Indian Plate, Earthquake Hazard And Insurance

 

A.B. Pandey

Research Associate,

National Insurance Academy, Pune

 

 

Introduction

 

An earthquake is a sudden movement of Earth, caused by the abrupt release of strain that has accumulated over a long period of time. Sudden slip on a fault results into ground shaking. Earthquake as a geophysical event is measured in terms of magnitude of energy release, duration and intensity of ground shaking. Earthquake as a human event is measured in terms of loss of life and property.

 

Due to suddenness of the event and unavailability of technology to predict the occurrence, earthquakes are the most devastating natural hazard faced by the mankind. India has faced many earthquakes of varying degrees in the past claiming huge property losses as well as high casualties. Bhuj earthquake of 26th January 2001 claimed over 20000           lives and caused injuries to more than 165000. The total estimated loss amounts to approximately Rs.212, 620 million.

 

Disasters cannot be avoided but managing the disasters prior to occurrence, during their operation and mitigating the losses after the event may reduce their effect on the mankind. Insurance serves as a tool of risk transfer in the process of mitigating the losses, but the assessment of future earthquake losses and pricing of insurance is a difficult task.

 

The present paper is an attempt to discuss:

 

  1. Plate tectonics and movement of Indian plate
  2. General geology of India and its relation to Earthquakes
  3. Official seismic map of India and TAC earthquake zones
  4. Issues regarding earthquake insurance before insurers
  5. Status of earthquake insurance in India, and
  6. Concept of Vulnerability Assessment

 

Plate Tectonics and Earthquakes

 

The interior of the earth is divided into three shells, Core, Mantle and Crust. The crust is the outermost shell of the earth, below which lies the mantle. Crust and upper most solid part of the mantle join together to form lithosphere. The lithosphere consists of many irregular plates comprised primarily of cool, solid rock upto 100 km thick. These enormous blocks of lithosphere vary in size and shape, and have definite borders that cut through continents and oceans. Below the outer part of the mantle lies the asthenosphere, which is semi liquid in nature and serves as a lubricant over which the lithosphere slides.

 

 

 

 

 

 

 


Fig: Interior of the Earth

There are nine large plates and a number of smaller plates identified by the scientists. While most plates comprise of both continental and oceanic crust the giant Pacific Plate is almost entirely oceanic. Of the nine major plates, six are named after the continents embedded in them, namely, the North American, South American, Eurasian, African, Indo-Australian, and Antarctic. The other three are oceanic plates: the Pacific, Nazca, and Cocos.

 

 

 

 

 

 

 

Fig: Plates on the Earth’s crust

There are many evidences that suggest that around 225 million years ago all of the landmasses of earth were locked together in the form of a supercontinent called Pangaea and the resulting ocean was called Panthalessa. Tectonic forces caused the break-up of Pangaea, leading to the current arrangement of oceans, continents, and other landmasses, which are still undergoing changes. Existence of mid oceanic ridges in the ocean floor, presence of geomagnetic anomalies, presence of deep-sea trenches and island arcs are some of the evidences of plate movements.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Fig: Breaking of Pangaea and movement of landmasses

 

Scientists now have a fairly good understanding of how the plates move and how such movements relate to earthquake activity. Most movement occurs along narrow zones between plates where the results of plate-tectonic forces are most evident. There are four types of plate boundaries, namely

 

 

Divergent boundaries occur where plates are moving apart and new crust is created by magma pushing up from the mantle. Convergent boundaries are those where plates are moving toward each other, and sometimes one plate sinks under the other. The location where sinking of a plate occurs, is called a subduction zone. The zone between two plates sliding horizontally past one another is called a transform-fault boundary, or simply a transform boundary. In some regions, the boundaries are not well defined because the plate-movement deformation occurring there extends over a broad belt called a plate-boundary zone.

 

Movement of the Indian Plate

 

India was a large island situated off the Australian coast dating back to about 225 million years ago. Tethys Sea, a vast ocean, separated India from the Asian continent. When Pangaea broke apart about 200 million years ago, India started drifting northward. About 80 million years ago, India was located roughly 6,400 km south of the Asian continent, moving northward at a rate of about 9 m a century. The Indian plate collided with the Asian plate about forty to fifty million years ago slowing down its northward advance by about half. The collision marks the beginning of uplift of the Himalayas.

 

 

Fig: Northward Drift of Indian Plate

 

Boundary of the Indian plate

 

The boundary of the Indian plate which meets the Eurasian plate can be marked by the Himalayas in the north, Burma-Andaman arc in the east and transverse fault systems such as the Chaman fault in the northwest. The continued northward collision of the Indian plate against the Eurasian landmass causes intense seismic activity.

