Overview

Earthquakes are one of the most powerful natural forces on earth and regularly affect people around the world. Earthquakes can have a range of magnitudes with the strongest having devastating consequences for the areas where they are centered, nearby areas, and even some far away in the case of earthquake-gen erated tsunamis.

Most earthquakes are quite small but are not readily felt. Larger and more violent earthquakes are those that occur in a release of energy as the plates slide past or collide into one another.

Large earthquakes can focus on the boundaries where two plates meet, but they are not limited to these areas. As the plates move, fractures in the earth's crust develop and earthquakes are often located on them. These fractures are referred to as faults,and all generate earthquakes when they move.

Cause & Impacts

The most damaging earthquakes happen when the blocks of rock become locked together due to the intense friction created when they move. As they continue attempting to move once they are locked, pressure continues to build until it has enough energy to move the rock and the blocks move, creating an earthquake. The point where the energy is released is called the focus and the focal depth is the depth beneath the earth's surface where the energy release originates. The epicenter is another term used in studying earthquakes and this is the point on the earth's surface directly above the focus. From here, the energy released spreads out in rings moving across the surface - not unlike those caused when a rock hits still water. In addition to the main shaking created by an earthquake, there are often foreshocks and aftershocks.Foreshocks generally increase in magnitude leading to the main earthquake, whereas aftershocks happen after the main event and decrease in strength. Once an earthquake finishes its movement, its magnitude (strength) is determined with the Richter scale measures the energy released by an earthquake while the Mercalli scale measures the felt or observed intensity at a particular location.

The impacts of earthquakes vary based on their energy and intensity. The strongest earthquakes that occur can result in ground rupture, causing damage to bridges, dams, roads, railroad tracks, and the foundations of buildings. They can also cause landslides and avalanches as a result of the shaking. Intense shaking can also cause liquification of ground built on landfill when water mains break. The shaking of an earthquake is increased in areas of landfill.

Another major cause of damage is the fires that ignite when power lines fall and gas lines rupture. In addition, undersea earthquakes can generate tsunamis that are capable of traveling great distances from the epicenter and cause significant damage to coastal communities. In an effort to reduce the impacts of earthquakes, areas prone to them have taken steps to retrofit buildings and educate citizens (in schools and the media for example) on what they should do when an earthquake occurs. Knowledge of what to do and always being prepared for an earthquake is the best way to reduce injuries, deaths, and damage to cities and towns when they occur.

Seismic Hazard in West Bengal

The seismic hazard map of India was updated in 2000 (6) by the Bureau of Indian Standards (BIS). There are no major changes in the zones in West Bengal with the exception of the merging of Zones I and II in the 1984 BIS map. Western sections of the northern districts of Jalpaiguri and Kooch Bihar lie in Zone V. The remaining parts of these two districts, along with the districts of Darjeeling, Uttar Dinajpur, Dakshin Dinajpur, Maldah, 24 North Parganas and 24 South Parganas lie in Zone IV. The rest of the state along with the city of Kolkata, lies in Zone III. Since the earthquake database in India is still incomplete, especially with regards to earthquakes prior to the historical period (before 1800 A.D.), these zones offer a rough guide of the earthquake hazard in any particular region and need to be regularly updated

Largest Instrumented Earthquake in West Bengal

15 April 1964 - Sagar Island, West Bengal, Mb 5.2 (4). 21.600 N, 88.700 E, D=036.0 kms, OT=08:35:27 UTC (4) Felt in southern West Bengal and eastern Orissa including at Kolkata and Hugli. Damage in areas near the epicentre such as at Contai and Diamond Harbour. The maximum intensity in Kolkata was V.

Statistical Data of Earthquakes in West Bengal

Acronyms Used:

D=Depth, OT =Origin Time, Mw =Moment Magnitude, Ms =Surface Wave magnitude, Mb =Body Wave Magnitude, ML=Local Magnitude, M?=Magnitude Type unknown