完形填空 At present groups of scientific workers, often 【26】 by their governments, spend their time in 【27】 the 【28】 of earthquakes. It is very 【29】 that they will be able to 【30】 earthquakes or influence the direction of their movements, which are determined by events far below the surface of the earth. But the facts they report and the theories 【31】 by those facts may enable scientists to 【32】 the centers of future earthquakes and even 【33】 the time of their 【34】 . It is only when the 【35】 of events that we call an earthquake can be seen as a pattern that predictions can be made. Part of this pattern is external and 【36】 or capable of being recorded by 【37】 instruments above the earth'' s surface. Scientists and engineers studied the 【38】 of the San Francisco earthquake. The city was rebuilt, and new features were 【39】 to strengthen buildings and maintain a 【40】 water supply in the event of another earthquake. The water mains ( = main pipes) were fitted with control valves which would enable water to travel by different routes round broken places. Large underground tanks were constructed to supply water if 【41】 supplies could not be tapped. Special measures were 【42】 to prevent fires, which often do more damage than earth- quakes themselves. The San Francisco earthquake 【43】 scientists with important information. Since the effects of the break were visible at the surface, scientists could 【44】 what actually happened, and reports of the incident were an important 【45】 to the world'' s store of knowledge bout earthquakes.