Simple, non-electronic seismometers produce a seismogram using a recording device, such as a pen, attached to a heavy weight. The weight and pen are suspended from a frame. A powered, rotating cylinder is also attached to the frame, which is secured to the ground. As the Earth shakes, the weight and pen remain still, but record the movement as the frame and the cylinder are shaken.
To understand the way in which simple seismometers work, tie a small, heavy weight, such as a pocket-sized plastic bottle full of liquid, to one end of a 1 meter (40 in) length of string. Hold the string at the other end and lift the weight off the floor. If you move your hand rapidly backward and forward, the weight will remain almost stationary. During an earthquake the weight and pen of a seismometer act in this way.
Locating earthquakes
Within minutes of an earthquake occurring, observation stations are alerted as seismometers register the arrival of shock waves. At the instant an earthquake occurs, shock waves speed out in all directions from the focus. They travel through and around the Earth and are detected by seismometers. Because different types of shock waves travel at different speeds, it is possible to calculate how far they have travelled and how far away the epicentre is. This is done by measuring the time interval between the arrival of each different type of shock wave. The task of locating the earthquake may then be necessary as information may not be received directly from the affected area. This could be because it is an isolated region, or because its communication links, such as telephone cables, have been damaged by the earthquake. Information from several different observation stations is needed to locate an earthquake's epicentre
