Nowadays groups of scientific workers, often【C1】______ by their governments, spend their time【C2】______ the occurrence of earthquakes. It is very likely that they will be able to【C3】______ earthquakes or influence the direction of their movements,【C4】______ determined by events far below the surface of the earth. But the facts they report and the theories【C5】______ by those facts may enable scientists to【C6】______ the centers of future earth quakes and even【C7】______ the time of their【C8】______ . It is only when the【C9】______ of events that we call an earthquake can be seen as a pattern that【C10】______ can be made. Part of its pattern is external and visible, or capable of being【C11】______ by sensitive instruments above the earth's surface. Scientists and engineers studied the shock waves of the San Francisco earthquake. The city was rebuilt and new features were【C12】 ______ to strengthen buildings and【C13】______ a constant water supply【C14】______ the event of another earthquake. Large underground tanks were constructed to supply water if【C15】______ supplies could not be tapped. Special measures were【C16】______ to prevent fires, which often do more damage than earth quakes themselves. The San Francisco earthquake【C17】______ scientists with valuable information. Since the effects of the break were【C18】______ at the surface, scientists could【C19】______ what actually happened, and reports of the incident were an important【C20】______ to the world's store of knowledge a bout earthquakes. 【C1】