Discuss The Structure of The Earth as Revealed by Seismic Waves

.   Discuss the  structure of the earth as revealed by seismic waves

P-wave and s-waves are the two types of waves propagated through the earth, both the two waves has distinctive characteristics that gives a picture of how the structure of the earth is. The presence of either p-waves or s-wave at any region will reveal what features present at a particular region within the earth mass.

the velocity of both types of waves continue to increase through the mantle until at a point where there is an sharp decrease in the velocity of the p-wave at about a distance of 2900km down and a sudden disappearance of s-wave. This region is known as the outer core and this is where mantle and the core separate, absence of s-wave shows that the region is liquid, because s-wave does not propagate in a liquid.

 The presence of s-wave in the inner core reveals that it is solid; this evidence was established in 1972.

 The decrease in velocity between the mantle- core boundary cause waves entering the core to be refracted to the verticals and as a result p-waves cannot reach the earth surface. the increase in velocity of p-waves in the inner core reveals that it is solid .

At the mantle the velocity continue to increase with depth , as the depth increase the velocity of both s and p waves increase, there is also a low velocity zone characterized by a remarkable decrease in velocity at about 60 to 250km , this low velocity allow  isostatic adjustment of land mass to occur. There is also a point called the Moho, located 35km beneath land mass and 5 to 10 km beneath sea- levels. Seismic velocity increase as one descends through the Moho but it varies above and below the Moho. This region defined the boundary between the mantle and the crust.

Generally as revealed by the seismic waves the earth consist of the three main region , the mantle, the outer core and the inner core, the mantle has some regions as the crust, the upper mantle lower and the transition zones. The mantle is solid and both p-waves and s-waves propagate through it. The outer core is next after the mantle, it is in liquid cause s-waves disappear in it and p-waves drastically reduced, last is the inner core which is solid causes s-waves reappear in it and p-waves velocity also increases.