Assess quake risks with microzonation
Earthquakes cannot be prevented but damage can be limited if geologists arrive at near-accurate predictions
The recent consecutive earthquakes t hat hit Nepal have put the spotlight back on a series of questions - Why does the earth ‘quake’ (shake) periodically? And how can people fortify their homes to protect themselves? Technically, earthquakes are the sudden release of an enormous amount of pressure from the rocks contained in the uppermost part of the earth which is called the crust. These pressure build-ups are a natural and inevitable phenomenon. The earth’s crust is made up of a combination of rocks and most of these (except the softer ones like shales) are strong enough to withstand a significant amount of pressure. However, if the pressures acting upon the rocks are stronger than their physical constitution they disintegrate and develop faults. When faults occur the rock layers shift (refer to image). Such faults are the cause of earthquakes.
Despite ongoing research in several earth quake-prone countries like USA, Japan and India, there is no mechanism to predict the time of an earthquake. There is no way to determine the buildup of pressure and its subsequent release. Usually a major quake is preceded by minor tremors lasting a few months and followed by similar minor shocks persisting for a few months afterwards. This is because the entire pressure may not get relieved at one go but through small installments before and after the main event.
However, all is not bleak in the domain of prediction and risk mitigation. Microzonation is one geological theory that is a positive development. Seismic microzonation is defined as the process of subdividing a seismic zone into smaller microzones on the basis of several parameters so that the risk quotient at different locations can be correctly identified. Recently a team of six scientists of the Earthquake Risk Evaluation Centre, India Meteorological Department, New Delhi have presented their findings in the subject of micro- zonation in a research publication which carries their first level map for the NCR.
The zone of weathered rock in Central Delhi and the plains of northwest Delhi and South Najafgarh are zones of moderate hazard. The Chattarpur Basin, the area to the west of the Delhi Ridge and the stretch of river sediments adjoining the Yamuna River are all zones of high hazard. The centre has proposed to take up further stud- ies for earthquake microzonation with higher precision on a scale of 1:10,000. These will be immensely useful to the people of Delhi in understanding the risk quotient of the areas in which they reside. They can strengthen their buildings in accordance with the ‘hazard’ factor and make sure that their new houses are built in conformity with the specified/ standard building codes.
In simple terms, these microzones help us undestand the relative hazard/risk in different parts of the same seismic zone. Delineating these microzones is a major exercise that involves the collection of interdisciplinary data by experts in various fields, like geology, geophysics, seismology, earthquake engineering, etc.