320 TECTONICS/Earthquakes Table The Richter scale Average number per year Average intensity equivalent close to epicentre 1.9 700 000 2.9 300 000 I V; recorded but not felt I V; recorded but not felt I V; felt by some I V; felt by many V VII; slight damage VII; damaging IX XI; destructive XII; widely devastating Magnitude Qualitative description 3.9 4.9 5.9 Minor Light Moderate 6.9 7.9 8.9 Strong Major Great 40 000 6200 800 120 18 every 10 20 years The Importance of Seismological Records Figure The components of a seismograph designed to record vertical ground movement Figure The way in which earthquake waves spread through the globe and are reflected at boundaries, returning to the sur face Measurement of the speed of such return is used to delin eate the materials of the inner Earth according to density and physical state Reproduced from Van Andel TJ (1994) New Views on an Old Planet Cambridge: Cambridge University Press comparative intensity at the focus by the moment magnitude scale, but the principles are the same – the moment magnitude scale allows more refined methods of comparison We cannot directly study the rocks of the crust below the limits of borehole drilling (a few kilometres), though ancient rock systems expose sections of the ancient deep crust (as in the Kapuskasing Belt, Ontario, Canada) and perhaps even the crust–mantle contact (as in Oman and Western Newfoundland) The behaviour of earthquake waves, however, provides us with invaluable evidence about the nature of the lower crust, mantle, and core because the velocities of P and S waves are functions of the density of the material through which they pass Knowledge of rock density can tell us much about the physical state of the materials deep within the Earth, and the behaviour of S waves tells us that the outer core is molten In Figure 5, the different densities of common rock materials are plotted against the P-wave velocity It is fair to say that earthquake waves form the basis of our knowledge of the mantle and core Artificially produced seisms can also be picked up by seismographs, and, thus, nuclear explosions can be globally monitored The explosion in the submarine Kursk in 2000 was picked up by distant seismograph stations in Africa, and this provided valuable evidence of what happened Tomographic methods have recently been developed, producing threedimensional images of the deep Earth, including subducted slabs of crust, using a technique akin to the use of tomography in medicine Earthquakes occur in sequences: slight foreshocks may give warning of a major earthquake, and aftershocks occur for long after the main shock Foreshocks and aftershocks are generally of lower intensity than the main shock, but sometimes very