地理卷一 Q6 詳盡評改報告

There are three types of plate boundaries on earth which acted with different internal forces. Various plate movements will lead to the occurence of tsunamis in different ways, which will be illustrated in the following paragraphs. Starting with, tsunamis are formed by the uplifting and displacement of seafloor or sea bed, which is formed ^with a large succession of waves washing towards the shore. It is mostly formed by submarine earthquakes, submarine landslides and submarine volcanic eruptions. Submarine earthquakes caused tsunamis which are majorly found along the convergent and transform plate boundaries. Earthquakes occurs when the stress and pressure imposed on the rock exceeds its internal strength which cause a sudden release of energy and shaking of land. It is easily triggered along convergent and transform plate boundaries due to plate movements. At convergent plate boundary, two plates collided and subducts and forming compressional force. The subduction of plates puts stress and energy on the rocks and thus breaking the rocks to release pressure and forms earthquake, which submarine earthquake is common mostly along ocean-ocean collision. Similarly, along transform plate boundaries, like the movement of Pacific plate and North American plate, two plates slide past each other and forms tensional force that causes friction and stress on the rock, thus resulting in earthquakes. Submarine earthquakes causes a ^vertical displacement of the sea floor and so forming a bulge of water. The bulge of water will gradually collapses ^(due to gravity) form large succession of waves washing in different directions and thus tsunamis are formed when reaching the land. Examples brought by submarine earthquakes included ^of tsunamis ^the tsunami the Tohoku Earthquake in Japan in 2011 and ^the tsunami the Chile brought by [unclear: Chile Earthquake] caused by the collision of South American plate and Nazca plate. Moving on, tsunamis are also brought by submarine volcanic eruptions along divergent plate boundaries and convergent plate boundaries. At divergence plate boundaries, two plates move apart, forming tensional forces. Fissure is created and caused a release of pressure of the asthenosphere. It will then lower its melting point and so produce magma, and eventually rises through the fissure and erupts at the ocean surface as lava under ocean-ocean divergence. Submarine volcanoes are thereby formed like those at the Mid-Atlantic zone. Similarly, along convergent plate boundaries, submarine volcanoes will also be formed. Two plates collides and the denser plate will subduct to form compressional force. Cracks are created on the crust and the subducted plate releases water to the asthenosphere to lower its melting point to form magma. It then rises through the cracks and erupts on the surface as lava, forming submarine volcanoes, typically under ocean-ocean collision like that of the collision of Indo-Australian plate and the Eurasian plate. When submarine volcanoes empts, it will emit volcanic materials like volcanic ashes, gases, lava, rock fragments, etc, which will collapse on the ocean floor and again forming large succession of waves, thus resulting in the occurence of tsunamis. Violent volcanic eruptions under convergent plate boundaries to trigger tsunamis is more common as vent eruption is frequently found at convergent plate boundaries with acid lava zones, thus tsunamis are more oftenly found along the Pacific Ring of Fire with most covered by compressional forces. Examples of tsunamis caused by volcanic eruptions included that in Tonga (2022) ^submarine and Indonesia caused by the collision of Indo-Australian plate and Eurasian plate. Lastly, tsunamis may be formed by submarine landslides that could occur in any type of plate boundaries. The falling and collapse of landslide materials and rock fragments causes a splash on the ocean floor, forming a bulge that collapses and thus large succession of waves which results in tsunamis. The tsunami in Indonesia in 2014 is a typical example caused by submarine landslides. To add on, it can be shown that tsunamis are mostly found on convergent plate boundaries due to a more violent volcanic eruptions and commonly found ^submarine submarine earthquakes when plates collide, thus showing the major distribution of tsunamis along the Circum-Pacific Belt which accounts for 90% of tsunamis on earth. Consequently, various plate movements along different plate boundaries leads to the occurance of tsunamis with the triggering of submarine earthquakes, submarine volcanic erupt[?]ns and submarine landslides.

Moving on to the next point, tsunamis may bring huge casualties and economic losses to the affected areas and thus should have measures to prevent. Early warning systems are a kind of precautionary measures yet not too effective in reducing casualties. Starting with, early warning systems are majorly used to inform the occurence of hazards before its approach to provide sufficient evacuation time for residents to escape to prevent loss of lives. For instance, tsunami guages are used to predict the occurence of tsunamis and tsunami warning system, like that of the Indian Ocean warning centres and Pacific tsunami warning systems were made to provide adequate information and time for evacuation to reduce casualties. When early warnings are established, people may flee from dangerous areas like going up on highlands to save their lives. Nevertheless, it is mostly ineffective in reducing casualties in the areas. First, the effectiveness of warning systems depends on the accuracy of its prediction. Without accurate prediction, the time for people to escape will be insufficient and thus failing to reduce casualties. Tsunamis are oftenly hard to predict due to its sudden large sussection of waves, and also its immensely fast speed of over 10?km/h [unclear: 100km/h], hence failing to accurately predict its occurence to issue early warning systems will limit its effectiveness. Moreover, the effectiveness of warning systems depends on the economic level and technological development in regions. Setting up monitoring centres and ocean bottom tsunami buoys to monitor the ocean activity requires higher level of technology and also large amount of capital investment. Less developed countries like India, Chile, Phillipines are also prone to tsunamis yet with insufficient capital and technology input to invest in the warning systems, hence limiting the effectiveness of the early warning systems in reducing casualties. Additionally, the level of awareness of hazards and education also causes constraints that make the warning systems ineffective. For example, less developed countries like Indonesia and Tonga are also prone to tsunamis yet is less developed with low education level and awareness of preparation and evacuation during hazards. Even if early warning systems were posed, citizens don't know how to prepare or evacuate to protect themselves hence making the warning systems very ineffective in reducing losses of life due to a low awareness. Besides, tsunamis have a very short onset time that hinders the effectiveness of early warning systems. For example, the Tohoku earthquake 2011 with magnitude of 9.1 has triggered a large tsunami within just two hours. Such short onset time makes it hard to predict and for citizens to evacuate even with a warning system posed, thereby limiting the effectiveness of tsunami warning systems to reduce casualties. Lastly, secondary hazards may be resulted after tsunamis which warning systems could not prevent its loss of life. For example, tsunamis caused a contamination of water that poses health diseases and death, while also breaking roads and infrastructures that hinder rescuing, that will lead to more casualties, hence limiting the effectiveness of preventing casualties in the areas. Other measures could therefore be adopted together with warning systems to reduce casualties, like frequent tsunami evacuation drills. This could increase the awareness of people during or before hazards and so raising the effectiveness of the warning systems. Moreover, land use zoning shall be implementated together with warning system to reduce casualties and losses. For example, buffer zones with mangroves shall be grown to reduce energy and slow down its onset, while housing should no longer be developed in coastal low-lying areas and also establishing partial evacuation route to raise the effectiveness of warning systems. Consequently, with early warning systems on its own, it is mostly ineffective in reducing casualties in hazard prone areas.