Shaken Beliefs: Seismic Lessons from Japan's Tohoku Earthquake
By Marcia Bjornerud
Five years after the 9.0-magnitude earthquake in Tohoku, geologists are still learning from the disaster. Credit Illustration by Daniel Zender 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.
To understand why Tohoku defied conventional geophysical wisdom, consider the paradox inherent in an earthquake. One of the prerequisites is a strong fault surface—a plate boundary or rock fracture with a high coefficient of friction, like a skid-resistant pad under an area rug. (A weak fault will slip continuously, never storing up much elastic energy.) But the strong fault must also be capable of slipping, all at once and over a wide area, because this is what causes the tremor. In technical terms, its dynamic friction must be lower than its static friction. This can happen when opposing rock surfaces scrape against each other, because the resultant heat and pulverization act as a kind of lubricant, making the fault surface suddenly, fleetingly slick. The traditional style of cross-country skiing works on the same principle—the downward kick stroke requires high static friction, while the glide is possible thanks to low dynamic friction.
Prior to the Tohoku event, the prevailing assumption was that a subduction zone could generate megathrust earthquakes at only limited depths—specifically, between about five and twenty-five miles down. On the deep end of this range, the temperature is high enough that the rock, softened by heat from Earth's interior, is malleable. On the shallow end, the rock is relatively weak and watery, because it is made up of seafloor sediment. In either case, the fault surface is all glide and no kick: it ought to be too slippery to generate a big earthquake. But Tohoku ignored that idea. The greatest lurching occurred along the shallowest part of the fault. The coastal area near Sendai, where the subduction zone is about fifteen miles down, was shoved an astonishing fourteen feet eastward. At the uppermost part of the plate boundary, though, the figure was closer to a hundred feet. This, in turn, displaced an immense volume of overlying seawater, triggering the tsunami that caused most of the disaster's sixteen thousand deaths.
Most seismologists now agree that Tohoku should be considered a composite event. What started as a large but otherwise unremarkable earthquake unleashed a chain reaction of secondary effects, and it morphed into a monster. One hypothesis is that the initial jolt was strong enough to dislodge a particularly high-friction patch of the subduction zone, which then activated the otherwise stable, water-rich upper section. Once this began slipping, a process called thermal pressurization—heating and expansion of fluids on the fault—may have led to runaway failure at shallow depths. Like the thin film of water beneath the blade of a skate, the heated fluids would have kept the rock surfaces out of contact with each other, reducing the friction between them virtually to zero. Tragically, because the early stages of the fault slip looked like an ordinary earthquake, Japan's sophisticated automatic early-warning systems initially underestimated the magnitude of the event. As a result, some people ignored the first alerts that were issued. The arcane details of rock friction, then, are of not only geophysical but also humanitarian concern.
Whatever the cause of its anomalously large and shallow slip, the Tohoku earthquake has forced the seismological community to abandon the characteristic-earthquake concept, at least for the biggest events, and to confront the unsettling reality that even an incipient earthquake doesn't know at the outset whether it will become a magnitude nine or not. Rather, its propagation and eventual size will depend on a cascade of effects that play out over seconds or minutes. Earlier this month, a group of four scientists published a paper in Nature addressing the question of why the Japanese earthquake did not in fact become even larger. In particular, they focus on why the slip apparently stopped abruptly at a line that runs across the overriding plate—the upper slice in the subduction sandwich, which carries Japan itself. Combining geologic maps, high-resolution depth and gravity readings, and historical patterns of earthquakes in the region, the authors argue that this sudden halt was due to a geologic discontinuity in the plate, where Japan's oldest rocks (to the north) are juxtaposed against more recent, mainly volcanic rocks. They suggest that the contrasting rock types, with their distinct geologic histories, created differences in friction along the subduction zone. They behaved like a patch of grass in the snow, catching the ski as it whizzed over.
For seismologists, the Tohoku earthquake was a humbling reminder that our geophysical records offer only a peephole view of Earth's behavior over time, and that our most advanced models for geological phenomena are cartoonish oversimplifications of nature. A hundred years of cutting-edge seismology can be undermined in a hundred seconds.
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