When the Earth Moves

What are earthquakes? Why do earthquakes occur? Where do they occur? What is the damaged caused by earthquakes? Can we predict earthquakes?

These were some of the few questions, which were raised by the participants during the very first session of the Freedom University at Siolim on 3^rd November.

Gordon Anthony Mendonca, a young geologist from Raint in Moira, provided cogent answers with diagrams and illustrations wherever possible. One unnerving piece of information he gave us was: "Scientists have warned of the strong likelihood of a big earthquake of a magnitude exceeding 8 on the Richter scale in the Tehri region."

Gordon, who has worked as a Geo-logging Engineer for Oilfield Instrumentation in Thane and has been working as a Mud-logging Engineer for Geoservices Eastern Inc (Singapore), was a bright student of Bombay’s St Xavier’s College, securing the first rank at M.Sc.(Geology) with 69% aggregate marks. At the moment Gordon is at Lagos in Nigeria.

Technically speaking, earthquakes are vibrations induced in the earth’s crust due to the internal/external causes, which virtually shake up a part of the crust and all the structures, objects—living and non-living, and things existing on it.

For the common man’s point of view, earthquakes are a shake, roll or produce sudden shocks on the earth’s surface.

There may be as many as a million earthquakes in a single year. Most of them take place beneath the surface of the sea. However, merely a few of these earthquakes cause any serious damage. But earthquakes which occur near the large cities and in highly populated areas can cause extensive damage and loss to life, especially if the cities rest on soft ground.

The energy released by a large earthquake may be equivalent to that of 180 million metric tonnes of TNT. In other words, it may be 10,000 times greater than that of the first atomic bomb dropped on Hiroshima.

The strength of the earthquake is measured commonly on the Richter Scale. The system was developed by Charles F Richter, an American seismologist, in 1935.

Scientists calculate the Richter magnitude by using the information obtained from a seismograph, an instrument which records and earthquake’s ground motion. An earthquake of magnitude 7 or more can cause serious damage to property and kill many people if situated in a populated area. The highest magnitude ever recorded was 8.9 in the Pacific Ocean, near the Colombia-Ecuador border in 1906 and in Japan in 1933.

Although each earthquake has only 1 magnitude, its damage varies from place to place. Seismologists commonly use the Modified Mercalli Intensity Scale, which classifies earthquakes into 12 categories.

TERMS

Focus: The centre of an earthquake where the energy resulting from the earth’s plates grinding and pushing against one another is converted into ripples and waves. It is the source of seismic waves produced during an earthquake.

Epicentre: The point on the earth’s surface situated directly above the focus of an earthquake.

Fault : A fracture is the earth’s crust along the plane of which there has been displacement of rock on one side relative to the other, either in a horizontal, vertical or oblique sense.

Seismic Waves : Waves generated by an explosion or earthquake within the earth’s surface.

Seismograph : An instrument for recording ground (velocity) motions as a function of time.

Plate Tectonics: A theory according to which the outer, uppermost 70-100 km of the earth made up of the crust and upper mantle (lithosphere) is divided into several large and small plates; the interacting margins of which are regions of severe deformations, causing a high incidence of earthquakes and volcanic activity.

Major plates: Antarctica, Africa, Eurasia, India, Australia, Arabia, Philippines, N America, S America, Pacific, Bazca and Cocos.

Earthquakes are not distributed evenly over the surface of the globe but occur in well-defined narrow zones which are oayan belt and Mid-Oceanic Ridges. The Indian plate is moving North at the rate of 5.5 cm/year.

Location: Seismologists locate earthquakes by studying the time intervals at which the different seismic waves reach a number of seismographic stations. They draw circles on a map to show the distance of the earthquake from each of the stations. The earthquake is located where the circles intersect each other. The focus of most earthquakes occur less than 25 miles (40 kms) beneath the surface of the earth. Though some may take place at a greater depth.

Most earthquakes occur along the boundaries where plates separate, collide or slide past each other. These are the earth’s most geologically active regions.

Damage/Effects: Landslides, fires, human lives lost, tsunanis.

PREDICTION

The prediction of earthquakes is not yet possible. Based on past records, scientists know the regions where earthquakes are most likely to occur. Some seismologists have experimented with devices that record small movements along a fault. Others have measured the speed of seismic waves produced by small quakes/explosive charges.

As reported by
Sebastian Rodrigues