Earthquakes generate a variety of effects. Some are temporary, such as the shaking ground, swinging objects, rattling windows and oscillating trees. Permanent effects include damage to buildings, transportation, water supply systems and the landslides.
Till today there is no perfect method to forecast earthquakes. There are some methods for long, medium and short range earthquake forecasting. My article is more concentrated on medium and short range earthquake forecasting.
Medium term prediction means forecast of an earthquake a few months to a year or more ahead, while short term prediction implies forecast ranging from a few hours to some day in advance. The medium and short range stages in earthquake prediction are concerned with forecasting the occurrence of an earthquake of a particular intensity over a specified locality within a specified time limit. Satisfying this criterion, a few earthquakes have been successfully predicted in Japan, USA former USSR, India and China. It is actually these two stages of earthquake prediction which save the largest population from disaster in terms of life or property, and is more often demanded by public as well as Government. Even though the medium and short range prediction techniques are broadly similar using several disciplines of geophysics, some simple observations like earthquakes lights or sounds, unusual behaviour of animals, changes in the level and colours of well water, hydrochemical changes and foreshocks can be of great assistance from the point of view of short range prediction of large earthquakes.
Unusual animal behaviour: Unusual behaviour of animals prior to earthquakes received wide publicity after the Haichang earthquake of February 4, 1975 was successfully predicted in China. The official report was presented by the Chinese delegation at the Inter-governmental meeting convened at UNESCO, Paris in February 1976 which stimulated considerable scientific interest. Prior to this, however, several instances of abnormal animal behaviour were noticed before occurrence of some of the damaging earthquakes in different parts of the world, but they were considered more as historical legend. In Japan, innumerable rats were seen every day in a restaurant in Nagoya city, which suddenly disappeared on the evening prior to the Nobi earthquake of 1891.
Hyodrochemical precursors: Regular observations of the chemical composition of underground water were taken during 1997 in seismically active regions of Tadzhik, and Uzbekistan. The water samples were analyzed in the following two ways:
· The concentration levels of dissolved mineral components like sodium and calcium ions, bicarbonate and chloride ions were measured before, during and after the earthquakes.
· The gaseous components of water like helium and hydrogen sulfide were analyzed at various intervals of time.
The following results were obtained:
· During seismically inactive period, the concentration levels of dissolved minerals and gaseous components remained almost constant.
· About 2 to 8 days before an earthquake, appreciable increase in the concentration for dissolved minerals was noticed. Also, the maximum volume of helium gas in thermal water occurred 3 to 5 days before the increase in seismic activity.
Significant pre-disaster and post disaster hydro geo-logical changes rendering the groundwater turbid were observed during Jabalpur earthquake, 1997 (I.M.D. Report 1998).
The mechanism of the behaviour of these hydro geochemical precursors is attributed to the upsetting of balance in the rock/interstatial solution/ underground water system prior to earthquake. This is due to increase of stress and the consequent appearance of permeable fissures through which an increased inflow underground fluid from the subsurface zones of the earth’s crust takes place.
Temperature changes: A rise of temperature by 10 degree c. and 15 degree c. was reported before earthquakes in Lunglin, China (1976) and Przhevalsk, Russia (1970). Same relationship between magnitude and geothermal anomaly has been found for earthquakes in China.
Water level: Unusually muddy and fall in the level of water was reported in several wells a few days before the great Nankai earthquake (1946) in Japan. However, rise of water level by 3 and 15 cm was also reported before the Lunglin (China) and Przhevalsk (Russia) earthquakes. Similarly, water level rose by 3 cm a few hours before the earthquakes in Meckering, Australia (1968). In general a pre-seismic variations at observation wells follows this sequence:1) A gradual lowering of water levels of a period of months or years2) An accelerated lowering of water levels (rate often exponential) in the final few months or weeks preceding the earthquake. 3) A “rebound” where water levels begin to increase rapidly in the last few days or hours before the main shock.In the monitoring of water levels in deep wells, care must be taken to correct the data for “earth tides”. This is due either to volume changes caused in fractured aquifers by tidal strain, or perhaps by changes in gravitational acceleration alone. In either case, it is important that data is corrected for this phenomenon. In addition water extraction from the aquifer must also be considered. In many part’s of the planet the water table is falling due to water abstraction for drinking and irrigation. It is quite possible that such drops could be mistaken for a long-term seismic precursor.
Radon gas: The first evidence of a correlation between radon and earthquake came from Tashkent Basin prior to destructive earthquake in 1966. Radon observations revealed many precursory changes in its concentration as far as 1800 km from their respective epicenters. The measured radon in soils could be strongly disturbed by meteorological parameters, seasonal factors as well as a deeper phenomenon of seismic activity. Variety of studies which use complex mathematical methods have been done in order to distinguish between the variations of radon caused by environmental factors.
Work carried out in this direction was based upon the assumption that significant changes take place in the emission of gases such as radon and trapped in the earth crust before the arrival of a 'physical jolt' of an earthquake. This change takes place because of the physical stresses which are built up within the earth crust to trigger an earthquake. Work so far done has indicated the existence of a relationship between earthquake producing processes and radon movement. It has been noted that variation in radon levels is related to the intensity of an approaching earthquake.
Radon is a radioactive gas with a half-life of about 2.5 days. It is discharged from rock masses prior to an earthquake and dissolves in the well water which shows increase in its concentration.
Oil wells: some cases of sharp fluctuations in the oil flow prior to earthquakes were reported for wells in Israel, northern Caucasus and China. It is argued that when the tectonic stress accumulates to certain level, the pore pressure within a deep oil bearing strata may reach its breaking strength causing oil to spout along the oil wells.
Changes in the Electrical Resistivity of Rocks - Electrical resistivity is the resistance to the flow of electric current . In general rocks are poor conductors of electricity, but water is more efficient a conducting electricity. If microcracks develop and groundwater is forced into the cracks, this may cause the electrical resistivity to decrease (causing the electrical conductivity to increase). In some cases a 5-10% drop in electrical resistivity has been observed prior to an earthquake.
Ground Uplift and Tilting of the ground - Measurements taken in the vicinity of active faults sometimes show that prior to an earthquake the ground is uplifted or tilts due to the swelling of rocks caused by strain building on the fault. This may lead to the formation of numerous small cracks (called microcracks). This cracking in the rocks may lead to small earthquakes called foreshocks.
Srivastava, H.N. 1983. Earthquakes, Forecasting and Mitigation. National Book Trust, India, New Delhi.
Gupta, D. and Shahani, D.T. 2011. Estimation of Radon as an Earthquake Precursor: A neural network approach. Jr. of Geol. Soc. Of India, Vol.78, pp. 243-248.