Broad cracks in the ground. XII Damage total. Waves seen on ground surfaces. Objects thrown upwards into the air. The place where an earthquake occurs inside the earth is called the focus , and the place on the earth's surface directly above the focus is called the epicentre .
Seismic waves radiate out from the focus.
How a seismograph works is based on the pendulum theory. As the mass of the ground moves during an earthquake the pendulum will stay in the same place. Attached to the pendulum is a marker that touches a paper surface. As an earthquake occurs the paper is dragged across under the pendulum. The marker on the pendulum records the earthquake on the paper.