Detection and recording Detection and recording Earthquakes vary in size. Those that do the most damage are extremely large, but some are so small they are almost undetectable.
Geologists use seismographs to record the surface and body waves.
Inside a seismograph designed to measure horizontal motion, a weight is freely suspended. As waves from earthquakes reach the seismograph the mass stays in relatively the same place, while the ground and the support move around it. This movement is recorded on magnetic tape by a pen attached to the mass. In a seismograph designed to measure vertical motion, the mass is connected to a spring, so as the ground and support move up and down, the pen on the mass measures the vertical motion. The metal tape which the motion is recorded on is marked with lines that correspond to one minute intervals.