Like dormant volcanos, earthquakes, too, can strike in areas considered seismically inactive.

The Latur quake occurred in a< IT DID not happen in Latur alone. Earthquakes can and do occur in what geologists generally consider peaceful regions of the earth. The 1819 Kutch earthquake and the 1886 Charleston earthquake in South Carolina, USA, are two famous examples of such rare errant behaviour of the earth. Both quakes were greater in magnitude (8 and 7.8 on the Richter scale, respectively) than the Latur earthquake (6).

The Kutch earthquake was a spectacular one and thrust up a scarp or wall about nine metres high and at least 90 kilometres long. Locals christened it the Allah Bund or "Wall of God". Sindree fort, which was built on an elevation near the coast, was besieged by the sea when the quake depressed the land to the south of the fort.

The rare occurrence of such earthquakes and the fact that the major ones took place before good instruments were developed have made their study a seismological backwater. But the mere occurrence of these unlikely quakes is both a scientific puzzle and a source of concern in siting such vulnerable facilities as dams and nuclear power plants.

Earthquakes are known to occur where continental plates meet -- areas where tectonic activity is concentrated. There are three types of continental movements: collision, separation and side-by-side grind. As plate boundaries reorient themselves over aeons, the interior regions once deformed by compression or stretching become stable. But they continue to experience stresses -- generally compressive ones -- transmitted from distant edges of the plates and can get deformed by perhaps a millimetre over a year.

Scientists studying quakes in stable interiors are looking for answers to two key questions: How much seismic activity takes place in these parts and what are the specific geological features that make them susceptible to earthquakes?

Experts believe a prerequisite for large earthquakes in stable crust areas is the existence of specific weak zones, chiefly in the nature of the rocks it is made up of. K V Subba Rao of the Indian Institute Technology in Bombay says Latur stands over a fragile zone, which is constantly fed by compressive stresses from the colliding Indian and Eurasian plates near the Himalaya and from the Karlsberg ridge in the Arabian Sea.

Scientists reason that past tectonics may have weakened such regions, which cause stresses built up over time to be released as earthquakes. Crusts dissected by old faults, such as an ancient mountain belt (the Western Ghats, for example) or a hidden rift (one beneath the Kutch area) might be reactivated by the pervasive compression within the continental crust. T Chandrashekhar, also of IIT, Bombay, believes the rocks separated by the Kurudwadi rift near the Western Ghats may have cracked because of excessive strain, causing the Latur earthquake. This view is supported by the earthquakes in Kutch and Charleston (which lies on an extended passive margin of a continental plate).

Mark D Zoback of Stanford University offers an alternative model of stable-continent quakes. He argues that compressive stresses found in the brittle upper crust of continents also exist in the lower crust, at depths of more than 20 km. B K Rastogi of the National Institute of Geophysical Research in Hyderabad told Down To Earth, "At those depths, high temperatures and differences in rock composition make the crust ductile rather than brittle. Zoback suggests that instead of cracking or bending elastically under stress, the lower crust gets squeezed slowly, adding to the stress on the brittle crust above. If the ductile deformation somehow becomes concentrated in a small region, the stresses might build up in the upper crust and cause earthquakes."

Forecasting stable-continent earthquakes is very difficult because, unlike the San Andreas fault in California that is visible on the surface, faults in continental plate interiors are buried and cannot be easily detected.

Whether or not stable-continent earthquakes follow a periodic pattern is also an open question. Scientists believe their recurrence interval is longer than those at plate margins because of crust deformation is stable areas is extremely slow. This implies that great earthquakes could recur at a place only after thousands of years. On the other hand, if intra-plate deformation is concentrated in a few zones of weak crust, as in the Latur area, earthquakes can recur even more frequently.

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