ABSTRACT
Earthquakes do not occur in all geographical locations around the world. Rather, they take place within certain limited areas. Fortunately, earthquakes have been recorded instrumentally since the beginning of the twentieth century at a large number of seismographic stations distributed throughout the world (see Box 2.1). Therefore, it is now possible to know where and when earthquakes have occurred in the past, how large they have been, and where in the world earthquakes are likely to occur again. The description of the time, location, size, and frequency of the earthquakes that have occurred in a region is referred to as the description of the seismicity of the region. Conventionally, this seismicity is portrayed in the form of seismicity maps. Seismicity maps show the geographical location where earthquakes have occurred during a specifi ed time interval and describe the intensity of these earthquakes by means of circles or dots of different sizes. A seismicity map may describe the geographic distribution of earthquakes around the world, a country, or a specifi c region. Seismicity maps, the cooperative work of hundreds of seismologists throughout many years, have contributed in a fundamental way to defi ne the seismicity of the earth and are nowadays an essential tool for the planners, geologists, engineers, and government offi cials involved in earthquake mitigation activities. Seismicity maps have also played a major role in the evolution of the plate tectonic theory described in Chapter 3.
