A train tipped on its side by the 1906 earthquake in San Francisco.

Although we like to think of the Earth as very stable and fixed, we know now that the continents move on plates. Still, this movement is too slow for us to notice. Every once in a while, however, the Earth literally does move beneath us. This is exactly what author Charles Dickens experienced in Hereford, England, while sleeping in 1863. He said that he "was awakened by a violent swaying of my bedstead from side to side" as if "some great beast ... was ... trying to rise." We call this movement an earthquake and we know that it is not caused by some great beast. An earthquake is a vibration of the Earth's crust. This vibration may be very small so that it is hardly noticed: or it may be quite large and catastrophic.