This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
The earliest seismographs worked on a simple mechanical idea: hang a heavy mass from a spring or a pendulum inside a frame bolted to the ground, and when the ground shakes, the frame moves with it while the mass, held back by its own inertia, lags behind. Measuring the relative motion between the frame and the mass, traced by a pen onto a slowly rotating drum of paper, gives a record of the ground's own motion. The modern, widely adopted horizontal-pendulum design that made this practical for routine earthquake monitoring is credited to the English seismologist John Milne, though simpler earthquake-detecting devices predate his work. Modern broadband seismometers keep the same basic physics but change what actually gets measured. Rather than letting the inertial mass swing freely and recording its displacement directly, an electronic feedback system detects the moment the mass begins to move relative to the frame and immediately applies an opposing electromagnetic force to hold it almost perfectly still. It is the strength of that corrective force, rather than the mass's own displacement, that gets recorded, which lets the instrument stay accurate and linear across a far wider range of ground motion than a simple swinging pendulum ever could. Vertical instruments often use a specialized constant-force, zero-length spring suspension, a design associated with the geophysicist Lucien LaCoste, to reach a sensitivity an ordinary spring cannot match. Recording has changed just as much as sensing has. Where Milne's stations once produced ink traces on paper drums that had to be collected and read by hand, a modern seismic station digitizes its signal through an analog-to-digital converter and streams it over the internet, letting a single earthquake be compared, in near real time, across a worldwide network of stations.