Shaken beliefs: Seismic lessons from Japan’s Tohoku earthquake
Japan could well be called the cradle of seismology. It occupies one of Earth’s most precarious tectonic settings, at the nexus of four plates, and its written record of earthquakes extends back to 599 A.D., during the reign of Empress Suiko. Japanese seismologists of the early twentieth century made a number of significant contributions to geophysics, some of which anticipated plate-tectonic theory by decades. In 1899, several years after a powerful tsunami struck the Sanriku Coast, a man named Akitsune Imamura correctly inferred the type of fault slip (now called a megathrust event) that generates such waves. In the nineteen-twenties, another Japanese scientist, Kiyoo Wadati, collected seismic data that became critical to the development of the Richter scale and, eventually, the discovery of the process of subduction, in which old ocean crust sinks back into Earth’s mantle. Today, Japan spends more money per capita than any other country on earthquake research, engineering, and preparedness.
And yet the disaster of five years ago—a magnitude-9.0 megathrust earthquake off Tohoku, followed by a major tsunami and nuclear accident—came as a surprise. Until March 11, 2011, the consensus among seismologists was that a particular stretch of fault would observe certain rules, rupturing at consistent intervals in events of similar size. These events are known as characteristic earthquakes. For the subduction zone east of northern Honshu, where the Tohoku quake originated, the characteristic earthquake was considered to be in the lower magnitude-eight range. Japan’s civil-defense strategies, including its tsunami walls, were designed in accordance with that view. As it turned out, though, the Tohoku earthquake wasn’t characteristic. It was the fourth-largest seismic event ever recorded, outranked only by the 2004 Indonesian and 1964 Alaskan earthquakes (both 9.2) and the enormous but less well-known 1960 Chilean earthquake, a 9.5 event whose resulting tsunami devastated Hilo, Hawaii, some six thousand miles away.