Context: Recent years have seen a series of minor and major earthquake in Afghanistan and adjoining areas. This tries to explain the reasons for the increased seismicity in Afghanistan region.
What is Earthquake?
Earthquake, any sudden shaking of the ground caused by the passage of seismic waves through Earth’s rocks.
Earthquake waves or seismic waves are vibrations generated by an earthquake, explosion, or similar energetic source and propagated within Earth or along its surface. Earthquakes generate four principal types of elastic waves; two, known as body waves (P and S), travel within Earth, whereas the other two, called surface waves (Love and Rayleigh), travel along its surface.
P Waves: It causes the ground to compress and expand, that is, to move back and forth, in the direction of travel.
- They are called primary waves because they are the first type of wave to arrive at seismic recording stations.
- P waves can travel through solids, liquids, and even gases.
S Waves: It shakes the ground in a shearing, or crosswise, motion that is perpendicular to the direction of travel.
- These are the shake waves that move the ground up and down or from side to side.
- S waves are called secondary waves because they always arrive after P waves at seismic recording stations.
- Unlike P waves, S waves can travel only through solid materials.
L Waves: Love waves have a horizontal motion that moves the surface from side to side perpendicular to the direction the wave is traveling.
Rayleigh Waves: It travels along the free surface of an elastic solid such as the Earth. Their motion is a combination of longitudinal compression and dilation that results in an elliptical motion of points on the surface.
Causes of earthquake
Major causes of earthquake are natural and occur mostly around the plate boundaries due to continuous interaction of plates.
1.Tectonics: tectonic movements associated with plate boundaries are ruptures and faults along the constructive plate boundaries, folding and faulting along the destructive plate boundaries ,this sort of disequilibrium caused due to different types of plate movement and consequently earthquakes of varying degree are caused.
2.Volcanic: Explosive violent gases during process of vulcanicity try to escape upwards and hence they push the crustal surface from below with great force and thus is caused severe earth tremor of high magnitude. Example-Krakatoa volcano of Indonesia caused intense earthquakes.
Earthquakes are induced by human activities causes earth tremors of serious consequences.
- Fracking: Injection of fluids into deep wells (Fracking)- can lubricate existing faults and crack rocks, triggering earthquakes.
- Underground nuclear explosions: Detonation of large underground nuclear explosions-will accentuate the fault lines leads to earthquakes. Case study– North Korea’s 2017 nuclear bomb test set off aftershocks that lasted for about eight months after the explosion. The bomb was set off near a previously unmapped fault line, and a first produced a 6.3 earthquake, followed by a magnitude 4 quake just minutes later.
- Mining: rocks and other materials are being removed from the Earth that instability in the ground is occurring, which is triggering earthquakes.
- Reservoir induced earthquake: Earthquake triggered by the impoundment of water behind the dam of enormous quantity causes isostatic disequilibrium of already adjusted rocks or further augment the already isostatically adjusted rocks below the reservoir or further augment the already fragile structures due to faults and fractures underneath. Examples Koyna dam in India, Hoover dam in USA.
Impact of Earthquake
Geomorphological impact-slope instability and failures leading to landslides, mudflows, and changes river courses.
Structural impact-Failure of roads, bridges, dams, pipelines, railways and embankment, subsidence of lands.
Economic impact-Loss of life and property, cost in rehabilitation and disaster risk reduction.
Tsunami- earthquake on ocean floor leads to Tsunami.
Why Afghanistan and adjoining Himalayan regions witnessing frequent earthquakes.
- Afghanistan is located over multiple fault lines in the region where the Indian and Eurasian tectonic plates meet and collides often causing tectonic activity like earthquakes, folding and faulting.
- Afghanistan is located on Eurasian plate, towards its west, Arabian plate move northwards and subducts under Eurasian plate whereas towards its east, Indian plates move northward to subduct under Eurasian plate. In southern Afghanistan, Indian and Arabian plate adjoins and both subducts northward under Eurasian Plate.
- The collision and convergence of the Indian plate and the Eurasian plate result in the folding and faulting of the Earth’s crust.
- The ongoing northward movement of the Indian plate towards the Eurasian plate results in compression, leading to uplift of Himalayas and the transmission of tectonic stress across the entire region, including Afghanistan.
- These compression causes the crust to deform and creates faults and fractures that can slip and generate earthquakes.
- Afghanistan is also crisscrossed by various active fault systems like the Chaman fault, and the main Pamir thrust. These faults are the source of many earthquakes in the region.