Calculation of true thickness of the bed

Gate_2017_23

23) The Percentage increase in P-wave Velocity (km/sec) across the Mohorovicic discontinuity from the lower crust to the upper mantle beneath a carton is approximately__________(%).

(Thanks to Chandrasekhar, ANU)

Solution:

As we all know the P-wave Velocities crust and mantle are as follows

Upper crust—5.6 km/sec

Lower crust-6.29 km/sec

Upper mantle-8 km/sec

The difference in the P-wave velocity between lower crust and upper mantle is

=8-6.29

= 1.71 km/sec

The percentage of this difference across the moho discontinuity is

= 1.71*100/8

=0.21375*100

=21.375%

Extra information:

• The Upper Crust is also called Granitic layer and the P and S waves are called $P_g and S_g$ waves

• The lower crust is also called Basaltic layer and the P and S waves are called $P^* and S^*$ waves

• The P- wave in the Upper mantle is called the Head Wave which is denoted by $P_n$ wave.

Reference : Fundamentals of Geophysics by William Lowrie.