 

 

 

Fig: Collision of India and Eurasian plates and making of Himalayas

 

 

The major tectonic features in the Himalayan region include Main Boundary Thrust (MBT), the Main Central Thrust (MCT) and the Indus Tsangpo Suture Zone (ITSZ). The Burma-Andaman arc marks the eastern margin of the Indian plate. The Hindukush Syntaxis and the Pamir knot border the northwestern Himalayan region. This region is characterized by the junction of several tectonic features and experiences high level of seismicity varying from shallow to intermediate-depth earthquakes.

 

Fig: Generalized tectonic map of India and adjoining regions.

 

Abbreviations: ANR: Adaman Nicobar Ridge, CB: Cuddapah Basin, DF: Dharwar Fold, GG: Godavari Graben, HF: Herat Fault, IBR: Indo Burma Ranges, ITSZ: Indus Tibetan Suture Zone, KF: Kunlun Fault, KKF: Karakuram Fault, MBT: Main Boundary Fault, MCT: Main Central Thrust, MG: Mahanadi Graben, SDG: Satpura Damodar Graben, NH: Naga Hills, NSL: Narmada Son Lineament, PF: Panvel Flexure, RRFZ: Red River Fault Zone, SF: Sagaing Fault , SR: Sulaiman Range, SP: Shillong Plateau, WAF: West Adaman Fault, WCF: Wang Chao Fault, XF: Xian Shui He Fault, YZS: Yarlung Zangpo suture.

 

Indian Shield region

 

Several rift zones and thrust zones mark the Indian shield region. Although considered to be a stable continental region (SCR), this region has experienced many earthquakes of varying magnitude in the past. The Narmada-Son Lineament (NSL) is a prominent tectonic feature of the Indian shield, which divides the shield into two sectors, namely, the northern sector and the southern sector. 

 

The northern sector of Indian shield has a relatively lower level of seismicity. The major tectonic constituents in the southern sector of the Indian Shield include the massive Deccan Volcanic Province (DVP), the South Indian Granulite Terrain (SIGT), the Dharwar craton (DC), the Cuddapah basin (CB), the Godavari Graben (GG) and the Mahanadi Graben (MG), the Eastern and the Western Ghats on the east and west coast of India, respectively.

 

The Eastern Ghat region in general is a quiet zone and the Western Ghat region of the Indian shield also depicts very low seismicity, except for some clusters, the prominent one being the Koyna- Warna region. The Koyna reservoir region has been experiencing induced earthquakes right from the date of its first filling in 1962.

 

In the central shield region south of the Narmada Son Lineament, the seismic activity is considerable although diffused. The Latur region in central India experienced an earthquake of 6.1 magnitude in 1993.

 

The northwestern corner of Indian shield shows relatively higher level of seismicity. This region experienced the well-known 7.8 magnitude Kutch earthquake of 1819 and   recent Bhuj earthquake (6.9 as measured by IMD).

 

Seismic Zones of India

 

History of classification of earthquake zones in India dates back to the year 1956 when for the first time Indian subcontinent was divided into three zones based on geotectonics and distribution of the earthquakes in the historical times (Tandon, 1956). The three zones thus divided were Severe, Light and Minor. Severe related to the plate boundaries, Minor related to the shield region and the Light was the transitional zone between the two.

 

Bureau of Indian Standards is the official agency for publication of the Seismic maps in India. BIS came out with a seismic map containing six zones in the year 1962, which was changed to a seven-zone map in 1966. A five-zone map was developed in the year 1970/84 based on MM intensities recorded in various parts of the country. Khattri et al, 1984 published a map of seismic hazard in units of ‘g’, for 10% probability of exceedance in 50 years.

 

 

 

 

Details of the five seismic zones are being given below:

 

Zone V: Very High Damage Risk Zone - Covers the areas liable to seismic intensity IX and above on Modified Mercalli Intensity Scale.

 

Zone IV: High Damage Risk Zone - Gives the area liable to MM VIII.


Zone III: Moderate Damage Risk Zone - The associated intensity is MM VII.

 

Zone II: Low Damage Risk Zone - The probable intensity is MM VI. This zone is

 

Zone I:    Very Low Damage Risk Zone - Here the maximum intensity is estimated as MM   V or less.

 

 

Fig: Seismic zones of India

 

 

Intensity refers to the effects of earthquakes and Modified Mercalli scale is the standard measurement for these effects. The MM scale is divided into twelve MMI values. The damage is near total at MM XII.

 

This seismic zone map has further been revised with only four zones. Zone I and Zone II have been merged and hence Zone I does not appear in the new zoning. The Killari area has been included in Zone III and the Bellary isolated zone has been removed. The parts of eastern coast area which were earlier in the level of Zone II and have shown similar hazard to that of the Killari area, have been enhanced to Zone III and connected with Zone III of Godawari Graben ar