Rethinking Layers of Earth

Rethinking Layers of Earth

Kamaraj IAS Academy | Rethinking Layers of Earth
  • February 25, 2019, 11:33 am

In NEWS

Scientists have discovered massive mountains in the Earth’s mantle, an advance that may change our understanding of how the planet was formed. Earth has three layers: a crust, mantle and core, which is subdivided into an inner and outer core.

While that is not wrong, it does leave out several other layers that scientists have identified within the Earth.

An earthquake is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth's lithosphere that creates seismic wave

Earthquake data


In a study published in the journal Science, scientists used data from an enormous earthquake in Bolivia to find mountains and other topography on a layer located 660 km straight down, which separates the upper and lower mantle.

Lacking a formal name for this layer, the researchers simply call it “the 660-km boundary.”
Data from earthquakes that are magnitude 7.0 or higher send out shockwaves in all directions that can travel through the core to the other side of the planet  and back again.

For this study, the key data came from waves picked up after a magnitude 8.2 earthquake  the second largest deep earthquake ever recorded  that shook Bolivia in 1994.
Scientists used powerful computers to simulate the complicated behaviour of scattering waves in the deep Earth.

The technology depends on a fundamental property of waves: their ability to bend and bounce.Just as light waves can bounce (reflect) off a mirror or bend (refract) when passing through a prism, earthquake waves travel straight through homogeneous rocks but reflect or refract when they encounter any boundary or roughness.That’s why we can see these objects the scattering waves carry the information about the surface roughness .

The researchers were surprised by just how rough that boundary is rougher than the surface layer that we all live on.In other words, stronger topography than the Rocky Mountains or the Appalachians is present at the 660-km boundary.Their statistical model did not allow for precise height determinations, but there’s a chance that these mountains are bigger than anything on the surface of the Earth.

The roughness was not equally distributed, either; just as the crust’s surface has smooth ocean floors and massive mountains, the 660-km boundary has rough areas and smooth patches. The presence of roughness on the 660-km boundary has significant implications for understanding how our planet formed and evolved.

 

Basics of quake:

 

The point within the earth’s crust where an earthquake originates is called as the focus or hypocenter or seismic focus.
It generally lies within a depth of 6 kms in the earth crust.The point vertically above the focus on the earth’s surface is called as the epicenter.The intensity of earthquake will be highest in the epicenter and decreases as one moves away.All natural earthquakes take place in the lithosphere i.e, the region which constitutes the earth’s crust and rigid upper part of the mantle.Earthquake waves are basically of two types – body waves and surface wave

 

 

Body waves
They are generated due to the release of energy at the focus and moves in all directions traveling through the body of the earth. Hence, the name body waves.
They travel only through the interior of the earth.Body waves are faster than surface waves and hence they are the first to be detected on a seismograph.


There are two types of body waves as primary waves and secondary waves.


Primary waves (p-waves):
Primary waves are the fastest body waves (twice the speed of s-waves) and are the first to reach during an earthquake.They are similar to sound waves, i.e, they are longitudinal waves, in which particle movement is in the same direction of wave propagation.
They travel through solid, liquid and gaseous materials.They create density differences in the earth material leading to stretching and squeezing.
diagram of primary waves

Secondary waves (s-waves):


They arrive at the surface with some time-lag after primary waves.
They are slower than primary waves and can pass only through solid materials.
This property of s-waves led seismologists to conclude that the earth’s outer core is in a liquid state. (the entire zone beyond 105o from the epicenter does not receive S-waves)
They are transverse waves in which directions of particle movement and wave propagation are perpendicular to each other.
secondary waves

Surface Waves


When the body waves interact with surface rocks, a new set of waves is generated called as surface waves.These waves move along the earth's surface.
Surface waves are also transverse waves in which particle movement is perpendicular to the wave propagation.
Hence, they create crests and troughs in the material through which they pass.
Surface waves are considered to be the most damaging waves.
 

Two common surface waves are Love waves and Rayleigh waves.


Love waves:


This kind of surface waves causes horizontal shifting of the earth during an earthquake.
They have much slower than body waves but are faster than Rayleigh.
They exist only in the presence of semi-infinite medium overlain by an upper finite thickness.
Confined to the surface of the crust, Love waves produce entirely horizontal motion.
love waves propagation diagram

Rayleigh waves:
These waves follow an elliptical motion.
A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean.
Because it rolls, it moves the ground up and down and side-to-side in the same direction that the wave is moving.
Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves.


Shadow regions of waves


We already discussed that p-waves pass through all medium while S-waves passes only through solid medium.With the help of these properties of primary waves, seismologists have a fair idea about the interior of the earth.Even though p-waves pass through all mediums, it causes reflection when it enters from one medium to another.The variations in the direction of waves are inferred with the help of their record on seismographs.
The area where the seismograph records no waves is called as ‘shadow zone’ of that wave.
Accordingly, it is observed that the area beyond 1050 does not receive S-waves and the area in between 1050 to 1400  does not receive P-waves.
shadow zones of seismic waves

Measuring earthquakes
Seismometers are the instruments which are used to measure the motion of the ground, which including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources.A Seismograph is also another term used to mean seismometer though it is more applicable to the older instruments.
The recorded graphical output from a seismometer/seismograph is called as a seismogram. There are two main scales used in the seismometers:

 (1) Mercalli Scale:
The scale represents the intensity of earthquake by analyzing the after effects like how many people felt it, how much destruction occurred etc. The range of intensity is from 1-12.

(2)Richter Scale:
The scale represents the magnitude of the earthquake. The magnitude is expressed in absolute numbers from 1-10. Each whole number increase in Richter scale represents a ten times increase in power of an earthquake.

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