Beneath the surface

Print edition : January 19, 2002

THE war in Afghanistan has created a variety of problems. Reports on Afghanistan occupy much space in the media. These look at the problems from the archaeological, historical, social, military and economic points of view. A country that has by and large remained far away from international attention except during the Soviet intervention in 1979, Afghanistan has finally come into the limelight.

Afghanistan, the Hindukush range in particular, is among the most active seismic regions of the world. Three to five earthquakes of magnitudes of around 6.0 on the Richter scale occur in this region every year. Earthquakes have constituted big obstacles for military operations. Present-day Afghanistan, once known as Gandhar, has suffered badly from earthquakes in the past. There is a mention of such an earthquake in the Mahabharata. A reference to an earthquake is found also in the accounts of Alexander the Great. During his journey through Iran (then Persia) and Afghanistan an earthquake occurred between Quetta and Karat. The time of this event is given as November 326 B.C. Alexander began his journey from Attock along the path of the river Indus and wanted to return to Greece by sea. However, the earthquake, of a magnitude of around 8.0 on the Richter scale, shattered his plans. The drastic change the earthquake caused to the geography of the area forced the king to take a land route.

According to historical records, invaders such as Genghis Khan, the Huns and the Mughals have suffered as a result of earthquakes. The fissures they create on the ground, often one to two metres deep, are too wide for the horses, camels and elephants to cross, thus forcing the marching army to take a detour.

However, present-day war is an entirely different affair. In ancient days, the power of an army was measured by the number of soldiers, elephants, horses and camels. Today, a missile can be fired from hundreds or even thousands of kilometres away from the target. It is guided with the help of GPS (Global Positioning System) and can hit the target with an accuracy of up to one metre. The GPS and high-tech inputs, such as infrared or visible pictures of the target and its geographical co-ordinates, guide the course of the missile. Before the missile hits its target, some illuminating chemicals, which look like fireballs, are used to attain clarity in the picture of the target.

Meteorological phenomena such as winds, hail storms, snow, rain, lightning and cyclones affect the path of a missile. So do sub-surface conditions. The basic parameters are the geo-magnetic field (usually known as the magnetic field) and the gravity field. The values of both these for the entire globe are so accurately recorded by the meteorological satellites that if there are any deviations in the path of the missile owing to any of the atmospheric parameters the path gets corrected automatically. However, if there are changes in the sub-surface parameters, it is difficult to correct the path immediately.

Prior to the occurrence of any medium to large earthquake (magnitude 6.0 to 6.5 or more), the sub-surface temperature usually shows a remarkable increase. The rise in temperature causes a reduction in the value of the geo-magnetic field in the vicinity of an active seismic area. The rise in temperature also changes to some extent the density of sub-surface rocks. Consequently, the gravity value changes at a micro level. Both these changes affect the path of the missile.

In the mid-1960s, a commercial aircraft flying from Singapore to Jakarta happened to fly over a potential volcanic area. The aircraft suddenly lost height by about 1,200 metres and the instruments in its cockpit began malfunctioning. The pilot finally made an emergency landing. The reason was that a few kilometres below the surface of the earth, the entire rock was in a molten state. As molten rock has no magnetic field, this affected the plane flying above it.

Similarly seismic movements affect objects above them, fired missiles for example. A missile could be travelling over a potential or vulnerable seismic area and it is possible that its path could be affected. The effect is in evidence during both pre-seismic and post-seismic periods. After an earthquake the geology, including fountains and hillocks and the paths of rivers, undergoes macroscopic change.

Humankind may make tremendous progress in science and technology but cannot overcome nature and its omnipotent forces, which are manifested in various natural phenomena.

Arun Bapat is a research seismologist based in Pune.

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