Earthquake in Afghanistan

Geography Disasters and Management issues

21-Dec-19

Context-A massive earthquake of 6.3 on the Richter scale hit Afghanistan. Tremors were felt in Delhi-NCR and other parts of north India.

Tremors were felt in parts of northern India including Delhi and the National Capital Region on Friday evening following an earthquake in the Hindukush region of Afghanistan, and in Pakistan
The earthquake hit 51 km southwest of Jurm, northern Afghanistan, and was more than 200 km deep

What is an earthquake?

An earthquake is what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is called the fault or fault plane. The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter.
Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens. The largest, main earthquake is called the mainshock. Mainshocks always have aftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock!

What causes earthquakes and where do they happen?

The earth has four major layers: the inner core, outer core, mantle and crust. The crust and the top of the mantle make up a thin skin on the surface of our planet.
But this skin is not all in one piece – it is made up of many pieces like a puzzle covering the surface of the earth. Not only that, but these puzzle pieces keep slowly moving around, sliding past one another and bumping into each other. We call these puzzle pieces tectonic plates, and the edges of the plates are called the plate boundaries. The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake

Why does the earth shake when there is an earthquake?

While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released.
The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us!

How are earthquakes recorded?

The cartoon sketch of the seismograph shows how the insrument shakes with the earth below it, but the recording device remains stationary (instead of the other way around). (Public domain.)

Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram. The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free.
When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.

How do scientists measure the size of earthquakes?

The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but that’s not something scientists can simply measure with a measuring tape since faults are many kilometers deep beneath the earth’s surface. So how do they measure an earthquake?
They use the seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake was (figure 5). A short wiggly line that doesn’t wiggle very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.
The size of the earthquake is called its magnitude. There is one magnitude for each earthquake. Scientists also talk about theintensity of shaking from an earthquake, and this varies depending on where you are during the earthquake.

How can scientists tell where the earthquake happened?

Seismograms come in handy for locating earthquakes too, and being able to see the P wave and the S wave is important.
Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. There are two types of body waves. They are called P and S-waves.

P-waves: are the first to arrive at the surface. They travel through gaseous, liquid and solid materials. P-waves vibrate parallel to the direction of the wave.
S-waves: arrive at the surface with some time lag. S-waves can travel only through solid materials. The direction of vibrations of S-waves is perpendicular to the wave direction in the vertical plane. Hence, they create troughs and crests in the material through which they pass.Surface waves are considered to be the most damaging waves.
Surface waves: The body waves interact with the surface rocks and generate new set of waves called surface waves. These waves move along the surface. The surface waves are the last to report on seismograph. They cause displacement of rocks, and hence, the collapse of structures occurs. Surface waves are divided into two different categories: Love and Rayleigh.

Love waves: have a particle motion, which, like the S-wave, is transverse to the direction of propagation but with no vertical motion. Their side-to-side motion causes the ground to twist from side to side.
Rayleigh waves: create a rolling, up and down motion with an elliptical and retrograde particle motion confined to the vertical plane in the direction of propagation.

The shadow zone is the area of the earth from angular distances of 105° to 145° from a given earthquake that does not receive any direct P waves or S waves. The entire zone beyond 105° does not receive S- waves. The shadow zone of S-wave is much larger than that of the P-waves.

The shadow zones have led seismologists to conclude that interior of the earth is not homogeneous. The concepts of change in medium and density has been identified with the help of shadow zones. The shadow zone of P waves led to identification of boundary between mantle and core. Further, it has also been deduced that outer core is made of liquid, as S waves cannot travel through liquid mediums and P waves are refracted by the liquid core.

छोटे Courses बड़े Results

Master Disasters and Management issues

Prepare through Micro-courses