Seismometers measure motion in the Earth caused by seismic waves from earthquakes or explosions. They consist of a frame and an inertial mass, with the latter measuring the waves by moving relative to the frame. The Richter Magnitude Scale is used to measure the strength of the waves. Modern seismometers are electrical devices classified as broadband, short term or long term.
A seismometer is a device designed to measure motion in the Earth. Seismometers are typically used to measure seismic waves that originate from an earthquake or explosion and travel through the ground as force waves in rocks and soil. The basic design of seismometers is typically based on two objects, or masses. The first object is called a frame and moves with any terrain movement. The second object is usually referred to as the inertial mass and this mass tends not to move when the ground and chassis move.
Typically, the inertial mass is attached to the frame such that it can move relative to the frame. By measuring the movement of the inertial mass with respect to the chassis, it is possible to calculate the magnitude of the waves in the ground which triggered the movement. Usually, the motions of the inertial mass are very small, so often a seismometer will include a mechanism to amplify the motion of the inertial mass in order to facilitate more accurate measurements.
An early example of a seismometer was a device used in 1906 to analyze an earthquake that occurred in San Francisco. This seismometer had a fixed 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 swung the pendulum, the stylus recorded the movement on the glass. Devices like this one, called an earthquake seismometer, could help scientists map out where an earthquake might have originated.
A seismometer measurement is often based on a measurement scale called the Richter Magnitude Scale. This is a logarithmic scale where each unit of the seismometer 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 using electrical sensors, highly effective amplifiers, and electrical recording of output measurements. These tools are generally classified as broadband, short term or long term. A broadband seismometer is usually the more flexible device, as it typically has the ability to measure a very wide range of different seismic waves. Short- and long-period seismometers are usually limited to measuring waves in a certain range, but they can be extremely sensitive.
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