3.b. earthquakes generate distinctive hazards

ground shaking and ground displacement

this is vertical and horizontal movement of the ground. the severity of ground shaking depends on: earthquake magnitude; distance from the epicentre; local geology

locations close to epicentre of high magnitude quake and where the surfec layers are relatively unconsolidated and have high water content= extreme ground shaking. these conditions exist in parts of mexico city and the japanese port of kobe hit by devasting earthquakes in 1985 and 1995. in general buildings can withstand vertical movements better than horizontal- swaging of structures is so dangerous to their stability. ground movements that cause displacement of rocks along fault lies can rip apart pipelines and sewers, sever rigid structures such as railway tracks and roads and cause building to collapse. the displacement of the surface can also disrupt natural drainage, divert streams/rivers and affect the movement of groundwater in aquifers- serious implications for public water supplies and irrigation for agriculture

liquefaction

an area with surface materuals of fine grained sands, alluvium and landfil with high water content= vibrations can cause these materials to behave like liquids= these materials lose their strength; slopes such as riverbanks collapse and structures tilt and sink as their foundations give way. liquefaction= major issue during the kobe earthquake as much of the port had been built on reclaimed land in osaka bay. just under 200 berths in the port were destroyed, affecting not just the japanese economy but trade throughout the world

landslides and avalanches

both ground shaking and liquefaction can cause slope failure. some earthquake prone regions are especially vulnerable to it. steep slopes in mountainous regions like himalayas are notoriously unstabke and vulnerable to landslides. their vulnerability is increased by deforestation and heavy monsoon rains, so even small tremors can trigger landslides. the nepalese earthquake 2015 triggered large numbers of landslides and avalanches caused by ground shaking. landslides block transport routes in mountainous regions where accessibility is already difficult. movements of soil and rock on slopes can also block rivers. these natural dams create temporary lakes, which can threaten areas downstream with catastrophic floods where the dams fail. cases in several locations in kashmir in 2005 and sichuan in 2008. #

upland valleys are often favoured sites for reservoirs. earthquakes creating landslides on slopes above a reservoir can cause the displacement of water and the waves generated could weaken and overtop the damn. an earthquake in northern italy in 1963 did this- collpase of a hillside above the vaiont reservoir, generating a 100m wave which swept over the dam and down the valley of the piave river= drowning 3000 people

tsunamis

underwater earthquake can cause the seabed to rise vertically. this displaces the water above, producing powerful waves at the surface which spread out at high velocity from the epicentre. tsunami waves= low height <1m and very long wavelengths up to 200km. however wave height increases greatly as they approach the shore and enter shallow water. before the wave breaks, water in front of the wave is pulled back out to sea, a process= drawdown. finally the tsunami wave rushes in as a wel of water that can exceed 25m in height. tsunami generated by earthquake off the coast of Aceh province in sumatra in dec 2004 is estimated to have delivered about 1000 tonnes of water per metre of shoreline. local height of a tsunami is also affected by the shape of the seabed and coastline. depending on the relief of the coastal zone, tsunami can spread variable distances inland

underwater landslides caused by earthquakes can also dispalce water and create tsunami waves. when a large volume of rock is shaken and slides downslope, water is dragged in behind from all sides and collides in the centre. this can gernate a tsunami wave which radiates outwards. while the resulting wave may not have enough power to cross oceans, the local effects can be devasting eg 1998 2200 villagers living in coastal communities in papua new guinea were killed by a local tunsami generated from an underwater landslide triggered by an earthquake. because such events are local, warning times are short making them particularly hazardous. much research is currently being undertaken to map the seabed along vulnerable coastlines, such as southern california to identify potential underwater landslide hazards