AT U R EÊSF U N R Y What Is an What Is an Earthquake Earthquake In an earthquake the ground moves or shakes sometimes very violently It can bring buildings crashing to the ground and twist or break up roads railway lines and bridges For people who experience an earthquake it can be a terrifying dangerous event A major quake that hits a densely populated area can kill hundreds of thousands of people Rippling ground Shock waves from an earthquake can travel a long way spreading through the ground just like ripples spread through water This means that even people living a long way from where the earthquake starts can be affected Each year the earth experiences more than 50,000 earthquakes although A massive earthquake rocked Tokyo in 1650 most of these are so small they are completely unnoticed by most people and are only detected by scientists using sensitive measuring instruments Earthquakes in history Earthquakes have happened ever since the earth formed a solid surface 3 8 billion years ago We have historical records of earthquakes throughout human history and geological evidence from earlier times People long ago did not know what caused earthquakes and often made up legends about them or believed their gods were responsible for the terrifying events 4
AT U R EÊSF U N R Y Danger area Earthquakes are most dangerous for people when they strike built-up areas Falling buildings and roads breaking up cause many injuries and deaths Although there is less danger from an earthquake itself if it happens in open countryside it can trigger other hazards such as landslides floods avalanches and giant waves called tsunamis These can strike an area already devastated by an earthquake making rescue operations difficult and killing survivors of the original quake THE LEGEND OF NAMAZU Japan is an area often troubled by earthquakes and people living there have kept records of them for many centuries Long ago Japanese people accounted for earthquakes with the story of Namazu a giant catfish According to the legend Namazu lives in the mud under the islands of Japan Most of the time the fish is kept still by the god Kashima pressing down on him with the weight of a huge stone But when Kashima fails to hold him down Namazu thrashes around and Japan is shaken wildly Around 10,000 people may have died when Mexico City was struck by an earthquake in 1985 W h a t I s a n E a r t h q u a k e 5
AT U R EÊSF U N R Y Patchwork of Plates Patchwork of Plates If the surface of the earth were a single piece like the skin of an orange we probably wouldn’t have any earthquakes But it is actually many pieces fitted together like a jigsaw puzzle more like the skin of an orange that has been peeled off and then pieced back together Tectonic plates The hard rocky surface of the earth is only the very top layer It is called the crust Beneath it is slowly moving semi-molten hot rock thousands of miles deep The crust is divided into lots of chunks that fit together to cover the whole surface of the earth The chunks are called tectonic plates There are seven large plates and many smaller ones Faults The lines where tectonic plates meet are called faults It is along the faults that most earthquakes happen Most volcanoes are found near faults too The red dots on this map are often the sites of major earthquakes Most of these occur at the edges of tectonic plates shown here in gray 6
AT U R EÊSF U N R Y Earth and sea Six of the large tectonic plates carry land or a mix of land and sea but much of the Pacific Ocean is on a single vast plate of its own The crust that holds land is thicker and older than the crust that carries ocean Land-bearing crust is also lighter so it floats higher on the semi-molten rock underneath The thinner but denser rock of the ocean-bearing crust sinks farther into the semi-liquid rock Because these areas are lower water runs into them from the higher ground forming the oceans Beneath the thin surface of the earth are thousands of miles of scalding rock and metal LAYERS OF THE EARTH The crust makes up less than 1 percent of the earth’s thickness Beneath the crust is a layer of very hot rock called the mantle The upper mantle descends roughly 400 miles 640 km into the earth and is almost solid Below this is the lower mantle which spans the region between 400 miles and 1,800 miles 2,900 km deep and contains weaker more-fluid rocks Beneath the lower mantle is the outer core a thick layer of liquid metal At the center of the earth is the inner core a ball of metal hotter than the surface of the sun Despite its heat the inner core is kept solid by the immense pressure of the earth pressing down on it The top part of the upper mantle and the crust together are called the lithosphere the part that makes up the tectonic plates P a t c h w o r k o f P l a t e s 7
