Rocks can be subject to two kinds of stresses: the “clamping” stress that pushes them together, and the “shear” stress they undergo as they slide past each other. Together, these stresses are known as Coulomb stress, named for Charles-Augustin de Coulomb, an 18th-century French physicist. Coulomb calculations had been used for years in engineering, to find the failure points of various building materials, but they’d never been applied properly to faults. It turned out, though, that faults in the ground behave much like rocks in the laboratory: they come unglued when shear stress exceeds the friction and pressure (the clamping stress) holding them together. When Stein, Lin, and King applied the Coulomb model to the California sequence, they found that most of the earthquakes had occurred in areas where the shifting of the ground had caused increased stress.
In 1997, Stein and two other geologists using the model found that there was a 12 percent chance that a magnitude 7 or greater would hit near Izmit, Turkey, within 30 years; two years later, on August 17, 1999, a magnitude 7.4 destroyed the city, which wasn’t designed to withstand such a tremor. A Turkish geologist named Aykut Barka quickly wrote up a paper warning that Coulomb stress from the Izmit quake could trigger a similar rupture near Düzce, a town roughly 60 miles east. His work persuaded authorities there to close school buildings damaged during the Izmit shaking. On November 12, a segment of the North Anatolian Fault gave way, in a magnitude 7.2. The empty school buildings collapsed.
Lin and Stein both admit that Coulomb stress doesn’t explain all earthquakes. Indeed, some geophysicists, like Karen Felzer, of the U.S. Geological Survey, think their hypothesis gives short shrift to the impact that dynamic stress—the actual rattling of a quake in motion—has on neighboring faults.
In the aftermath of the disastrous March 11 Tōhoku quake, both camps are looking at its well-monitored aftershocks (including several within 100 miles of Tokyo) for answers. Intriguingly, it was preceded by a flurry of earthquakes, one as large as magnitude 7.2, that may have been foreshocks, although no one thought so at the time; the researchers are trying to determine what those early quakes meant.
When I ask Lin whether California, where I live, is next, he laughs. “I understand that the public now thinks that we’ve entered a global earthquake cluster. Even my own mother in China thinks that. But there’s no scientific evidence whatsoever to suggest that the earthquake in New Zealand triggered the earthquake in Japan, or Japan will trigger one in California.” Still, Lin and his colleagues do wonder whether Tōhoku has pushed neighboring faults closer to rupture. “I am particularly interested in how this earthquake might have changed the potential of future earthquakes to the south, even closer to Tokyo,” Lin tells me. “There, even a much smaller earthquake could be devastating.”