SEISMOLOGY TOPICS
Earthquakes result from the rapid release of energy stored in various forms in earth materials. Most earthquakes result from the release of energy stored in elastically deformed rocks usually associated with faults (see the section in rock mechanics for a brief discussion of elastic deformation). Other earthquakes, generally small, are associated with volcanic activity where magma under high pressure moves in a system of shallow conduits causing rocks to fracture, perhaps also influenced by the release of gas dissolved in the magma. Earthquakes also result from large-scale collapse such as landslides and from human activities such as commercial and military explosive detonations.
Large earthquakes almost always result when rocks which are elastically bent by movement along faults "snap back" or recover from undergoing long-term deformation (strain). Look at the avi file (reid.avi - - -not ready yet) which represents a hypothetical portion of the San Andreas fault. Notice the elastic strain or bending of the rocks between "earthquakes". After the 1906 San Francisco earthquake H.F. Reid was assigned, the task, among others, of investigating cause of earthquakes. One of the approaches he took was to compare the positions of triangulated benchmarks before and after the earthquake. After analysis of the data he found that benchmarks closest to the fault were displaced farther than more distant benchmarks. Benchmarks to the east of the fault were displaced to the south and benchmarks to the west of the fault were displaced to the north. He inferred that the rocks were being elastically bent or dragged along the fault as displacement occurred. He proposed the "Elastic Rebound Theory" to explain the origin of earthquakes.
Shadow Zones
A major advance in our understanding of the nature of the deep interior of the Earth was made in the 1930s by a consortium of U.S. and European universities. In a monumental undertaking of data analysis seismologists from these institutions began to recognize that fist generation earthquake waves from large earthquakes did not arrive in certain regions on the opposite side of the Earth. These regions were recognized to fall into belts at constant angular distances from the position of the earthquake and were called shadow zones. After detailed analysis two shadow zones were discovered.
If you've read about earthquakes, you know the stored elastic energy released by the earthquake travels thru the Earth as waves, similar to giant three dimensional pond ripples. Two kinds of waves are P waves and S waves (for Primary and Secondary). P waves because of the geometry of their vibrations can be propagated thru solids and fluids and are like sound waves. S waves on the other hand can be propagated only thru solids. Using this knowledge and their new knowledge about shadow zones seismologists recognized that part of the interior of the earth was fluid and based upon the size of the shadow zones they calculated that that fluid body- which they called the core- was about 3500 km is radius. The S waves were completely blocked by the fluid core and the P waves were bent or refracted causing the shadow zones. Later work showed that the very interior of the core is solid.
Look at the avi to see how the waves are either bent or completely blocked to form the shadow zones.