Part of a lecture about earthquakes and what causes them

Section 4. You will hear part of a lecture about earthquakes and what causes them. First, you have some time to look at questions 31 to 35. Now listen carefully and answer questions 31 to 35. WOMAN: Good morning. Today we're turning to earthquakes, the sudden shaking of the ground that can level a city in seconds. To understand them, we have to start not at the surface but deep below it, with the structure of the Earth itself. WOMAN: The key idea is that the Earth's rigid outer shell is not a single, continuous layer. Instead it is broken into a number of huge, slowly moving pieces that we call plates. These plates float on the hotter, softer rock beneath and drift at roughly the speed your fingernails grow, only a few centimetres a year. Most earthquakes happen not in the middle of a plate but at the edges, where two plates meet, a line we call a fault. WOMAN: Now, why does this cause shaking? The plates are trying to move past one another, but they don't slide smoothly. Friction locks the two edges together, and while they're stuck the pressure keeps building, year after year. Eventually the stress becomes too great, the rock suddenly slips, and all that stored energy is released at once. That release is the earthquake. Before you hear the rest of the lecture, you have some time to look at questions 36 to 40. WOMAN: The energy travels outwards from the point of rupture as waves, and here's a distinction worth noting. The exact point underground where the rock breaks is called the focus. The place directly above it, on the surface, is the epicentre, and that's usually where the damage is greatest. WOMAN: How do we measure them? For a long time the public knew the Richter scale, but seismologists today mostly use a newer measure called the magnitude scale, which works better for very large events. The crucial thing to understand is that the scale is not linear, each whole number up represents a roughly thirty-fold jump in the energy released. So a magnitude seven is vastly more powerful than a six, not just a little. WOMAN: Finally, a word on why some earthquakes are far deadlier than others of the same size. It's often not the shaking itself that kills, but collapsing buildings, so construction standards matter enormously. And a large quake under the sea can be doubly dangerous, because it can displace a huge volume of water and trigger a tsunami. This is why, in earthquake zones, engineering and early warning save more lives than anything else. We'll stop there. That is the end of Section 4. You now have some time to check your answers.