Comprehensive Notes on Earthquakes, Seismic Waves, and Geomorphological Earth Movements

Mechanisms of Earthquake Generation

Earthquakes are characterized by the abrupt release of energy along a fault, which generates earthquake waves. A fault is defined as a sharp break within the crustal rock layer. Under normal conditions, rocks situated along a fault tend to move in opposite directions; however, the tremendous friction exerted by the overlying rock strata prevents immediate movement. Over time, significant pressure builds up within these layers. When this intense pressure at a specific point overcomes the friction offered by the overlying layer, an abrupt movement occurs. This movement generates shockwaves and releases energy that travels in all directions from the source.

The specific point where this energy is released within the Earth is known as the focus or the hypocentre. The point on the Earth's surface that is vertically nearest to the focus is designated as the epicentre. These energy waves, or seismic waves, are recorded by seismographs stationed globally at various seismic stations.

Seismic Wave Classifications and Characteristics

Seismic waves are broadly categorized into Body Waves and Surface Waves. Body waves are generated by the initial energy release at the focus and travel through the interior (body) of the Earth in all directions. There are two primary types: P-waves and S-waves. The velocity of these waves varies based on the elasticity (stiffness) and density of the material they traverse. Generally, higher elasticity leads to higher velocity. As these waves encounter materials of different densities, they undergo reflection (rebounding) or refraction (changing direction). For example, cooler, rigid areas of the Earth transmit waves at higher velocities compared to hotter, less rigid areas. Surface waves are generated when body waves interact with surface rocks.

Primary Waves (P-waves) are longitudinal or compressional waves, meaning the particles of the medium vibrate parallel to the direction of the wave's propagation. They are the fastest seismic waves and the first to arrive at a seismograph. P-waves can travel through all mediums—solids, liquids, and gases—though their velocity is highest in solids and lowest in gases ($\text{Solids} > \text{Liquids} > \text{Gases}$). The shadow zone for P-waves exists between $103^{\circ}$ and $143^{\circ}$ from the epicentre on both sides, a phenomenon that provides evidence for the existence of a solid inner core.

Secondary Waves (S-waves) are transverse or distortional waves, similar to light waves or water ripples. In these waves, particles move perpendicular to the direction of wave propagation. S-waves arrive at the surface after a time lag following the P-waves. Crucially, S-waves can only travel through solid materials and cannot pass through liquids or gases. Their shadow zone is significantly larger than that of P-waves, extending from $103^{\circ}$ to $180^{\circ}$ from the epicentre. This lack of transmission through the Earth's core led to the discovery of a liquid outer core.

Surface waves include Love waves (L-waves) and Rayleigh waves (R-waves). L-waves are the fastest surface waves and move in a transverse (shear) motion, causing significant horizontal ground shaking and structural collapse. They are the third to reach the seismograph. R-waves are the slowest surface waves and exhibit an elliptical or rolling motion, combining vertical and horizontal particle movement similar to ocean waves. R-waves are primarily responsible for the generation of Tsunamis due to the vertical shift they induce.

The Scientific Significance of Shadow Zones

Shadow zones are specific areas on the Earth's surface where seismic waves are not detected by seismographs. While seismographs within $103^{\circ}$ of the epicentre record both P and S-waves, those located beyond $143^{\circ}$ only record P-waves. The zone between $103^{\circ}$ and $143^{\circ}$ is the shadow zone for both types of waves. The S-wave shadow zone is much more extensive, covering a little over $40\%$ of the Earth's surface (the entire area beyond $103^{\circ}$).

By studying these zones and the behavior of seismic waves, scientists can infer the properties of the Earth's interior. Velocity changes allow for density estimation, while the emergence of shadow zones allows for the identification of different layers. Specifically, the refraction and reflection patterns help distinguish between the solid mantle, the liquid outer core (which blocks S-waves), and the solid inner core.

Distribution and Measurement of Earthquakes

Earthquakes are primarily distributed along plate boundaries, concentrated in two major belts: the Circum-Pacific belt and the Mid-continental belt. The depth of the focus varies by boundary type: convergent boundaries typically experience deep-focussed earthquakes, while diverging boundaries are associated with shallow-focussed earthquakes.

Earthquakes are measured using three primary scales:

1. Richter Scale: A logarithmic scale that measures magnitude based on the amplitude of surface waves. On this scale, a magnitude 6 earthquake has an amplitude $10$ times greater than a magnitude 5 earthquake. It ranges from $0$ to $10$.
2. Mercalli Scale: Measures the intensity of an earthquake based on observed destruction. It ranges from $1$ to $12$.
3. Moment Magnitude Scale: Currently the most widely used scale. It is also logarithmic but calculates the total energy released. It factors in the area of the fault rupture, the average slip along the fault, and the rigidity of the involved rocks.

