Introduction to earthquake

Seismology

1. Introduction to Earthquakes

2. Seismology Seismology is the study of the generation, propagation and measurement of seismic waves through earth and the sources that generate them . The word seismic is commonly used to qualify anything related to an earthquake, such as seismic waves , seismic intensity, seismic zoning, seismic region and so on.

3. Earth The earth’s shape is an oblate spheroid with a diameter along the equator of about 12740 km with the polar diameter as 12700km . The interior of the earth can be classified into three major categories as Crust, Mantle and Core

4. Continental Drift Theory German scientist Alfred Wegener, in 1915, proposed the hypothesis that the continents had once formed a single landmass before breaking apart and drifting to their present locations . He proposed that a large continent termed Pangea existed in earth around 200 million years ago and was surrounded by an ocean called Panthalassa.

5. This theory requires a source that can generate tremendous force is acting on the plates . The widely accepted explanation is based on the force offered by convection currents created by thermo-mechanical behavior of the earth’s subsurface. The variation of mantle density with temperature produces an unstable equilibrium. The colder and denser upper layer sinks under the action of gravity to the warmer bottom layer which is less dense. The lesser dense material rises upwards and the colder material as it sinks gets heated up and becomes less dense. These convection currents create shear stresses at the bottom of the plates which drags them along the surface of earth.

6. T heory of Plate T ectonics P resented in early 1960’s , explains that the lithosphere is broken into seven large (and several smaller) segments The upper most part of the earth is considered to be divided into two layers with different deformation properties. The upper rigid layer, called the lithosphere, is about 100 km thick below the continents, and about 50 km under the oceans, and consists of Crust and rigid upper-mantle rocks. The lower layer, called the asthenosphere , extends down to about 700 km depth . The rigid lithospheric shell is broken into several irregularly shaped major plates and a large number of minor or secondary plates. The lithospheric plates are not stationary, on the contrary, they float in a complex pattern, with a velocity of some 2-10 cm/year on the soft rocks of the underlying asthenosphere like rafts on a lake.

7. Plate Tectonics

8. Epicentres of 30000 earthquakes recorded during 1961-1967

9. Movement of Plate Boundaries Divergent/Spreading Ridges - Spreading ridges or divergent boundaries are areas along the edges of plates move apart from each other. Convergent boundaries - The convergent boundaries are formed where the two plates move toward each other . In this process, one plate could slip below the other one or both could collide with each other . Transform boundaries - Transform boundaries occur along the plate margins where two plate moves past each other without destroying or creating new crust

10. Convergence Boundaries Oceanic-continental convergence - W hen oceanic lithosphere subducts beneath continental lithosphere Ocean-ocean convergence - These boundaries are created when either oceanic lithosphere subducts beneath oceanic lithosphere. Continental-continental convergence - When two plates with continental lithosphere collide, subduction ceases and a mountain range is formed by squeezing together and uplifting the continental crust on both plates,

11. The movement of Indian Plate After splitting with ‘Pangea’ Indian plate started moving towards Eurasian plate at a speed of 9 m a century. Both continents had same rock density so one plate could not be subducted under the other. Immense Himalayan mountain range started forming 40 to 50 million years ago .

12. Fault A fault is a fracture within some particular rocky mass within the earth's crust. The depth and length of faults vary greatly. Faults may range in length from few meters to many kilometers and are drawn on a geological map as continuous or broken lines . Earthquakes are caused by active faults , that is, faults along which the two sides of the fracture move with respect to each other. So, an earthquake is caused by the sudden movement of the two sides of a fault with respect to another .

13. Types of Faults

14. Earthquake Terminology Earthquake is the vibration of earth’s surface caused by waves coming from a source of disturbance inside the earth. Most earthquakes of engineering significance are of tectonic origin and is caused by slip along geological faults . The point on the fault where rupture initiates is referred to as the focus or hypocenter of an earthquake . The hypocenter of an earthquake is described by its depth in kilometers, its map location in latitude and longitude , its date and time of its occurrence, and its magnitude. The term epicenter is the point on the earth’s surface directly above the hypocenter

15. Elastic Rebound theory As the plate try to move relative to each other, strain energy gets built up along the boundaries . When the stress buildup reaches the ultimate strength of rock, rock fractures and releases the accumulated strain energy The nature of failure dictates the effect of the fracture. If the material is very ductile and weak, hardly any strain energy could be stored in the plates due to their movement. But if the material is strong and brittle, the stress built up and subsequent sudden rupture releases the energy stored in the form of stress waves and heat. The propagation of these elastic stress waves causes the vibratory motion associated with earthquakes.

