EARTHQUAKE :INTRODUCTION

An earthquake is a series of vibrations on the earth's surface caused by the generation of elastic (seismic) waves due to sudden rupture within the earth during release of accumulated strain energy. Earthquakes are one of the natural hazards and are well known to us. They are happening for centuries. India has had a long history of earthquake occurrences. Earthquake can be described as a sudden or transient motion or series of motion of  Earth's surface originating in a limited underground region, due to disturbance of the elastic equilibrium of the rock mass and spreading from there in all directions. Earthquakes occur due to slippage of rocks in the earth's crust or in the upper part of the mantle, consequent to these sudden movements strong vibrations occur on the ground in a short span of time. The tremendous amount of energy suddenly releases during an earthquake which accumulates slowly due to geological process.

Earthquakes are identified by their location (Longitude and Latitude), depth of the focus and the energy released/size of the earthquakes. The most common measures of the size of the earthquakes are magnitude and intensity. The intensity of the earthquake is measured by an instrument called Seismograph.

In the past, India has experienced several devastating earthquakes resulting in a large number of deaths and severe damage to property. During the last century, four great earthquakes struck different parts of the country. These are:

1.     Great Assam Earthquake in 1898

2.     Kangra Earthquake in 1905

3.     Bihar-Nepal Earthquake in 1934

4.     Assam Earthquake in 1950

 In the recent past, damaging earthquakes experienced by our country are

1.     Bihar-Nepal Earthquake in 1988

2.     Uttarpradesh Earthquake in 1991

3.     Killari Earthquake in 1993

4.     Chamoli Earthquake in 1999

5.     Jabalpur Earthquake in 1997

6.     Bhuj Earthquake in 2001

7.     Tsunami (Indian Ocean earthquake) in 2004 in the southern part of India

 The frequent occurrence of damaging earthquakes clearly demonstrates the high seismic hazards in India and highlights the need for a comprehensive earthquake disaster risk management policy. Earthquake hazard maps are developed by Building Material and Technology Promotion Council (BMTPC) New Delhi are based on Seismic Zoning Map of India contained in Indian Standard Code, IS: 1893-2002. Epicen years of occurrence of earthquakes (>5.0 intensity) are also mentioned. It is important to note that Bureau of Indian Standards (BIS) is the official agency for publishing the Indian Standard (IS) Codes and seismic hazard maps for India. BIS in its code IS: 1893 (Part-1)-2002 has revised the seismic zoning map of India, and seismic zoning map now contains only four zones as compared to earlier five zones. Relative values of zone factors are now different and the same should be referred while using Vulnerability Atlas of India. There is a need to assess the seismic vulnerability of buildings in urban areas of India as they are causing more loss to human lives.

 SEISMIC ZONES

 The seismic zones as shown in seismic zoning map of India contained in IS: 1893 (Part-1) 2002 fifth revision named these zones as II to V. A seismic zone map is based on a statistical compilation of the number and the magnitude of the past earthquakes. Therefore, the seismic zone map is an indication of the probable occurrence of an earthquake of a specific magnitude. There is no direct correlation between seismic zone and Richter scale, except past experience shows that the worst earthquakes occur in the higher seismic zones. According to above seismic code, the general basis for the zones is as follows:

 Zone V: Covers the areas liable to seismic intensity IX and above on Modified Mercalli (MM) Intensity scale. This is the most severe seismic zone and is referred as Very High Damage Risk Zone (maximum damage during earthquake may be of MSK intensity IX or greater).

 Zone IV: Gives the area liable to MM VIII (maximum damage during earthquake may be up to MSK intensity VIII).

 Zone III: The associated intensity is MM VII. This is termed here as Moderate Damage Risk Zone (maximum damage during earthquake may be up to MSK intensity VII).

 Zone II: The probable intensity is MM VI. This zone is referred to as Low Damage Risk Zone (maximum damage during earthquake may be up to MSK intensity VI).

It is also noted that Zone I of IS 1893-1984-(fourth revision) where the maximum intensity was estimated as MM or less is abolished and also merged in zone II. The new intensity scale, called as MSK Intensity scale 1964, is much more detailed and quantitative in nature. Its intensity levels are almost the same as Modified Mercalli (MM) Intensity Scale;