INDIAN GEOGRAPHY

Detailed explanation-1: -Figure 19.7: Beneath the crust, seismic waves increase abruptly indicating a sharp boundary between the crust and upper mantle. This is due to the compositional change from granite, or basalt, to peridotite that comprises the upper mantle. The boundary between the crust and upper mantle is called the Moho.

Detailed explanation-2: -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-3: -First note that in several large regions such as in the lower mantle, the outer core, and inner core, the velocity smoothly increases with depth. The increase is a result of the effects of pressure on the seismic wave speed.

Detailed explanation-4: -Because the earth’s mantle becomes more rigid and compressible as the depth below the asthenosphere increases, P-waves travel faster as they go deeper in the mantle.

Detailed explanation-5: -The p-wave velocity at the upper most mantle is 7.97 km/s, and the S-wave 4.55 km/s. The low velocity layer in the upper mantle occurs approximately at 140 km deep with a thickness of about 55–62 km.