BUILDING HIGHER EARTHQUAKES edge: bordo fault: faglia to occur: verificarsi plate tectonics: tettonica a placche/zolle to roll: far ballare, rollare seismic waves: onde sismiche shear waves: onde di taglio to shift: scivolare slide: scivolare to squeeze: comprimere to stretch: allungare, tirare to sweep over: devastare An earthquake is a sudden shaking and rolling of the earth s surface. There are millions of earthquakes every year, but most of them take place beneath the surface of the sea. Few of these cause any damage but sometimes they can cause huge sea waves that sweep over the land. Such waves often occur in the Pacific Ocean because of the great number of earthquakes there. These destructive waves are called by the Japanese word, tsunami. When earthquakes occur on land near large cities or in densely populated areas, they can cause great damage and loss of life. According to the plate tectonics theory, the surface of the Earth consists of 20 rigid plates that move slowly. The movements of these plates squeeze and stretch rocks at the edges of the plates. If the force is too great, the rocks break and shift, causing an earthquake. These fractures are called faults. Most faults lie underground, but some are visible, for example, the San Andreas Fault in California. Much of the energy released in an earthquake travels away from the fault in seismic waves. The site of the movement is called the focus of the quake. The point on the surface above the focus is the epicentre. Seismic waves include shear waves, which shake buildings vertically and compressional waves, which move them horizontally. Most earthquakes take place where the plates collide or slide past each other. The Earth s most geologically active regions are the Circum-Pacific belt and the Alpide belt. The Circum-Pacific belt lies along the west coast of North and South America and on the other side of the ocean from Japan to Indonesia. It is also known as the Ring of Fire . The Alpide belt follows the plate boundaries from Europe to Asia. Earthquakes are recorded with a seismograph. The magnitude1 of an earthquake is conventionally reported on the Richter Scale. With magnitude 3 (or lower), earthquakes may be imperceptible, but magnitude 7 causes serious damage over large areas. The intensity2 of shaking is measured on the Mercalli Scale and commonly abbreviated MCS (Mercalli, Cancani, Sieberg). 1. magnitude: a measured value of the earthquake size. The Richter magnitude scale assigns a single number to quantify the amount of seismic energy released by an earthquake. It is a base-10 logarithm, so, for example, an earthquake that measures 5.0 on the Richter scale has a shaking amplitude 10 times larger than one that measures 4.0. 2. intensity: the measure of the shaking created by the earthquake. This value may vary with location. The Mercalli Scale is rather arbitrary because it is based upon what people in the area feel, and their observations of damage to buildings around them.