Earthquakes in India; Seismic Zoning and Major Quakes

India has a very high frequency of great earthquakes (magnitude greater than 8.0) in comparison to the moderate earthquakes (magnitude 6.0 to 7.0). For example, during 1897 to 1950, India was hit by four great earthquakes. However, since 1950, only moderate size earthquakes have occurred in India which should be no reason to assume that the truly great earthquakes are a thing of the past.

The reasons of high magnitude earthquakes in India are hidden in the tectonic setting of India. India is currently penetrating into Asia at a rate of approximately 45 mm/year and rotating slowly anticlockwise.  This rotation and translation results in left-lateral transform slip in Baluchistan at approximately 42 mm/year and right-lateral slip relative to Asia in the Indo-Burman ranges at 55 mm/year. At the same time, deformation within Asia reduces India’s convergence with Tibet to approximately 18 mm/year. Since Tibet is extending east-west, there Is a convergence across the Himalaya that results in the development of potential slip available to drive large thrust earthquakes beneath the Himalaya at roughly 1.8 m/century.

Seismic Zoning of India

Indian subcontinent has a long history of devastating earthquakes, partially due to the fact that India is driving into Asia at a rate of approximately 47 mm/year. More than 50% area of Indian Subcontinent is vulnerable to earthquakes. According to the IS 1893:2002 ^{(It is the latest code of Bureau of Indian Standards (BIS) which lays down the criteria of for earthquake resistant design of structures)}, India has been divided into four seismic zones viz. Zone-II, -III, -IV and -V unlike its previous version which consisted of five zones for the country. After some revisions in the previous zoning, Zone I was altogether removed.

This zoning has been done on the basis of MSK-64 scale and a IS code Zone factor has been assigned by the BIS to each of them. The zone factor of 0.36 is indicative of effective (zero period) peak horizontal ground acceleration of 0.36 g (36% of gravity) that may be generated during MCE level earthquake in this zone. They are presented in the following table with IS code.

Some Great Indian Earthquakes

India has suffered some of the greatest earthquakes in the world with magnitude exceeding 8.0. For instance, in a short span of about 50 years, four such earthquakes occurred: Assam earthquake of 1897 (magnitude 8.7), Kangra earthquake of 1905 (magnitude 8.6), Bihar-Nepal earthquake of 1934 (magnitude 8.4)  and the Assam-Tibet earthquake of 1950 (magnitude 8.7).

Katch Earthquake of 1819

This 8.3 magnitude earthquake took place on the west coast of India and caused ground motion which was perceptible as far as Calcutta. It created a fault scarp of about 16 mile long and about 10 foot high which was later named as “Allah Bund”.

Assam earthquake of 1897

This 8.7 magnitude earthquake caused severe damage in an area of about 500 km radius and caused extensive surface distortions in the area.  The earthquake caused extensive liquefaction in the alluviated plains of Brahmaputra.

Bihar – Nepal Earthquake Of 1934

This 8.4 magnitude earthquake caused wide-spread damage in the northern Bihar and in Nepal. Due to extensive liquefaction, most buildings tilted and slumped bodily into the ground in an area of about 300 km long and of irregular width. This area was termed as the “slump belt”.

Koyna Earthquake Of 1967

This 6.5 magnitude earthquake occurred close to 103 metre concrete gravity dam at Koyna. Prior to this earthquake, the area used to be considered aseismic. However, after the construction of dam and filling up of reservoir in 1962, the seismic activity increased significantly.

The main shock of December 10, 1967 caused widespread damage, killing about 200 persons and injuring more than 1500 persons. This was an example of the reservoir-induced seismicity in India.

The dam, designed  keeping in mind the possible seismic activity, performed quite well with only nominal damage to the dam. This earthquake lead to the revision of Indian seismic zone map wherein the area around Koyna was brought in zone IV from zone I, and seismic zone for Bombay was upgraded from zone I to zone III.

Uttarkashi Earthquake Of 1991

This 6.6 magnitude earthquake shook the districts of Uttarkashi, Tehri, and Chamoli  of current Uttarakhand.

Killari (Latur) earthquake of 1993

This was a magnitude 6.4 earthquake that shook the area near village Killari in Latur district killing about 8,000 persons. Until this earthquake the area was considered non-seismic and placed in the lowest seismic zone (zone I) by the Indian code (IS:1893-1984).

The affected area did not have any modern towns, modern buildings or major industries. In some of the villages more than 30% of the population was killed. This earthquake will be known for outstanding rescue, relief and rehabilitation.

Jabalpur Earthquake Of 1997

This magnitude 6.0 earthquake is only example of such earthquakes which occurred close to a major Indian city in recent times.

2004 Indian Ocean earthquake and tsunami

The 2004 Indian Ocean earthquake was an undersea megathrust earthquake with an epicentre off the west coast of Sumatra, Indonesia, and it is known as Sumatra–Andaman earthquake or 2004 Indian Ocean tsunami or South Asian tsunami, Indonesian tsunami, and the Boxing Day tsunami. It killed 230,000 people in fourteen countries, and inundating coastal communities with waves up to 30 meters.

Impact of Earthquakes – Liquefaction

Earthquakes can cause soil liquefaction where loosely packed, water-logged sediments come loose from the intense shaking of the earthquake. The liquefaction is more prominent in areas such as river valleys, river plains and deltas.  The randomly bunched together soil particles have spaces have formed between them. These spaces, called pores, can be filled with water or air. The pressure of the material in the spaces holds the particles apart and stabilizing the soil in its present configuration.

The effect of a seismic wave on granular soil and pore pressure is that it increases the water pressure and forces the particles apart as well as disrupts  the contact point of the particles themselves. At this point in time the soil will flow like a liquid. The end product is the collapse of the particles so that there is less space between them. The water that was in that space is then forced upward. Liquefaction should have the following conditions for it to take place:

• Water table is less deep
• The soil has pore spaces
• The intensity of shaking in that area is viii or greater

The impacts of the Liquefaction are as follows:

The underlying layer of water rich sand compacts and sends a column of water and fine sand up and out onto the surface. This phenomenon is called Differential Compaction. At the same time, depth of lakes, ponds, borrow areas, and other depressions becomes lower, because the sand is pushed through the ground.  The buildings sink into the ground after the earthquake.