After two tectonic plates gave way off the eastern coast of Japan, gravity wobbled. The Earth's tug is the result of a distribution of matter that is tighter and denser. The distribution of the earth and water changes when they are displaced. The forces that hold the moon close, keep the atmosphere thick, and tie our feet to the ground jerked into a new alignment. Japan really shook after the earthquake and the whole world tripped.

Not that anyone noticed. The Tohoku earthquake has a subtle effect on gravity. Seismologists are used to listening closely to the Earth's rumblings and have long thought about an earthquake signal that is practically instantaneous, spreading through the globe at the speed of light. Scientists have been looking at data from big earthquakes for signs of gravity changes. They are still controversial in seismology. The hunters have begun to find them with the help of more sensitive instruments and better computer models.

They are getting closer to putting that data to use. Researchers describe an earthquake early warning system that relies on gravity-derived signals alone. The model they tested was able to detect the Tohoku earthquake in eight seconds, giving a better estimate of its size than previous methods, and it was also able to give a better estimate of the time it took for the earthquake to hit. The work looks back on a single event. The method is being tested to see if it can be used in the future.

The existing signals are mainly P-waves, which are ripples in the rock caused by a sudden shock. When the waves reach the seismology stations, the software quickly estimates the size of the earthquake. The goal is to give people a heads-up before the up-and-down motion of S-waves, a slower type of tremor that causes the most damage. In recent years, better instruments have resulted in more reliable warning systems. The theoretical limit on the speed of detection is put at a few kilometers per second.

Martin Vall, a seismologist at the University of Paris, says that gravity perturbations are quicker than any other method. They're not as strong as P-waves, making them difficult to pick out from seismologists. The early signs of a major earthquake are hard to hear because of the constant din of the earth. Seismologists want a clear signal. Millions of city dwellers may end up flooding the streets or cowering in door frames if they don't hear the noise.

Seismologists have debated if a clear detection is possible. The LIGO facilities in Louisiana and Washington can be used to observe the waves directly. They aren't practical for picking up the small shifts caused by earthquakes. Seismometers pick up the response of the Earth as it pushes and pulls away to counteract the shift in mass. The elastic responses to them mostly cancel each other as the gravity changes. This leaves a very faint signal, known as a elastogravity signal, or PEGS.