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  3. Tsunamis [A]Up until December of 2004, the phenomenon of tsunami was not on the minds of most of the world’s population. Tha

Tsunamis [A]Up until December of 2004, the phenomenon of tsunami was not on the minds of most of the world’s population. Tha

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问题     Tsunamis
    [A]Up until December of 2004, the phenomenon of tsunami was not on the minds of most of the world’s population. That changed on the morning of December 24, 2004 when an earthquake of moment magnitude 9.1 occurred along the oceanic trench off the coast of Sumatra in Indonesia This large earthquake resulted in vertical displacement of the sea floor and generated a tsunami that eventually killed 280,000 people and affected the lives of several million people.
    [B]Although people living on the coastline near the epicenter of the earthquake had little time or warning of the approaching tsunami, those living farther away along the coasts of Thailand, Sri Lanka, India, and East Africa had plenty of time to move to higher ground to escape.
    [C]But, there was no tsunami warning system in place in the Indian Ocean, and although other tsunami warning centers attempted to provide a warning, there was no effective communication system in place. Unfortunately, it has taken a disaster of great magnitude to point out the failings of the world’s scientific community and to educate almost every person on the planet about tsunami.
How Tsunamis Are Generated
    [D]There is an average of two destructive tsunamis per year in the Pacific basin. Pacific wide tsunamis are a rare phenomenon, occurring every 10-12 years on the average. Most of these tsunamis are generated by earthquakes that cause displacement of the seafloor.
    [E]Earthquakes cause tsunami by causing a disturbance of the seafloor. Thus, earthquakes that occur along coastlines or anywhere beneath the oceans can generate tsunami. The size of the tsunami is usually related to the size of the earthquake, with larger tsunami generated by larger earthquakes.
    [F]But the sense of displacement is also important. Tsunamis are generally only formed when an earthquake causes vertical displacement of the seafloor. The 1906 earthquake near San Francisco California had a Richter Magnitude of about 7.1, yet no tsunami was generated because the motion on the fault(断层)was strike-slip motion with no vertical displacement. Thus, tsunamis only occur if the fault generating the earthquake has normal or reverse displacement.
    [G]Because of this, most tsunamis are generated by earthquakes that occur along the subduction boundaries of plates, along the oceanic trenches. Since the Pacific Ocean is surrounded by plate boundaries of this type, tsunamis are frequently generated by earthquakes around the margins of the Pacific Ocean.
    Examples of Tsunamis Generated by Earthquakes
    [H]May 22, 1960—A moment magnitude 9.5 earthquake occurred along the subduction zone off South America. Because the population of Chile is familiar with earthquakes and potential tsunamis, most people along the coast moved to higher ground. 15 minutes after the earthquake, a tsunami with a run-up of 4.5m hit the coast. The first wave then retreated, dragging broken houses and boats back into the ocean. Many people saw this smooth retreat of the sea as a sign they could ride their boats out to sea and recover some of the property swept away by the first wave. But, about 1 hour later, the second wave traveling at a velocity of 166 km/hr crashed in with a run-up of 8 m. This wave crushed boats along the coast and destroyed coastal buildings. This was followed by a third wave traveling at only 83 km/hr that crashed in later with a run-up of 11 m, destroying all that was left of coastal villages. The resulting causalities listed 909 dead with 834 missing.
    [I]However, the situation was quite different in Hawaii. A tsunami warning system had been in place and the tsunami was expected to arrive at 9:57 AM. It hit at 9:58 AM and 61 people died, mostly sightseers that wanted to watch the wave roll in at close range(obviously they were too close). The tsunami continued across the Pacific Ocean, eventually reaching Japan where it killed an additional 185 people.
    [J]Another example. On March 11, 2011 a moment magnitude 9.0 earthquake struck off the northern coast of Japan. The earthquake generated a tsunami that rose up to 135 feet above sea level and killed over 20,000 people. Besides that high death toll, the tsunami caused one of the worst nuclear disasters in history. The Fukushima nuclear power plant, located on the coast was hit by a 49 feet tsunami wave that overtopped the tsunami protection walls that were only 19 feet high, and flooded the backup generators for the plant that were somehow placed on the first floor in a known tsunami zone!
Prediction and Early Warning
    [K]For areas located at great distances from earthquakes that could potentially generate a tsunami there is usually plenty of time for warnings to be sent and coastal areas evacuated, even though tsunami travels at high velocities across the oceans. Hawaii is a good example of an area located far from most of the sources of tsunami, where early warning is possible and has saved lives. For earthquakes occurring anywhere on the subduction margins of the Pacific Ocean there is a minimum of 4 hours of warning before a tsunami would strike any of the Hawaiian Islands.
    [L]The National Oceanic and Atmospheric Administration(NOAA)has set up a Pacific warning system for areas in the Pacific Ocean, called the Pacific Tsunami Warning Center. It consists of an international network of seismographic stations, and tidal stations around the Pacific basin that can all send information via satellite to the Center located in Hawaii.
    [M]When an earthquake occurs somewhere in the region, the Center immediately begins to analyze the data looking for signs that the earthquake could have generated a tsunami. The tidal stations are also monitored, and if a tsunami is detected, a warning is sent out to all areas on the Pacific coast. It takes at least 1 hour to assimilate all of the information and issue a warning. Thus for an average velocity of a tsunami of 750 km/hr, the regional system can provide a warning sufficient for adequate evacuation of coastal areas within 750 km of the earthquake.
    [N]In order to be able to issue warnings about tsunami generated within 100 to 750 km of an earthquake, several regional warning centers have been set up in areas prone to tsunami generating earthquakes. These include centers in Japan, Kamchatka, Alaska, Hawaii, French Polynesia, and Chile. These systems have been very successful at saving lives. For example, before the Japanese warning system was established, 14 tsunamis killed over 6000 people in Japan. Since the establishment of the warning system 20 tsunamis have killed 215 people in Japan.
    [O]Like all warning systems, the effectiveness of tsunami early warning depends strongly on local authority’s ability to determine that there is a danger, their ability to spread the information to those potentially affected, and on the education of the public to heed the warnings and remove themselves from the area.
Pacific wide tsunamis occur once every 10-12 years averagely.
选项
答案D
解析 根据Pacific wide tsunamis和every 10-12 years定位到D段。第2句提到，波及到整个太平 洋的海啸很少见，平均大约每隔10-12年发生一次。本题句子的averagely对应原文的on the average。
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