Strong earthquakes in quick succession in the country have heightened the sensitivity of the common man towards this natural phenomenon. Effects of earthquakes can be so severe as to cause social and political upheavals apart from property damage.
Thus mitigation of seismic risk in big cities is a matter of concern to engineers and administrators alike. All the current scientific approaches the world over, depend on historical records for estimating the seismic hazard in a given region.
Unfortunately for the Indian subcontinent, reliable data on place and date are available for the past two hundred years only. A recent work undertaken by Iyengar et al.1 has led to a reliable identification of another twenty earthquake occurrences in the medieval period (1218th century AD). This still leaves the ancient period almost blank except for stray references. However, this does not mean that ancient Indians were not fascinated or not affected by earthquakes.
The Vedas, Puranas and the epics contain many references to earthquakes and allied phenomena. But what may be of interest in the contemporary context, are the writings of persons who were acclaimed scholars of their days. There exist a large number of writings in Sanskrit on natural phenomena. Among these, at least two are available in print namely, Brihat Samhita (B.S.) of Varaha Mihira2 (56th century AD) and Adbhuta Sagara (A.S.) of Ballala Sena3 (1011th century AD).
Discussion on causes of earthquakes had been a perennial topic in ancient Indian literature. According to A.S., the opinion of Kashyapa was that earthquakes were due to movement of sea creatures, whereas Garga opined that it was due to the sigh of elephants carrying the earth.
According to Vasishta, tremors were due to interaction of two strong winds which eventually impacted the oceans and shook the earth. Another opinion was that earthquakes occurred due to chance or unseen forces. Finally, both B.S. and A.S. give an explanation due to Parashara that once upon a time mountains could fly and move. Thus they were frequently falling on the earth causing earthquakes continuously. At the request of the earth, the Creator ordered Indra (thunder) to cut the wings of the mountains so that the earth became stable.
Yet, the four elements namely Wind, Fire and Water along with Indra cause the earth to shake. This explanation originating from the Rig Veda has been given in the two texts as a rudimentary theory on formation of stable continents and not in any religious sense. Ballala Sena quotes another writer Ushanas, who was categorical that the four elements shake the earth in the four quarters east, south, west and Parashara was of the opinion that eclipses and planetary aberrations could also cause earthquakes. Over the centuries this idea might have been given up since A.S. does not list this reason.
After discussing the causes, both the books turn their attention to classification of earthquakes into four groups, depending on the time of occurrence and the reigning stellar constellation. Thus, an earthquake which occurs from north in the last quarter of the night or in the first quarter of the day under any of the stars Asvini, Mrigasira, Punarvasu, Hasta, Chitta, Svati or Uttarapalguni belongs to the wind group (Vayavya).
The directions of occurrence are not explicitly mentioned in B.S. These were given by Ushanas as quoted in A.S. It is also mentioned that many scholars do not accept the classification based on the time of occurrence. Similarly, earthquakes of the Agni (fire), Indra and Varuna (water) group originate from south, east and west, respectively and are governed by specific star groups. Neither of the books indicate how the directions are to be fixed. It is observed that so far the effort has been only to group the earthquakes after their occurrence, according to some recognizable attributes.
About the effects of earthquakes, Varaha Mihira does not mention about damages to buildings. However other writers are categorical that Vayu-type earthquakes lead to extensive destruction of houses, monasteries, temples, palaces, towers and forts. The after-effects mentioned are occurrences of windstorms within a week after such an earthquake. Similarly, the Agni-type earthquake induces surface fires engulfing villages and towns.
Rivers and water sources dry up. Indra-type tremors, lead to rains and elimination of pest colonies. The Varuna-type earthquake kills people living along river and sea coasts. Earthquakes were also supposed to be a portent for impending death of kings, outbreak of wars and epidemics. In B.S. the list of regions affected, is short, whereas information given in A.S. from various sources including those of B.S. is more exhaustive. The geographical regions disturbed by earthquakes according to both the books are as follows.
It was recognized by all the authors that earthquakes may occur under any star at any time. Thus, combination-type earthquakes became possible. The places affected by such mixed-type tremors as mentioned by Parashara, are reported in A.S.
The effects of these earthquakes were supposed to be a combination of the effects of the primary types described earlier. Many of the places in the list given earlier are easily identifiable with their present