What is an earthquake?

An earthquake is a shaking of the earth crust caused by a rapid release of energy.

•A result of motions of the lithospheric plates.

•Much larger areas are effected.

•Energy released radiates in all directions from its source, the focus.

•Energy is in the form of waves.

•Sensitive instruments around the world record the event.

The sudden slip that is an earthquake results from a gradual build-up of stress inside the Earth. When the stress in a particular location is great enough to overcome the forces holding together the rocks below us, something, effectively, “breaks”, and an earthquake begins. It is easier for them to snap if certain places are already weakened.The forces needed to cause this stress and move such large masses of rock are immense “What causes these forces?” The answer can be found in the theory of plate tectonics,

Continental drift and Plate Tectonic

The Earth’s crust is sectioned into great slabs, known as plates. These plates drift very slowly & steadily with relation to each other as they “float” on the more fluid material (the mantle) beneath them. At their edges, they may be colliding/separating/moving laterally past each other. The nature of these plate-plate boundaries can have a tremendous effect on the geology & types of volcanic & seismic activity found along the edges of each plate. Six continental plates, there are some smaller plates as well. The relative deformation between plates occurs only in narrow zones near the boundary and can be slowly & continuously (a seismic deformation) Or can occur spasmodically in the form of earthquake A radical concept when first suggested which has since gained acceptance as an overwhelming amount of supporting data was gathered by researchers in several fields states that
The cause of movement: requirement of the romemechanicaleqbr–associated convection.

Reid’s Elastic Rebound Theory

If a stretched rubber band is broken or cut, elastic energy stored in the rubber band during the stretching will suddenly be released. This theory supposes: The crust of the Earth in many places is being slowly displaced by underlying forces. Differential displacements set up elastic strains that reach levels greater than can be endured by the rock. Ruptures (faults) then occur, and the strained rock rebounds along the fault under the elastic stresses until the strain is partly or wholly relieved. The strain slowly accumulating in the crust builds a reservoir of elastic energy, in the same way as a coiled spring, so that at some place, the focus, within the strained zone, rupture suddenly commences, and spreads in all directions along the fault surface in a series of erratic movements due to the uneven strength of the rocks along the tear. This uneven propagation of the dislocation leads to bursts of high-frequency waves, which travel into the Earth to produce the seismic shaking.

Earthquake mechanism

•Slippage at the weakest point (the focus) occurs

•Vibrations (earthquakes) occur as the deformed rock “springs back” to its original shape (elastic rebound)

•Earthquakes most often occur along existing faults whenever the frictional forces on the fault surfaces are overcome

Foreshocks and aftershocks

•Adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks

•Small earthquakes, called foreshocks, often precede a major earthquake by days or, in some cases, by as much as several years.

Collisions between adjacent lithosphericplates, destruction of the slab-like plate as it descends or subductsinto a dipping zone beneath island arcs and spreading along midoceanicridges are all mechanisms that produce significant straining and fracturing of crustalrocks. Thus, the earthquakes in these tectonically active boundary regions are called plate-edge earthquakesInterplate Earthquakes(1897 Assam earthquake).Many large earthquakes are produced by slip along faults connecting the ends of offsets in the spreading oceanic ridges and the ends of island arcs or arc-ridge chains. In these regions, plates slide past each other along what are called transform faults-transform faults or ocean-plate subductiontectonics, such as Japan, Alaska, Chile and Mexico. The Himalaya, the Zagrosand Alpine regions are examples of mountain ranges formed by continent-to-continent collisions. These collision zones are regions of high present day seismic activity. The estimation of seismic hazard along continental collision margins at tectonic plates has not as yet received detailed attention.

An overwhelming percentage of earthquakes are the direct result of tectonic activity. A notable fraction of world-wide seismic activity, however, is either not directly related to tectonics, or related only in a secondary manner. Plate-tectonic does not explain all seismicity in detail, for within continental regions, away from boundaries, large devastating earthquakes sometimes occur away from the plate boundaries (1993 Laturearthquake) these are called Intra-plate Earthquakes. These intraplate earthquakes can be found on nearly every continent. Such major internal seismic activity indicates that lithospheric plates are not rigid or free of internal rupture. The occurrence of intraplate earthquakes makes the prediction of earthquake occurrence and size difficult in many regions where there is a significant seismic risk.

Explosions

Ground shaking may be produced by the underground detonation of chemicals or nuclear devices. Underground nuclear explosions fired during the past several decades at a number of test sites around the world have produced substantial artificial earthquakes (up to magnitude 6.0). Volcanic Earthquakes Despite these tectonic connections between volcanoes and earthquakes, there is no evidence that moderate to major shallow earthquakes are not essentially all of tectonic, elastic-rebound type. Those earthquakes that can be reasonably associated with volcanoes are relatively rare and fall into three categories: (i) volcanic explosions, (ii) shallow earthquakes arising from magma movements, and (iii) sympathetic tectonic earthquakes. Collapse Earthquakes Collapse earthquakes are small earthquakes occurring in regions of underground caverns and mines. An often observed variation is the mine burst. An intriguing variety of collapse earthquakes is sometimes produced by massive landsliding.

Seismic waves Body waves propagate within a body of solid rock The faster of these body waves is called the primary or P wave. The slower wave through the body of rock is called the secondary or S wave. The motion of individual particle is per-perpendicular to direction of s-wave travels: SV & SH waves. P-wave moves faster than s-waves. Surface waves: result from interaction between body waves and surface of earth. Its motion is more near the ground surface and decrease exponentially with depth. Love wave Rayleigh wave interaction of SH and p waves, motion is essentially the same as that of S waves that have no vertical displacement interaction of p & SV waves, Like rolling ocean waves, the pieces of rock move both vertically and horizontally in a vertical plane pointed in the direction of travel.

These P waves produces Successive compression & rarefaction, Analogous to sound wave can travel through solids & fluids such as granite mountains, and liquid material, such as volcanic magma or the water of the oceans .The S waves cannot propagate in the liquid parts of the Earth, such as the oceans and their amplitude is significantly reduced in liquefied soil Surface waves travel more slowly than body waves and, of the two surface waves, Love waves generally travel faster than Rayleigh waves.