Tag: our changing earth

• Volcanic activities refer to pouring out of hot magma from weak layers of the earth's crust while seismic occurrences are the shaking of the earth's crust due to the release of energy at plate boundaries. Both the phenomenon occur at the plate boundaries of the earth crust, and they draw energy from the same sources. Thus both are interrelated.

An earthquake is a moment of tremor of the earth's crust. It originates naturally and below the surface. It sometimes causes a permanent change of level at the surface of the earth.

According to the theory of plate tectonics, the world has been divided into six major plates and several smaller movable plates. Each of the plates is composed of crust up to a depth of 100 km from the surface of the earth. Due to the forces at the earth's interior, these plates move.

The epicenter of an earthquake  is the point on the earth's surface vertically above the focus. Maximum damage is caused at the epicenter. 

Following activities of humans can trigger an earthquake:

P- Waves or the primary waves are also known as longitudinal or compressional waves. It can pass through solid, liquid and gas. P- waves travel at the fastest speed through solid materials. Though these also pass through liquid and gaseous materials their speed is slowed down. 

S-Waves or secondary waves are also called transverse or distortional waves. These are analogous to water ripples or light waves wherein the particles move at right angles to the rays. S- Wave cannot pass through liquid materials. It can only pass through solids.  

Krakatoa is a volcanic island situated in the Sunda Strait between the islands of Java and Sumatra in the Indonesian province of Lampung. The name is also used for the surrounding island group comprising the remnants of a much larger island of three volcanic peaks which was obliterated in a cataclysmic 1883 eruption, unleashing huge tsunamis (killing more than 36,000 people) and destroying over two-thirds of the island. The explosion is considered to be the loudest sound ever heard in modern history, with reports of it being heard up to 4,800 km from its point of origin. The shock waves from the explosion were recorded on barographs worldwide for days afterward.

The Focus or Seismic Focus is also called the hypocenter of an earthquake. The vibrating waves travel away from the focus of the earthquake in all directions. The waves can be so powerful they will reach all parts of the Earth and cause it to vibrate like a tuning fork. The focus is the exact point inside the crust of the earth where the quake begins.