Earthquake occurrence, terminology, causes, effects

How an earthquake occurs

Plate motion slowly loads rocks along a fault. Friction holds the fault, so strain energy is stored in the rock. When the stress exceeds the strength, the rock suddenly breaks and slips (elastic rebound theory), releasing energy as seismic waves that shake the ground.

Terminology

• Focus (hypocentre): point of rupture inside the Earth.
• Epicentre: point on the surface directly above the focus.
• Focal depth: distance from focus to epicentre.
• Epicentral distance: distance from a station to the epicentre.
• Magnitude: energy released (Richter scale).
• Intensity: damage and effects at a place (Mercalli scale).
• Isoseismal line: line joining points of equal intensity.
• Foreshock / aftershock: smaller shocks before / after the main shock.
• Seismograph: instrument recording ground motion.

   Epicentre *
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             * Focus

Causes

1. Tectonic: movement of plates and slip on faults (most earthquakes, including Himalayan ones).
2. Volcanic: movement of magma and gases.
3. Isostatic adjustment and landslides, rockfalls and cave collapse.
4. Man-made: reservoir impoundment, deep mining, underground explosions, deep well injection.

Effects

• Ground shaking, cracks and surface rupture.
• Collapse of buildings, bridges and dams, with loss of life.
• Landslides, rockfall and avalanches in hills.
• Liquefaction of saturated sandy soil and ground settlement.
• Tsunamis along coasts and seiches in lakes.
• Fire, floods from dam failure, and disease.
• Change in river courses and groundwater level, and economic loss.