It was early morning, January 17, 1994. John Evans and his wife were asleep in their house in Northridge, California. Suddenly, the loudest thunder they'd ever heard seemed to explode right out of the ceiling, the floor, and the walls. They were thrown onto the floor. The dresser, on the other side of the bed, crashed onto the bed, which was shaking and bouncing violently. Paintings and a mirror flew off the bedroom walls. Their bedroom windows broke and the walls cracked as the foundation of the house moved.
The Northridge quake was "only" 6.8 on the Richter scale (10 is maximum). There have been many earthquakes stronger than the Northridge one. The strongest quake ever recorded, 9.5, occurred in Chile in 1960.
Each whole number on the Richter scale equals ten times the energy of the previous number. That is, a 3.0 quake is ten times as powerful as a 2.0 quake. Quakes are measured on instruments called seismographs.
Earthquakes usually occur where tectonic plates meet. About 30 of these plates cover the Earth. They are several miles thick and huge in area—most of the Pacific Ocean sits on just one plate.
Earthquakes occur when one plate strikes another or slides beneath another. Either action produces a huge amount of energy that travels upwards to the surface of the land or upwards to the surface of the ocean floor. A powerful earthquake on the ocean floor produces a tsunami.
It is well-known that various kinds of animals act oddly just before an earthquake occurs. Researchers hope to discover how these animals can actually detect an earthquake. They'll use that knowledge to try to create an early warning system to save human lives. We'll never be able to take the power out of an earthquake, but maybe someday we can take out the surprise.