24 luglio 2016

Effects caused

The effects of an earthquake on the environment can be divided into direct, in other words those that bring about deformation of the ground in the fault where the tremor originated (surface fault) and secondary, namely those not directly caused by the movement of the fault, but by the propagation of the seismic waves at a distance from the source area or hypocentre (landslides, liquefaction and Tsunamis).

Secondary effects
Earthquakes produce elastic waves that propagate, making ground “vibrate” and the landslides they can cause may mean permanent variations in the landscape. Shaking may also cause an unexpected increase in the pressure of water with the loss of ground cohesion, leading to liquefaction. These events are similar to what happens with “quicksand” and buildings on this ground either sink or topple over.
Another effect caused by earthquakes is the Tsunami (Japanese for “harbour wave”) or tidal wave and this is used to describe any waves that are violent enough to move the water inside ports. The most frequent causes of Tsunamis are earthquakes that originate underwater, but they can also be caused by volcanic eruptions or undersea landslides. In Italy the most destructive Tsunamis in this century occurred as a result of the earthquake in Calabria in 1783 and in Messina in 1908.

Local conditions at the site
The effects of an earthquake on buildings are influenced, as well as by the quality of the buildings, also by local geological and morphological characteristics, like the depth of the surface ground, the presence of abandoned areas, the lie of the land and steep slopes. These characteristics as a whole make up the picture of the “local conditions at the site”, determined by the type and entity of the effects caused by an earthquake (seismic response).
Stronger shaking (amplification) is generally observed in the areas that have soft ground cover (like recent deposits made up of flooding from rivers or lakes. Furthermore, seismic waves are elastic waves that move ground particles with variable frequencies. Seismic wave frequencies depend not only on the geological characteristics of the ground, but also on the morphology of the surface they meet, rather like what happens with sound waves that in certain conditions are subject to echoes.
If oscillation of such an intensity occurs that it “resounds” with a building (in other words echoes with it), very serious damage may occur or the building may even collapse. This is the reason why often the most damage is observed on buildings at the top of ridges, along the edges of slopes, but also on those areas where grounds with different seismic response characteristics are in contact (rigid ground in contact with soft ground).