Waves can travel through solids as well as liquids and gases. Here are just a few examples: You can often feel fast cars and heavy lorries passing close to a house; It was said that American Indians used to put their ear to the ground to hear approaching horse riders; By putting a glass nest to your ear and pacing it on an adjoining wall, you can hear what is being said in the nest room. The ability of the wave to travel through the material depends very much upon its properties. By study Earthquake waves, seismologists are able to indirectly ‘see’ into the depths of the Earth and tell us about its structure. Earthquakes Earthquakes happen when rocks suddenly fracture and move. The tremendous energy released during the fracturing process makes the ground vibrate and it is these vibrations that travel away from the focus of the quake. Obviously the passage of the earthquakes can cause serious damage. Often the ground can be seen moving, if enough movement occurs then building can fall and mountains can break up. Earthquakes can also occur under the sea. The ground movements form big waves that get even higher as they get closer to the coast. These vibrations are picked up by seismometers, which measure the strength of the waves. Seismographs show that there are three main types of waves: 1. P-Waves Primary waves are the fastest waves and are therefore the first to be recorded. They are longitudinal push-pull waves and can travel through liquids as well as solids. 2. S-Waves- Secondary waves are slower than P waves. They are transverse shake and shear waves and can only travel through solid materials. Surface Waves- These waves are the slowest waves and only travel along the surface or crust of the Earth. There are two types of surface waves – longitudinal and transverse waves. These are the waves that do the most damage when the earthquakes occur. The black dots show the distribution of the focus of Earthquakes around the world. The pattern of Earthquakes is evidence of the crust of the Earth being split up into tectonic plates. Questions The following sketch shows a seismograph record of an earthquake. Start of P Waves Start of S Waves Start of surface Waves Time in Minutes P Waves P and S P+S+Surface 1. Which wave arrived first? ______________________________________ 2. Both P and S waves travelled through the Earth to arrive at the station. Which wave travelled faster? __________________________________ 3. What was the time delay between: (i) P and S waves? _____________________________________ (ii) P and Surface waves? ________________________________ (iii) S and Surface waves? ________________________________ 4. An elapse time of 1 minute corresponds to a distance of approximately 800 km between the epicentre (point on the Earth’s surface directly above focus) and the recording station. If the recording station records an elapse time of 3 ½ minutes, how far away is the epicentre? _________________________________________________________ _________________________________________________________ _________________________________________________________ 5. Why is it not possible for one seismograph to pinpoint the epicentre of an earthquake? ___________________________________________________________ ___________________________________________________________ ________________ 6. How is it possible to find the epicentre of an earthquake? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 7. A powerful earthquake has struck somewhere in Europe. The relief agencies want to get help to the area as quickly as possible, but they have not located the epicentre yet. Help them to locate its position on the map from the information gained by some recording stations. Elapse times between P and S waves: London 1 ½ min, London Madrid Naples 0 0 400 800 1 min 1200 1600 2 min 2000 2400 km Madrid 2 min, Naples ¾ min. 3 min Time between arrival of P and S waves 7. Seismic waves are caused by __________________________________ 8. Seismic waves start in the Earth’s ___________ from a point called the __________ 9. The point directly above this on the Earth’s surface is called the ______________ 10. Seismic waves are detected using a ____________________________ 11. The _____ waves are the fastest and can travel through _________ and _________ 12. The _____ waves are slower, they can only travel through ___________ 13. Which regions of the Earth get the most earthquakes and why? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ Seismic records have led geologist to believe that the Earth has a layered structure, and can be thought of as being a bit like a cracked egg. The solid crust is broken up into smaller bits, called plates, which float on a dense mantle. Parts of the mantle are molten liquid and movements in this liquid cause the plates to drift into one another. It is the meeting of the plates that causes the earthquakes. The density of the material forming the Earth increases with depth. Nuclear reactions within the solid inner core maintain the high temperatures of the body of the Earth. The outer core is liquid and large convection currents within this layer give rise to the Earth’s magnetic field. P-Waves The speed of both types of waves increase as they travel through the Earth’s interior. However, the speed rapidly changes when they cross between layers of material. S-Waves The speed of the p waves decrease considerably when encountering the outer core, but S waves are stopped altogether. A B Direct P-wave shadow zone The diagram above shows the paths taken by P and S waves as they travel through the interior of the Earth. Point A - Both P and S waves are gradually refracted as they travel and therefore follow curved paths as they spread out from the epicentre. This indicates that the density of the material within a layer gradually increases with depth. P and S waves are detected up to angles of 1030 from the epicentre. Point B - At certain points the refraction becomes more pronounced as the waves meet a definite boundary between layers. No S waves can travel through angles greater than 1030 hence the boundary at B must separate solid from liquid. This is the evidence that the outer core is made of a dense liquid. P waves entering the liquid outer core slow down and are focused (like waves travelling through a lens) into the region at the bottom of the diagram (angles greater than 1400). A shadow zones for direct travelling P waves exists between 1030 and 1400, however, weak P waves are detected in this region due to the refraction of the inner core.