Answer :


Speed of longitudinal wave in solid = 8 kms-1

Speed of longitudinal wave in liquid = 5 kms-1

When the wave travels across the earth towards the core, it will pass the liquid region and the two solid regions. The first solid region starting from the crust has a radial distance of

For the molten region the radial distance is

And for the solid region of the core the distance is

The time taken to travel to the centre of earth from the crust is therefore,

where vs and vl are the velocities of the longitudinal wave in solid and liquid respectively.

To travel to a diametrical opposite point the time will be equal to 2t = 1975 s. Therefore, the seismometer will record the earthquake after 1975 seconds after the earthquake initiates.

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