Earthquakes & Volcanoes

Earthquake is the sudden shaking of the Earth's crust caused by the movement of tectonic plates, resulting in the release of sudden energy and the formation of seismic waves.

• When a part of Earth's surface moves upward and backward, tremors are felt on the ground — this shaking is called an Earthquake.
• The Earth's crust is made up of different parts called plates or tectonic plates.
• Seismic waves created by the energy released travel through the Earth's surface, causing shakes and vibrations felt as an earthquake.

Earthquakes are primarily caused by the continuous movement of tectonic plates building up stress on both sides of plate boundaries until the stress is released in a sudden jerky movement.

• Earth's crust consists of several tectonic plates that make continuous movements.
• The pressure builds up on both sides of boundaries until it is released suddenly — generating seismic waves that travel through the Earth's surface.
• Other causes include volcanic activity, underground mine collapses, and artificial nuclear explosions.

Tectonic Earthquake is the most common type, caused by the movements of tectonic plates present in the Earth's crust.

• Most common form of earthquake.
• Caused by the movements of tectonic plates in the Earth's crust.

• Quite less common compared to tectonic earthquakes.
• Occurs before or after the explosion of a volcano.
• Caused by magma leaving the volcano, which is filled with rocks being pushed to the surface.

• Occurs in underground mines.
• Main cause is the pressure generated within the rocks underground.

• Artificial in nature — generated by man-made activities.
• High-density explosions such as nuclear explosions on the ground are the primary cause.

Epicentre is the point on the ground surface closest to the Focus/Hypocentre — it is where the maximum destruction is felt first.

• Seismology — the branch of Geology dealing with the study of earthquakes.
• Seismic Waves — waves of energy caused by the earthquake that travel through the layers of the Earth.
• Focus / Hypocentre — the place where the origin of seismic waves takes place, located beneath the Earth's surface.
• Epicentre — the point on the ground surface that is closest to the Focus/Hypocentre; directly above the focus.
• Seismograph — the instrument on which seismic waves are recorded.

``` [Focus/Hypocentre] — deep underground origin of the earthquake

v [Epicentre] — point on Earth's surface closest to focus; maximum shaking felt here

v [Seismic Waves] — travel through Earth's layers causing ground shaking ```

P-waves (Primary waves) are the fastest seismic waves and the first to be recorded on a seismograph; they are longitudinal in nature and can travel through solids, liquids, and gases.

• There are mainly 3 types of seismic waves: Primary waves (P-waves), Secondary waves (S-waves), Surface or Long waves (L-waves).
• S-waves cannot travel through liquids — this property helped scientists discover Earth has a liquid outer core.
• L-waves travel along the surface of the Earth and cause the most damage to buildings and structures.

The Richter Scale measures the magnitude of an earthquake (the amount of energy released at the focus).

• Richter Scale — used to measure the magnitude of an earthquake; it is a logarithmic scale (each whole number = 10× more amplitude, ~31× more energy).
• Mercalli Scale — used to record the intensity of an earthquake (the degree of shaking and damage felt at a specific place).
• Seismograph — the instrument that records seismic waves and is used to determine both magnitude and intensity.

Approximately 54% of India's land area is prone to earthquakes; India is divided into 4 seismic zones (Zone II to Zone V) based on seismicity, past earthquakes, and tectonic setup.

• The Indian subcontinent is divided into 4 seismic zones: Zone II, Zone III, Zone IV, and Zone V.
• Zone V covers the areas of maximum risk of earthquake — Very High Risk.
• Zone II covers the areas of minimum risk of earthquake — Low Risk.

• About 54% of India's land is prone to earthquake damage.
• Zone V has the highest seismic hazard; Zone V and Zone IV together cover the most vulnerable areas.

Volcanoes are openings in the Earth's crust through which magma, gases, and ash erupt from the interior of the Earth.

• They are formed when molten rock (magma) rises to the Earth's surface through cracks or weak zones in the crust.
• Once magma reaches the surface, it is called lava.
• Volcanoes can be active, dormant, or extinct depending on their eruption activity.
• Closely linked to tectonic plate boundaries — most volcanoes occur along convergent or divergent plate margins.

Active Volcanoes are those that erupt very frequently — once in 10,000 years is the general threshold used to classify a volcano as active.

• Barren Island in Andaman & Nicobar Islands is the only active volcano in India.
• Mt. Etna in Italy is one of the most active volcanoes in the world.
• Mt. Krakatoa (Indonesia) had a devastating eruption in 1883.
• Mt. Fuji (Japan) is an extinct volcano and is considered a sacred mountain.

More than 90% of the world's volcanoes lie in the circum-pacific belt, near convergent or divergent plate boundaries, also known as the 'Ring of Fire'.

• The Ring of Fire is a horseshoe-shaped zone around the Pacific Ocean where the majority of the world's earthquakes and volcanic eruptions occur.
• It is associated with convergent and divergent plate boundaries.
• Countries along the Ring of Fire include USA, Japan, Philippines, Indonesia, New Zealand, Chile, and Peru.
• The Pacific Ring of Fire accounts for about 75% of the world's active volcanoes and 90% of the world's earthquakes.
• Volcanoes also occur along mid-ocean ridges (divergent boundaries) such as the Mid-Atlantic Ridge.