text: The simplest explanation:\n\nA seismograph is just basically a pen on a spring (although now days this is replaced by a sensor) It measures ground motion. Earthquakes, nuclear weapons tests, the impact of waves striking the coast, the rumble of trucks on a highway, volcanoes and many other things can result in ground motion that a seismograph can measure. Seismologists looking for oil can use vibrating trucks or underground explosions to send waves into the ground that seismographs can measure, and turn into pictures of what the earth looks like underground.  \n\nOf course, this can get far more complicated. Some seismographs can measure lower frequencies, even using superconductivity, etc. Seismographs really measure ground acceleration, which is the second derivative of ground displacement.