Part 2 About Tsunami 关于海啸

    A tsunami is a wave train, or series of waves, generated in a body of water by an impulsive disturbance that vertically displaces the water column. Earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis.

    Tsunamis can savagely attack coastlines, causing devastating property damage and loss of life.

    Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that is associated with the earth's crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created.

    A tsunami can be generated by any disturbance that displaces a large water mass from its equilibrium position. In the case of earthquake-generated tsunamis, the water column is disturbed by the uplift or subsidence of the sea floor. Submarine landslides, which often accompany large earthquakes, as well as collapses of volcanic edifices, can also disturb the overlying water column as sediment and rock slump down slope and are redistributed across the sea floor.

    Similarly, a violent submarine volcanic eruption can create an impulsive force that uplifts the water column and generates a tsunami. Conversely, supermarine landslides and cosmic-body impacts disturb the water from above, as momentum from falling debris is transferred to the water into which the debris falls. Generally speaking, tsunamis generated from these mechanisms, unlike the Pacific-wide tsunamis caused by some earthquakes, dissipate quickly and rarely affect coastlines distant from the source area.

    As a tsunami leaves the deep water of the open ocean and travels into the shallower water near the coast, it transforms. A tsunami travels at a speed that is related to the water depth - hence, as the water depth decreases, the tsunami slows. The tsunami's energy flux, which is dependent on both its wave speed and wave height, remains nearly constant. Consequently, as the tsunami's speed diminishes as it travels into shallower water, its height grows. Because of this shoaling effect, a tsunami, imperceptible at sea, may grow to be several meters or more in height near the coast. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore.

    Just like other water waves, tsunamis begin to lose energy as they rush onshore - part of the wave energy is reflected offshore, while the shoreward-propagating wave energy is dissipated through bottom friction and turbulence. Despite these losses, tsunamis still reach the coast with tremendous amounts of energy. Tsunamis have great erosional potential, stripping beaches of sand that may have taken years to accumulate and undermining trees and other coastal vegetation.

    Capable of inundating, or flooding, hundreds of meters inland past the typical high-water level, the fast-moving water associated with the inundating tsunami can crush homes and other coastal structures. Tsunamis may reach a maximum vertical height onshore above sea level, often called a runup height, of 10, 20, and even 30 meters. Throughout recorded history tsunamis have occurred that have significantly impacted coastal communities.

    The Following are some important facts to know about tsunamis:

    Tsunamis that strike coastal locations in the Pacific Ocean Basin are most always caused by earthquakes. These earthquakes might occur far away or near where you live.

    Some tsunamis can be very large. In coastal areas their height can be as great as 30 feet or more (100 feet in extreme cases), and they can move inland several hundred feet.

    All low-lying coastal areas can be struck by tsunamis.

    A tsunami consists of a series of waves. Often the first wave may not be the largest.

    The danger from a tsunami can last for several hours after the arrival of the first wave.

    Tsunamis can move faster than a person can run. Sometimes a tsunami causes the water near the shore to recede, exposing the ocean floor.

    The force of some tsunamis is enormous. Large rocks weighing several tons along with boats and other debris can be moved inland hundreds of feet by tsunami wave activity. Homes and other buildings are destroyed. All this material and water move with great force and can kill or injure people.

    Tsunamis can occur at any time, day or night.

    Tsunamis can travel up rivers and streams that lead to the ocean.

    海啸是一种具有强大破坏力的海浪。这种波浪运动引发的狂涛骇浪，汹涌澎湃，它卷起的海涛，波高可达数十米。这种“水墙”内含极大的能量，冲上陆地后所向披靡，往往造成对生命和财产的严重摧残。智利大海啸形成的波涛，移动了上万公里仍不减雄风，足见它的巨大威力。

    海啸是一种灾难性的海浪，通常由震源在海底下50千米以内、里氏震级6.5以上的海底地震引起。水下或沿岸山崩或火山爆发也可能引起海啸。在一次震动之后，震荡波在海面上以不断扩大的圆圈，传播到很远的距离，正象卵石掉进浅池里产生的波纹一样。海啸波长比海洋的最大深度还要大，轨道运动在海底附近也没受多大阻碍，不管海洋深度如何，波都可以传播过去。

    剧烈震动之后不久，巨浪呼啸,以催枯拉朽之势，越过海岸线，越过田野，迅猛地袭击着岸边的城市和村庄，瞬时人们都消失在巨浪中。港口所有设施，被震塌的建筑物，在狂涛的洗劫下，被席卷一空。事后，海滩上一片狼藉，到处是残木破板和人畜尸体。 地震海啸给人类带来的灾难是十分巨大的。目前，人类对地震、火山、海啸等突如其来的灾变，只能通过预测、观察来预防或减少它们所造成的损失，但还不能控制它们的发生。

    这次印度洋海啸也是由地震引起的。地震是发生在板块边缘的逆冲型地震，它所释放出的能量在近几十年来是较大的。逆冲型地震的成因是地层断层的上部上移。苏门答腊以北地区位于印度板块边缘，板块边缘的一个长距离破裂带通过长时间积累，蓄积了巨大能量，最后这些能量在26日集中释放出来，这就是此次印度洋海啸大地震的直接原因。

    虽然此次地震震级很高，使周边地区都有震感，但对人类威胁最大的是这次地震引起的海啸。在这次地震中，斯里兰卡东部和南部、泰国、印度尼西亚和印度南部地区等都受到海啸的侵袭。地震中的绝大部分人员伤亡都与海啸有关。

    此次地震中断层移动导致断层间产生一个空洞，当海水填充这个空洞时产生巨大的海水波动。这种波动从深海传至浅海时，海浪陡然升到十几米高，并以每秒200米的速度传播。海浪冲到岸上后，造成重大破坏。

    目前一些国家在海啸预报方面已经做得不错。当地震发生后，有关部门将地震的位置、震级和类型输入电脑，即可分析出它是否会造成海啸、海水波动程度及其传播方向，然后就可尽快向可能受影响的地区发出预警，通知居民撤离。另外，在易受海啸侵袭的沿岸地区可提前构筑能阻挡海浪的防护设施，以减少损失。