Question

Where do you think, FOCUS is located?
It is located just below the epicentre
It is located above the magma chamber
It is located below the volcanic vent
It is located below the crater
​

Answer 1

Answer:

Ok your answer was

Explanation:

Indirect Sources

Analysis of properties of matter indirectly

provides information about the interior. We

know through the mining activity that

temperature and pressure increase with the

increasing distance from the surface towards

the interior in deeper depths. Moreover, it is

also known that the density of the material also

increases with depth. It is possible to find the

rate of change of these characteristics. Knowing

the total thickness of the earth, scientists have

estimated the values of temperature, pressure

and the density of materials at different depths.

The details of these characteristics with

reference to each layer of the interior are

discussed later in this chapter.

Another source of information are the

meteors that at times reach the earth. However,

it may be noted that the material that becomes

available for analysis from meteors, is not from

the interior of the earth. The material and the

structure observed in the meteors are similar

to that of the earth. They are solid bodies

developed out of materials same as, or similar

to, our planet. Hence, this becomes yet another

source of information about the interior of the

earth.

The other indirect sources include

gravitation, magnetic field, and seismic activity.

The gravitation force (g) is not the same at

different latitudes on the surface. It is greater

near the poles and less at the equator. This is

because of the distance from the centre at the

equator being greater than that at the poles.

The gravity values also differ according to the

mass of material. The uneven distribution of

mass of material within the earth influences

this value. The reading of the gravity at different

places is influenced by many other factors.

These readings differ from the expected values.

Such a difference is called gravity anomaly.

Gravity anomalies give us information about

the distribution of mass of the material in the

crust of the earth. Magnetic surveys also

provide information about the distribution of

magnetic materials in the crustal portion, and

thus, provide information about the

distribution of materials in this part. Seismic

activity is one of the most important sources of

information about the interior of the earth.

Hence, we shall discuss it in some detail.

Earthquake

The study of seismic waves provides a complete

picture of the layered interior. An earthquake

in simple words is shaking of the earth. It is a

natural event. It is caused due to release of

energy, which generates waves that travel in

all directions.

Why does the earth shake?

The release of energy occurs along a fault. A

fault is a sharp break in the crustal rocks.

Rocks along a fault tend to move in opposite

directions. As the overlying rock strata press

them, the friction locks them together. However,

their tendency to move apart at some point of

time overcomes the friction. As a result, the

blocks get deformed and eventually, they slide

past one another abruptly. This causes a

release of energy, and the energy waves travel

in all directions. The point where the energy is

released is called the focus of an earthquake,

alternatively, it is called the hypocentre. The

energy waves travelling in different directions

reach the surface. The point on the surface,

nearest to the focus, is called epicentre. It is

the first one to experience the waves. It is a point

directly above the focus.

Earthquake Waves

All natural earthquakes take place in the

lithosphere. You will learn about different

layers of the earth later in this chapter. It is

sufficient to note here that the lithosphere refers

to the portion of depth up to 200 km from the

surface of the earth. An instrument called

‘seismograph’ records the waves reaching the

surface. A curve of earthquake waves recorded

on the seismograph is given in Figure 3.1. Note

that the curve shows three distinct sections

each representing different types of wave

patterns. Earthquake waves are basically of two

types — body waves and surface waves. 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. Hence, the name