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Figure 3. The upper sketch depicts a seismograph. The left hand seimograph records horizontal movement, the right hand records vertical movement. The lower sketch depicts a typical earthquake trace as recorded at Cambridge, Massachusetts, with epicentre at Erzincan, Turkey, showing the small deflections made by the P-waves (which arrive first), and larger deflections made by P & S and P, S & L -waves. In this example the time difference between the arival of the P and S-waves was 10 minutes and 45 seconds, indicating that the source of the earthquake was about 8,550 km away (after Rahn 1986, Open University Press 1981). |