Earthquakes and volcanoes

A major success of the theory of plate tectonics is an explanation of why many of the volcanoes and earthquakes of the world occur where they do. For example, the ``Ring of Fire'' around the Pacific Ocean contains numerous volcanoes, and results from the motion of the Pacific plate, particularly the appearance of subduction zones near the boundaries. Another example is the San Andreas fault, which is a transform boundary between two plates, which of course is the location of numerous earthquakes. Much effort has gone into predicting volcanoes and earthquakes, with mixed results.

 
• Volcanoes: Generally speaking, scientists can predict with a relatively good degree of certainty when a volcano will erupt. This is because most volcanoes follow a regular pattern of increasing seismic activity as the eruption approaches, usually in the form of small earthquakes.  
• Earthquakes: On the other hand, earthquakes have proven to be notoriously difficult to predict because of the lack of regular patterns - sometimes they are preceded by a series of small earthquakes, and other times not. Scientists measure the severity of an earthquake on a (modified) Richter scale, where an increase of 1 unit represents 10 times more ground motion and 30 times more energy released. A magnitude 5 earthquake will be felt by most people in the vicinity, but do little damage to well-made buildings. A magnitude 7-8 earthquake will do major damage. No earthquake larger than a magnitude 9 has ever been recorded. The motion that this Richter scale measures is due to seismic waves that are generated at the center and subsequently travel through the Earth. Much like the use of radiation (electromagnetic waves) that are used in medicine to diagnose diseases, the way that these waves travel through the Earth can tell us a great deal of the structure of the inner Earth.