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Earthquake terms

Earthquakes have several terms associated with them.

india Updated: Mar 13, 2003 15:10 IST

The focus is the point within the Earth's crust from where seismic waves originate. The focus is also called the hypocentre and the shortest distance between the focus and the Earth's surface is called the focal distance.

The epicentre is the point on the surface of the earth vertically above the place of origin of an earthquake. It is this point that is usually referred to in news reports as the point of origin of an earthquake.

Magnitude is the amount of shaking of the ground when an earthquake affects an area. It measures the size of an earthquake in terms.

A seismograph is an instrument that measures the magnitude of an earthquake. Seismographs record the vibrations caused by the earthquake. This is recorded in a zigzag manner that shows the varying amplitude of ground oscillations beneath the seismograph. Some highly sensitive seismographs can detect strong earthquakes from sources anywhere in the world.
The time, locations, and magnitude of an earthquake can be determined from the data recorded by seismograph stations.
Inside a seismograph are levers that scratch marks on the surface of smoked paper fixed to a constantly moving drum. When the marks become irregular, it indicates the occurrence of an earthquake.

Richter Scale
Magnitude is measured on the basis of ground motion recorded by a seismograph, which then gives a number based on the severity of the quake.

Intensity also measures the earthquake, only in terms of the effects of an earthquake at a particular site based on the observations of the affected areas, essentially measuring the effects on humans, other living beings and built structures. It is measured using a descriptive scale called Modified Mercalli Scale.

Mercalli Scale
The Mercalli Scale measures the intensity of an earthquake. First developed by an Italian geologist, Guiseppe Mercalli, in 1095, the scale was modified in 1931. It is now referred to as Modified Mercalli Scale. The intensity of a quake may range from I to XII [Measure of Intensity is always written in Roman numerals]

A fault is a fracture in the rock layers within the surface of the Earth. According to type, faults are classified as strike, normal and reverse.

Seismic Waves
An earthquake generates elastic waves that travel from the source to the surface of the Earth. Seismologists are concerned with p, s and surface waves. A p wave is a primary wave that travels away from the focus and consists of a series of compressions and dilations. An s wave is the secondary wave and travels more slowly than the s wave. It consists of elastic vibrations transverse to the direction of travel. The longer the distance of a distance of a place from the focus, the greater is the arrival time between the p and s waves. Surface waves, which consist of Rayleigh Waves and Love Waves, and are even slower and follow the surface of the Earth.

The outer layer of Earth, it is about 10 to 65 kilometres thick. The crust is brittle and broken into various pieces. A lot of the earthquakes originate in this layer.

An earthquake is usually a series of quakes. While the largest vibration is termed the earthquake, smaller shocks after the quake are called aftershocks. Similarly, there may be foreshocks, which are vibrations of lesser magnitude before the quake. Aftershocks can continue to happen even hours or days after the main quake. They have occasionally been known to happen even after years or months.

The innermost part of the earth, divided into two layers. The outer core extends from 2500 to 3500 miles below the earth's surface and is liquid metal. The inner core is the central 500 miles and is solid metal.

The angle that a planar geologic surface (for example, a fault) is inclined to the horizontal surface of the Earth.

A fault is a fracture in the Earth's surface, the rocks on the either side of which have moved with respect to each other. Faults can be of strike-slip or dip-slip.
Strike-slip faults are vertical fractures where the blocks have mostly moved horizontally. Dip-slip faults are inclined fractures where the blocks have mostly shifted vertically. If the rock mass above an inclined fault moves down, the fault is termed normal, whereas if the rock above the fault moves up, the fault is termed reverse (or thrust).

Fault plane
The flat surface along which there is slip during an earthquake.

The outer solid layer of the earth. The lithosphere includes the crust and the mantle. The lithosphere is about 100 km thick, although its thickness is age dependent (older lithosphere is thicker).

Seismic gap
A section of a fault that has produced earthquakes in the past but is now quiet.

Seismicity maps
Maps that plot the location of earthquakes, big and small.

A very large wave, caused by the occurance of an earthquake below the surface of the ocean bed. Capable of reaching great heights and lengths, it can wreak extensive damage to coastal areas.