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Earthquakes produce seismic waves that travel through the ground and make it vibrate Figure 9 shows a seismograph S, that is used to record these vibrations. A heavy mass M is attached to a rigid case by a frictionless hinge. M is supported by a spring that has a low stiffness. The case is bolted to the ground.

During an earthquake the ground surface moves up and down but the centre of mass of M stays almost stationary. Figure 10 shows S, bolted to the ground before the earthquake, with M at its equilibrium position. Figures 11 and 12 show S, during the earthquake. In Figure 11 the ground has moved down but M has stayed in its initial position. In Figure 12 the ground has moved up and M remains in its initial position.

The pen records the movement of the seismograph on graph paper attached to a rotating drum. Explain why M stays almost stationary as a seismic wave passes $₁. Figure 13 shows a magnified image of a trace from S₁. At time t=0 the pen is at position P.

The drum has a circumference of 500 mm and a period of rotation of 1000 s.Each square on the graph paper is 1.0 mm wide. Estimate the frequency of the seismic waves recorded by S₁.

At time=0 the pen is at position P. S₂ is further from the origin of the earthquake than S₁. The distance between S, and S₂ is d. The speed of the seismic waves is 7.0 km s¹. Deduce d. 3The time at which the trace starts is different in Figure 14 compared with Figure 13. Explain one other way in which Figure 14 demonstrates that S₂ is further than S, from the origin of the earthquake.[2 marks]

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