Ruptures in Earth’s crust—earthquakes—generate elastic waves that travel through the crust and upper mantle at speeds of several kilometers per second. When observed at seismic stations, those waves provide information about the interior structure of the planet and the magnitude of the quake that generated them.

The earthquake’s initial rupture and subsequent seismic waves redistribute mass and thus alter Earth’s gravitational acceleration, or field. The change in the field, like an electromagnetic disturbance, propagates at the speed of light, and a seismometer feels it almost immediately. The perturbed gravitational field also induces small mass redistributions, which generate their own mini seismic waves that jiggle the ground at seismic stations before the arrival of the main elastic waves. The two tiny effects combine to give a seismometer reading of something like 1 nm/s2 for the largest of earthquakes. If only its magnitude mattered, such a small gravitational effect would be...

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