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. So, how are these measurements recorded? And how is their size determined?

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. When motion is recorded a seismogram is created, which tells about the waves--how big they were and how long they lasted. P waves are recorded first, followed by S waves and then surface waves. While surface waves are the last to reach the seismograph, they last the longest time.

Using the information from the seismogram, the epicenter and focus of the earthquake can be determined. The focus is the point on the fault at which the first movement or break occurred. The epicenter is the point on the surface directly above the focus. Once several seismograph stations have determined their distance from the epicenter, the actual epicenter can be located, using triangulation, on a map.

What are earthquakes? Types of earthquakes Forces What causes stress? Elasticity Waves Detection and recording Measurement A new type of measurement