When the ground in Japan started shaking on March 11, 2011, the Japanese, who are well-accustomed to earthquakes, knew this time was different. They weren't surprised--the fault that ruptured has a long record of seismic activity. But this time the trembling continued for six minutes. When it finished, many turned their eyes to the sea off the country's craggy and quake-scarred coast, as they are taught, and waited for the waves to come.
But the last time something remotely similar had happened was more than 1,000 years ago and, even in a country that prides itself on its shared cultural memory of the distant past, that event had been largely forgotten. Since that time, much has changed. People and development have sprung up on the coast, along with a string of nuclear reactors. Everything, it seemed, had changed in the intervening millennium--except the ocean.
Compared with other large earthquakes in recent memory, the magnitude-9.0 Tohoku earthquake, as it became known, was different in many ways. Temblors off Chile in 2010 (magnitude 8.8) and Sumatra in 2004 (magnitude 9.1) involved faults that extended partly onto land, but the Tohoku earthquake occurred entirely under the ocean--nearly 19 miles below the seafloor in some places. In Japan, the combination of natural forces and greater human presence created a domino-like sequence of events, from earthquake to tsunami to the release of radiation from the mangled nuclear power plant near Fukushima. (See Page 37.)
The majority of these events played out in the ocean, noted Jian Lin, a seismologist at Woods Hole Oceanographic Institution (WHOI). The Tohoku earthquake also triggered a scientific cascade, as geologists, geophysicists, chemists, modelers, physical oceanographers, and marine biologists mobilized to understand the quake's causes and consequences.
Where the fault lies
The undersea fault that ruptured along 300 miles on March 11 extends north-south, roughly parallel to the northeast coast of Japan. It is a mega-thrust fault in the Japan Trench, where the massive Pacific Plate pushes westward, and beneath, the continental Eurasian Plate. Where the two tectonic plates grind against each other, gargantuan stress builds over time in the seafloor crust. Hundreds of large and small earthquakes dot the fault each year as stress exceeds the breaking point of rocks and suddenly releases.
The immense forces of two colliding plates, said Lin, have compressed the Japanese island of Honshu like an accordion, pushing up the mountain ranges that fill much of the island's interior and creating a spider web of smaller, land-based faults that periodically rupture. As a result, the Japanese have made living with seismic activity part of their daily life and national culture. Each year, schools and businesses across the country mark Disaster Prevention Day on Sept. 1, the anniversary of the 1923 earthquake that devastated Tokyo, by participating in drills and other activities to prepare for a large earthquake. Building codes are laden with requirements intended to prevent high-rise buildings from collapsing during violent shaking. Even the iconic bullet train is connected to a network of seismic sensors designed to automatically stop any moving train before shaking from a large offshore earthquake can reach shore.
When the shaking on March 11 stopped, the city of Sendai (population 1 million), the largest urban area near the epicenter, was largely spared--even though the temblor turned out to be the world's fifth largest ever recorded. In Tokyo, less than 200 miles away, frightened office workers safely evacuated or rode out the six-minute quake as buildings swayed, but did not fall. On the busy railway corridor running north of Tokyo, trains slowed and stopped with very few derailments. In fact, the earthquake produced surprisingly little damage, despite the fact that national hazard maps for the region, based on data from only the past few hundred years, directed officials to prepare for a maximum magnitude of 8. …