322 TECTONICS/Earthquakes Figure The global distribution of earthquakes that occurred in 1994 Reproduced from US National Earthquake Information Center of the most destructive and costly in living memory (55 000 houses destroyed as well as freeway, rail, and port installations), had its epicentre 20 km from the city and had a magnitude of only 6.8 The 1960 Agadir earthquake had a magnitude of only 5.8, but the focus was shallow and right beneath the city Thus, magnitude, though a valuable absolute measurement, tells us little about the degree of damage and the loss of property and life, even at the epicentre The nature of the subsurface rocks can have a significant effect, especially if shock-induced liquefaction occurs, and thus we need another measurement scale The Mercalli intensity scale measures the relative intensity of the effects felt at a specific site (the intensity will commonly decrease away from the epicentre, but secondary effects such as subsurface variation and shock liquefaction complicate this relationship) In Europe, a modification of the Mercalli scale, the MSK scale (named after Medvedev, Sponheuer, and Karnik), is now used This scale is given in Table Tectonic Earthquakes Earthquakes may be divided into tectonic earthquakes, volcanic earthquakes, and man-made earthquakes In considering the natural-hazard aspect, it is the tectonic earthquakes that are by far the most destructive natural hazards This hazard largely affects urban populations, and human design and construction has a unique role in mitigating this hazard The actual physical process of ground motion presents little threat to humans in the open: most casualties (other than the casualties of secondary tsunamis) occur inside buildings that partially or totally collapse The correct design of buildings and constructions such as bridges and viaducts can thus greatly mitigate the damage and casualties resulting from an earthquake The vulnerability of a building to earthquake damage varies according to many factors Vertical ground motion is the principal damaging component causing collapse, burial of people, and death Lateral ground motion breaks or deforms power lines, pipelines, water pipes and sewers, roadways, railways (Figure 8), and bridges Quite small lateral offsets can be very damaging In the Mexico City earthquake of 1985, much damage was caused by adjacent highrise buildings swaying with different wave motions and knocking each other down It is noticeable that in Beijing, an earthquake-prone city, the high-rise buildings are widely spaced, with intervening areas of lowrise buildings, so that they cannot interact in this way In the case of Kobe, the sixth floor of one high-rise building pancaked (Figure 9) The lower floors were built of steel-encased reinforced concrete and the upper floors of pure reinforced concrete; the junction on the sixth floor acted as an element of weakness Though earthquakes are mainly an urban hazard, catastrophic earthquakes may strike village populations where low-rise housing is substandard – as in