Endogenic Earth Movements: Diastrophism and Sudden Forces

Endogenic forces originate from the Earth's internal energy, fueled by radioactivity, rotational/tidal friction, and primordial heat. These forces induce diastrophism and volcanism.

Diastrophism involves the warping, bending, folding, and fracturing of the crust and is divided into two processes:

• Orogenic processes: These are mountain-building processes involving severe folding of long, narrow belts. They involve horizontal forces and have vertical effects.
• Epeirogenic processes: These are continent-forming movements that act along the radius of the Earth (radial movements). They involve vertical forces and have horizontal effects. They can result in upliftment (e.g., Colorado Plateau, Brazilian Highlands, Australian Shield, Siberian Platform) or subsidence (e.g., Siberian Basin, Konkan region).

Sudden movements include earthquakes and volcanism. While localized, their sudden nature causes massive destruction and qualifies them as natural disasters. Examples of permanent changes include the submergence of Indira Point during the 2004 Indian Ocean earthquake, a $3\,m$ uplift in New Zealand in 1885, and a $6\,m$ subsidence in Japan in 1891.

Exogenic Forces and Denudation Processes

Exogenic forces result from stress induced by the sun's heat, primarily controlled by temperature and precipitation. These processes, collectively known as denudation, include weathering, erosion, deposition, and mass movement. Denudation is significant for soil formation, the creation of landforms, and the enrichment of valuable mineral ores like iron, manganese, aluminium, and copper. Mass movements are distinct in that the material moves solely under the impact of gravity.

Detailed Classification of Weathering

Weathering is categorized into chemical, physical, and biological types:

Chemical weathering changes the rock's chemical composition through:

• Solution/Hydration: Affects water-soluble minerals like nitrates and potassium.
• Carbonation: Carbonic acid (formed from $CO_2$ and water) reacts with rocks.
• Oxidation/Reduction: Oxidation occurs when minerals (like Iron or Sulphur) combine with oxygen. Reduction occurs in oxygen-poor environments, changing mineral colors (e.g., red iron oxides turning greenish-grey).

Physical weathering occurs without chemical change through:

• Unloading and Expansion: Removal of overlying weight causes rocks to expand and crack.
• Granular Disintegration: Coarse grains in sedimentary rocks fall apart grain by grain.
• Exfoliation (Onion Peeling): In dry climates with high diurnal temperature ranges, surface layers expand and contract more than deeper layers, causing rounded fracturing parallel to the surface.
• Block Separation: Rocks with existing joints (from cooling or pressure) break into blocks.
• Shattering: Thermal changes or waves cause rocks to break into sharp, angular pieces.
• Frost Wedging: Water in pores freezes, expands, and exerts pressure that tears rocks apart during freeze-thaw cycles.
• Salt Weathering: Crystallization and thermal expansion of salts (calcium, sodium, etc.) in desert pores cause individual grains to fall off.

Biological weathering involves living organisms:

• Physical: Burrowing animals and plant roots expose or wedge rock surfaces.
• Chemical: Decaying matter produces humic and carbonic acids, and algae/plants remove minerals, enhancing decay.

Questions & Discussion

Q1.) Consider the following statements:

1. In a seismograph, P waves are recorded earlier than S waves.
2. In P waves, the individual particles vibrate to and fro in the direction of Wave propagation whereas, in S waves, the particles vibrate up and down at right angles to the direction of wave propagation. Which of the statements given above is/are correct? (2023) (a) 1 only (b) 2 only (c) Both 1 & 2 (d) Neither 1 nor 2

Answer: (c) Both 1 & 2. Explanation: P (Primary) waves are longitudinal and the fastest, reaching the seismograph first. S (Secondary) waves are transverse, moving the particles perpendicular to the wave direction.

Q2.) Consider the following statements: (2024) Statement-I: Rainfall is one of the reasons for weathering of rocks. Statement-II: Rain water contains carbon dioxide in solution. Statement-III: Rain water contains atmospheric oxygen. Which one of the following is correct in respect of the above statements? (a) Both Statement-II and Statement-III are correct and both of them explain Statement-I (b) Both Statement-II and Statement-III are correct, but only one of them explains Statement-I (c) Only one of the Statement II and III is correct and that explains Statement-I (d) Neither Statement-II nor Statement-III is correct

Answer: (a) Both Statement-II and Statement-III are correct and both of them explain Statement-I.