GEOLOGICAL HAZARDS applications
Earthquakes are recurrent demonstrations that the Earth is indeed an active planet. These sudden releases of energy pose several hazards to our natural and built environment. There are different hazards depending on the local conditions of the rupture and the geographic area: buildings may crumble, cracking in bridge supports or even casualties among population. In coastal areas, powerful offshore earthquakes could trigger a tsunami, a series of massive waves that would flood the shoreline within minutes; damage or destroy roads, bridges and utility lines; and likely cause many injuries and deaths.
The first main earthquake hazard (danger) is the effect of ground shaking. Buildings can be damaged by the shaking itself or by the ground beneath them settling to a different level than it was before the earthquake (subsidence).
Buildings can even sink into the ground if soil liquefaction occurs. Liquefaction is the mixing of sand or soil and groundwater (water underground) during the shaking of a moderate or strong earthquake. When the water and soil are mixed, the ground becomes very soft and acts like quicksand. If liquefaction occurs under a building, it may start to lean, tip over, or sink several feet. The ground firms up again after the earthquake has past and the water has settled back down to its usual place deeper in the ground. Liquefaction is a hazard in areas that have groundwater near the surface and sandy soil.
The term site effect is generally used to refer to wave propagation in the immediate vicinity of the site, as opposed to the propagation effects, which refer to the complete path from the source to receiver. Site effects can include modification of seismic waves by the local sedimentary cover, particularly where this local cover is not representative of the total path from the epicenter, the effect of alluvial valleys or basins, effect of local topography, and effects of the water table.
Site effects play a very important role in characterizing seismic ground motions because they may strongly amplify seismic motions just before reaching the surface of the ground or the basement of man-made structures.
Recently, a large number of studies have focused in developing tools to asses potential site effects using as a proxy the shear-wave velocity of the topsoil (first tens of meters). In this regard, passive seismic methods (H/V spectral ratio, MASW) have proven their usefulness to derive such results in urban areas.