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Attempts have been made to predict earthquake occurrences, and some successes have also been achieved. In 1975 lives were saved when the Haichang earthquake of February 4 was predicted in time. The Loma Prieta earthquake of October 17, 1989 is the best-studied earthquake so far in earthquake history, because its occurrence was predicted, though not its day and time. Large-scale preparations had been made for studying this earthquake. Substantial instrumental and observational data on damage were collected on this earthquake. There are several instances where earthquakes were predicted with varying degree of success.

Earthquake prediction is based on certain precursors, which were identified after studies of data on past earthquakes. Some changes were observed before the arrival of a strong earthquake, and the duration for which the change existed, was found to be dependent on the magnitude of the earthquake. These changes, called earthquake precursors, fell into different categories, namely:

(ii) Changes in Isotopic composition of potassium, argon and sulfur in well water

Abnormal animal behavior before occurrence of an earthquake has caught maximum public attention. It might appear mysterious, to begin with, but scientific explanations have already been advanced s for a variety of such behaviors. Some of these explanations are based on laboratory experiments. Different types of abnormality in behaviors of animals have been reported in cases of different earthquakes. This reporting has been large in China and Japan, where it had long been recognized as potential earthquake prediction tool. It has been observed that prior to earthquake rats, cats, dogs, birds and fish suddenly disappeared from their usual places, and migrated else where in large numbers to find shelters. Monkeys and horses became restless. Chickens did not enter their dens, yaks did not eat, and sheep and deer gathered together, snakes came out of the ground. Hens and cocks started crying, and snakes came out of the ground. One or more of these type of behaviors were reported in several hundred earthquakes, the world over. These behaviors have been attributed to extra ordinary powers of the animals to sense minor tremors (foreshocks), emission of gasses, changes in electric properties of the ground, heat flow etc.-different animals being affected differently depending on which factors were more dominant in a certain situation.

The attempts to identify precursors of earthquakes are based on the recognition that fracturing of rocks, at some depth below the Earth’s surface, is involved in every earthquake, and the rocks fractured under stress. Investigations of the earthquake precursors were, therefore, based on the observed behavior of rocks under applied stress. Laboratory experiments showed that a uniform homogeneous rock breaks, once its strength is exceeded, without any microfractures in it. In such an environment of rock behavior (termed as Type-I behavior) no foreshocks appear before a strong earthquake. For a heterogeneous rock with uniformly distributed stress microcracks are produced in rock, which increase in number with increasing stress, before the rock finally breaks (Type-II behavior). In such a situation the foreshocks will precede the main shock. As the heterogeneity increases, and stress gets more concentrated (loss of uniformity in stress) microcracks continue to increase with increase in stress, and it becomes difficult to precisely pin point the breaking point (Type-III behavior). In such situations earthquakes continue to occur, without allowing a distinction between the foreshocks, main shock and the aftershocks. The three types of behaviors are illustrated below.

Figure- : Different types of behavior of rocks under stress. The vertical axis represents the number of shocks at any time. The thick vertical line indicates the main shock in type-I and Type-II behavior. The main shock cannot be identified in case of Type-III behavior.

Research has been undertaken on investigating the changes, which take place in the Earth’s interior leading to the various earthquake precursors, including those related to animal behavior. It is surmised that when stress approaches the breaking strength, rocks dilate[1]^,[2]. Small open space (voids) are created in the rocks due to the cracks. This results into increase in volume. For saturated rocks the water content decreases due to increased porosity. This leads to decrease in P-wave velocity, which continues to decrease as porosity increases. At a certain stage porosity becomes maximum, following which water starts to enter from outside, and the P-wave velocity begins to increase again. In this process S-velocity remains unaffected, so that at the beginning of dilatation the Vp/Vs ratio also decreases, and with recovery in the P-wave velocity Vp/Vs ratio begins to restore. Finally, the rock is saturated. Further increase of pore pressure decreases the effective pressure, and the strength of the rock. The rocks break and the result is an earthquake. The time required for completion of this process depends on the size of the zone of dilatancy, and consequently the magnitude of the earthquake. The time, when the Vp/Vs ratio begins increase is the time when the earthquake and its magnitude is predicted. Elastic resistivity, which depends on the water content, is expected to decrease due to dilatancy. Reports of decrease in resistivity to the tune of 10-20 % have been reported prior to strong earthquakes. These changes have been attributed to increase in stress in the source region affecting the properties of the soil right up to the surface.

Generally, concentration of minerals and gases in well waters remains constant. It has, however, been found that a few days before an earthquake the concentration changes. Increase in Helium and Radon content has been observed, frequently. Changes have also been observed in water levels in wells and discharging rates, temperature, pressure etc. have also been observed.

In some cases changes in the horizontal and vertical intensity of the geomagnetic field has been reported several years before a strong earthquake. The changed intensity returned to normal a few hours after the earthquake. Interpretations of the geomagnetic changes as precursor, however, become difficult because relatively larger changes in the geomagnetic field occur due to inospheric and magnetospeheric effects.

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[1] Rikitake,T.(1976). Earthquake Prediction, Elsevier, Amsterdam.

[2] Sivastava, H.N. (1983). Forecasting Earthquakes, National Book Trust, New Delhi.