What Measures Vibrations in the Earth?

Types Of Ground Vibrations

• Earthquakes release energy in the form of ground vibrations, called seismic waves. These waves radiate outwards in all directions. There are three types of seismic waves, or vibrations. The fastest are pressure waves, called P waves. These waves are also known as compression waves. Slower than P waves, S waves are vertical and horizontal waves that run perpendicular to their direction of travel. Finally, surface waves, the slowest moving vibrations, follow behind the P and S waves.

Devices Used To Measure Ground Vibrations

• There are two types of devices used to measure ground vibrations: seismometers and accelerometers. Accelerometers are used to record very strong vibrations, while seismometers are used to detect weaker vibrations. A seismometer is a sensor used to detect ground motion, containing a mass or pendulum mounted on a spring. When the ground moves, the seismometer moves with it; however, the mass remains stationary. A recording device, called a seismograph, records the relative motion between the mass and the unit, producing a record of the movement. Modern seismometers utilize electronic circuits to measure the electrical voltage created by this motion rather than mechanical means. These modern devices are capable of detecting vibrations as small as semidiurnal gravitational tide movements and distances almost as small as atomic spacing.

Ground Vibration Magnitude

• Magnitude is the most common measure used to describe the amount of energy released by an earthquake at its source. In 1935, Charles F. Richter developed a method for comparing the magnitudes of earthquakes, named the Richter scale. This scale is commonly referred to as running from zero to 10; however, in reality, it is infinite in terms of its upper limit. The Richter scale is a base-10, logarithmic scale; as you increase magnitude by one whole number, the amplitude of the ground motion (size of vibration) increases 10 times and the amount of energy released increases by around 31 times.

Classification Of Ground Vibrations

• Based on the Richter scale, seismologists have developed a system to classify ground vibrations. Vibrations of less than three on the Richter scale are considered micro earthquakes. Vibrations between three and 3.9 are considered minor earthquakes. Vibrations between four and 4.9 are considered light earthquakes. Vibrations between five and 5.9 are considered moderate earthquakes. Vibrations between six and 6.9 are considered strong earthquakes. Vibrations between seven and 7.9 are considered major earthquakes. Vibrations over eight are considered great earthquakes.

Frequency Of Ground Vibrations

• Each year, seismometers detect approximately 1.3 million incidents of ground vibrations that register less than three on the Richter scale. On average, there are 130,000 minor earthquakes, 13,000 light earthquakes, 1,319 moderate earthquakes, 134 strong earthquakes, 17 major earthquakes and one great earthquake. These numbers represent a statistical average and in any given year the actual numbers may fluctuate greatly.