E) Earthquakes have caused a great deal of property damage over the years, and they have claimed many lives. In the last hundred years alone, there have been more than 1.5 million earthquake-related fatalities. Usually, it’s not the shaking ground itself that claims lives; it’s the associated destruction of man-made structures and other natural disasters it causes, such as tsunamis, avalanches (雪崩)and landslides.
F) The biggest scientific breakthrough in the history of seismology—the study of earthquakes—came in the middle of the 20th century, with the development of the theory of plate tectonics(筑造学).Scientists proposed the idea of plate tectonics to explain a number of peculiar phenomena on earth, such as the apparent movement of continents over time, the clustering of volcanic activity in certain areas and the presence of huge ridges at the bottom of the ocean.
G) The basic theory is that the surface layer of the earth—the lithosphere—is comprised of many plates that slide over the lubricating (润滑的)asthenosphere layer. At the boundaries between these huge plates of soil and rock, three different things can happen.
H) Plates can move apart. If two plates are moving apart from each other, hot, molten rock flows up from the layers of mantle below the lithosphere. This magma (岩浆) comes out on the surface (mostly at the bottom of the ocean), where it is called lava (熔岩).As the lava cools, it hardens to form new lithosphere material, filling in the gap. This is called a divergent plate boundary.
I) Plates can push together. If the two plates are moving toward each other, one plate typically pushes under the other one. This plate below sinks into the lower mantle layers, where it melts. At some boundaries where two plates meet, neither plate is in a position to push under the other, so they both push against each other to form mountains. The lines where plates push toward each other are called convergent plate boundaries.
J) Plates slide against each other. At other boundaries, plates simply slide by each other—one moves north and one moves south, for example. While the plates don’t drift directly into each other at these transform boundaries, they are pushed tightly together. A great deal of tension builds at the boundary.
K) We understand earthquakes a lot better than we did even 50 years ago, but we still can’t do much about them. They are caused by fundamental, powerful geological processes that are far beyond our control. These processes are also fairly unpredictable, so it’s not possible at this time to tell people exactly when an earthquake is going to occur. The first detected earthquake waves will tell us that more powerful vibrations are on their way, but this only gives us a few minutes’ warning, at most.
L) So what can we do about earthquakes? The major advances over the past 50 years have been in preparedness, particularly in the field of construction engineering. In 1973, the Uniform Building Code, an international set of standards for building construction,7 added7 specifications7 to7 strengthen7 buildings7 against7 the7 force7 of7 earthquake7 waves.7 This7 includes7 strengthening7 support7 material7 as7 well7 as7 designing buildings so they are flexible enough to absorb vibrations without falling or deteriorating. It’s very important to design structures that can undergo this sort of attack, particularly in earthquake -prone areas.