Earthquakes

Notes

An earthquake is the shaking of the Earth's surface caused by the sudden release of energy in the lithosphere. The point of origin below the surface is the focus (hypocentre); the point directly above it on the surface is the epicentre, where the shaking is strongest.

Causes

Natural: tectonic movements along faults (elastic rebound theory, H.F. Reid, after the 1906 San Francisco earthquake); volcanic eruptions; landslides.
Anthropogenic: reservoir-induced seismicity (Koyna, 1967), mining, nuclear explosions, fracking.

Types by focal depth: shallow (0-70 km, most destructive), intermediate (70-300 km), deep (300-700 km).

Measurement

Seismograph records the waves.
Richter scale (1935): measures magnitude, a logarithmic scale; each step is a tenfold increase in amplitude (about 32 times the energy). The Moment Magnitude Scale is now preferred for large quakes.
Mercalli scale (Modified Mercalli, I-XII): measures intensity (damage) at a place.

Distribution

Circum-Pacific belt (Ring of Fire): the largest share of earthquakes and volcanoes.
Mediterranean-Himalayan (Alpine) belt: about 17%.
Mid-Atlantic Ridge and other zones.

Effects: ground shaking, ground rupture and fissures, landslides, liquefaction, fires, tsunamis, and loss of life and property.

Earthquakes in India

BIS seismic zones (2002): Zones II, III, IV and V (Zone I merged with Zone II). Zone V is the most severe: Kashmir, parts of Himachal and Uttarakhand, North-East India, Rann of Kutch, and the Andaman and Nicobar Islands. About 59% of India's land is prone to moderate-to-severe shaking.
Major events: Shillong (1897), Kangra (1905), Bihar-Nepal (1934), Assam-Tibet (1950), Koyna (1967), Latur (1993), Uttarkashi (1991), Chamoli (1999), Bhuj (2001), Indian Ocean tsunami (2004), Kashmir (2005), Nepal (2015).
Institutions: National Centre for Seismology (Ministry of Earth Sciences), IMD, NDMA; the Indian Tsunami Early Warning Centre (INCOIS, Hyderabad).

Key Points

Focus vs epicentre; shallow quakes are the most destructive.
Richter: magnitude (logarithmic); Mercalli: intensity.
Belts: Circum-Pacific, Mediterranean-Himalayan, Mid-Atlantic.
Zone V is India's most severe seismic zone.
Koyna (1967) was reservoir-induced.

MCQs

Q1. The Richter scale measures:

(a) Intensity of damage
(b) Magnitude of energy released
(c) Depth of focus
(d) Speed of waves
Ans: (b).

Q2. The point on the Earth's surface directly above the focus is the:

(a) Hypocentre
(b) Epicentre
(c) Isoseismal line
(d) Seismic gap
Ans: (b).

Q3. The most seismically active zone in India according to BIS is:

(a) Zone II
(b) Zone III
(c) Zone IV
(d) Zone V
Ans: (d).