16. Seismic Waves Earthquake vibrations originate from the point of initiation of rupture and propagates in all directions. These vibrations travel through the rocks in the form of elastic waves .

17. Primary (P) waves These waves propagate by longitudinal or compressive action , which mean that the ground is alternately compressed and dilated in the direction of propagation Particle motion for P-wave is in the direction of wave propagation. P waves are the fastest among the seismic waves and travel as fast as 8 to 13 km per second

18. Secondary (S) waves These are also called shear waves, secondary waves, transverse waves, etc. Compared to P waves, these are relatively slow. These are transverse or shear waves , which mean that the ground is displaced perpendicularly to the direction of propagation. In nature, these are like light waves, i.e., the waves move perpendicular to the direction of propagation. Hence, transverse particle motion is characteristic of these waves.

19. Rayleigh Waves Lord Rayleigh(1885) described the propagation of wave as retrograde ellipse in the vertical plane with its major axis vertical and minor axis in the direction of wave propagation. The particle motion can regarded as the combination of P and SV-waves.

20. Love Waves A.E.H. Love (1911 ) When the angle of reflection at the base of the soil layer is more than the critical angle, SH waves are trapped in the soil layer. The particle motion is in the horizontal plane and transverse to the direction of wave propagation.

21. Recording Earthquakes The vibratory motion produced during an earthquake could be measured in terms of displacement , velocity or acceleration. A seismologist is interested in even small amplitude ground motions (in terms of displacement ) that provides insight into the wave propagation characteristics and enables him to estimate the associated earthquake parameters.

22. Intensity of Earthquake The intensity of an earthquake refers to the degree of destruction caused by it. Numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, the most popular is the Modified Mercalli Intensity (MMI) Scale

23. Magnitude of Earthquake The magnitude of an earthquake is related to the amount of energy released by the geological rupture causing it, and is therefore a measure of the absolute size of the earthquake , without reference to distance from the epicenter . Richter scale Where, A = A denotes the amplitude in micron (10-6m) recorded by the Wood-Anderson (WA) seismograph instrument located at an epicentral distance of 100 km  

24. Relationship between Magnitude and intensity of Earthquake Comparisons between magnitude and intensity are fraught with difficulty. Firstly, intensity varies with distance from the epicentre . Secondly , a large earthquake may occur away from inhabited areas and therefore cause little apparent damage. Focal depth , ground conditions and quality of building construction can have a considerable effect on subjective assessments of damage. Magnitude-intensity relationships are not favoured for engineering purposes. However, intensity could be the only information available for large historical earthquakes and the inputs from intensity measurements would be necessary in estimating the maximum earthquake potential of the region.

25. Seismic Zoning of India Under the initiative of the Ministry of Urban Development, a Vulnerability Atlas of India was prepared in which the earthquake, cyclone and flood hazard maps for every state and Union Territory of India The seismic zoning map was periodically updated and the latest (2007) map is shown in figure.

26. Seismic Zones of India Zone 5 Zone 5 covers the areas with the highest risks zone that suffers earthquakes of intensity MSK IX or greater. The IS code assigns zone factor of 0.36 for Zone 5. Structural designers use this factor for earthquake resistant design of structures in Zone 5. The zone factor of 0.36 is indicative of effective (zero period) level earthquake in this zone. It is referred to as the Very High Damage Risk Zone. The region of Kashmir, the Western and Central Himalayas, North and Middle Bihar, the North-East Indian region, the  Rann of Kutch and the Andaman and Nicobar group of islands fall in this zone. Zone 4 This zone is called the High Damage Risk Zone and covers areas liable to MSK VIII. The IS code assigns zone factor of 0.24 for Zone 4 Jammu and Kashmir, Himachal Pradesh, Uttarakhand , Sikkim, the parts of Indo-Gangetic plains (North Punjab, Chandigarh, Western Uttar Pradesh,  Terai , North Bengal, Sundarbans) and the capital of the country Delhi fall in Zone 4. In Maharashtra, the  Patan area ( Koynanagar ) is also in zone no-4. In Bihar the northern part of the state like  Raxaul , Near the border of India and Nepal, is also in zone no-4. Zone 3 This zone is classified as Moderate Damage Risk Zone which is liable to MSK VII. and also 7.8 The IS code assigns zone factor of 0.16 for Zone 3. Mumbai comes in Zone 3 Zone 2 This region is liable to MSK VI or less and is classified as the Low Damage Risk Zone. The IS code assigns zone factor of 0.10 (maximum horizontal acceleration that can be experienced by a structure) for Zone 2. Zone 1 Since the current division of India into earthquake hazard zones does not use Zone 1, no area of India is classed as Zone 1. Future changes in the classification system may or may not return this zone to use.