The major cause of earthquakes is shifting tectonic plates, the dozen or so fragments of crust that float on the Earths thick mantle. Most occur at the boundaries, for example the Ring of Fire along the margins of the Pacific Ocean. Mid-plate earthquakes, like the one at New Madrid, are usually large and destructiveand less well understood.
Damage from earthquakes is not limited to buildings, bridges, and dams. They can trigger fires and landslides: When loose soil, such as landfill, loses its ability to bear loads, the ground behaves like quicksand. Buildings can sink and even disappear.
When they occur at sea, earthquakes can generate seismic waves called tsunamis, which travel great distances at speeds equaling those of commercial jetliners. Tsunamis are barely noticeableoften just ripples on the surfaceuntil they approach shores, when they become monsters.