A seismograph is a mass on a spring with a pen on the end, and a drum turning underneath it. The mass wants to stay still while the ground moves, so the pen draws the difference — and that difference is the whole of the record. Everything seismology knows was read off strips of paper like this one.
The first thing to arrive is the P wave — primary, a push, compressional, and fast. Then a gap, and then the S wave — secondary, a shake, shear, slower and much bigger. The gap between them is the distance to the epicentre: the S wave has travelled further and slower, so the difference in arrival time multiplied by about eight kilometres per second gives the distance.
After the S wave come the surface waves, the long rolling ones that travel along the Earth's surface and cause most of the damage, because they last longer and move the ground sideways. And through everything runs the background noise — the microseism, which is mostly ocean swell hitting the coast somewhere else entirely.