Dishes, windows, doors disturbed walls make cracking sound. Vibration similar to the passing of a truck.įelt indoors by many, outdoors by few during the day. Many do not recognise it as an earthquake.

Delicately suspended objects may swing.įelt quite noticeably by persons indoors, especially on the upper floors of buildings. Not felt except by a very few under especially favourable conditions.įelt only by a few persons at rest, especially on upper floors of buildings. Table 2 Modified Mercalli intensity scale Magnitude The related Mercalli scale in Table 2 shows the effects of quakes on structures and people. The scale is defined strictly by reference to a set of tables, using data such as the maximum lateral amplitude of the vibrations recorded by a pen on a moving graph and the distance of the listening station to the epicentre of the quake. The severity is described on a logarithmic scale from 1 to 9, as shown in Table 1. Such phenomena are now well studied, and the magnitude of earthquakes is classified in terms of the Richter scale – after the Californian seismologist who devised the scale in 1935. This positive geographical correlation between zones of earthquake activity and volcanoes becomes apparent in places like the Mediterranean, the Andes and Japan.Įarthquakes themselves often involve movement of large bodies of rock, which, if occurring on the seabed, will create a sudden movement of the sea above and hence a tidal wave that propagates to land, often over vast distances. Volcanic explosions, such as Krakatoa (1883) and Mount St Helens (1980) are also manifestations of activity in the earth's crust, where pools of hot magma build up under the surface and erupt explosively. These natural phenomena are now known to be interconnected: earthquakes result from vast plates of the earth's crust meeting and moving against one another. The disasters that first come to mind are those where the earth itself changes in an unpredictable and sudden way: Natural phenomena that can cause disasters are inherently uncontrollable, but yet attempts can be made to limit their effects on human populations by planning, or by engineering structures to make them more resistant to natural forces. They remain fixed in time and place, and are endlessly re-analysed for any clues as to their cause or causes with the quest for understanding, and for ways of forestalling or preventing future catastrophes of a similar nature. Great disasters are always traumatic, especially for those who endure them and come through alive. Certainly, the aspect of suddenness is one that features in many catastrophes, and indeed, it is this feature by which a catastrophe is defined. Why are disasters important? They attract public attention because there is great loss of life, or because the event happened suddenly and quite unexpectedly, or because the accident occurred to a new project that had been regarded as completely safe. When structures such as ships, aircraft or spacecraft are made specifically to withstand extreme environments, they should be able to resist those forces safely.įind out more about studying with The Open University by visiting our online prospectus.ġ Disasters of natural origin 1.1 Overview It follows immediately that designers need to know what magnitude of force to expect for the lifetime of their particular product.

Structures should therefore be designed to withstand such forces. The great forces unleashed by natural effects can make a structure unstable and hence unsafe, or even destroy it entirely. The division cannot be enforced rigorously, however, because the one can cause or interact with the other. One way of examining such events is by dividing them into two categories, those of natural origin, and those occurring to manufactured structures. We cannot therefore neglect discussing such problems in failure cases. They may involve the failure by engineers, designers or managers to recognise faults in safety-critical products, or managers overriding the design team for other reasons – such as keeping to a deadline or keeping costs within a predetermined budget. Inevitably, human factors emerge as important in many major disasters. It then analyses the Tay Bridge disaster, which was caused by mechanical failure. This unit starts by giving an overview of the two main categories of disasters: disasters of natural origin and disasters of human origin.

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