Natural geohazards

Natural geohazards are geological processes that become a hazard when they have an adverse impact on the environment. Their hazardous nature depends upon their reaction with the environment, in particular when there is destruction of property, or worse, loss of life. The damage to property brought about by a natural geohazard is dependent not just on the geohazard but also on the vulnerability of the property. Also important are the magnitude of the event, its duration, the area affected and the frequency with which the event occurs. Furthermore, one type of hazard may trigger another, as when landslides are caused by an earthquake event.
Some geohazards are less spectacular and may occur over a longer time. Examples are provided by soil erosion and by the occurrence of subsidence depressions in karstic areas. Similarly, ground movements attributable to expansive/shrinkable clays could not be described as spectacular but may cause more damage in terms of cost to a community than more dramatic events.
The more spectacular natural geohazards are rapidly occurring events such as volcanic eruptions, landslides and earthquakes.  Some seismicity is caused by volcanic activity, but most is due to movements along faults within the Earth’s crust. In fact, earthquakes have been reported from all parts of the world but they are primarily associated with the margins of the crustal plates that move with respect to each other. Earthquakes are a manifestation of this movement – differential displacements giving rise to elastic strains, which eventually exceed the strength of the rocks involved. The strained rocks rebound along the fault, elastic strain energy being released in the form of seismic waves.
Initially, movement may occur over a small area of a fault plane, to be followed by slippage over a much larger surface. The initial movements give rise to the foreshocks that precede an earthquake. These are followed by the principal movements, but complete stability is not restored immediately. The shift of the rock masses involved in faulting relieves the main stress but new stresses develop in adjacent areas. Because stress is not relieved evenly everywhere, minor adjustments arise along a fault plane generating aftershocks. The decrease in strength of the aftershocks is irregular and occasionally they may continue for a year or more.
The duration of an earthquake is one of the most important factors as far as damage or failure of structures, soils and slopes are concerned. The magnitude of an earthquake affects the duration much more than it affects the maximum acceleration, since the larger the magnitude the greater the length of ruptured fault and the more extended the area from which the seismic waves are emitted. With increasing distance from a fault, the duration of shaking is longer but the intensity of shaking is less, the higher frequency waves being attenuated more than the lower frequencies.
Ground vibrations caused by earthquakes often lead to compaction of sandy soil and associated settlement of the ground surface. Loosely packed saturated sands and silts tend to lose all strength and behave like fluids during strong earthquakes. When such materials are subjected to shock, densification occurs. During the relatively short time of an earthquake, drainage cannot be achieved and this densification therefore leads to the development of excessive pore water pressures that cause the soil mass to act as a heavy fluid with practically no shear strength. Water moves upward from the voids to the ground surface where it emerges to form sand boils. If liquefaction occurs in a sloping soil mass, the entire mass begins to move as a flow slide. Loose saturated silts and sands often occur as thin layers underlying firmer materials. In such instances liquefaction of the silt or sand during an earthquake may cause the overlying material to slide over the liquefied layer. Structures on the main slide frequently are moved without suffering damage. However, a graben-like feature often forms at the head of the slide and buildings located in this area are subjected to large differential settlements and often are destroyed. Buildings near the toe of the slide are commonly heaved upwards or are even pushed over by the lateral thrust.

Extract from Basic Environmental and Engineering Geology by F. G. Bell
Whittles Publishing 2007 (RRP$150.00)
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