单项选择题

Everyone has heard of the San Andreas fault, which constantly threatens California and the West Coast with earth- quakes. But how many people know about the equally serious New Madrid fault in Missouri.’
Between December of 1811 and February of 1812, three major earthquakes occurred, all centered around the town of New Madrid, Missouri, on the Mississippi River. Property damage was severe.
Buildings in the area were almost dest oyed. Whole forests fell at once, and huge cracks opened in the ground, allowing smell of sulfur to filter upward.
The Mississippi River itself completely changed character, developing sudden rapids and whirlpools. Several times it changed its course, and once, according to some observers, it actually appeared to run backwards. Few people were killed in the New Madrid earthquakes, probably simply because few people lived in the area in 1811; but the severity of the earth- quakes are shown by the fact that the shock waves rang bells in church towers in Charleston, South Carolina, on the coast. Buildings shook in New York City, and clocks were stopped in Washington D.C. Scientists now know that America’s two major faults are essentially different. The San Andreas is a horizontal boundary between two major land masses that are slowly moving in opposite directions. California earthquakes result when the movement of these two masses suddenly lurches forward.
The New Madrid fault, on the other hand, is a vertical fault; at some point, possibly hundreds of millions of years ago, rock was pushed up toward the surface, probably by volcanoes under the surface. Suddenly, the volcanoes cooled and the rock collapsed, leaving huge cracks. Even now’, the rock continues to settle downwards, and sudden sinking motions trigger earthquakes in the region. The fault itself, a large crack in this layer of rock, with dozens of other cracks that split off from it, extends from northeast Arkansas through Missouri and into southern Illinois.
Scientists who have studied the New Madrid fault say there have been numerous smaller quakes in the area since 1811; these smaller quakes indicate that larger ones are probably coming, but rite scientists say they have no method of predicting when a large earthquake will occur.
We may conclude from the passage that ______.