GEOLOGY

Detailed explanation-1: -Seismic velocities tend to gradually increase with depth in the mantle due to the increasing pressure, and therefore density, with depth. However, seismic waves recorded at distances corresponding to depths of around 100 km to 250 km arrive later than expected indicating a zone of low seismic wave velocity.

Detailed explanation-2: -Seismic waves travel faster in denser rocks; temperature tends to lower the speed of seismic waves; and pressure tends to increases the speed.

Detailed explanation-3: -Body waves in the earth’s interior lose their amplitude rapidly as they get farther from the earthquake because they spread out inside the volume of the earth. Surface waves, however, spread out more slowly and only on the earth’s surface.

Detailed explanation-4: -It increases to about 11 km (6.8 miles) per second near the centre of the Earth. The speed increase with depth results from increased hydrostatic pressure as well as from changes in rock composition; in general, the increase causes P waves to travel in curved paths that are concave upward.