1.2 Seismicity of the World
Seismicity is a description of the relationship of time, space, strength, and frequency of earthquake occurrences within a certain region, and its
understanding is the foundation of earthquake study. Since there is still no practical way to control earthquakes, one can only try to understand
and follow their nature wisely to prepare for possible strong earthquakes through prediction, earthquake engineering, and society or
governmental efforts of disaster reduction (Bangash, 2011). Dowrick (2009) defined seismicity as the frequency of occurrence of earthquakes
per unit area in a given region and is illustrated in non-numerical terms by the seismicity map of the world shown below in Fig 1.1.
Fig. 1.1 Seismicity of the world (courtesy of Cartographic Data Atlas version 1.02, British Geological Survey Worldwide Earthquake Database)
Please see MODULE 1 (5-23-2020) rev-1.pdf (https://tip.instructure.com/courses/25333/files/8971541/download?wrap=1)
(https://tip.instructure.com/courses/25333/files/8971541/download?download_frd=1) for the regions of the world where earthquake activity is
widespread. Note the number of casualties, the need for the earthquake resistant design of the platform, infrequent and dispersed seismicity.
Earthquake focus is the point from which the waves first emanate. The point on the ground surface directly above the focus is called the
earthquake epicenter.
Foci are classified into two namely shallow and deep focus. earthquakes with foci from 70 to 300 kilometers deep are called intermediate
focus and those below this depth are termed deep focus. Some intermediate and deep focus earthquakes are located away from the Pacific
region, in the Hindu Kush, in Romania, in the Aegean Sea, and under Spain. The shallow-focus earthquakes (< 70 Kms depth) is the most
deadly and contribute about three-quarters of the total energy released in earthquakes throughout the world.
Aftershocks are numerous earthquakes, usually smaller that follow most moderate to large shallow earthquakes in the ensuing hours and even
in the next several months. Aftershocks are sometimes energetic enough to cause additional damage to already weakened structures. A few
earthquakes are preceded by smaller foreshocks from the source area, and it has been suggested that these can be used to predict the main
shock.
Reference:
BOLT, B. A. 2008. Chapter 01 The nature of earthquake ground motion. In: NAEIM, F. (ed.) The Seismic Design Handbook. US: Springer US