SEISMICITY

The seismicity or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. Seismic waves are elastic waves that propagate in solid or fluid materials. They can be divided into body waves that travel through the interior of the materials; surface waves that travel along surfaces between materials; and normal modes, a form of standing waves.

Types of Seismic Waves:

Body waves

There are two types of body wave, P-waves and S-waves (both body waves). Pressure waves or Primary waves (P-waves), are longitudinal waves that involve compression and rarefaction (expansion) in the direction that the wave is traveling. P-waves are the fastest waves in solids and are therefore the first waves to appear on a seismogram. S-waves, also called shear or secondary waves, are transverse waves that involve motion perpendicular to the direction of propagation. S-waves appear later than P-waves on a Seismogram. Fluids cannot support this perpendicular motion, or shear, so S-waves only travel in solids. P-waves travel in both solids and fluids.

Surface waves

The two main kinds of surface wave are the Rayleigh wave, which has some compressional motion, and the Love Wave, which does not. Such waves can be theoretically explained in terms of interacting P- and/or S-waves. Surface waves travel more slowly than P-waves and S-waves, but because they are guided by the surface of the Earth (and their energy is thus trapped near the Earth's surface) they can be much larger in amplitude than body waves, and can be the largest signals seen in earthquake seismograms. They are particularly strongly excited when their source is close to the surface of the Earth, as in a shallow earthquake or explosion.

Normal modes

The above waves are traveling waves. Large earthquakes can also make the Earth "ring" like a bell. This ringing is a mixture of normal modes with discrete frequencies and periods of an hour or longer. Motion caused by a large earthquake can be observed for up to a month after the event. The first observations of normal modes were made in the 1960s as the advent of higher fidelity instruments coincided with two of the largest earthquakes of the 20th century – the 1960 Great Chilean Earthquake and the 1964 Great Alaskan Earthquake. Since then, the normal modes of the Earth have given us some of the strongest constraints on the deep structure of the Earth

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Reproduced from: www.wikipedia.org