Since the beginning of time, earthquakes have been among the most destructive of natural phenomena. Single earthquakes have leveled cities and killed hundreds of thousands of people. Tidal waves and landslides caused by and associated with earthquakes have destroyed cities, coastlines and fleets. Even today, despite all of our technological advances, we are still helpless in the face of these events. 

Part of the problem lies in the awesome amount of energy that is released by such natural disasters. It has been estimated that the earthquake that rocked Anchorage, Alaska on Good Friday, 1964, released as much energy as 50 (and perhaps as much as 200) megatons (that's millions of tons) of TNT. In comparison, large nuclear fusion warheads rarely exceed 5 megatons in yield. 

All earthquakes are a series of waves (disturbances) traveling through the earth. They are caused by the elastic rebound of rock layers. Modern understanding of the earth recognizes that its upper layers are broken into gigantic pieces, plates, that move. Where plates interact, the rock layers are subjected to enormous forces and deform. If the forces are sufficient to cause a rupture or to reactivate an older rupture, rocks suddenly return to an unstressed attitude, they rebound. The energy released in the rebound travels outward and dissipates as waves which we call an earthquake. More mobile areas are therefore constantly racked with quakes, whereas more stable areas are relatively earthquake free. 

The actual location from where the waves originate is called the focus of the earthquake. This focus may occur less than 60 km below the surface, in which case we speak of shallow (focus) earthquakes. Earthquakes whose focus lies between 60 km and 300 km are classified as intermediate (focus) quakes, and those whose focus occurs beyond 300 km (to as deep as 1100 km.) are deep (focus) earthquakes. The point on the surface of the earth directly above the focus is called the epicenter. 

Permanent records of earthquakes are made with machines called seismographs, and the tracing or the record of the motion is called a seismogram. These records provide us with information about the actual motion of the ground during the passage of the various waves, and the timing and pattern of wave arrivals can tell us the point of origin, as well as the nature and the configuration of the layers that the waves pass through.