Varying geological conditions at different locations of the country may have at any time damaging earthquakes to occur. Thus there is a need for seismic zone map of the country so as to design structures taking into effect the magnitude of earthquake likely to occur at a particular location.
Figure 29.6 Location of epicentre from travel-time records (Source: IS: 1893–1984)
The zone map (IS: 1893–1984) sub-divides India into five zones, I, II, III, IV and V (Fig. 29.6). The corresponding intensity and acceleration are shown in Table 29.5 which is based on Mercali scale. Mercali scale is shown in Table 29.6.
Table 29.5 Intensity of earthquake
Table 29.6 Mercali scale
Seismic zone maps are to be revised periodically with the better understanding gained with time. For instance, the Koyna earthquake classified under Zone I in 1966 was changed to Zone IV in 1970.
Epicentre is the point on the earth’s surface vertically above the focus of an earthquake. Shaking is highest at the epicentre and gradually decreases outwards. The difference in primary waves (P – waves) and secondary waves (S – waves) may be used to determine the epicentre.
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