Earthquake Terminology

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Definitions:

Seismology (derived from Greek word Seismos meaning Earthquake and Logos meaning science) is the science of Earthquakes and related phenomena.

Earthquakes are the manifestations of sudden release of strain energy accumulated in the rocks over extensive periods of time in the upper part of the Earth.

Aftershock

An earthquake that follows a large magnitude earthquake called, ‘main shock’ and originates in or around the rupture zone of the main shock. Generally, major earthquakes are followed by a number of aftershocks, which show a decreasing trend in magnitude and frequency with time.

Fault

A weak plane in the Earth's crust and upper mantle along which two blocks of rock mass rupture or slip past each other. Faults are caused by earthquakes and earthquakes are likely to reoccur on pre-existing faults, where stresses are accumulated.

Seismograph/ Seismogram

Seismograph is an instrument that records the ground motions. Seismogram is a continuous written record of an earthquake recorded by a seismograph.

Tsunami

A system of gravity waves formed in the sea due to large scale disturbance of sea level over a short duration of time. Tsunamis are caused by earthquakes under the sea bottom, submarine volcanic eruptions, displacement of submarine sediments, coastal landslides and meteor impact. However, not all coastal earthquakes produce Tsunamis.

Classification of earthquakes
Category	Magnitude on Richter Scale
Slight	Upto 4.9
Moderate	5.0 to 6.9
Great	7.0 to 7.9
Very Great	8.0 and more

Epicentre: It is the point on the surface of the earth, vertically above the place of origin (hypocentre) of an earthquake. This point is expressed by its geographical Coordinates in terms of latitude and longitude. Hypocentre or Focus: It is the point within the earth, from where seismic waves originate. Focal depth is the vertical distance between the Hypocentre (Focus) and Epicentre. Magnitude: It is a quantity to measure the size of an earthquake and is independent of the place of the observation. Richter Scale: The local magnitude is defined as the logarithm of the maximum amplitude measured in microns on a seismogram written by Wood-Anderson seismograph with free period of 0.8 second, magnification of 2,800, damping factor of 0.8 calculated to be at a distance of 100 kms. The relative size of events is calculated by comparison to a reference event of ML=0,using the formula, ML=log A-log Ao where A is the maximum trace amplitude in micrometer recorded on a standard seismograph and Ao is a standard value which is a function of epicentral distance (Δ) in kilometers.