Earthquakes Flashcards
describe ground motion and give an example
ground motion- ground motion caused by surface seismic waves can damage or destroy buildings. the severity of earthquake is dependant on the amplitude and wave frequency. rayleigh waves causes ground to move up and down as particles follow an elliptical path while love waves causes horizontal shifting of earth. resonance amplifies the vibration of a building until it shakes itself apart. a building will sway back and forth at a frequency depending on the height and type of construction. when natural frequency is similar to frequency of ground motion, resonance occurs and buildings will amplify vibrations. 1985 Mexico city earthquake. about 70% of 10-20 storey buildings were damagd as they had similar natural frequency to ground motion however fewer than 20% of buildings more than 2o storeys tall were damages as they were not in resonance with ground motion hence did not shake so violently
tsunamis
tsunamis are a series of large waves caused by the vertical displacement of water when a submarine earthquake causes vertical displacement of seafloor along a fault line. tsunami starts at about 1m travelling at 800km/h but as it approaches shallower water, it slows down to about 30-50km/h but increases in height to 15m. when tsunami comes onshore, low wave trough arrives first exposing the sea bed then high wave crest arrives flooding coastal areas. 2004 26 dec Indian ocean earthquake resulted in about 230000 deaths among 14 countries as fault motion raised seafloors and ocean surfaces about 5m above, over an area of 400km long and 50km wide. this lifted about 100 billion metric tons of water and resulting tsunami crossed Indian ocean in about 10 hours and penetrated the island as afar as 50km
liquefaction
this happens in soft water saturated sendimets. when grain settles during strong ground shaking, water is displaced upwards into pore spaces. water pressure moves grain apart hence sendiment water mixture loses strength. soil in state of liquefaction has no strength and cannot bear any load which causes ground surface to subside, buildings found on liquefied ground will lean or topple. the kobe port constructed on two artificial islands made of relatively lose sendiments suffered liquefaction during great kobe earthquake in 1995
describe
rapid downslope movement of soil, rocks and vegetation. Earthquakes are common in mountains and found along convergent plate boundary. steep slopes in these environments are prone to landslides when shaken during earthquake. Sichuan earthquake in 2008 occurred in mountainous central region of Sichuan province. caused by the collision of indian Australian and Eurasian plates. 90,000 dead and 375000 injured by falling debris or collapsed building. whole villages and towns were destroyed
describe fires
fires can easily start during earthquake when stoves are overturned, gas pipes are fractured or electrical cables exposed by earthquakes ignite flammable items such as gas, oil chemical and plastics. this hazard is often eggravated by difficulty in managing the fires due to blocked streets and damaged water pipes.
1995 great kobe earthquake caused extensive fires that raged on and off for two days. hundres of fires were ignited by toppled gas cookers and kerosene stove. fireman were unable to control the fire as ruptured water pipes means that there were no water supply. strong winds also caused fire to spread.
