Seismograph
A seismograph is an instrument that detects and records ground motion from seismic waves, producing a seismogram used to locate and measure earthquakes.
Definition
A seismograph is a machine that feels the ground shake and draws a record of it, using a heavy weight on a spring that tends to stay still while the ground and its frame move beneath it, so the difference between the two gets traced onto paper or, in modern instruments, recorded electronically. The resulting seismogram shows the arrival of P-waves, S-waves, and surface waves, and the time gap between the P- and S-wave arrivals at a station gives the distance to the epicenter, while readings from at least three stations pin down its exact location by triangulation. Modern broadband seismometers use a suspended mass with electronic feedback that holds it fixed and measures the force needed to do so, giving a flat response from about $0.01$ to over $100$ seconds and sensitivity to ground velocities below a nanometer per second.
Example
The North Carolina Geological Survey and university networks operate seismometers across the state that recorded the 2020 Sparta earthquake, and a global network of thousands of stations can locate any significant earthquake within minutes. The 1935 Richter scale was originally defined from a Wood-Anderson torsion seismograph, with magnitude taken as the logarithm of the maximum trace amplitude in micrometers at $100\ \text{km}$ distance, and USArray's Transportable Array crossed the continental United States with a $70\ \text{km}$ station grid from 2004 to 2015, imaging the crust and mantle beneath North Carolina.
Key Insight
One seismograph tells how far away an earthquake was but not which direction, which is why three stations are needed: the circles of possible distance from each one cross at a single point, the epicenter. Seismographs also record far more than earthquakes, since ocean waves create a constant background hum called microseisms, and seismometers placed on the Moon and Mars have revealed the interior structure of those worlds using the same basic physics.