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Seismology studies elastic waves generated by earthquakes, impacts and controlled sources, and uses them to investigate planetary interiors. A seismometer converts ground motion into a recorded seismogram. Arrival times, waveforms and amplitudes then constrain both the source and the material traversed by the waves.

P waves respond to compression and S waves to shear. Their different speeds and their reflections or conversions at interfaces help distinguish layers. Locating an earthquake requires several stations because its origin time and location must be estimated along with the propagation model. A single late arrival can reflect a deeper source, a slower path or a mistaken seismic phase assignment.

A seismometer compares ground motion with the motion of an inertial test mass. A mechanical seismograph records their relative displacement. Its frequency response depends on the suspension and on which relative or absolute velocity enters the damping law.