Earthquakes often happen near volcanoes, but this is not always true. The centers of some are (1) . The bottom of the sea suddenly (2) . The powerful forces inside the (3) break the rocks. The coast is shaken and great waves appear. These waves travel (4) and rush over the land when they reach it. They are p enough to (5) houses and other buildings; sometimes they break (6) than the earthquake itself.
Very often fires followed the (7) earthquakes. In 1906, the great earthquake at San Francisco broke the gas-pipes. The gas (8) , and soon large numbers of fires were burning in the city. The water-pipes were also (9) ; so it was not possible to put the fires out. There was (10) . The Tokyo earthquake of (11) happened just before the middle of the day. People were (12) on their fires at that time. When the ground shook, the fires shook too. (13) were thrown on the different parts of the houses, some of which were (14) . Soon 134 fires were burning in the city. What kind of building (15) in an earthquake A building with concrete walls is perhaps the best. A (16) will make it even per. The frame holds the (17) together, and the walls do not easily fall. There is (18) of fire because concrete and steel do not burn. The Americans (19) the results of the earthquake in San Francisco, and they are convinced that this kind of building is the (20) .
的不可导的点的个数为