Question

During an earthquake, seismic energy is released and it can travel long distance from source (location of the earthquake) to a seismic station. Path taken by a wave is similar to a ray (line with an arrow) which travels through the earth and arrives at a station. In the last assignment, we look into a case where earthquake is right beneath the station. In this assignment, we are going to investigate a case in which the station is far away from the earthquake, as seen in the diagram below. The ray bends this way because wave speed is generally slower at shallow depth compared to deeper part. Questions 8.1: Why the ray path is bending to be nearly vertical when it approaches the station near the surface? (Hint: think about how horizontal rock layers are stacked on top of each other in terms of wave speed? Which layers have faster wave speed, which ones have slower wave speed? What is the consequence?)

Answer 1

As the earthquake waves propagate upward through the various lithologies and approaches towards the surface (base station), their velocities keep on decreasing. Velocity is directly proportional to the square root of Tension in the medium. thus with decreasing tension, the elasticity of the medium decrease which also causes the velocity in the medium to decrease. The Eath's interior is made up of less dense rocks at shallow depth and the density of rocks keeps on increasing with increasing depth.

Now, for waves passing from a denser to less dense medium.i.e. while traveling up the surface of the earth, the wave paths move towards the normal direction and starts to become near vertical (Snell's law)

Snell's Law: (Sin i/Sin r)=n1/n2, where n1 and n2 are the values of refractive indexes of each layer.