An unfamiliar lineament is among four factors behind frequent earthquakes in northern Assam’s Sonitpur area. A lineament is a linear feature in a landscape dictated by an underlying geological structure such as a fault.

     The northeast is demarcated as Seismic Zone V, which indicates a zone with high vulnerability. The Indian plate is moving northeast toward the Eurasian plate in the Himalayan region, their oblique collision and release of stress and strain accumulated in the local tectonic or fault environments lead to earthquakes.

          LEARNING WITH TIMES /WITHOUT CLASSES : BASICS

An earthquake is the result of a sudden release of energy, an intense shaking of Earth’s surface, in the Earth’s crust that creates seismic waves.The shaking is caused by movements in Earth’s outermost layer(Crust).

The Earth is made of four basic layers: a solid crust, a hot, nearly solid mantle, a liquid outer core and a solid inner core. The solid crust and top, stiff layer of the mantle make up a region called the lithosphere. The lithosphere isn’t a continuous piece that covers around the whole Earth like an eggshell. It’s actually made up of giant tectonic plates.

Tectonic plates are constantly shifting as they drift around on the viscous, or slowly flowing, mantle layer below. This non-stop movement causes stress on Earth’s crust. When the stresses get too large, it leads to cracks called faults. When tectonic plates move, it also causes movements at the faults. An earthquake is the sudden movement of Earth’s crust at a fault line.

The location where an earthquake begins is called the epicenter. An earthquake’s most intense shaking is often felt near the epicenter.

The energy from an earthquake travels through Earth in vibrations called seismic waves. Scientists can measure these seismic waves on instruments called seismographs. The magnitude of an earthquake, total amount of energy released by earthquake, is conventionally reported using the Richter scale or a related Moment scale (with magnitude 3 or lower earthquakes being hard to notice and magnitude 7 causing serious damage over large areas)

Another way to measure the strength of an earthquake is to use the Mercalli scale. Invented by Giuseppe Mercalli in 1902, this scale uses the observations of the people who experienced the earthquake to estimate its intensity based on amount of damage to various types of structures.

Three main types of waves are produced during the earthquake: Primary waves or P, Secondary waves or S and Long waves or surface waves or L. L waves are responsible for most of the the destruction caused by an earthquake. P and S waves in the study of earth’s interior.

 The  P waves travel fastest and first to reach seismic station, and after that the S waves which are the slowest arrive. P waves is a longitudinal wave: in which the particle displacement is parallel to the direction of wave propagation. P wave is also called compression -dilation. Like the sound waves, the P waves pass through solids, liquids, and gases alike.

   The S waves are Transverse waves: the particle displacement is perpendicular to the direction of wave propagation. The S waves pass through only  solids.