Plate quiz 1 answers 11th Sep

Class Quiz (Plate Tectonics)

Name:                                                             Index:                                   Class:                                 Marks:

1) Study Fig.1, which shows volcanic activity in 2007.

Fig. 1

        Outline why there are volcanoes at plate boundary Y.                                                                                                [4]

·       Oceanic-continental convergence between the Nazca Plate (oceanic) and the South American plate (continental)

·       When the oceanic plate converges with the continental plate, the denser oceanic Nazca plate subducts beneath the less dense continental South American plate.

·       **not necessary (An oceanic trench is formed at the point fo subduction and fold mountains form on the continental plate.)

·       At the subduction zone, solid mantle material melts and forms magma.

·       The magma rises through the crust, giving rise to volcanoes and volcanic eruptions.

2)      With the help of Fig. 2, describe the phenomenon and the formation of its resultant landforms.                                                                                                                          [5] Fig. 2 ·       The oceanic crust and the continental crust are converging/moving towards each other. ·       Fractures are formed at the convergence zone. ·       The denser and heavier oceanic crust sinks/subducts into the mantle and melts into magma under the high heat. ·       A long, narrow and deep depression called an oceanic trench is formed at the subducted zone. ·       Magma rises up the factures caused by the plate convergence and cools and solidifies to form volcanoes. Over time, more and more volcanoes build up to form a volcano arc. 3)      “The extent of damages caused by an earthquake is solely determined by an area’s proximity to the epicentre.” How true is this statement? Give reasons and relevant examples in your answer. [6]
		1. Magnitude of the Earthquake The strength of an earthquake, based on the energy released from the focus Measured using the Richter Scale, 0 being the lowest and 9 the highest The Great Kobe Earthquake in 1995 (7.2 on Richter Scale; lasted 20 secs, 5000 people died, 300000 people became homeless The Great East Japan Earthquake in 2011 (8.9 on Richter Scale; 16000 deaths; tsunami with wave height up to 10m) The higher the magnitude of an earthquake, the higher the possibility of infrastructure collapsing, as they are unable to withstand the force exerted by the earthquake, the greater the extent of damages. This may result in the death of more people who are buried under the debris, and the country’s economy will be crippled as resources will be diverted to rebuild the country while investors lose confidence in the country. 2. Population Density Refers to the ratio of the number of people living within an area. Earthquake in Anchorage, Alaska in 1964. Magnitude 9.2 on the Richter Scale Only 115 people died as the area had a small and sparse population The higher the population density of an earthquake-prone area, the higher the chances of people getting killed or injured. When people are living in close proximity, the destruction of an infrastructure in an area will have an impact on a larger number of people. A stampede may also occur when so many people are trying to escape from an earthquake-prone area at the same time. All these may lead to a greater extent of damages as lives are loss when high-rise buildings topple over. Hence, a magnitude 9.0 earthquake will only cause extensive damages if the area is densely populated. 3. Level of Preparedness Refers to how well the people know what to do in the event of an earthquake, such as having evacuation plans and trained rescue workers. In Japan, students are taught how to react when the earthquake warning signal goes off They are to crouch under the nearest table, so as to prevent falling objects from injuring them People who are prepared will know of a variety of measures to protect themselves in an event of an earthquake. They will not panic during an earthquake but will move to a safe area in an orderly manner. This will reduce the number of casualties that may arise from a stampede, and will also ensure that those who are injured receive timely medical assistance. Hence, a magnitude 9.0 earthquake may not cause extensive damages if the people know what to do and are able to evacuate safely before getting injured. 4. Distance from the Epicentre Epicentre is the point on the Earth’s surface that lies directly above the Focus Generally, the nearer to the epicentre, the stronger the shockwaves receive During the 2011 Christchurch earthquake in New Zealand, the epicentre was in a town a few km away from the city centre. Thus, the city centre suffered more damages than areas further away from the city. The energy of the earthquake is absorbed and weakened as it radiates outwards from the epicentre to the surrounding land. Thus, the nearer to the epicentre, the stronger the shock waves experienced at the place, resulting in a greater extent of damages, as infrastructure of the area may not be able to withstand the impact of the shock waves and topple over, causing the loss of lives and damages to properties. Hence, a magnitude 9.0 earthquake may not cause extensive damages if the area involved is located far away from the epicentre. 5. Time of Occurrence The time of the day during which the earthquake occurs determines where people are and what they are doing, which will affect their chances of survival. More than 2400 people died when an earthquake occurred a couple of hours after midnight in the Sun Moon Lake Region in Taiwan in 1999. If the earthquake occurs when most people are sleeping, there is a higher chance that these people will be trapped in their houses and more deaths may occur. This may reduce the workforce of the country, thus reducing the revenue earned by the country, which will hamper the country’s development of infrastructure, thus attracting less investors into the country. Hence, a magnitude 9.0 earthquake may not cause extensive damages if the earthquake occurs during the day and people are more aware and thus have sufficient time to escape. 6. Type of Soil Different soil properties can affect the shock waves as they pass through a soil layer Where sediments are loose and unconsolidated, the seismic waves are amplified Structures built on saturated and unconsolidated sediments can be affected by liquefaction, where the ground becomes unstable and the saturated soil flows like a liquid In the 2011 Christchurch earthquake in New Zealand, many houses and buildings had to be abandoned because of liquefaction after the earthquake. When sediments are loose and unconsolidated or saturated, the effects of the earthquake are amplified, which could lead to the collapse of infrastructure. This may result in the death of more people who are buried under the debris, and the country’s economy will be crippled as resources will be diverted to rebuild the country while investors lose confidence in the country. Hence, a magnitude 9.0 earthquake may not cause extensive damages if the type of soil found in that area is well-consolidated and more resistant to the movements of the ground. Conclusion The extent of damages experienced by an area is not just dependent on the magnitude of the earthquake, but also other factors as mentioned above. These factors are interlinked and will determine how badly affected an area is. However, men can reduce the extent of damages experienced by adopting the various adaptation measures like earthquake resistant buildings or installing earthquake monitoring and warning systems.