When a massive earthquake struck Japan in 2011, it created a tsunami that killed thousands. But seismologists didn’t have a sense of the quake’s true magnitude for hours—and by that time, the immense wave had already inundated many areas (shown). Now, researchers have come up with a way to more quickly gauge a big quake’s magnitude and thus provide faster, more accurate tsunami warnings: by measuring the miniscule changes in Earth’s gravitational field that are generated when massive slabs of the planet’s crust shift by dozens of meters over the course of a few minutes. The signals generated by the magnitude-9.1 quake that struck Japan were barely one-billionth g, the amount of Earth’s gravitational field at sea level, but they traveled at the speed of light and were detected at seismometers hundreds of kilometers away, the researchers report today in Science. In fact, the team notes, the best measurements of these so-called elastogravity signals can be made by instruments between 1000 and 2000 kilometers away. That’s because at seismometers closer to a massive quake’s epicenter, the slowly moving vibrations of typical seismic waves often arrive at instruments before Earth’s tectonic slabs have stopped shifting, thus masking the signals generated by the temblor’s final spasms. If the scientists’ approach had been available in 2011, they suggest, the quake’s true magnitude could have been estimated within minutes rather than hours. Quakes smaller than magnitude 8 probably don’t generate elastogravity signals large enough to be measured by current instruments, but such temblors are also not as prone to trigger large tsunamis, the researchers note.