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between 1836 and 1994,san francisco had 11 earthquakes.earthquakes happen when rocks under the earth move. 1836到1994年间，三藩市共发生了11次地震。当地底下的石头移动时地震就会发生。 when they move a little. there is a small space. a line, between them. 当石头移动一点，就有产生一点空间，断层。 T