What is Seismograph

Some natural phenomenon of Class 8

SEISMOGRAPH

Seismograph or seismometer is an instrument used to measure earthquake waves or seismic waves. It consists of a ball having pen attached to it, suspended with a string. Such that the tip of the pen touches a sheet of paper (like a graph sheet) placed on a fixed base. The shaking of the earth shakes the fixed base (as it is fixed on the ground) and the suspended ball moves with respect to the fixed base. The pen tip records this movement on the sheet of paper. This is the output of the seismometer. The seismological output, looks like a series of waves and is called as seismogram.

MAGNITUDE (OR INTENSITY) OF AN EARTHQUAKE :

The data recorded by seismograph can be used to determine the magnitude of an earthquake. The scale used to measure the magnitude of an earthquake is the Richter scale. On Richter scale and earthquake of 2 to 4 is not damaging, 4-8 is a moderate to severe earthquake and greater than 8-9 is very severe. The amount of damage cause to human being by earthquake can be determined by the location of earthquake, that is if it is in highly populated area or in a remote area.

Each increase of 1 on a Richter scale means a 30-fold increase in energy. This means a magnitude of 6 on this scale represents 30 times more energy than a magnitude of 5.

HAZARDS OF EARTHQUAKE

Hazard is the possible source of danger. Earthquake hazards are the damage cause to human life and property due to various destruction agents.

An earthquake hazard may be defined as any event or process associated with an earthquake that may effect the normal activities of people.

Some of the earthquake hazards are

Vibratory ground motion may result in the collapse of buildings.
Flood into inhabited area due to tsunamis and dam breaches.
Ground failure due to liquefaction.

Liquefaction is the process by which water saturated sand or soil temporarily loses strength and acts as a fluid.

The impact of liquefaction on old bricks building and those constructed on poor soil may be devastating in the event of a severe earthquake.

If there is a beach in your city, you could try wiggling your toes in the wet sand near the water at the beach. Does the sand act like a fluid? This can be likened to the effect caused by liquefaction.

DAMAGE CAUSED BY EARTHQUAKES:

The effects of an earthquake are strongest in a broad zone surrounding the epicenter. Surface ground cracking associated with faults that reach the surface often occurs, with horizontal and vertical displacements of several yards common. Such movement does not have to occur during a major earthquake; slight periodic movements called fault creep can be accompanied by micro earthquakes too small to be felt. The extent of earthquake vibration and subsequent damage to a region is partly dependent on characteristics of the ground. For example, earthquake vibrations last longer and are of greater wave amplitudes in unconsolidated surface material, such as poorly compacted fill or river deposits; bedrock areas receive fewer effects. The worst damage occurs in densely populated urban areas where structures are not built to withstand intense shaking. Therefore, L waves can produce destructive vibrations in buildings and break water and gas lines, starting uncontrollable fires.

Damage and loss of life sustained during an earthquake result from falling structures and flying glass and objects. Flexible structures built on bedrock are generally more resistant to earthquake damage than rigid structures built on loose soil. In certain areas, an earthquake can trigger mudslides, which slip down mountain slopes and can bury habitations below. A submarine earthquake can cause a tsunami, a series of damaging waves that ripple outward from the earthquake epicenter and inundate coastal cities.