A seismometer is a device designed to measure movement in the Earth. Seismometers are typically used to measure seismic waves originating from an earthquake or explosion, and traveling through the ground as waves of force in the rocks and soil. The basic design of seismometers is typically based on two objects, or masses. The first object is called the frame, and it moves along with any movements of the ground. The second object is usually referred to as an inertial mass, and this mass tends not to move when the ground and the frame move.
Typically, the inertial mass is attached to the frame in such a way that it can move relative to the frame. By measuring the movement of the inertial mass relative to the frame, it is possible to calculate the magnitude of the waves in the ground that triggered the movement. Usually, the movements of the inertial mass are very small, so often a seismometer will include some mechanism to amplify the movement of the inertial mass in order to facilitate more accurate measurements.
An early example of a seismometer was a device that was used in 1906 to analyze an earthquake that occurred in San Francisco. This seismometer had a stationery frame from which a pendulum was suspended. Attached to the pendulum was a stylus, which rested on a glass plate that had been covered in soot. As movements in the ground caused the pendulum to swing, the stylus recorded the movement on glass. Devices like this one, called an earthquake seismometer, could help scientists to map out the likely point where an earthquake might have originated.
A seismometer measure is often based on a scale of measurement called the Richter Magnitude Scale. This is a logarithmic scale where each unit of seismometer measure indicates a seismic wave ten times stronger than the previous unit. For example, an earthquake measuring a six on the Richter scale is ten times more powerful than an earthquake measuring a five on the Richter scale.
Modern seismometers are typically highly sensitive electrical devices that utilize electrical sensors, highly effective amplifiers, and electrical recording of the output measurements. These instruments are usually categorized as broadband, short period, or long period. A broadband seismometer is usually the most flexible device, as it typically has the ability to measure a very large range of different seismic waves. Short period and long period seismometers are usually restricted to measuring waves in a certain range, but can be extremely sensitive.