(a) What is earthquake? How is the epicenter of an earthquake determined? Discuss on the disaster risk associated with…

(a) Earthquake, location of the epicentre and disaster risk (20 marks)

Definition. An earthquake is a sudden release of accumulated elastic strain energy in the crust or upper mantle, usually by slip on a fault, that radiates as seismic waves and shakes the ground. The point of rupture is the focus (hypocentre) and the point on the surface directly above it is the epicentre.

Location of the epicentre. Seismographs record the arrival of P waves and then the slower S waves. The S–P interval at a station increases with distance, and travel-time curves convert it to the epicentral distance (roughly 8 km for every second of interval). A circle of this radius is drawn on a map around each station; the circles from three or more stations intersect at the epicentre (triangulation). Focal depth is found from depth phases such as pP, and today the location is computed from many stations by inversion.

Disaster risk with magnitude. Magnitude is logarithmic: each unit means ten times the amplitude and about 32 times the energy.

• Below 3: recorded only by instruments, no risk.
• 3 to 4.9: felt, with minor or no damage.
• 5 to 5.9: damage to poorly built structures near the epicentre.
• 6 to 6.9: destructive within about 100 km, as at Latur (1993, magnitude 6.2, shallow, about 10,000 deaths).
• 7 to 7.9: heavy damage over a wide area, as at Bhuj (2001, magnitude 7.7).
• 8 and above: devastation over hundreds of kilometres, often with tsunami, as at Sumatra (2004, magnitude 9.1).

The actual risk also depends on focal depth, distance, soft soil (which amplifies shaking and liquefies), the quality of construction, population density and secondary effects such as landslides, tsunami and fire. India is divided into seismic zones II to V, with the Himalaya and the north-east in zone V.

(b) Origin of the Earth and interior of the Earth (15 marks)

Origin. The most accepted hypothesis is the modern nebular (solar-nebula) model. About 4.6 billion years ago a fragment of a molecular cloud collapsed under gravity, and, because of its spin, flattened into a rotating disc with the proto-Sun at its centre. Dust in the disc condensed and stuck together into planetesimals, which merged into protoplanets. The inner disc gave rocky planets, and the cold outer disc gave gas and ice giants. Earth then heated by accretion, impacts and radioactivity, and the molten iron sank to form the core, leaving the mantle and crust; a giant impact formed the Moon.

Support. The planets orbit in the same direction and nearly in one plane; meteorite ages are 4.56 billion years; the composition changes from rocky to gaseous with distance; and discs are observed around young stars.

Interior. Seismic waves show layers (figure): the crust, the mantle down to 2900 km and the core, of which the outer core is liquid, since S waves do not pass through it, and the inner core is solid.

(c) Foliation: types and processes of formation (15 marks)

Definition. Foliation is a planar fabric formed in deformed and metamorphic rocks by the parallel alignment or layering of minerals, and usually lies perpendicular to the direction of maximum shortening.

Types.

• Slaty cleavage: very fine, closely spaced, giving splitting into thin plates in slate; the grains are microscopic.
• Phyllitic foliation: slightly coarser grains, giving a silky sheen on the surface.
• Schistosity: platy or elongate minerals, such as mica, visible to the naked eye, in schist.
• Gneissic banding: coarse alternating light and dark mineral bands in gneiss.
• Spaced (crenulation) cleavage: discrete cleavage planes separated by uncleaved rock, and mylonitic foliation in shear zones.

Processes of formation.

• Mechanical rotation: platy grains are rotated into the plane perpendicular to the maximum compressive stress.
• Pressure solution: soluble minerals such as quartz and calcite dissolve at grain contacts and move away, concentrating insoluble mica and clay in cleavage seams.
• Recrystallisation and neocrystallisation: new mica and other minerals grow with preferred orientation under stress and heat.
• Flattening of grains and dynamic recrystallisation, and metamorphic differentiation, which segregates minerals into bands (gneissic banding).
• Shearing in ductile shear zones, which produces mylonitic foliation.