot in the tectonic neighborhood, where a thin finger of the North American Plate extends down between the Eurasian and Pacific Plates. The largest of the islands, Honshu, is in an especially precarious position, straddling the boundary between the Eurasian and North American Plates. To the east of the island, the North American and Pacific Plates meet in a section of the seafloor called the Japan Trench. There, the Pacific Plate slides beneath the North American Plate in what’s known as a subduction fault.
Mount Fuji, Japan’s tallest peak, is one of more than a hundred active volcanoes in the country.
The tectonic plates surrounding the Pacific Ocean are converging, or moving toward each other, resulting in a rough arc of subduction faults around the edge of the ocean. Because these faults produce volcanic activity, the arc has become known as the Ring of Fire. On the other side of the world, beneath the Atlantic Ocean, the tectonic plates are moving apart, or diverging, which creates a more stable seismic environment.
It sounds harmless enough, and even more so when you realize that the Pacific Plate creeps westward at only about three and a half inches per year. But tectonic plates don’t slide smoothly—they stick. And on a scale as large as a tectonic plate, a little bit of motion combined with a lot of stickiness can build up an enormous amount of energy.
As the Pacific Plate slides beneath the North American Plate, it catches the North American Plate’s edge. Over time, the movement of the Pacific Plate pulls the North American Plate downward, like the bucket of a catapult that is being readied to fire. The upper plate begins to bend, curving at the fault and forming a deep ocean trench. Tension builds between the two. Potential energy grows. When the potential energy in the fault becomes greater than the force, called friction, that holds the plates together, the top plate breaks free and springs upward. The subducting plate surges forward. The catapult has been released. The strength of the resulting earthquake depends on how much energy has built up in the fault before it budges, how much of the fault slips, and how far it moves.
Japan’s landscape has been shaped over millennia by the subduction fault off its eastern coast. Rock from the subducting plate melts when it reaches the mantle, creating hot spots of magma.
The many faults surrounding Japan are constantly slipping. In fact, the country experiences a tremor somewhere within its borders every five minutes or so. Most of those are too small for humans to feel. Of the two thousand or so each year that are strong enough to be felt, most are small tremors, which do little more than rattle dishes and set off car alarms. But the Great Tohoku Earthquake, as it came to be known, was far more powerful than any that had been measured in Japan before.
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Scientists rate the strength of an earthquake using the moment magnitude scale, a system that gauges the amount of energy released by the quake and assigns it a number. Each number on the scale is ten times more powerful than the one before it. An earthquake that rates a 5 on the scale is strong enough to be felt by everyone, rattling dishes and waking sleepers in their beds. An earthquake that rates a 6 is ten times more powerful than a 5. A 7 is 100 times more powerful than a 5, toppling furniture and shaking loose plaster and bricks in older buildings. The Tohoku quake was measured at 9.1.
Determining the magnitude of an earthquake is a tricky business. After the fact, seismologists study measurements of the ground’s movement taken by seismographs and evaluate the damage done by the quake to decide what magnitude to assign to the event.
Before 2011, most scientists believed that the Japan Trench could not produce an earthquake stronger than magnitude 7.5. That’s because it’s not a very sticky fault—it moves forward pretty smoothly, without building up too much friction between the plates. And less sticking means that less potential energy builds up in the fault.
But in the end, it was the fault’s slipperiness that proved the scientific predictions wrong. The upper plate lurched forward on March 9, causing a magnitude 7.2 earthquake—on its own a major event. Three more large quakes, each larger than magnitude 6, followed that same day. Then the fault ruptured again.
Copyright © 2021 by Deirdre Langeland
