Earthquakes: Risk in Nepal

SUVECHA PANT

Nepal is an earthquake-prone country because it is located at the junction between the two continent-to-continent collision (the Indian Continent and the Asian Continent), the Himalayas being a manifestation of this. Historical records shows that Nepal has experienced several great earthquakes in the years 1200, 1253, 1255, 1259, 1407, 1680, 1809, 1823, 1833, 1866, 1869, 1897, 1916, 1934, 1936, 1960, 1980, 1988 and 1991 A.D. which all had damaging effects to the environment.

The greatest one in recent history was the Nepal-Bihar, 15 January 1934, earthquake that reached 8.4 on the Richter scale. The earthquake induced a destructive Intensity X (an earthquake of intensity X creates immense public panic and is very destructive, many well-built buildings collapse). Although the Valley was 170 kms away from the epicentre, the capital was reduced to rubble due to severe shaking and no structure whether palace, temple or shrine was left unscathed. Over 8000 people died, approximately 1500 were injured and 200,000 houses destroyed.

The 1988 earthquake, which measured 7.7 on the Richter scale and had an Intensity IX (an earthquake of intensity IX causes public panic and is destructive, well-built buildings show very heavy damage) killed 1000 people and caused severe destruction. The distribution and the rate of earthquake occurrence in the past is an important indicator to show expected earthquakes. Worth noting is that Nepal lies in seismic zone where frequent earthquakes of 8 on the Richter scale is possible in the future.

The geology of the Kathmandu valley is fragile and the soil still unconsolidated. The topmost soft-sediment layer contains many silt and fine sands. Experts have come with evidence that the Valley was settled after it was drained thousands of years ago and the bottom contains deposition of the sediments of the Kathmandu lake. During a major earthquake, ten or more times greater ground shaking is possible on a lake-belt, which can result in disastrous effects. Ground slumping or soil liquefaction could also occur, leading to the temporary conversion of unconsolidated soils in behaving like water. This could result in a quicksand like condition where the buildings sink or tilt causing massive destruction.

Seismic hazard describes chances of occurrence of potentially damaging natural phenomena as soil amplification. Liquefaction at a certain site within a certain time interval is expected during a major earthquake. Seismic hazard analysis is an integrated effort of scientists, engineers, politicians, planners and public. It is successful if they are all aware that they are related to one another and are tune-in with the whole process. Vulnerability is the expected loss within a defined area resulting from the occurrence of a major earthquake. For example, if two buildings are subjected to exactly the same earthquake shaking, and one performs better than the other, than it can be said that the buildings that were less damaged had lower earthquake vulnerability than the ones that were more damaged. The seismic risk analysis is the product of seismic hazard and vulnerability. The risk analysis is a continuing job and is not finished until the decision maker, engineer, scientists, and public all feel comfortable. However, the whole process becomes meaningless if the common people fail to implement the information provided.

A catastrophe in a major earthquake is because of buildings. The failure of buildings occurs due to the state of a building. For example, quality of construction, irregularity of the building shape, and the level of earthquake resistant design. Careful consideration and aspects of cost, conservatism, ‘cultures’ of science and engineering, skilled workmanship, high-quality engineering design and appropriate site location are needed if safety is to be maximised. The general philosophy established for earthquake resistant buildings designs is to sustain structural damage without loss of human lives, structural integrity and stability in a major earthquake that may rarely occur in the lifetime of the structures.

Much is known about earthquakes and much is not. Earthquakes are not killers, but uncontrolled human activity kills and causes major damage during an earthquake. In Nepal, observing the present scenario of poorly constructed buildings, haphazard rapid urban expansion (growth rate of four percent), dense population (urban population of three million), and alarming rate of migration from rural areas – changes induced by uncontrolled human activity are occurring at an accelerating pace. Many of these are unbeneficial and combine to create a catastrophe that is waiting to happen in event of a major earthquake. We – in Nepal live every moment of our lives at a very high seismic risk.

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