agate from the earthquake, the gravity perturbations travel at the speed of light. Vallée et al. have finally observed these gravity perturbations in seismometer records from the great Tohoku earthquake in Japan in 2011. The signal would have allowed an accurate magnitude estimation in minutes, rather than hours, for this catastrophic earthquake.
After an earthquake, the earliest deformation signals are not expected to be carried by the fastest (P) elastic waves but by the speed-of-light changes of the gravitational field. However, these perturbations are weak and, so far, their detection has not been accurate enough to fully understand their origins and to use them for a highly valuable rapid estimate of the earthquake magnitude. We show that gravity perturbations are particularly well observed with broadband seismometers at distances between 1000 and 2000 kilometers from the source of the 2011, moment magnitude 9.1, Tohoku earthquake. We can accurately model them by a new formalism, taking into account both the gravity changes and the gravity-induced motion. These prompt elastogravity signals open the window for minute time-scale magnitude determination for great earthquakes.Japan has been expecting and preparing for the “big one” for more than 30 years. But the magnitude-9.0 temblor that struck March 11 — the world’s fourth biggest quake since 1900 — wasn’t the catastrophe the island nation had in mind. The epicenter of the quake was about 80 miles east of the city Sendai, in a strip of ocean crust previously thought unlikely to be capable of unleashing such energy.
“This area has a long history of earthquakes, but [the Sendai earthquake] doesn’t fit the pattern,” says Harold Tobin, a marine geophysicist at the University of Wisconsin–Madison. “The expectation was high for a 7.5, but that’s a hundred times smaller than a 9.0.”
Understanding where big earthquakes will emerge is extraordinarily difficult, and nowhere more so than Japan. The northern part of the island nation sits at the intersection of four moving pieces of the Earth’s crust. Where one tectonic plate slides beneath another, forming a subduction zone, sudden slippages can unleash tremendous amounts of energy.
The Sendai earthquake occurred at the Japan Trench, the junction of the westward-moving Pacific plate and the plate beneath northern Japan. Historical records, one of seismologists’ best tools for identifying areas at risk, suggest that this segmented fault has produced several earthquakes bigger than 7.0 in the 20th century alone — but none bigger than 8.0.
That’s why the Japanese government has long focused on the nation’s southern coast and the northward-moving Philippine plate, which has a proven ability to generate large quakes. Quakes larger than 8.0 tend to strike the Tokai region in central Japan every 150 years or so, with the last big one appearing in 1854. In 1976 researcher Katsuhiko Ishibashi of Kobe University warned that Suruga trough, a subduction zone just off the coast of Tokai, was due for a big one. In the years since, the Japanese government and research community have braced for this predicted Tokai earthquake — deploying GPS systems to monitor the movements of islands on the Philippine plate and even generating computer simulations of how crowds in train stations might behave during such an event.
Current thinking about the mechanisms that govern megaquakes also favored the Philippine plate as the site of greatest risk. About 80 percent of all earthquakes above magnitude 8.5 occur at the edges of such geologically young, warm tectonic plates. Kilometer-thick sediment layers carried by these plates are thought to grind smooth patches that allow long stretches of fault to rupture at once. The Pacific plate, some of the oldest ocean crust on the planet, doesn’t fit this description.
You must be logged in to post a comment.